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, 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 Euphrasia andAconitum (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. Senecio
eboracensis , 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).