Origin and evolutionary dynamics of Australian Elaeocarpaceae
A project undertaken at the Royal Botanic Gardens and Domain Trust, Sydney, and supervised by M. Rossetto and D. Crayn.
How and why did some representatives of the original Gondwanan flora survive within the small remaining pockets of Australian rainforest, while others disappeared or adapted to arid conditions and radiated more widely? This project aims to investigate the phylogeny, biogeography and within-species diversity of the plant family Elaeocarpaceae in order to understand some of the evolutionary mechanisms that have influenced speciation and distribution patterns within the Australian flora.
This project aims to investigate two broad themes on the biogeography and evolution of the Elaeocarpaceae. The first theme explored phylogenetic, biogeographic and evolutionary relationships among the Australian representatives of the family based on between-species DNA differentiation. Based on this phylogenetic framework, a second set of questions investigated in more detail the factors influencing the distribution of rainforest trees by measuring within-species differentiation in a representative genus, Elaeocarpus. Answers to these questions provided insights on how ecological traits, evolutionary potential, biogeographic origin and post-glacial survival have shaped current floristic distributions in Australian rainforests. Understanding the evolutionary basis that enabled the perseverance of taxa despite extreme environmental pressures will help us develop better conservation and management strategies for the future.
Phylogenetics and evolution of Elaeocarpaceae – Divergence time analysis using molecular phylogenies and fossil-calibrated evolutionary rates can distinguish between competing historical biogeographic scenarios (e.g. dispersal and vicariance). In a detailed study of Elaeocarpaceae sensu lato (including Tremandraceae) we used plastid trnL-trnF and nuclear ITS sequence data and parsimony and Bayesian methods to demonstrate the monophyly of all recognised genera, infer their relationships, and estimate the minimum divergence dates (Crayn et al. 2006). The Tremandraceae clade, which consists of three genera and about 49 species of shrubby, dry-adapted Australian plants was nested within the widespread predominantly rainforest tree family Elaeocarpaceae (nine genera, over 500 species) as sister to a clade comprising Aceratium, Elaeocarpus and Sericolea. These two lineages diverged during the Palaeocene, after which the evolutionary rate accelerated markedly in former Tremandraceae. Extant members of former Tremandraceae trace their origins to the Oligocene but the major diversification occurred during the late Miocene, when environments in Australia underwent progressive aridification accompanied by rapid diversification in several sclerophyllous groups. The importance of dispersal in explaining the current geographical distribution of this family was illustrated by Aristotelia. The distribution of this genus, previously thought to be consistent with Gondwanan vicariance, was shown to be a result of dispersal, the ancestor of the two New Zealand species arriving from Australia at least 6–7 Ma.
Why is a once-widespread lineage now confined to a narrow distribution? –To assess the status of a putative new species Elaeocarpus L. (Elaeocarpaceae) from northeastern New South Wales (NSW) with respect to the morphologically similar E. blepharoceras Schltr. from New Guinea, we undertook morphometric analysis of 11 vegetative attributes measured on 11 specimens of the putative new species and nine of E. blepharoceras (Maynard et al. 2009). Cluster analysis (Flexible UPGMA) and ordination (PCC) separated highland specimens of E. blepharoceras from the NSW material plus lowland E. blepharoceras. Furthermore, the ordination showed some separation of the NSW material into Koonyum Range and Nightcap Range groups. Although it is not clearly differentiated from lowland E. blepharoceras on morphometric analysis, description of the NSW material as E. sedentarius D.J.Maynard & Crayn was justified by a number of morphological and distributional factors. Nevertheless the question remained as to why E. sedentarius has such a narrow current distribution.
The use of multi-species approaches to discover the current and historical factors shaping the distribution of rainforest trees – Rainforest contractions caused by the aridification of the continent and the recent glacial cycles have left discrete genetic signatures on modern-day populations of rainforest trees. The nature of between-population differentiation is likely to have been influenced by a range of ecological and environmental factors. We used microsatellites to examine range-wide population genetic structure in two congeneric rainforest trees, Elaeocarpus angustifolius and E. largiflorens (Rossetto et al. 2007), with similar habitat preference and dispersal potential. The aim was to investigate the relationships between genetic structure, geographic disjunction and morphological differentiation and attempt to clarify the likely evolutionary processes responsible for the observed patterns. We found substantial differences in the amount and type of genetic differentiation within the two co-distributed species. E. largiflorens revealed an abrupt genetic-disjunction front between two subspecies separated by a biogeographic barrier. While for E. angustifolius, instead of a localised and sharp disjunction, we found a gradient of genetic differentiation across a much wider geographic area. Our findings suggest that biogeographic features may have different impacts on related species, and that generalisations on evolutionary patterns can be untenable without considering a range of factors.
Main publications arising from this project