Low but structured chloroplast diversity in Atherosperma moschatum (Atherospermataceae) suggests bottlenecks in response to the Pleistocene glacials

Abstract

The cool temperate rainforests of Australia were much reduced in range during the cold and dry glacial periods, although genetic evidence indicates that two key rainforest species, Nothofagus cunninghamii and Tasmannia lanceolata, survived within multiple locations and underwent only local range expansions at the end of the Last Glacial. To better understand the glacial response of a co-occurring but wind-dispersed and less cold-tolerant rainforest tree species, Atherosperma moschatum, a chloroplast phylogeographic study was undertaken. A total of 3294 bp of chloroplast DNA sequence was obtained for 155 samples collected from across the species' range. The distribution of six haplotypes observed in A. moschatum was geographically structured with an inferred ancestral haplotype restricted to Tasmania, while three non-overlapping and endemic haplotypes were found on the mainland of south-eastern Australia. Last glacial refugia for A. moschatum are likely to have occurred in at least one location in western Tasmania and in Victoria and within at least two locations in the Great Dividing Range of New South Wales. Nucleotide diversity of A. moschatum was lower (π = 0·00021) than either N. cunninghamii (0·00101) or T. lanceolata (0·00073), and was amongst the lowest recorded for any tree species. This study provides evidence for past bottlenecks having impacted the chloroplast diversity of A. moschatum as a result of the species narrower climatic niche during glacials. This hypothesis is supported by the star-like haplotype network and similar estimated rates of chloroplast DNA substitution for A. moschatum and the two more cold tolerant and co-occurring species that have higher chloroplast diversity, N. cunninghamii and T. lanceolata.