Seed development following reciprocal crossing among autotetraploid and diploid Acacia mangium and diploid A. auriculiformis

As part of a program to breed sterile triploid varieties of tropical Acacia, a series of inter-and intra-specific crosses were made among clones of neo-tetraploid A. mangium (AM-4x) and diploid A. mangium (AM-2x) and A. auriculiformis (AA-2x). The present paper reports variation in seed-crop development from anthesis to harvest, in comparison with that after open pollination of the respective parent trees. Abscission of spikes and pods within spikes commenced soon after anthesis and was more rapid in inter-cytotype crosses than in open-pollinated controls. Less than 12% of spikes were retained to maturity in either cytotype, emphasising the likely importance of resource competition during development. Inter-cytotype crosses showed higher levels of abnormal ovule development at 7 weeks after pollination and more undeveloped seeds in those pods which did develop to maturity. No inter-cytotype combination produced more than one viable seed per pod on average, and all needed to be germinated in vitro to survive. A directional effect was apparent in the inter-cytotype crosses within AM but this was not obvious when the cross was inter-specific. The study contributes new knowledge of the post-anthesis timeline for ovule, pod and spike abscission and discusses the likely genetic and environmental causes of observed differences between inter-and intra-cytotype crosses as well as the implications for breeding.