The major Australian cool temperate rainforest tree Nothofagus cunninghamii withstood Pleistocene glacial aridity within multiple regions: evidence from the chloroplast

Abstract

Summary• Glacial aridity of the Pleistocene was inhospitable for the cool temperate rainforest tree Nothofagus cunninghamii over most of its current range in southeastern Australia, particularly in eastern Tasmania. A chloroplast DNA phylogeographic study was undertaken to investigate whether this species was likely to have survived in situ or conforms to a dispersal model of postglacial recovery.• Twenty-three chloroplast haplotypes were identified by PCR-RFLP and direct sequencing of 2164 base pairs from 213 N. cunninghamii individuals collected in a range-wide survey. Fine-scale haplotype distribution was investigated using PCR-RFLP in eastern Tasmania.• Deep chloroplast divergence occurred in N. cunninghamii. The single haplotype of the sister species, N. moorei, was nested among N. cunninghamii haplotypes. The distribution of N. cunninghamii haplotypes supports: multiple glacial refugia in coastal and inland western Tasmania, the centre of haplotype diversity; glacial survival in the central highlands of Victoria, corroborating pollen data; and the long-term occupation of eastern Tasmania because of the presence of a unique deeply diverged chloroplast lineage.• Nothofagus cunninghamii withstood glacial aridity within multiple regions in apparently nonequable climates. This finding contributes to a growing understanding of how the resilience of temperate species during glacial periods has shaped modern biota.

Genetic differentiation in spite of high gene flow in the dominant rainforest tree of southeastern Australia, Nothofagus cunninghamii

“…This gene flow is most likely occurring through pollen dispersal because chloroplast DNA evidence indicates low gene flow by seed in this species (Worth et al, 2009). Nonetheless, the presence of significant genetic patterns at the nuclear level across the species range can be explained by the combined effects of geographic isolation and small population size.…”

“…Chloroplast evidence implies long-term survival of the species on both sides of Bass Strait: in the Central Highlands of Victoria and in multiple locations in Tasmania (Worth et al, 2009). In contrast, the Bayesian coalescent analysis estimates that the mean time of genetic isolation across Bass Strait was o25 000 years ago and older than 3000 years ago, a time period that includes the Last Glacial Maximum when sea levels were sufficiently low for Victoria and Tasmania to be connected by land.…”

“…Under this scenario the populations diverged 226 generations ago (T1, median value) or 22 600 years ago. However, scenario 2 is difficult to reconcile with the presence of much greater diversity of chloroplast haplotypes in Tasmania than in Victoria (Worth et al, 2009). When Ne was constrained scenario 1, where Victorian populations diverged from Tasmanian ones, had the highest posterior probability (0.54, 95% confidence interval = 0.39-0.69).…”

“…The strong geographic structuring of chloroplast haplotypes in N. cunninghamii provides evidence both that this species persisted in multiple glacial refugia across all major parts of its range and, assuming that the chloroplast genome is dispersed by seed, that seed-mediated gene flow has been restricted since the last glacial (Worth et al, 2009). Furthermore, the species shows significant morphological and physiological variation across its range for important functional traits such as leaf thickness, area, specific leaf area and stomatal density, and some of this variation is genetic (Hovenden and Vander Schoor, 2006).…”

“…This is because the species is wind pollinated and is strongly represented in modern pollen counts even at tens of kilometres distance from source trees (Fletcher and Thomas, 2007). Thus, it is unclear whether substantial geographical barriers to seedmediated gene flow created by major areas of inhospitable habitat, such as Bass Strait and the dry Midlands region of Tasmania (Worth et al, 2009), are in fact barriers to gene flow via pollen.…”