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, Euc alyptus argutifolia Grayling and Brooker. Multilocus estimates of the outcrossing rate were high in most populations (t(m) = 0.79-0.96), and no relationship between population size and the outcrossing rate was evident. In addition, significant amounts of interspecific hybridizati on were found in some small populations (up to 47 per cent of the seed s 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. arguti folia is self-compatible, and therefore the higher than expected outcr ossing rates were attributed to inbreeding depression. This view was s upported by substantial levels (over 50 per cent) of seed abortion. Se lection against homozygotes was also evident during later stages of de velopment, and this resulted in adult populations having genotypic pro portions similar to those expected under random mating (complete outcr ossing). 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 load s. Similar explanations may be applied to other mass-flowering eucalyp ts that maintain strong inbreeding depression despite a mixed-mating s ystem.