For a long time, changes in ploidy level were assumed to essentially create new species, because newly-developed polyploids often looked different (phenotypic change), acted different (phenological and physiological change), couldn't mate with their diploid relatives (prezygotic barriers). or made offspring that didn't survive or were sterile (postzygotic barriers).
My work, along with contemporaneous work in other systems, shows that this assumption of "instant" speciation works for some polyploids, but not others. Generally, diploids and tetraploids have difficulty hybridizing (although there are exceptions!). However, tetraploids and hexaploids can often successfully create pentaploid offspring, and those pentaploids are often fertile!
Figure 1: In artificially-constructed but naturally-pollinated arrays, very few triploid hybrids were produced when diploids and tetraploids were in contact, and then only when maternal plants were outnumbered by the other cytotype (a), however, pentaploids were common from both tetraploid and hexaploid mothers is mixed-ploidy arrays. Adapted from Sutherland et al., 2020
Interploidy hybrids: Bridge or Sink?
If pentaploids are successful in greenhouse and array studies, are they also successful in nature? And if they are, how do they affect polyploid complexes when other cytotypes come together? To answer that question, I surveyed mixed-ploidy contact populations to determine individual cytotypes and genotypes. I found no triploids in places where diploids and tetraploids were common, but many pentaploids where tetraploids and hexaploids meet. Preliminary genotyping suggests pentaploids may serve as a conduit for gene flow, but more work needs to be done to confirm this and determine how there odd-ploidy plants persist!
Figure 2: Histograms of estimated genome sizes in mixed-ploidy populations of C. rotundifolia. Diploid plants have an average estimated genome size around 2.2 Gb, tetraploids around 4.4 Gb, and hexaploids around 6.6 Gb. In mixed diploid-tetraploid populations, only plants consistent with diploids and tetraploids are found. However, lots of variation exists between the two dominant cytotypes in tetraploids-hexaploid populations, suggesting that pentaploids and possible even aneuploids are present!
Adapted from Sutherland and Galloway, 2021, in press.