The mating system and spatial genetic structure of the rare and endangered bird-pollinated mallee Eucalyptus rhodantha were investigated in a remnant stand, using progeny arrays and pollen assayed at four polymorphic allozyme loci. Comparisons of the genetic diversity within and between the pollen pools and maternal parents of two arbitrary subpopulations indicated the presence of spatial genetic heterogeneity which was not broken down by pollen flow. It was suggested that this is the result of a high level if inbreeding and limited pollen dispersal by birds. Estimates of outcrossing rate ranged between I = 0-59 and I = 0-67 and were at the low end of the range reported for other eucalypts. It was concluded that E. rhodantha has a mixed mating system with a significant proportion of selfpollination. Biparental inbreeding within small neighbourhoods probably also contributed to the high level of inbreeding. The low level of outcrossing observed in E.rhodantha was not consistent with the hypothesis that bird pollination leads to high levels of outcrossing in the Australian flora. However, the level of outcrossing achieved through bird pollination together with high levels of gene flow between populations contribute to the maintenance of the relatively high levels of diversity which characterise the dissected populations of this species.
Outcrossing rates, seed yields and the incidence of seed abortion were estimated in differentsized populations of a rare clonal mallee, Eucalyptus argutifolia Grayling and Brooker. Multilocus 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.
The orchid Leporella fimbriata is pollinated by pseudocopulation with winged males of the ant Myrmecia urens. This recently studied interaction provides a unique opportunity to examine the two current hypotheses concerning the apparent rarity of ant pollination systems worldwide. The first hypothesis requires a series of specialized growth forms and floral characteristics regarded as adaptations to ant pollination. L. fimbriata does not possess them. The second considers the pollenicidal effects of secretions from the metapleural gland of ants. These glands are absent in M. urens males and it may be that the occurrence of ant pollination requires the absence of metapleural glands in the vector.
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