Genetic differentiation and diversity of Acacia koa populations in the Hawaiian Islands

Acacia koa A. Gray (koa) is a leguminous tree endemic to the Hawaiian Islands and can be divided into morphologically distinguishable groups of A. koaia Hillebrand, A. koa and populations that are intermediate between these extremes. The objectives of this investigation were to distinguish among divergent groups of koa at molecular levels, and to determine genetic diversity within and among the groups. Phylogenetic analyses using the ITS/5.8S rDNA and trnK intron sequences did not separate the representative koa types into distinct clusters. An unweighted pair group method with arithmetic mean cluster analysis and principal coordinate analysis, based on allele profiles of 12 microsatellite loci for 215 individual koa samples, separated the population into three distinct clusters consistent with their morphology, A. koaia, A. koa and intermediate forms. There was an average of 8.8 alleles per polymorphic locus (AP) among all koa and koaia individuals. The intermediate populations had the highest genetic diversity (H′ = 1.599), AP (7.9) and total number of unique alleles (21), whereas A. koaia and A. koa showed similar levels of genetic diversity (H′ = 0.965 and 0.943, respectively). No correlation was observed between geographic distance and genetic distance as determined by a Mantel test (r = 0.027, P = 0.91). The data presented here support previous recommendations that morphological variation within koa should be recognized at the subspecific level rather than as distinct species.