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

Abstract

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 between ploidy and plant radiations could be the result of polyploid lineages exploiting broader niche space and novel niches due to their enhanced adaptability. The evolution of ploidy and its link to plant diversification across the Australian continent is not well understood. Here, we focus on the ploidy evolution of the Australasian Rhamnaceae tribe Pomaderreae.

Methods: We generated a densely sampled phylogeny (90 %, 215/240 species) 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 were 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 the majority of genera exhibiting this trait. We found a significant positive relationship between polyploidy and genus size (i.e. species richness), but a 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 transitions into novel wetter niches. Spatially, eastern Australia is the diversification hotspot for Pomaderreae in contrast to the species hotspot of south-west Western Australia.

Conclusions: The relationship between polyploidy and diversification is complex. Ancient polyploidization 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.

Keywords: Australian flora; Rhamnaceae; diversification; niche space; phylogenomics; polyploidy; turnover; woody.

Fig. 1.

Maps derived from cleaned herbarium occurrence data and modelled species distributions (Maxent) showing (A) species richness (high–low; red–blue), (B) net diversification rates (high–low; red–blue) and (C) residuals between the two for Pomaderreae (Rhamnaceae) across Australia. Left y-axis and x-axes respectively show latitude and longitude coordinates.

Fig. 2.

Proportion of different ploidy levels for Pomaderreae in (A) south-west Western Australia (SWA) and other regions across Australia, and (B) across different genera, numbers in brackets indicate species diversity for each genus. Ploidy levels were estimated using nQuire based on sequence data.

Fig. 3.

Dated treePL phylogeny showing tip-specific diversification rates estimated from MiSSE; heat colours show low–high (blue–red) diversification rates. Ploidy level and geographic range of each taxon are also shown with different colour schemes. The ‘SPB’ clade includes Serichonus, Papistylus and Blackallia. ‘Pol.’ represents Polianthion, and ‘T.’ represents Trymalium in the genus-delimiting box. All photographs taken by Francis J. Nge (unless stated otherwise). Top to bottom: Pomaderris lanigera, Pomaderris paniculosa, Pomaderris brevifolia (photo by K.R.Thiele), Spyridium ericoides*, Spyridium bifidum, Spyridium thymifolium, Serichonus gracilipes (photo by K.R.Thiele), Cryptandra amara, Cryptandra arbutiflora var. arbutiflora (photo by K.R.Thiele), Cryptandra myriantha (South Australian form), Stenanthemum humile, Stenanthemum pomaderroides, Trymalium wayi.

Fig. 4.

Violin plots of ploidy with different diversification metrics and WorldClim environmental variables (proxy for niche space). (A) Turnover rate, (B) diversification rate, (C) distributional range size, (D) distributional length, (E) annual precipitation, (F) precipitation of wettest month, (G) precipitation of wettest quarter, (H) precipitation of warmest quarter. Only WorldClim variables having significant associations with ploidy are shown. Values represent midpoints. The width of each violin plot indicates density of values, and the white circles the value of the median. Grey bars represent values within a factor of 1.5 of the upper and lower quartiles.