Extensive Genetic Connectivity and Historical Persistence Are Features of Two Widespread Tree Species in the Ancient Pilbara Region of Western Australia

Abstract

Phylogeographic studies can be used as a tool to understand the evolutionary history of a landscape, including the major drivers of species distributions and diversity. Extensive research has been conducted on phylogeographic patterns of species found in northern hemisphere landscapes that were affected by glaciations, yet the body of literature for older, unaffected landscapes is still underrepresented. The Pilbara region of north-western Australia is an ancient and vast landscape that is topographically complex, consisting of plateaus, gorges, valleys, and ranges, and experiences extreme meteorological phenomena including seasonal cyclonic activity. These features are expected to influence patterns of genetic structuring throughout the landscape either by promoting or restricting the movement of pollen and seed. Whilst a growing body of literature exists for the fauna endemic to this region, less is known about the forces shaping the evolution of plant taxa. In this study we investigate the phylogeography of two iconic Pilbara tree species, the Hamersley Bloodwood (Corymbia hamersleyana) and Western Gidgee (Acacia pruinocarpa), by assessing patterns of variation and structure in several chloroplast DNA regions and nuclear microsatellite loci developed for each species. Gene flow was found to be extensive in both taxa and there was evidence of long-distance seed dispersal across the region (pollen to seed ratios of 6.67 and 2.96 for C. hamersleyana and A. pruinocarpa, respectively), which may result from flooding and strong wind gusts associated with extreme cyclonic activity. Both species possessed high levels of cpDNA genetic diversity in comparison to those from formerly glaciated landscapes (C. hamersleyana = 14 haplotypes, A. pruinocarpa = 37 haplotypes) and showed evidence of deep lineage diversification occurring from the late Miocene, a time of intensifying aridity in this landscape that appears to be a critical driver of evolution in Pilbara taxa. In contrast to another study, we did not find evidence for topographic features acting as refugia for the widely sampled C. hamersleyana.

Limited phylogeographic and genetic connectivity in Acacia species of low stature in an arid landscape

“…hilliana and A . spondylophylla were comparable to those found for two other widespread Acacia species with populations in the Pilbara bioregion, A. ancistrocarpa (Levy et al, 2016) and A. pruinocarpa (Nistelberger et al, 2020). This result supports our first prediction regarding genetic diversity and is consistent with population genetic theory that plant species with widespread ranges maintain greater diversity than species with restricted range sizes, as they are expected to be less impacted overall by stochastic environmental and genetic effects that may negatively affect population sizes (Allendorf et al, 2013; Ellstrand & Ellam, 1993; Godt & Hamrick, 1993).…”

“…ancistrocarpa (Levy et al, 2016) and A . pruinocarpa (Nistelberger et al, 2020). A notable exception is a shared haplotype (H1) of A. spondylophylla that occurs in single populations at the north‐eastern (BAT) and north‐western (PAN) geographical extremes.…”

“…Greater longevity may provide greater potential for expansion by providing greater opportunity for spatial and temporal seed dispersal. A more complex network of dispersal and significant test of non-neutrality, also suggesting a scenario of population expansion have been observed for the tall tree species, A. pruinocarpa, in the same landscape (Nistelberger et al, 2020). Acacia pruinocarpa is also long lived (50+ years) and sparsely scattered (Maslin et al, 2010), and these traits would also provide increased opportunity for spatial and temporal dispersal of pollen and seed.…”

“…Previous studies of three Acacia present in the Pilbara have shown evidence for congruent patterns of historical population isolation and persistence at the broad regional scale (Levy et al, 2016;Nistelberger et al, 2020). However, patterns of phylogeographic and contemporary population genetic diversity and differentiation have also been shown to vary with differences in taxon-specific life history and ecological traits known to impact pollen and seed dispersal, including differences in species geographic range size, and growth form or stature.…”

“…However, patterns of phylogeographic and contemporary population genetic diversity and differentiation have also been shown to vary with differences in taxon‐specific life history and ecological traits known to impact pollen and seed dispersal, including differences in species geographic range size, and growth form or stature. For example, species with widespread ranges, A. ancistrocarpa and A. pruinocarpa , show greater levels of chloroplast haplotype diversity, haplotype network complexity, phylogeographic structure, and contemporary genetic diversity and connectivity (Levy et al, 2016; Nistelberger et al, 2020), compared to the range restricted A. atkinsiana (Levy et al, 2016). Also, the tallest of the three species, A. pruinocarpa (to 12 m), has increased haplotype diversity, a more complex haplotype network, and some evidence for population expansion (Nistelberger et al, 2020) in comparison to the smaller shrub species (both to 4 m) that show more limited haplotype network diversity, and evidence for population persistence (Levy et al, 2016).…”