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).