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).