STUDIES OF MATING SYSTEMS IN SEED STANDS SUGGEST POSSIBLE CAUSES OF VARIABLE OUTCROSSING RATES IN NATURAL POPULATIONS OF ACACIA MANGIUM

摘要

Acacia mangium, a tropical tree species planted widely for pulpwood production, occurs naturally on the margins of rainforest in north-eastern Australia and in the tropical lowlands of New Guinea. It is a pioneer species that establishes from seed after fire or other disturbance. The species has low levels of genetic diversity but high genetic differentiation among populations in different geographic regions. Outcrossing rates in natural populations, estimated using micro satellite markers, range from complete selfing in outlying populations with low genetic diversity to complete outcrossing in the more variable New Guinea populations. This is in contrast to other diploid acacias that are predominantly outcrossing. Studies in A. mangium seed orchards have shown that outcrossing rates are determined not only by the evolutionary history of the natural population used to establish the orchard but also the flowering patterns in the orchard. While the species is preferentially outcrossing, reductions inthe number of flowering trees will result in a marked increase in the level of selfing and a decline in genetic diversity in the next generation. This clearly demonstrates the impact of a genetic bottleneck and explains the correlation between outcrossing rate and genetic diversity in natural populations. The adaptability of the breeding system in A. mangium may explain its success as a poineer species.