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: Kolář 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 & Köhler, 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 (>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).