Genetic variation in 26 populations of Racosperma auriculiforme and Racosperma mangium using allozymes

Racosperma auriculiforme (Cunn. ex Benth.) Pedley and Racosperma mangium (Willd.) Pedley are two fast-growing multipurpose leguminous species that have great potential for reforestation in the subhumid and humid tropics. The level and distribution of genetic variability were evaluated among and within 13 populations of each species at 18 loci encoding 10 enzymes, using starch gel electrophoresis. At the population level, the mean number of alleles per locus, the mean percentage of polymorphic loci, and mean expected heterozygosity were, respectively, 1.9, 52.1%, and 0.122 for R. auriculiforme, and 1.5, 24.3%, and 0.064 for R. mangium. Therefore, R. mangium appeared genetically depauperate compared with R. auriculiforme. The proportion of the total genetic diversity that resided among populations in R. auriculiforme (18%) was twice that in R. mangium (9%). No linear relationships were detected between genetic and geographic distances among populations. Cluster analysis of Nei's genetic distances and discriminant analysis did not show any significant geographic pattern of population differentiation for R. mangium. These same analyses revealed two distinct clusters of populations for R. auriculiforme, one in Queensland, Australia, and Papua New Guinea, and the other in the Northern Territory of Australia and Indonesia. For both taxa, seed of unknown origin of some introduced populations in Zaire and Congo could be identified. The genetic distance between the two species (D = 0.097) was, as expected, much larger than the average intraspecific genetic distance derived from comparisons of conspecific populations (D = 0.034 for R. auriculiforme and D = 0.007 for R. mangium). However, the amplitude of the interspecific genetic distance calculated is representative of divergence levels usually observed among subspecific taxa. Based on this, and the lower levels of genetic diversity found in R. mangium, it is suggested that R. mangium derived recently from R. auriculiforme