Distribution of Veronica polyploids is influenced by both habitat differentiation and historic glaciation

Polyploidy, the phenomenon in which an organism has more than two copies of every chromosome, is common in plants and plays a key role in angiosperm evolution. A parallel process, genome downsizing, in which the amount of DNA per cell is reduced without chromosome loss, is correlated with greater diversification in polyploid groups. The adaptive significance of polyploidy may be that it allows polyploids to expand into new and different habitats compared to diploids (plants containing the standard two copies per chromosome), though this hasn’t been shown to be uniformly the case. Few studies have examined habitat differences and genome size among polyploid complexes of closely related species on an intercontinental scale.

In a recent article published in Annals of Botany, Blanca M. Rojas-Andrés and colleagues set out to do just this using 20 species of Veronica subsection Pentasepalae, a closely-related group of European and North African plants comprising ploidy levels of 2x (diploids), 4x, 6x, and 8x. The authors determined the ploidy level of 680 individuals from over 200 populations and looked for relationships between ploidy and environmental variables. They also determined the mass of nuclear DNA per cell to determine if genome downsizing has taken place in this group.

The study found that the different polyploids do occupy different habitats, with hexaploids and octoploids occurring in colder, wetter, more seasonal regions compared to diploids. The tetraploids, which were the least common of the ploidy levels, occurred only in restricted areas, and did not show any clear biogeographical distribution pattern. Genome downsizing does occur among polyploid populations of Veronica, and the hexaploid and octoploid genomes have downsized more than the tetraploids.

Considered in the context of historical climate patterns, the Veronica polyploids have an overall north-south pattern, with diploids occurring in southern glacial refuges, while octoploids are found further north, above the Quaternary permafrost boundary. A similar range pattern has been found in other European polyploid complexes. Most of the populations studied contained only a single ploidy level, but the researchers found an area of convergence where mixing was more common. “Cytotype coexistence is mainly detected in the western Balkans, indicating that it is an important contact zone among cytotypes,” the authors write. “Future work should focus on these contact areas at a finer scale including higher intra-population sampling, reciprocal transplants, common garden experiments and synthetic polyploids, together with genomic analyses, to confirm if ecological differentiation of cytotypes is effectively caused by polyploidy.”

Taken together, the results suggest that polyploid distribution is influenced by both habitat differentiation and patterns of refugia and permafrost retreat during the last glacial maximum. “While many studies have demonstrated an influence of the ice retreat on the distribution patterns of polyploids,” write the authors, “more emphasis should be put on permafrost coverage as a driver that shaped the biogeographic patterns of polyploid groups in temperate regions of Europe.”