Conservation genetics and ecology of the endangered heathland shrub, Allocasuarina emuina

Robert W Lamont

doi:10.25907/00588

The subtropical coastal heathlands of south-east Queensland have undergone considerable habitat loss in recent years due to high levels of urbanisation. This study used morphological and microsatellite markers to investigate the reproductive ecology and population genetic structure of the endangered wind-pollinated shrub, Allocasuarina emuina, and its risk of genetic assimilation by a common congener, A. littoralis. Differences in breeding system were found between the dioecious population of A. emuina on Mt. Emu and the monoecious, wet heath populations of the coastal plain. Highly significant differences (p < 0.001) were detected among populations for all cone and seed-related traits and indicated that the reproductive viability of A. emuina is more related to fire interval than population size. In addition, species phenology and climate variables were strongly correlated (p < 0.001), suggesting that the spatial configuration of populations is non-random and influenced by the mechanical properties of seasonal weather patterns. The highest level of genetic diversity for the species was resolved in the dioecious population on Mt. Emu (AE4A, HE = 0.688), whereas all of the monoecious populations were found to share a limited set of identical multi-locus genotypes and were genetically depauperate in comparison (eg. Coolum Ridges, AE8, HE = 0.511). Contrary to the expectations of conservation genetic theory, no relationship was found between the level of genetic variation and population size, density, or degree of isolation, suggesting that genetic diversity in small populations is being maintained by polyploidy and apomixis. Estimates of gene flow in A. emuina were low (Nm = 0.117), consistent with the geographic and genetic differentiation of southern and northern populations (ΦPT = 0.682). However, neighbouring populations within sandplain formations exhibited effective levels of gene flow (Nm = 0.982 - 5.380). Genetic signatures in the northern populations indicated that limited, unidirectional dispersal of genetic material from the refugial population on Mt. Emu (AE4A) to the surrounding coastal plain has been occurring with the recession of sea-levels since the Last Interglacial, ~ 100 000 years B.P. The study found substantial phenological overlap between A. emuina and A. littoralis, and microsatellite evidence indicated that low levels of bidirectional hybridisation between sympatric populations are a natural occurrence, however fundamental breeding system differences between A. emuina and the obligate outcrosser, A. littoralis appear to be maintaining species boundaries. An examination of the genetic relationships between A. littoralis, A. emuina and three other restricted, 'dioecious' species from the region, A. thalassoscopica (E), A. rigida ssp. exsul (V), and A. filidens (R), found genotypes of the four threatened taxa to each be composed of a limited subset of A. littoralis alleles, with varying proportions of identical multilocus profiles, indicating both facultative apomixis and sexual reproduction. Chromosome determinations confirmed A. littoralis as an autopolyploid (2n = 22, 33, 44), whereas the four threatened species were found to be either diploid (A. rigida ssp. exsul, 2n = 22), triploid (A. filidens, 2n = 33), or tetraploid (A. emuina and A. thalassoscopica, 2n = 44). Negligible differentiation was detected among species at either the matK or the JLA chloroplast regions, implying that diversification from the putative progenitor, A. littoralis, has been relatively recent. Minimal morphological, cytological, or genetic differentiation was found between the single population of A. thalassoscopica and A. emuina, which suggests that the status of A. thalassoscopica as a separate taxon should be reviewed.

Conservation genetics and ecology of the endangered heathland shrub, Allocasuarina emuina

Files and links (1)

Abstract

Details

Metrics