Orchards established from New Guinea provenances have more than double the level of genetic diversity of those established from provenances in the Daintree-Cairns region in Queensland. The orchards also differ widely in their outcrossing rates. In one orchard, over 80% of the progeny were derived from self-pollination, while some other orchards produced 100% of out-crossed progeny. The lowest outcrossing rates were in an orchard with a low proportion of flowering trees.
The study is providing some clues to the variability in inbreeding that we have previously found in the natural populations of A. mangium. The level of inbreeding is affected not only by which natural provenances were used to establish a seed orchard, but also the proportion of trees flowering when the seed crop was initiated. It appears that A. mangium does not have a strong selfincompatibility system, and if pollen from other trees is not available, many individual trees will produce substantial seed crops by self-fertilisation. Such selfing may explain the rapid deterioration in genetic quality of some of the land races of tropical acacias that have developed in South-east Asia. However, in stands where many trees flower at the same time, pollen transfer between trees occurs (other workers have found that insects including flies and bees are the main pollinators of A. mangium). It is not known whether maternal resources are allocated preferentially to outcrossed embryos in A. mangium, but the high outcrossing rates in the majority of orchards suggest some mechanism that favours outcrossed over selfed seed.
Field trials have been established at Ba Vi in northern Vietnam and Dong Ha in central Vietnam to compare the performance of the inbred and outcrossed seedlings identified using microsatellite markers. Estimates of the level of inbreeding depression of tree growth obtained from these trials will help to quantify the benefits obtainable by using out-crossed seed from well-designed seed orchards. This should encourage the use of improved seed and increase the productivity of plantations in Vietnam and other tropical countries.
The microsatellite marker technology will also enable RCFTI to check the identity of clones of Acacia and other genera being mass-propagated for clonal forestry in Vietnam, and to verify the success of controlled crosses in breeding programs.
Source :
Australian Tree Resources News, June 2002
