Figure 4 – Distribution of cross-ploidy hybrids across the British and Irish flora. The number of cross-ploidy (dark bar) and intra-ploidy (light bar) hybrids are shown per family, in the context of phylogenetic relationships inferred from the plastid barcoding locimatK and rbcL (Jones et al. 2021). Red family names highlight those families which contain five or more different ploidy levels. Numbers in parentheses are the number of cross-ploidy hybrids formed per family. The phylogeny is based on Brown et al. (2023); here we subsample to one tip per family, and force the tree to be ultrametric for visualisation.
In animal groups where diploids and polyploids are both present there may be cross-ploidy hybridisation and subsequent introgression, though based on the published literature this is very uncommon, with only three well-studied examples (Table 1). In many other cases where taxa with contrasting ploidies mate introgression is limited, as the hybrid derivatives are hybridogenetic taxa which lack recombination. For example, the edible frog Pelophylax esculentus is an extremely ecologically successful and widespread hybrid species formed between the diploid taxa P. ridibundus and P. lessonae. It includes two cytotypes, a diploid and a triploid, with the triploid formed and maintained by haploid sperm fertilising unreduced eggs from a diploid hybrid female (Hoffmann et al., 2015). However, it appears to be in a state of flux, with no documented all-triploid populations, and tetraploids are extremely rare. Opportunities for novel allelic combinations and introgression are limited as the parental genomes rarely recombine.
Table 1 – Studies reporting cross-ploidy hybrids based on cytological and/or molecular genetic analyses. Details are provided of the family, hybridising species, broad geographic locality, and the direction of introgression (if known). Superscripts indicate whether the polyploids are allopolyploid (allo) or autopolyploid (auto). Note that the ploidy refers to evidence of cross-ploidy hybridisation based on material presented in the specific study; other ploidy levels may also be found for these species. Criteria for inclusion are listed in Table S2.