THE RELATIONSHIP BETWEEN DYSPLOIDY AND REPRODUCTIVE CAPACITY IN MYRTACEAE

Australian Myrtaceae show a positive correlation between base chromosome number and reproductive capacity. There is some evidence of a similar trend in other Australian plant families. Reduced chromosome numbers in the Myrtaceae arose in lineages with critically reduced reproductive capacities, occurring mainly in small shrub species with four or fewer ovules per flower and one seed per fruit or per fruit loculus. Dysploid reduction serves the short-term advantage of ensuring greater genetic uniformity through reduced recombination, therefore resulting in less seed wastage. However, lineages with reduced chromosome numbers (x=5-10) have reduced evolutionary flexibility, as evidenced by their failure to fully penetrate the arid zones, in contrast to the species with the primitive chromosome number or above (x=11 or 12). Trends for decreased ovule and seed numbers have apparently resulted partly from the selection for larger seed size. Among species with more than one ovule per flower but normally only one seed per fruit, there is evidence of physiological and genetic selection systems determining which of the fertilised ovules becomes the seed.