Figure 3 – Distribution of ploidy levels across the British and Irish flora. Shown are the number of species at each ploidy level which are not known to have multiple cytotypes. Odd ploidies are less frequent than even ploidies, resulting in a ‘saw tooth’ pattern. The most highly polyploid species is Leucanthemum maximum at 22-ploid.
Inferring hybridisation from morphology, geography, cytology and limited genetic data, as is the case with many hybrids in the British and Irish flora, overlooks cryptic hybridisation and introgression that can be detected with multiple nuclear genetic markers. Moreover, the extent of cross-ploidy hybridisation in this flora is likely to be affected by extensive habitat disturbance and recent postglacial divergence. A wider survey of published studies of hybridisation based on multiple genetic markers or strong cytogenetic evidence (criteria in Table S2), reveals 48 different parental species combinations from 58 studies resulting in cross-ploidy hybridisation, with such hybrids present in 35 genera from 16 angiosperm families, three fern families, and three animal families (Table 1). Diploid-tetraploid crosses are found in 35 of the 48 parental crosses, with the rest being higher ploidy crosses. This confirms that cross-ploidy hybridisation is likely to be much more common than is currently appreciated.
The taxonomic spread of cross-ploidy hybridisation is especially broad in angiosperms, as evidenced by data both from the British and Irish flora and the wider literature. For example, monocots are well represented (Liliaceae, Orchidaceae, Poaceae), as are basal eudicots (Ranunculaceae, Papaveraceae) and throughout the rest of the phylogenetic tree scattered in the Fabids, Malvids and Superastrids. This distribution indicates cross-ploidy hybridisation is very widespread and potentially abundant throughout the flowering plant phylogeny (Figure 4). On the other hand, the conspicuous absence of records from large, diverse families with variable ploidy, such as Rubiaceae, potentially indicate a phylogenetic skew in cross-ploidy hybridisation. Cases of such hybridisation are not just phylogenetically but also geographically widespread, with examples reported from across four continents, though tropical regions are poorly represented and most studies report hybridisation in large temperate or cosmopolitan plant families (e.g. Asteraceae and Orchidaceae; see Schley et al. 2022 for discussion of overlooked polyploidy and hybridisation in the tropics). In terms of life form, most well-documented cross-ploidy hybrids (with the notable exception of Euphrasia ) are perennial, a factor which is associated with hybridisation regardless of parental ploidy level (Mitchell et al., 2019).