Wild seedlings of a tree endemic on granite outcrops show no evidence of inbreeding depression

Abstract

Plant communities on granite outcrops are characterised by high biodiversity and endemism, and strong population genetic structuring. The floristic diversity and structure of inselberg flora receives much attention in international conservation research, yet our understanding of how recruitment influences population genetic structure of plants endemic to granite outcrops is limited. To determine the immediate genetic consequences of recruitment and to quantify mating and dispersal patterns by parentage analysis, I conducted an opportunistic study following a wildfire in a stand of the lignotuberous tree Eucalyptus caesia Benth. (Myrtaceae) at Boyagin Nature Reserve. The entire adult stand (n = 180) plus seedlings (n = 115) were genotyped with 14 microsatellite loci. Compared with adults, seedlings had low heterozygosity and a high inbreeding co-efficient, but similar levels of allelic diversity. There was no evidence of post-germination selection against homozygous seedlings over the 20 month survey period. Seedlings that resulted from cross-pollination were more heterozygous than seedlings resulting from selfing, but could not be distinguished based on height measurements or survivorship. Parentage analysis in CERVUS revealed mostly limited seed dispersal, assuming that candidate parents closest to seedlings were the maternal parent. By comparison, pollen movement was more extensive, yet still restricted to plants within the stand. Recruitment has increased the population size, but appears insufficient to enhance genetic diversity in the Boyagin stand of E. caesia. Nonetheless, E. caesia appears adept at persisting as extremely small populations.