The emerging importance of cross-ploidy hybridisation and introgression.

Natural hybridisation is now recognised as pervasive in its occurrence across the Tree of Life. Resurgent interest in natural hybridisation fuelled by developments in genomics has led to an improved understanding of the genetic factors that promote or prevent species cross-mating. Despite this body of work overturning many widely held assumptions about the genetic barriers to hybridisation, it is still widely thought that ploidy differences between species will be an absolute barrier to hybridisation and introgression. Here, we revisit this assumption, reviewing findings from surveys of polyploidy and hybridisation in the wild. In a case study in the British flora, 203 hybrids representing 35% of hybrids with suitable data have formed via cross-ploidy matings, while a wider literature search revealed 59 studies (56 in plants and 3 in animals) in which cross-ploidy hybridisation has been confirmed with genetic data. These results show cross-ploidy hybridisation is readily overlooked, and potentially common in some groups. General findings from these studies include strong directionality of hybridisation, with introgression usually towards the higher ploidy parent, and cross-ploidy hybridisation being more likely to involve allopolyploids than autopolyploids. Evidence for adaptive introgression across a ploidy barrier and cases of cross-ploidy hybrid speciation shows the potential for important evolutionary outcomes.