Polyploidy linked with species richness but not diversification rates or niche breadth in Australian Pomaderreae (Rhamnaceae).

Background and aims: Polyploidy is an important evolutionary driver for plants and has been linked with higher species richness and increases in diversification rate. These correlations of ploidy with plant radiations could be the result of polyploid lineages exploiting greater niche space and novel niches due to their enhanced adaptability. Ploidy evolution and how it links with diversification of plants across the Australian continent is not well understood. Here, we focused on the ploidy evolution of the Australasian Rhamnaceae tribe Pomaderreae.

Methods: We generated a densely-sampled phylogeny (90%, 215/240 spp.) of the tribe and used it to test for the evolution of ploidy. We obtained 30 orthologous nuclear loci per sample and dated the phylogeny using TreePL. Ploidy estimates for each sequenced species was obtained using nQuire, based on phased sequence data. We used MiSSE to obtain tip diversification rates and tested for significant relationships between diversification rates and ploidy. We also assessed for relationships between ploidy level and niche breadth, using distributional records, species distributional modelling, and WorldClim data.

Key results: Polyploidy is extensive across the tribe, with almost half (45%) of species and majority of genera exhibiting this trait. We found a significant positive relationship between polyploidy and genus size (i.e., species richness), but non-significant positive relationship between polyploidy and diversification rates. Polyploidy did not result in significantly wider niche space occupancy for Pomaderreae, however polyploidy did allow for transitions into novel wetter niches. Spatially, eastern Australia is the diversification hotspot for Pomaderreae in contrast to the species hotspot of southwest Western Australia.

Conclusions: The relationship between polyploidy and diversification is complex. Ancient polyploidisation events likely played an important role in the diversification of species rich genera. A lag time effect may explain the uncoupling of tip diversification rates and polyploidy of extant lineages. Further studies on other groups are required to validate these hypotheses.