Selection Against Homozygotes and Resource Allocation in the Mating System of Eucalyptus camaldulensis Dehnh

Abstract

Allozyme genotypes of seed progeny were determined for an essentially self-pollinating individual of E. camaldulensis heterozygous at the LAP and GPI-2 loci. The seed exhibited a normal 1:1 allelic recovery, and an excess of heterozygotes relative to Mendelian expectations that was significant at the GPI-2 locus (χ²[1] = 58.0, P<0.001). GPI-2 allozyme genotypes were also determined for the seed produced by an open-pollinated E. camaldulensis growing in a mixed clonal plantation. Most of the seed produced by this plant exhibited non-maternal alleles and was the result of cross-pollination. We conclude that post-zygotic seed selection was selectively eliminating genetic homozygotes from the selfed progeny and amplifying the frequency of cross-pollination products in the open-pollinated tree. We also found that the level of selection within fruits of the self pollinated tree was dependent upon the number of seeds and the number of ovules per capsule (P=0.019, r²=0.63 and P=0.008, r²=0.79 respectively using pooled classed data); selection intensities were found to be greater in fruits containing lower numbers of seeds or ovules. This may reflect the level of resources allocated to the fruit by a hormonally controlled positive feedback system. Incorporating the notions of heterosis or selection against recessive homozygotes and resource allocation allowed the presentation of alternative explanations for the low estimates of î found in eucalyptus producing low numbers of flowers and for the temporal heterogeneity in outcrossing rates reported in E. delegatensis.