Ecological genetics of the wind-pollinated, tetraploid, Allocasuarina emuina L. Johnson (Casuarinaceae) from southeast Queensland reveals montane refugia for coastal heath during the last interglacial

Abstract

The subtropical coastal heathlands of southeast Queensland contain many rare and threatened species and have undergone considerable habitat loss in recent years due to high levels of urbanisation. We used morphological and microsatellite markers to investigate the reproductive ecology, in relation to fire, and population genetic structure of the endangered shrub Allocasuarina emuina. Highly significant differences (P < 0.001) were detected among populations for all cone- and seed-related morphological traits, which revealed distinct northern and southern groups of populations, a pattern repeated in the genetic component of the study. Results also indicated that the reproductive viability of A. emuina is more related to fire interval than population size and confirmed that seed viability declines with time since fire. The highest level of genetic diversity for the species was observed in the population on Mt Emu (AE4A; HE = 0.688). Contrary to the expectations of population genetic theory, no relationship was found between the level of genetic variation and population size, density or degree of isolation, hinting that genetic diversity is being conserved as a result of polyploidy and apomixis. However, genetic signatures in the northern populations indicated that unidirectional dispersal of genetic material from the putatively refugial population on Mt Emu to the surrounding coastal plain has been occurring with recession of sea levels following the last interglacial. The findings of the study will aid in both the conservation of natural populations and translocations of A. emuina and have significant implications relating to the biogeographical history of a considerable number of co-occurring heathland taxa.