Rise and fall of a continental mesic radiation in Australia: spine evolution, biogeography, and diversification of Cryptandra (Rhamnaceae: Pomaderreae)

Abstract The Australian continent has experienced progressive aridification since the Miocene, spurring recent radiations of arid-adapted lineages and the likely decline of mesic biotic groups. While examples of the former have been relatively well-documented, post-Miocene declines of non-arid sclerophyllous floras are less well understood. Here, we present a well-sampled time-calibrated nuclear phylogeny (140 accessions representing 60/65 species) of an Australian plant genus (Cryptandra Sm.: Rhamnaceae) and using ancestral range reconstructions and diversification analyses, elucidate its evolutionary history through space and time. We used high-throughput sequencing to recover 30 orthologous nuclear loci and BioGeoBEARS to infer ancestral areas. We show that the present-day distribution of Cryptandra can be explained by multiple vicariance events followed by in situ diversification with little exchange between regions. All diversification models show a speciation rate decline in Cryptandra after its radiation in the Miocene (c. 23 Mya). This coincides with aridification episodes across Australia and indicates that diversification of this genus has been negatively affected by the expansion of aridity. We also show that there were no significant differences in diversification rates between spinescent and non-spinescent Cryptandra lineages, suggesting that spinescent lineages may be the legacies of selection from extinct megaherbivores.