The effect of small population size on the mating system of a rare clonal mallee, Eucalyptus argutifolia (Myrtaceae)

Outcrossing rates, seed yields and the incidence of seed abortion were estimated in different-sized populations of a rare clonal mallee, Eucalyptus argutifolia Grayling and Brooker. Multi-locus estimates of the outcrossing rate were high in most populations (tm = 0.79–0.96), and no relationship between population size and the outcrossing rate was evident. In addition, significant amounts of interspecific hybridization were found in some small populations (up to 47 per cent of the seeds assayed). These estimates were much higher than expected, as it was apparent that the potential for geitonogamous pollination far exceeded that of outcrossing. Pollination experiments indicated that E. argutifolia is self-compatible, and therefore the higher than expected outcrossing rates were attributed to inbreeding depression. This view was supported by substantial levels (over 50 per cent) of seed abortion. Selection against homozygotes was also evident during later stages of development, and this resulted in adult populations having genotypic proportions similar to those expected under random mating (complete outcrossing). It was suggested that this selection later in the life cycle offset the purging of early-acting lethal (and semi-lethal) recessives and may explain why even small populations maintain high genetic loads. Similar explanations may be applied to other mass-flowering eucalypts that maintain strong inbreeding depression despite a mixed-mating system.