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\begin{document}

\title{The emerging importance of cross-ploidy hybridisation and introgression}



\author[1]{Max Brown}%
\author[2]{Richard J Abbott}%
\author[1]{Alex Twyford}%
\affil[1]{The University of Edinburgh School of Biological Sciences}%
\affil[2]{Affiliation not available}%


\vspace{-1em}



  
  \date{June 18, 2024}


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\selectlanguage{english}
\begin{abstract}
Natural hybridisation is now recognised as pervasive in its occurrence
across the Tree of Life. Resurgent interest in natural hybridisation
fuelled by developments in genomics has led to an improved understanding
of the genetic factors that promote or prevent species cross-mating.
Despite this body of work overturning many widely held assumptions about
the genetic barriers to hybridisation, it is still widely thought that
ploidy differences between species will be an absolute barrier to
hybridisation and introgression. Here, we revisit this assumption,
reviewing findings from surveys of polyploidy and natural hybrids in the
wild. In a case study in the British flora, 203 hybrids representing
35\% of hybrids with suitable data have formed via cross-ploidy matings,
while a wider literature search revealed 48 studies (45 in plants and 3
in animals), where cross-ploidy hybridisation has been confirmed with
genetic data. These results show cross-ploidy hybridisation is readily
overlooked, and potentially common in some groups. General findings from
these studies include strong directionality of hybridisation, with
introgression usually towards the higher ploidy parent, and cross-ploidy
hybridisation being more likely to involve allopolyploids than
autopolyploids. Evidence for adaptive introgression across a ploidy
barrier and cases of cross-ploidy hybrid speciation show the potential
for important evolutionary outcomes.%
\end{abstract}%



\sloppy


\textbf{The emerging importance of cross-ploidy hybridisation and
introgression}

\textbf{Authors:}

Max R. Brown\textsuperscript{1} , Richard J. Abbott\textsuperscript{2},
Alex D. Twyford\textsuperscript{1,3,4}

\textsuperscript{1} University of Edinburgh, Institute of Ecology and
Evolution, Charlotte Auerbach Road, Edinburgh, EH9 3FL, UK

\textsuperscript{2} School of Biology, University of St Andrews, St
Andrews, Fife, KY16 9TH, UK

\textsuperscript{3} Royal Botanical Garden Edinburgh, 20A Inverleith
Row, Edinburgh, EH3 5LR, UK

\textsuperscript{4} Author for correspondence (e-mail
Alex.Twyford@ed.ac.uk)

Contact emails: Max Brown: Max\_Brown1000@hotmail.co.uk, Richard Abbott:
rja@st-andrews.ac.uk, Alex Twyford: Alex.Twyford@ed.ac.uk

Running title: Cross-ploidy hybridisation



\textbf{Abstract}

Natural hybridisation is now recognised as pervasive in its occurrence
across the Tree of Life. Resurgent interest in natural hybridisation
fuelled by developments in genomics has led to an improved understanding
of the genetic factors that promote or prevent species cross-mating.
Despite this body of work overturning many widely held assumptions about
the genetic barriers to hybridisation, it is still widely thought that
ploidy differences between species will be an absolute barrier to
hybridisation and introgression. Here, we revisit this assumption,
reviewing findings from surveys of polyploidy and natural hybrids in the
wild. In a case study in the British flora, 203 hybrids representing
35\% of hybrids with suitable data have formed via cross-ploidy matings,
while a wider literature search revealed 48 studies (45 in plants and 3
in animals), where cross-ploidy hybridisation has been confirmed with
genetic data. These results show cross-ploidy hybridisation is readily
overlooked, and potentially common in some groups. General findings from
these studies include strong directionality of hybridisation, with
introgression usually towards the higher ploidy parent, and cross-ploidy
hybridisation being more likely to involve allopolyploids than
autopolyploids. Evidence for adaptive introgression across a ploidy
barrier and cases of cross-ploidy hybrid speciation show the potential
for important evolutionary outcomes.

\textbf{Keywords}

Hybridization, Polyploidy, Introgression, Interploidy, Hybrid species,
Hybrid zone



\textbf{Introduction}

Climate change, habitat disturbance, and large-scale translocations
resulting from human activities are increasing contacts between species
previously isolated by geographical and ecological barriers, thus
raising their potential to hybridise (Brennan et al., 2014; Crispo et
al, 2011; Larson et al., 2019). Closely related species isolated only by
prezygotic barriers are more likely to hybridise (Vallejo-Marin \&
Hiscock, 2016), but even species isolated by very strong postzygotic
barriers do hybridise in some instances. Polyploidy (whole genome
duplication), which is particularly common in plants, creates a very
strong postzygotic barrier between species. Cross-ploidy hybridisation
is therefore usually considered rare because hybrids will have
unbalanced chromosome content and therefore irregular pairing of
chromosomes, rendering the hybrid infertile (Ramsey \& Schemske, 1998).
Should a triploid hybrid form, it is normally either completely or
partially sterile, due to the formation of malfunctioning gametes
containing unbalanced chromosome numbers. On occasion, however, some
species differing in ploidy do produce hybrid offspring. The importance
of such events is not to be underestimated; for example, cross-ploidy
hybridisation has led to some very recently originated plant species,
which are now models for the study of polyploid speciation
(Vallejo-Marin \& Hiscock, 2016), and also to the origin of some of our
most important crop plants, including wheat, sweet potato, sugar cane
and oilseed rape (Matsuoka, 2011; Yang et al., 2017; Wang et al., 2023;
Zhang et al., 2018). Nonetheless, the frequency of cross-ploidy (or
interploidy) hybridisation in the wild is a neglected topic, with
information scattered through the literature. Here, we bring this
information together and consider its biological significance.

The first known artificial hybrid from crossing two parents of differing
ploidy level was created by K\selectlanguage{ngerman}ölreuter in 1761 between
diploid\emph{Nicotiana paniculata} and allotetraploid \emph{N.}
\emph{rustica.}This hybrid was known as the first ``botanical mule'' due
to its shrivelled anthers and malformed ovaries, indicative of sterility
(Roberts, 1929). Further artificial crosses demonstrated the formation
of other cross-ploidy hybrids that were partially or completely sterile,
but nothing was discovered of the frequency or importance of the
phenomenon in the wild until much later (Lawrence, 1936). Beginning
around the mid-twentieth century, cytogenetic studies became more
frequent and revealed extensive ploidy variation both within and between
species, and which could be used to explain evolutionary relationships
(Love \& Love, 1943; Stebbins, 1956). However, it was with the
availability of multiple nuclear markers in the 1990s that researchers
reliably detected hybridisation and introgression between species of
differing ploidy (Abbott et al., 1992; Nason et al., 1992). Now, by
examining many thousands of genetic markers or the complete genomes of
target species, there is potential to detect cases of adaptive
introgression (Suarez-Gonzalez et al., 2018). Moreover, through focusing
on specific genes, examples are now known of cross-ploidy introgression
resulting in the transfer of particular traits that markedly affect the
biology and fitness of recipient species (Baduel et al., 2018; Chapman
\& Abbott, 2010; Kim et al., 2008; Monnahan et al., 2019)

While there have been many recent reviews on evolutionary mechanisms the
prevalence of polyploids in nature (e.g. Alix et al., 2017; Chen, 2010;
Kohler et al., 2010; Marques et al., 2018; Soltis et al., 2004), and on
the importance of natural hybridisation (Abbott et al., 2013; Moran et
al., 2021; Soltis \& Soltis, 2009; Suarez-Gonzalez et al., 2018; Taylor
\& Larson, 2019; Todesco et al., 2016), our aim is to reconcile early
work on cytological variation with recent work on genomics, to consider
whether cross-ploidy hybridisation may be more prevalent and important
than previously known. We first summarise the ways in which cross-ploidy
hybrids may form. Next, we review the prevalence of cross-ploidy
hybridisation, both in the case of the British and Irish flora, which
includes comprehensive data on hybridisation and ploidy, and in the
wider published literature, allowing us to generalise about the
occurrence in nature. Lastly, we explore the biology of cross-ploidy
hybrids and the potential long-term evolutionary outcomes, and discuss
how advances in sequencing technologies and analytical tools may aid
detection to assess more accurately the state of cross-ploidy
hybridisation in nature. We emphasise case studies in flowering plants,
where hybridisation and polyploidy are particularly prevalent and
well-documented, but also consider other organismal groups where
cross-ploidy hybridisation may occur.

\textbf{Mechanisms of cross-ploidy hybrid formation and persistence}

There are two main pathways to creation of cross-ploidy hybrids; either
through reduced or unreduced gametes from parents of differing ploidy.
Reduced (``normal'') gametes of both parental species results in the
generation of a hybrid with intermediate ploidy. These hybrids, usually
triploids derived from diploid-tetraploid crosses, are common and found
in a variety of taxa where congeners co-occur (Sutkowska et al., 2017,
Popelka et al., 2019, De Hert et al., 2012, Irwin and Abbott, 1992). A
barrier to the creation of these hybrids through reduced gametes is
known under the umbrella term `triploid block' (Ramsey and Schemske,
1998: Kolá\selectlanguage{polish}ř \selectlanguage{english}et al., 2017). Early work on experimental
diploid-autopolyploid crosses established the presence of a triploid
block and that direction of crosses was important (Stebbins, 1971;
Thompson, 1930; Valentine \& Woodell, 1960). The major cause of triploid
block is attributed to genomic conflict in the maternal endosperm, which
is usually triploid and composed of a ratio of two maternal and one
paternal genomes (Lafon-Placette \& K\selectlanguage{ngerman}öhler, 2016). Deviations from this
ratio cause the endosperm to malfunction in development and function
(Kohler et al., 2010). Reciprocal crosses differ in their likelihood of
success, and it is a general phenomenon that crosses where the higher
ploidy parent is female are more likely to produce viable offspring, due
to endosperm ratios which are better tolerated (Burton \& Husband, 2000;
Figure 1a \& b). Triploid block may also be caused by the action of
allelic incompatibilities at an early stage in development, although
this topic is little explored (Scott et al., 2013). A second possibility
in the creation of cross-ploidy hybrids is where the lower ploidy parent
produces unreduced (``polyploid'') gametes. Unreduced gamete production
is on average 0.1-2\% per individual, with rare individuals and hybrids
that produce considerably higher frequencies (\textgreater{}85\%,
Kreiner et al., 2017a, 2017b; Mason \& Pires, 2015). In addition, many
different taxa produce unreduced gametes, and their production also
varies with environmental variables (Baduel et al., 2018, Rice et al.,
2019). Successful crosses occur more readily in diploid-tetraploid
crosses when unreduced gametes are produced by the diploid parent, thus
restoring the gamete ploidy to that of the higher ploidy parent (Figure
1c; Ramsey and Schemske, 1998).

\textbf{Hosted file}

\verb`image1.emf`  available at \url{https://authorea.com/users/696915/articles/685209-the-emerging-importance-of-cross-ploidy-hybridisation-and-introgression}

\textbf{Figure 1 -- Potential outcomes of hybridisation between diploid
and tetraploid species.} In each panel, the top two circles refer to the
parental species, the middle two ellipses to the gametes produced from
each parent, the bottom left box to the F1 hybrid and the bottom right
bold box to the endosperm. a and b consider hybridisation with reduced
gametes and therefore generate triploid hybrids, while c and d consider
hybridisation where one parent produces unreduced gametes. In
particular\textbf{,} c illustrates that a fertile polyploid can be
generated in a single generation. Figure generated with graphviz (Ellson
et al., 2002), with an interactive versions available at
https://observablehq.com/@euphrasiologist/cross-ploidy-hybridisation

After a cross-ploidy hybrid has formed there are a number of possible
outcomes. Firstly, the hybrid may be ephemeral and go extinct,
especially if it is formed at low frequencies (i.e. low propagule
pressure, Fowler \& Levin, 2016) and if it wastes reproductive output by
mating with its parents (i.e. minority cytotype disadvantage, Levin,
1975; Fowler \& Levin 1984). The growth and development of the hybrid
can be affected by bringing together incompatible parental allelic
combinations, causing the hybrid to be unfit (e.g. hybrid necrosis,
Bomblies \& Weigel, 2007), further reducing its likelihood of survival.
Alternatively, if the hybrid persists it may have the opportunity to act
as a conduit to gene flow between ploidy levels. Ultimately, fertility
of an F1 hybrid will be a major determinant of the longer time outcomes.
If the hybrid is fertile, it may facilitate gene flow between ploidy
levels through backcrossing with parental species, with even low levels
of outcrossing being of evolutionary significance. For a triploid F1
hybrid created from a diploid-tetraploid cross, there are two pathways
to generate a backcross of equivalent ploidy to one of the parental
species. Firstly, the triploid F1 may produce reduced pollen which
combines with reduced pollen from the diploid male parent (Figure 2a)
which has been hypothesised to occur in \emph{Euphrasia}
and\emph{Aconitum} (Sutkowska et al., 2017; Yeo, 1956). Secondly, the
triploid F1 hybrid can produce unreduced or balanced gametes that can
either combine with reduced gametes from the tetraploid parent or
unreduced gametes from the diploid parent (Figure 2b; e.g. \emph{Senecio
eboracensis} , \emph{Lowe} and Abbott, 2004). Tetraploids therefore are
much more readily produced, as in addition to the two pathways
mentioned, tetraploids can be produced in a single generation following
cross-ploidy hybridisation (Figure 1c). The bias towards tetraploid
production has been known since the 1950s (Stebbins, 1956) and is the
reason why introgression in the direction of the tetraploid is more
common (Baduel et al., 2018).

\textbf{Hosted file}

\verb`image2.emf`  available at \url{https://authorea.com/users/696915/articles/685209-the-emerging-importance-of-cross-ploidy-hybridisation-and-introgression}

\textbf{Figure 2 -- Potential outcomes of a triploid F1 backcrossing to
the parental species.} In both panels, the schematic follows that of
Figure 1a. Interrupted lines indicate backcrosses to parental species.
In a the triploid F1 hybrid produces reduced gametes that combine with
reduced gametes from the diploid male parent. In b there are two
pathways to produce a tetraploid F1 backcross: firstly the unreduced
gametes from the triploid F1 can combine with reduced gametes from the
female tetraploid parent, secondly the unreduced gametes from the
triploid F1 can combine with unreduced gametes from the diploid male
parent. Figure generated with graphviz (Ellson et al., 2002).

\textbf{Frequency and occurrence of natural cross-ploidy hybrids}

Of major interest is how common cross-ploidy hybrids are in nature given
the varied constraints of both pre and postzygotic isolation in their
generation. In cross-ploidy hybridisation the usual reproductive
barriers to cross species mating apply, such as differences in
geography, phenology, morphology and mating system (Kay, 2006; Laport et
al., 2016; Martin \& Willis, 2007), along with specific factors
associated with ploidy level difference between parental species, such
as the sterility barriers outlined above. The evidence required to prove
cross-ploidy hybridisation is confirmation of parental ploidy
differences, which may come from chromosome counts (Rice et al., 2015),
genome size estimates (Leitch, 2019) or genomic information
(Ranallo-Benavidez et al., 2020), and evidence of hybridisation, which
may be from genetic data or from other sources such as morphology
(Rieseberg \& Ellstrand, 1993; though see issues with using
morphological data to detect hybrids, below).

There are increasingly comprehensive surveys of ploidy variation that
provide key contextual information as to where cross ploidy hybrids
could occur and set an upper boundary in term of their number.
Worldwide, the majority of plant species are diploid (Rice et al.,
2019), however extensive variability in ploidy levels exist at all
taxonomic levels and scales (Kolar et al., 2017; Soltis et al., 2010).
For example, the genus \emph{Sedum} in the Crassulaceae ranges from
diploid to 80-ploid, which is currently the highest known amongst
flowering plants (Bennett \& Leitch, 1997). Many commonly studied
species also exhibit ploidy variation such as \emph{Senecio
carolinensis} (diploid to 9-ploid; Kolá\selectlanguage{polish}ř \selectlanguage{english}et al., 2017), and this
variation has even been correlated with latitudinal gradients (Zhang et
al., 2019). Climatic effects, which include latitude, along with clade
specific effects are known to have a role in unreduced gamete formation,
a key factor in polyploid genesis (Bretagnolle \& Thompson, 1995;
Kreiner et al., 2017a, Rice et al., 2019). In contrast to flowering
plants, polyploidy in animals and fungi is thought to be rare, famously
so in mammals and birds, though many examples are known in certain
lineages of amphibians, teleost fish and reptiles (Spoelhof et al.,
2020).

While there are extensive estimates of ploidy variation across the Tree
of Life, the frequency of cross-ploidy hybridisation remains unknown.
Our best general estimate of this in plants may come from the British
and Irish flora, which contains a manageable number of native species
(\textasciitilde{}1500, excluding large taxonomically complex groups,
Stace, 2019), and is exceptional in having near complete information on
species chromosome counts (Henniges et al., 2022), and the extent of
natural hybridity (Stace et al., 2015). This extensive dataset has
previously been used to estimate that 25\% of plant species in the flora
hybridise (Mallet, 2005), providing a general estimate for the frequency
of hybridisation across diverse plant genera. More recently, a study
employing phylogenetic mixed models showed that species that differ in
ploidy are 35\% less likely to form a hybrid (Brown et al., 2023),
though there are still numerous cross-ploidy hybrids, which highlights
that ploidy level is far from an absolute barrier to hybridisation.

To further quantify the potential for cross-ploidy hybridisation, we
revaluated the available data for the British flora. Ploidy level
inferred from available genome size and chromosome counts for flowering
plant species (n = 1295 species with data, Brown et al., 2023), shows
most species are diploids (56\%), with higher ploidy levels becoming
exponentially less common (Figure 3). However, there is notable
variation when ploidy level is evaluated across the phylogeny for the
British flora, with some families showing much more ploidy variation
than others, altering the raw material for cross-ploidy hybridisation to
act on (Supplementary Figure 1). In terms of the frequency of
cross-ploidy hybrids, we analysed hybrids and their parentage identified
in the `Hybrid Flora of the British Isles' (Stace et al., 2015), coupled
with ploidy level estimates (Brown et al., 2023), to quantify their
occurrence. Of the 588 hybrids that have ploidy information for both
parents (321 hybrids lack appropriate data), 203 cross-ploidy hybrids
have formed (35\%; Supplementary Table 1), in comparison to 385
intraploidy hybrids (65\%). Cross-ploidy hybrids occur in 67 genera,
with over a quarter present in \emph{Rumex} (Polygonaceae,
24),\emph{Salix} (Salicaceae, 19) and \emph{Euphrasia} (Orobanchaceae,
13; Figure 4). The majority (55\%) of cross-ploidy hybrids involve
diploid-tetraploid crosses, with higher order ploidy crosses closely
following (43\%), and diploid-triploid crosses in the minority (2\%).
These results show that cross-ploidy hybrids are relatively common in
the British flora, and are present in many different plant groups but
overrepresented in few.

\textbf{Hosted file}

\verb`image3.emf`  available at \url{https://authorea.com/users/696915/articles/685209-the-emerging-importance-of-cross-ploidy-hybridisation-and-introgression}

\textbf{Figure 3 -- Distribution of ploidy levels across the British and
Irish flora.} Shown are the number of species at each ploidy level which
are not known to have multiple cytotypes. Odd ploidies are less frequent
than even ploidies, resulting in a `saw tooth' pattern. The most highly
polyploid species is \emph{Leucanthemum maximum} at 22-ploid.

Inferring hybridisation from morphology, geography, cytology and limited
genetic data, as is the case with many hybrids in the British and Irish
flora, overlooks cryptic hybridisation and introgression that can be
detected with multiple nuclear genetic markers. Moreover, the extent of
cross-ploidy hybridisation in this flora is likely to be affected by
extensive habitat disturbance and recent postglacial divergence. A wider
survey of published studies of hybridisation based on multiple genetic
markers or strong cytogenetic evidence (criteria in Table S2), reveals
48 different parental species combinations from 58 studies resulting in
cross-ploidy hybridisation, with such hybrids present in 35 genera from
16 angiosperm families, three fern families, and three animal families
(Table 1). Diploid-tetraploid crosses are found in 35 of the 48 parental
crosses, with the rest being higher ploidy crosses. This confirms that
cross-ploidy hybridisation is likely to be much more common than is
currently appreciated.

The taxonomic spread of cross-ploidy hybridisation is especially broad
in angiosperms, as evidenced by data both from the British and Irish
flora and the wider literature. For example, monocots are well
represented (Liliaceae, Orchidaceae, Poaceae), as are basal eudicots
(Ranunculaceae, Papaveraceae) and throughout the rest of the
phylogenetic tree scattered in the Fabids, Malvids and Superastrids.
This distribution indicates cross-ploidy hybridisation is very
widespread and potentially abundant throughout the flowering plant
phylogeny (Figure 4). On the other hand, the conspicuous absence of
records from large, diverse families with variable ploidy, such as
Rubiaceae, potentially indicate a phylogenetic skew in cross-ploidy
hybridisation. Cases of such hybridisation are not just phylogenetically
but also geographically widespread, with examples reported from across
four continents, though tropical regions are poorly represented and most
studies report hybridisation in large temperate or cosmopolitan plant
families (e.g. Asteraceae and Orchidaceae; see Schley et al. 2022 for
discussion of overlooked polyploidy and hybridisation in the tropics).
In terms of life form, most well-documented cross-ploidy hybrids (with
the notable exception of \emph{Euphrasia} ) are perennial, a factor
which is associated with hybridisation regardless of parental ploidy
level (Mitchell et al., 2019).

\textbf{Hosted file}

\verb`image4.emf`  available at \url{https://authorea.com/users/696915/articles/685209-the-emerging-importance-of-cross-ploidy-hybridisation-and-introgression}

\textbf{Figure 4 -- Distribution of cross-ploidy hybrids across the
British and Irish flora.} The number of cross-ploidy (dark bar) and
intra-ploidy (light bar) hybrids are shown per family, in the context of
phylogenetic relationships inferred from the plastid barcoding
loci\emph{matK} and \emph{rbcL} (Jones et al. 2021). Red family names
highlight those families which contain five or more different ploidy
levels. Numbers in parentheses are the number of cross-ploidy hybrids
formed per family. The phylogeny is based on Brown et al. (2023); here
we subsample to one tip per family, and force the tree to be ultrametric
for visualisation.

In animal groups where diploids and polyploids are both present there
may be cross-ploidy hybridisation and subsequent introgression, though
based on the published literature this is very uncommon, with only three
well-studied examples (Table 1). In many other cases where taxa with
contrasting ploidies mate introgression is limited, as the hybrid
derivatives are hybridogenetic taxa which lack recombination. For
example, the edible frog \emph{Pelophylax esculentus} is an extremely
ecologically successful and widespread hybrid species formed between the
diploid taxa \emph{P. ridibundus} and \emph{P. lessonae.} It includes
two cytotypes, a diploid and a triploid, with the triploid formed and
maintained by haploid sperm fertilising unreduced eggs from a diploid
hybrid female (Hoffmann et al., 2015). However, it appears to be in a
state of flux, with no documented all-triploid populations, and
tetraploids are extremely rare. Opportunities for novel allelic
combinations and introgression are limited as the parental genomes
rarely recombine.

\textbf{Table 1 --} Studies reporting cross-ploidy hybrids based on
cytological and/or molecular genetic analyses. Details are provided of
the family, hybridising species, broad geographic locality, and the
direction of introgression (if known). Superscripts indicate whether the
polyploids are allopolyploid (\textsuperscript{allo}) or autopolyploid
(\textsuperscript{auto}). Note that the ploidy refers to evidence of
cross-ploidy hybridisation based on material presented in the specific
study; other ploidy levels may also be found for these species. Criteria
for inclusion are listed in Table S2.\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
\textbf{Family} & \textbf{Hybridising species} & \textbf{Location} &
\textbf{Direction to} & \textbf{Reference}\tabularnewline
\midrule
\endhead
Animals & Animals & Animals & Animals & Animals\tabularnewline
Bufonidae & \emph{Bufo turanensis} (2n = 2x = 22) X \emph{Bufo pewzowi}
(2n = 4x = 44)\textsuperscript{allo} & Kyrgyzstan & Diploid & St\selectlanguage{ngerman}öck et
al. (2010)\tabularnewline
Cyprinidae & \emph{Squalius alburnoides} (2n = 2x = 50; 3n = 75; 4n =
100) X \emph{S. pyrenaicus} (2n = 2x = 50)\textsuperscript{allo} &
Iberia & - & Alves et al. (2001); Crespo-L\selectlanguage{ngerman}ópez et al.
(2007)\tabularnewline
Myobatrachidae & \emph{Neobatrachus sutor} (2n = 2x = 24) x \emph{N.
kunapalari} (2n = 4x = 48)\textsuperscript{auto} & Australia &
Tetraploid & Novikova et al. (2020)\tabularnewline
Plants & Plants & Plants & Plants & Plants\tabularnewline
Aspleniaceae & \emph{Asplenium scolopendrium} (2n = 2x = 72) x \emph{A.
adiantum-nigrum} (2n = 4x = 144) & Britain & - & Stace et al.
(2015)\tabularnewline
Cyatheaceae & \emph{Gymnosphaera denticulata} (2n = 2x = 138) x \emph{G.
metteniana} (2n = 4x = 274)\textsuperscript{allo} & China & Tetraploid &
Wang et al. (2020)\tabularnewline
Dryopteridaceae & \emph{Polystichum setiferum} (2n = 2x = 82) x \emph{P.
aculeatum} (2n = 4x = 164) & Britain & - & Manton (1950)\tabularnewline
Asteraceae & \emph{Achillea clype}olata (2n = 2x = 18) x \emph{A.
collina} (2n = 4x = 36)\textsuperscript{allo} & Bulgaria & Tetraploid &
Guo et al. (2005)\tabularnewline
Asteraceae & \emph{Achillea setacea} (2n = 2x = 18) x \emph{Achillea
collina} (2n = 4x = 36) & & & Ma et al. (2010)\tabularnewline
Asteraceae & \emph{Achillea asplenifolia} (2n = 2x = 18) x
\emph{Achillea collina} (2n = 4x = 36) & & & Ma et al.
(2010)\tabularnewline
Asteraceae & \emph{Centaurea pseudophrygia} (2n = 2x = 22) x \emph{C.
jacea} (2n = 4x = 44) & Czech Republic & - & Koutecky et al.
(2011)\tabularnewline
Asteraceae & \emph{Chyrsanthemum indicum} (2n = 4x = 36) and \emph{C.
vestitum} (2n = 6x = 54) & China & Both & Qi et al.
(2022)\tabularnewline
Asteraceae & \emph{Cirsium carniolicum} ssp. \emph{rufescens} (2n = 2x =
16) x \emph{C. palustre} (2n = 4x = 34) & France & Tetraploid &
Segarra-Moragues et al. (2007)\tabularnewline
Asteraceae & \emph{Ixeris repens} (2n = 2x = 16) x \emph{I. debilis} (2n
= 6x = 48)\textsuperscript{auto} & Japan & Hexaploid(?) & Denda \&
Yokota (2003)\tabularnewline
Asteraceae & \emph{Packera paupercula} (2n = 4x = 44) x \emph{P.
indecora} (2n = 8x = 88) & USA; Michigan & - & Kowal et al.
(2011)\tabularnewline
Asteraceae & \emph{Senecio madagascariensis} (2n = 2x = ?) x \emph{S.
pinnatifolius} (2n = 4x = ?) & Australia & - & Prentis et al.
(2007)\tabularnewline
Asteraceae & \emph{Senecio squalidus} (2n = 2x = 20) x \emph{S.
vulgaris} (2n = 4x = 40)\textsuperscript{allo} & Britain & Tetraploid;
chromosome doubling & Abbott et al. (2007); Irwin \& Abbott (1992);
Abbott et al. (1992); Chapman \& Abbott (2010); Abbott \& Lowe,
(2004)\tabularnewline
Betulaceae & \emph{Betula nana} (2n = 2x = 28) x \emph{B. pubescens} (2n
= 4x = 56)\textsuperscript{allo} & Britain & Tetraploid; both(?) & Wang
et al. (2014); Thorsson et al. (2007); Palme et al.
(2004)\tabularnewline
Betulaceae & \emph{Betula pendula} (2n = 2x = 28) x \emph{B. pubescens}
(2n = 4x = 56)\textsuperscript{allo} & Britain & Tetraploid & Zohren et
al. (2016)\tabularnewline
Betulaceae & \emph{Betula} \selectlanguage{ngerman}× \emph{purpusii} (2n = 5x = 70) x \emph{B.
alleghaniensis} (2n = 6x = 84)\textsuperscript{allo} & Michigan; USA &
Hexaploid & Barnes \& Dancik (1985)\tabularnewline
Brassicaceae & \emph{Cardamine apennina} (2n =2x = 16) x \emph{C.}
\emph{amporitana} (2n = 4x = 32) & Italy & Tetraploid & Lihova et al.
(2004)\tabularnewline
Brassicaceae & \emph{Cardamine} \selectlanguage{ngerman}× \emph{insueta} (2n = 3x = 24) x
\emph{C. pratensis} (2n = 4x = 32) & Switzerland & - & Mandakova et al.
(2013)\tabularnewline
Brassicaceae & \emph{Cochlearia officinalis} (2n = 4x = 24) x \emph{C.
danica} (2n = 6x = 42) & Britain & Tetraploid & Fearn
(1977)\tabularnewline
Brassicaceae & \emph{Draba incana} (2n = 4x = 32) x \emph{D. norvegica}
(2n = 6x = 48)\textsuperscript{allo} & Scandinavia & - & Brochmann et
al. (1992)\tabularnewline
Brassicaceae & \emph{Draba nivalis} (2n = 2x = 16) x \emph{D. daurica}
(2n = 8x = 64) & Scandinavia & - & Brochmann et al.
(1992)\tabularnewline
Brassicaceae & \emph{Draba arctica} (2n = 10x = 80) x \emph{D.
corymbosa} (2n = 16x = 128)\textsuperscript{allo} & Scandinavia & - &
Brochmann et al. (1992)\tabularnewline
Brassicaceae & \emph{Rorippa austraica} (2n = 2x = 16) x \emph{R.
sylvestris} (2n = 4x/6x = 32/48) & Germany & Both & Bleeker (2003), see
also Bleeker (2007)\tabularnewline
Fabaceae & \emph{Lotus stepposus} (2n = 2x = 12) x \selectlanguage{ngerman}\emph{L. ×
ucrainicus} (2n = 4x =24)\textsuperscript{allo} & Ukraine, Turkmenistan,
Kazakhstan, Mongolia & - & Kramina et al. (2018)\tabularnewline
Liliaceae & \emph{Erythronium mesochoreum} (2n = 2x = 22) x \emph{E.
albidum} (2n = 4x = 44) & Nebraska; USA & - & Roccaforte et al.
(2015)\tabularnewline
Orchidaceae & \emph{Dactylorhiza fuchsii} (2n = 2x = 40) x \emph{D.
praetermissa} (2n = 4x = 80)\textsuperscript{allo} & Belgium & - & De
Hert et al. (2012)\tabularnewline
Orchidaceae & Dactylorhiza incarnata (2n = 2x = 40) x \emph{D.
praetermissa} (2n = 4x = 80)\textsuperscript{allo} & Belgium & - & De
Hert et al. (2012); De Hert et al. (2011)\tabularnewline
Orchidaceae & \emph{Dactylorhiza incarnata} subsp. \emph{cruenta} (2n =
2x = 40) x \emph{D. lapponica} (2n = 4x = 80)\textsuperscript{allo} &
Norway & Tetraploid & Aagaard et al. (2005)\tabularnewline
Orchidaceae & \emph{Dactylorhiza incarnata} (2n = 2x = 40) x \emph{D.}
\emph{traunsteineri (}2n = 4x = 80)\textsuperscript{allo} & Sweden &
Tetraploid & Hedren (2003); see also Balao et al. (2017)\tabularnewline
Orchidaceae & \emph{Dactylorhiza fuchsii} (2n = 2x = 40) x \emph{D.
maculata} (2n = 4x = 80)\textsuperscript{auto} & Europe to Caucasus & -
& Shipunov et al. (2004)\tabularnewline
Orchidaceae & \emph{Epidendrum fulgens} (2n = 2x = 24) x \emph{E.
puniceoluteum} (2n = 4x = 52) & Brazil & Tetraploid & Pinheiro et al.
(2010)\tabularnewline
Orobanchaceae & \emph{Euphrasia anglica} (2n = 2x = 22) x \emph{E.
micrantha} (2n = 4x = 44)\textsuperscript{allo} & Britain & Diploid(?) &
Yeo (1956); French et al. (2008)\tabularnewline
Phrymaceae & \emph{Mimulus guttatus} (2n = 2x = 28) x \emph{M. luteus}
(2n = 4x = 60-2)\textsuperscript{allo} & Britain & Chromosome doubling &
Vallejo-Marin (2012)\tabularnewline
Plantaginaceae & \emph{Callitriche cophocarpa} (2n = 2x = 10) x
\emph{C.} \emph{platycarpa} (2n = 4x = 20)\textsuperscript{allo} &
Europe & - & Prancl et al. (2014)\tabularnewline
Poaceae & \emph{Miscanthus sacchariflorus (2n = 4x = 76)}
\textsuperscript{auto} \emph{x M. sinensis (2n = 2x = 38)} & Korea and
Japan & Tetraploid & Clark et al. (2019)\tabularnewline
Poaceae & \emph{Vulpia fasciculata} (2n = 4x = 28) x \emph{Festuca
rubra} (2n = 6x = 42) & Britain & Hexaploid(?) & Bailey et al.
(1993)\tabularnewline
Polygalaceae & \emph{Polygala calcarea} (2n = 2x = 34) x \emph{P.
vulgaris} (2n = 4x = 68) & Britain & Tetraploid & Lack
(1995)\tabularnewline
Polygonaceae & \emph{Fallopica sachaliensis} (2n = 4x = 44) x \emph{F.}
\emph{japonica} var \emph{japonica} (2n = 8x = 88) & Britain & - &
Bailey (2013); see also Bailey \& Wisskirchen, (2004) and Hollingsworth
et al. (1999)\tabularnewline
Polygonaceae & \emph{Rumex obtusifolius} (2n = 4x = 40) x \emph{R.
aquaticus} (2n = 20x = 200) & Britain & 20-ploid & Ruhsam et al.
(2015)\tabularnewline
Primulaceae & \emph{Dodecatheon frenchii} (2n = 2x = 44) x \emph{D.
meadia} (2n = 4x = 88) & Illinois; USA & Tetraploid & Oberle et al.
(2012)\tabularnewline
Rannunculaceae & \emph{Aconitum variegatum} (2n = 2x = 16) x \emph{A.
firmum} (2n = 4x = 32)\textsuperscript{allo} & Europe & Diploid? &
Sutkowska et al. (2017)\tabularnewline
Rannunculaceae & \emph{Ficaria calthifolia} (2n = 2x = 16) x \emph{F.
verna} subsp. \emph{verna} (2n = 4x = 32) & Europe & - & Popelka et al.
(2019)\tabularnewline
Rosaceae & \emph{Rosa rugosa} (2n = 2x = 14) x \emph{R. mollis} (2n = 4x
= 28) & Europe & Tetraploid & Kellner et al. (2012)\tabularnewline
Violaceae & \emph{Viola reichenbachiana} (2n = 2x = 20) x \emph{V.}
\emph{riviniana} (2n = 4x = 40)\textsuperscript{allo} & Germany & - &
Neuffer et al. (1999); see also Migdalek et al. (2017)\tabularnewline
Violaceae & \emph{Viola epipsila (2n = 4x = 24) x V. palustris (2n = 8x
= 48)}\textsuperscript{allo} & Poland & Putative F1s dominate & \selectlanguage{polish}Ż\selectlanguage{english}abicka
et al. (2020)\tabularnewline
\bottomrule
\end{longtable}

\textbf{Factors affecting cross-ploidy hybridisation and introgression}

Cross-ploidy hybrids can arise in a variety of situations. Many, but not
all, examples occur in contact zones between parental species with
contrasting ploidy, where hybrid zones and hybrid swarms may form. Some
of these hybrid zones have shifted over time (e.g. \emph{Betula} , Wang
et al., 2014), or are mosaic in structure (Popelka et al., 2019). In
addition, there are notable differences in genetic structure between
contact zones, with some comprising a swarm of F1, F2 and backcrossed
hybrids (Fearn, 1977), indicative of low genetic divergence between
parental species (Edmands, 2002), while others contain only a few early
generation hybrids, suggesting that parental species are more distantly
related, and show higher levels of reproductive isolation (Koutecky et
al., 2011). Hybrids may also occur in the absence of one or both
parents, normally where greater lifespans allow persistence long after
hybrid formation (Bailey, 2013; Preston \& Pearman, 2015). Where
cross-ploidy hybrids are present without their parents, they may
represent stable lineages that survive through asexual reproduction
(e.g. vegetative reproduction or apomixis), and are therefore different
to some ephemeral forms present in hybrid zones.

The direction of introgression in cross-ploidy hybrids is overwhelmingly
towards the higher ploidy parent (Table 1). This is unsurprising as the
union of an unreduced 2\emph{n} = 2\emph{x} gamete of a diploid and a
reduced n = 2\emph{x} gamete of a tetraploid provides a direct pathway
for introgression in this direction, whereas the alternative direction
is a two-step process via the triploid bridge (Baduel et al., 2018;
Stebbins, 1971). As such, only two plant studies and one animal study
report the opposite scenario (\emph{Aconitum} and \emph{Euphrasia}
,\emph{Neobatrachus} ; Sutkowska et al., 2017; Yeo, 1956), and a further
three studies report bidirectional introgression (in \emph{Betula,
Rorippa} , and \emph{Chrysanthemum} , Bleeker, 2003; Qi et al., 2022;
Thorsson et al., 2007). However, other factors may still pose limits for
introgression in the direction of the higher ploidy parent. Polyploids
evolve meiotic stability to ensure reliable segregation of additional
chromosomes at meiosis, with loci underlying tetraploid meiotic
stability shown to be under selection in natural populations of
autotetraploid \emph{Arabidopsis arenosa} (Hollister et al., 2012).
Cytogenetic evidence in \emph{Arabidopsis} suggests introgression from
diploids to tetraploids may introduce genetic variants that disrupt
regular meiosis in tetraploids (Morgan et al., 2020).

A key determinant of the outcomes of cross-ploidy hybridisation is the
ploidy of the parents, and the mode of ploidy (whether the parents are
auto or allopolyploids). In terms of ploidy, it is clear that successful
cross-ploidy hybridisation may occur more frequently between cytotypes
of higher ploidy (e.g. tetraploids and hexaploids) than of lower ploidy
(e.g. diploids and tetraploids, H\selectlanguage{ngerman}ülber et al., 2015; Sutherland et al.,
2020). However, despite the apparent weakening of postzygotic barriers
at higher ploidy levels, prezygotic barriers may be strong enough for
such cross-ploidy hybridization to remain relatively rare (Hülber et
al., 2015). In terms of mode of ploidy, in allotetraploid parents
characterised by disomic inheritance, preferential chromosome pairing
between the most similar, homeologous subgenomes, may lead to a subset
of polyploid variation introgressing. In contrast, in autotetraploids
with tetrasomic inheritance, free recombination between chromosomes may
allow any region of the tetraploid to introgress. According to our
literature survey, in 21 of 25 studies for which relevant information is
available the higher ploidy parent was an allopolyploid. While
allopolyploids garner more research interest than autopolyploids in
studies of hybridisation (Spoelhof et al., 2017), the higher number of
studies reporting allopolyploids may be biologically significant. For
example, chromosome pairing of an allotetraploid subgenome more related
to the diploid parent could lead to higher probabilities of successful
hybridisation than in diploid-autotetraploid hybridisation, where
chromosome pairing is disrupted.

\textbf{Evolutionary outcomes of cross-ploidy hybridisation}

Recent cross-ploidy hybridisation has led to notable examples of
speciation (\textless{}200 years). This has occurred in the plant
genera\emph{Senecio} (Abbott \& Lowe, 2004; Lowe \& Abbott, 2004)
and\emph{Mimulus} (Vallejo-Marin, 2012). These hybrids are also notable
in the context of the British Isles, as they involve alien species as
either one, or both parental species. Similarly, in \emph{Rosa} one of
the parents, while in \emph{Fallopia} both parents, involved in
cross-ploidy hybridisation are alien species (Table 1). Human mediated
translocations of species clearly have a profound effect on cross-ploidy
hybridisation. Older hybrid species (10,000+ years) have also originated
in a similar way to \emph{Senecio} and \emph{Mimulus} hybrid species,
with this inferred either through morphology and cytogenetic analysis,
or through sequence analysis showing `ghost' subgenomes of allopolyploid
species (e.g. \emph{Euphrasia} and \emph{Packera,} Kowal et al., 2011;
Yeo, 1956).

For a hybrid lineage to persist, reproductive isolation between the
newly formed hybrid and the parental progenitors is paramount. Unlike
cases of polyploid hybrid speciation where the hybrid is of differing
ploidy level to both parents, backcrossed F1 hybrids derived from
cross-ploidy hybridisation will match one parental ploidy and therefore
lack the strong reproductive barrier that polyploidy confers. In this
case, other factors must contribute to reproductive isolation, including
ecological selection, niche differentiation, selfing, and chromosomomal
or genetic sterility barriers (Grant, 1981; Gross \& Rieseberg, 2005;
Lowe \& Abbott, 2004; Rieseberg, 1997). Lastly, reproductive isolation
of a cross-ploidy hybrid can occur by the doubling of the triploid F1
chromosome complement to produce a fertile hexaploid that is isolated by
ploidy level from the parental species, as with \emph{Senecio
cambrensis} (Abbott and Lowe, 2004) and \emph{Mimulus
peregrinus}(Vallejo-Marin, 2012).

In addition to cross-ploidy hybridisation between species, much early
work, both theoretical and empirical, has explored crosses within
mixed-ploidy species complexes (Fowler \& Levin, 1984; Levin, 1975;
Lumaret \& Barrientos, 1990). The outcomes of crosses within (diploid x
autopolyploid) or between species (diploid x
autopolyploid/allopolyploid) are similar in many cases; with triploid
hybrids still formed (De Hert et al., 2012; Vandijk et al., 1992),
unreduced gametes remaining an important driver of hybridisation (Baduel
et al., 2018; Lihova et al., 2004), and the direction of introgression
usually being towards the higher ploidy parent (Table 1; Pinheiro et
al., 2010; Stebbins, 1956). On the other hand, between species
hybridisation can lead to higher levels of genetic variation through
fixed heterozygosity in hybrids, and backcrossing to parental species,
resulting in higher fitness (Ramsey \& Schemske, 2002). In addition, the
higher the divergence between species, the higher the likelihood of
whole genome duplication post hybridisation, and therefore the
generation of novel polyploid species (Paun et al., 2009).

More than 60 years ago, Stebbins (1956) proposed that within polyploid
complexes a widespread tetraploid could acquire genes via unilateral
introgression from ecogeographicaly isolated diploid taxa occurring
sympatrically with it in different parts of its range. In this way,
several different forms of a tetraploid might originate, with each one
bearing a close resemblance to the local diploid it hybridised with.
Based on cytotaxonomic evidence, Stebbins (1956, 1971) suggested this
has occurred in numerous polyploid complexes of a number of plant
genera, including \emph{Dactylis} , \emph{Knautia} , \emph{Grindelia}
,\emph{Phacelia} and \emph{Campanula} . Recently, genomic evidence has
been obtained to provide support for Stebbins' proposal from work
conducted on a polyploid complex comprising diploid and tetraploid forms
of \emph{Arabidopsis arenosa} in Europe (Arnold et al., 2015). Genomic
analysis indicates that autotetraploid \emph{A. arenosa} arose once
before splitting into five major lineages as it spread into different
parts of Central Europe (Arnold et al., 2015). For two of the lineages,
there is evidence that particular haplotypes, not found in any other
tetraploid lineage, are shared with proximal diploid forms of \emph{A.
arenosa} , indicating these haplotypes were acquired from the local
diploid type and are adaptive (Arnold et al., 2015). In addition, one of
the five tetraploid lineages is a ruderal form, widely distributed along
the railways of Central and Northern Europe. Subsequent analysis
indicates that the widespread lowland form of this early flowering and
rapid cycling ``railroad ecotype'' likely originated as a result of
introgression of genes from diploid \emph{A. arenosa} occurring on the
Baltic Coast of Germany and Poland into local populations of the
tetraploid (Baduel et al. 2018a; Monnahan et al. 2019).

\textbf{Future perspectives}

While cross-ploidy hybridisation is likely more common than once
thought, particularly in plants, there is still much uncertainty in our
understanding of the phenomenon. Key priorities should be to broaden the
taxonomic scope of study to understand the frequency of cross-ploidy
hybridisation across the Tree of Life and to reveal potential factors
that may promote or prevent it, and to employ new genomic sequencing and
analytical approaches to investigate the genomic basis of this
phenomenon.

In terms of establishing the frequency of cross-ploidy hybridisation,
there is currently a dearth of information on animal examples, even
though polyploid incidence can be high in some groups (e.g. insects,
decapods, fish, and amphibians, Otto \& Whitton, 2000). Further, while
we found many angiosperm examples, nearly half (17 out of 42) were
derived from the large families Asteraceae and Orchidaceae. A broader
scope will determine whether there is a phylogenetic signal to the
phenomenon, and which attributes, including ecological to genetic
factors, facilitate cross-ploidy hybridisation and introgression.

In terms of studying the genomics of cross-ploidy hybridisation, this
will allow us to more accurately understand the population dynamics of
cross-ploidy hybrid zones (Zohren et al., 2016), as well as precisely
determine parental genomic contributions to cross-ploidy hybrids and
hybrid species (Bertioli et al., 2016). The latter point is particularly
important, as hybrids may be introgressed at only a few loci in the
genome. Detecting these few loci requires a high contiguity polyploid
genome assembly, preferably with phase information, and new and emerging
sequencing methods such as long-read sequencing are beginning to address
these problems (Zhang et al., 2019). In addition, sequencing of diploid
relatives, and the application of more advanced approaches for
separating the two subgenomes, such as those based on characteristic
profile of different repeat content and transposable elements from each
parental progenitor (e.g. Cerca et al., 2022), will be instrumental for
understanding which subgenomes introgress.

Further genomic sequencing, aided by experimental work, will also
provide more detailed insights into the genetic mechanisms that allow
chromosomes to pair in newly formed polyploid hybrids (Morgan et al.,
2020), which is important in establishment and persistence of hybrids.
For example, in wheat (\emph{Triticum aestivum} )---an allohexaploid
composed of 3 related genomes---a single locus, \emph{Ph1} , enforces
strict homologous bivalent pairing. Suppression of \emph{Ph1} allows
meiotic pairing of homoeologous chromosomes, facilitating recombination
and introgression (Li et al., 2017). Whether similar mechanisms occur in
other polyploid systems remains largely uncharacterised.

Given the extensive ploidy variation throughout plants and animals, and
the high degree of hybridisation detected in these groups, cross-ploidy
hybridisation may be more important in plant and animal evolution than
is currently recognised.\emph{}

\textbf{Data Accessibility and Benefit-Sharing} : No new data are
included.

\textbf{Funding} : This work was developed from the doctoral
dissertation of M.R.B. published in 2020, which was supported by a
scholarship from the Biotechnology and Biological Sciences Research
Council (grant number BB/M010996/1). A.D.T. was supported by the Natural
Environment Research Council (grant number NE/L011336/1). The Royal
Botanic Garden Edinburgh acknowledges funding from the Scottish
Government's Rural and Environment Science and Analytical Services
Division.

\textbf{Conflict of interest disclosure} : No conflict to declare.

\textbf{Acknowledgements:} We thank Simon Martin, Duncan Cameron and
Anthony Hall for useful discussions.

\textbf{Author contributions:} All authors contributed to the writing of
the manuscript. \emph{}

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\textbf{Supplementary material}

\textbf{Table S1 -- Parental combinations of British flowering plant
species which are of different ploidy level and have produced putative
hybrids} . Hybrid combinations extracted from the `Hybrid Flora of the
British Isles' (Stace, 2015) and presented using the same taxonomic
system and in the same order, with ploidy and chromosome number compiled
in Brown et al (2023).

\textbf{Table S2 -- Search strings to find examples of cross-ploidy
hybridisation.} Searches were performed in Google Scholar. Examples from
other sources were added if they were deemed to be important and/or well
known.

\textbf{Figure S1 -- The distribution of ploidy levels across the
British and Irish flora between species in the four families with the
highest number of species.} Shown are Rosaceae, Poaceae, Asteraceae and
Fabaceae. Each family has distinct distributions of ploidy levels.

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