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 (Triticum aestivum )—an allohexaploid
composed of 3 related genomes—a single locus, Ph1 , enforces
strict homologous bivalent pairing. Suppression of 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.
Data Accessibility and Benefit-Sharing : No new data are
included.
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.
Conflict of interest disclosure : No conflict to declare.
Acknowledgements: We thank Simon Martin, Duncan Cameron and
Anthony Hall for useful discussions.
Author contributions: All authors contributed to the writing of
the manuscript.