Wj. Kennington et Sh. James, THE EFFECT OF SMALL POPULATION-SIZE ON THE MATING SYSTEM OF A RARE CLONAL MALLEE, EUCALYPTUS-ARGUTIFOLIA (MYRTACEAE), Heredity, 78, 1997, pp. 252-260
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.