Preferential Outcrossing in Eucalyptus regnans F. Muell

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Abstract
Seed production characteristics of Eucalyptus regnans following self (S), outcross (O), (S+O ) and open pollination were investigated as a contribution to understanding of breeding system control in this species. All five trees tested produced seed after self-pollination, although yield was reduced relative to outcrossing. Isozyme analysis was used to determine paternity of individual seeds produced by polli- nation with a 1 : 1 mixture of S and 0 pollens. Preferential outcrossing was demonstrated, with an average of 81% of seeds being outcrosses. S and 0 seeds were also found within the same open- pollinated capsules, confirming the experimental observation that receipt of outcross pollen does not per se preclude self-fertilisation. The independent probabilities of survival of S and 0 embryos, as determined from seed yield per 100 flowers after separate S and 0 pollination, accounted for much of the preferential outcrossing effect. However, some trees produced more outcrosses than expected and competitive interaction of embryo genotypes within a capsule cannot be discounted. Following open pollination, samples from 15 trees averaged 16.7 ovules and 1.48 full seeds per capsule, giving a mean seed : ovule ratio of 9.0%. The modal number of seeds per capsule was 1 and the maximum 9, while 21% of capsules yielded no full seeds. These observations and experiments suggest post-fertilisation control of the breeding system, dependent upon both embryo genotype and maternal resource allocation.