Quantitative Analysis of Self-Incompatibility in Trees of Seven Species of Acacia

Quantitative estimates of self-incompatibility in individual trees of natural populations of Acacia species have been achieved using the index of self-incompatibility (ISI, of the ratio of infructescences or pods set after self-pollination/cross-pollination). The ISI was 0 or very low for three species—A. mearnsii, A. pycnantha, and A. retinodes— indicating high levels of self-incompatibility. The ISI varied widely in three species— A. myrtifolia, A. paradoxa, and A. terminalis—indicating partial self-compatibility, and was high in one species—A. ulicifolia—which was self-compatible. This technique allows populations with different breeding systems to be identified and characterized. We propose that full and partial self-incompatibility in this genus may arise from a gametophytic S gene system overlying recessive postzygotic lethal genes that are expressed particularly in situations in which an Sc (compatible) allele is present and inbreeding occurs.