Phylogeographical patterns in chloroplast DNA variation within the Acacia acuminata (Leguminosae: Mimosoideae) complex in Western Australia

Abstract

The Acacia acuminata complex includes three taxa, A. acuminata ssp. acuminata, A. acuminata ssp. burkittii and A. oldfieldii, along with several informal variants of A. acuminata. It is widespread throughout southern Australia with the centre of diversity in south-west Western Australia. Phylogeographical patterns in the complex were investigated using a nested clade analysis of cpDNA RFLPs from 25 populations in Western Australia. Except for A. oldfieldii that was clearly identified as a distinct entity, haplotypes were not restricted to sub-specific taxa or variants within A. acuminata. There was significant association between phylogenetic position of many haplotypes and their geographical distribution. The fine-scale phylogeographical patterns were complex but at deeper levels in the phylogeny there was evidence of divergence between two lineages. The pattern of shared haplotypes between lineages suggests retention of ancestral polymorphism as a result of incomplete lineage sorting. The divergence of these lineages is consistent with fragmentation caused by climatic instability during the Pleistocene.

Phylogeography and divergence in the chloroplast genome of Western Australian Sandalwood (Santalum spicatum)

“…The identification of similar phylogeographic patterns over similar time frames suggests that they have occurred through the influences of broad biogeographic processes. The isolation and differentiation of lineages observed within S. spicatum, and other species that have been investigated (Byrne et al, 1999(Byrne et al, , 2001b(Byrne et al, , 2002, is consistent with the hypothesis that cyclic contraction and expansion of the arid region in the northeast, and the mesic region in the south-west, during the Pleistocene era led to fragmentation and isolation in the intermediate area between the arid and mesic zones (Hopper, 1979;Hopper et al, 1996). Comparative phylogeographic studies in other parts of the world have also demonstrated broad biogeographic influences, including common postglacial colonisation routes in Europe (Ferris et al, 1993Demesure et al, 1996;Dumolin-Lapègue et al, 1997;King and Ferris, 1998) and northern and southern glacial refugia in the Pacific North West of America (Soltis et al, 1997).…”

“…Using this estimate, the time of divergence between the two lineages is around one million years ago, in the middle of the Pleistocene era. Geographical structuring due to historical isolation has also been observed in a common host of S. spicatum, A. acuminata (Byrne et al, 2002) with a similar time frame for the divergence between lineages (approximately 800 000 years BP). The identification of similar phylogeographic patterns over similar time frames suggests that they have occurred through the influences of broad biogeographic processes.…”

“…The level of diversity in the chloroplast genome of S. spicatum is high compared to other woody perennials but not as high as has been identified in other Australian tree species, including the main host of S. spicatum, Acacia acuminata (Byrne et al, 2002). Significant structuring of the diversity occurs with two main clades that are geographically separated, one centred on the southern region and one centred in the north of the distribution.…”

“…There are some limitations in the use of cpDNA to investigate phylogeographic patterns in plants due to the often low level of variation within species and the slow rate of evolution compared to animal mitochondrial DNA. However, relatively high levels of cpDNA variation have been found in Australian plants (eg Butcher et al, 1995;Byrne and Macdonald, 2000;Byrne et al, 2002) in comparison to other species (Soltis et al, 1992). In addition, the southwest of Western Australia is an ancient landscape that has experienced geological stability with no major glaciation or extinction events, and where the pattern of variation is influenced by historic processes.…”

“…In particular, climatic instability during the Pleistocene resulting in cyclic expansion and contraction of the mesic and arid zones (Hopper, 1979;Hopper et al, 1996) has led to naturally fragmented population systems with a high level of endemism and both relictual and derived species. Hence, cpDNA analysis has proven useful in determining phylogenetic patterns in plant species in Western Australia, and studies of both widespread species and restricted species with disjunct distributions have revealed the presence of historical lineages that have been differentiated through isolation (Byrne et al, 1999(Byrne et al, , 2001b(Byrne et al, , 2002. The presence of two ecotypes in S. spicatum influenced by different genetic processes may represent similar patterns of historic separation in this species.…”