Diversification of the Malvales, Sterculioideae and Brachychiton

Abstract

The flora of Australia can be divided into five biomes and each of these has a distinct history that is a product of climate both past and present, tectonics and evolutionary dynamics. The history of the Australian monsoon biome is the least known of all the Australian biomes both from the standpoint of molecular phylogenetic reconstruction and the age and historical extent of the weather system that fundamentally defines this biome. To begin to unravel the history of the monsoon biome, Brachychiton (Sterculioideae: Malvales) was chosen as an exemplar group. To understand the evolutionary history of Brachychiton in the monsoon tropics it was first necessary to know the age and biogeographic relationships within the Sterculioideae and the Malvales. To achieve this multiple regions from both the chloroplast and nuclear genomes were sequenced. Phylogenies were reconstructed for the combined molecular data using likelihood and Bayesian methods. The molecular phylogeny for Brachychiton was compared with a phylogeny reconstructed using a morphological data set. Molecular dating analyses were conducted at the level of the Malvales and on a more densely sampled data set for the Sterculioideae, including the genus Brachychiton. At the level of the Malvales, it was found that there was some evidence for elevated but not exceptional diversification rates, lending some support to the notion that tropical groups may have faster rates of diversification. Within Brachychiton, results show a strong congruence between morphological and molecular data, validating the previously published sectional divisions in Brachychiton. The molecular dating results show a crown age of the Brachychiton clade that is of middle Miocene age, and suggest multiple radiations into different regions of the monsoon biome. Overall, interpretation of the data indicates that Brachychiton had an Australian rainforest ancestor and the extant taxa have recently moved into the monsoonal and rainforest regions of Australian and New Guinea.