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).