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.