Phylogenetic Relationship among Macadamia integrifolia and Macadamia tetraphylla Wild Accessions ^†

Two of Australia’s endemic rainforest species, Macadamia integrifolia and M. tetraphylla, produce edible nuts with high-value kernel. The majority of commercial varieties are based on the limited gene pool of M. integrifolia and hybrids between M. integrifolia and M. tetraphylla. An understanding of the existing genetic diversity within the species can be utilized for future genetic improvement. Aiming to identify the genetic diversity, we conducted phylogenetic analyses of 98 wild accessions of M. integrifolia collected from 23 population sites, and 94 accessions of M. tetraphylla from 21 sites using 2,872 high-quality DArTseq-based SNP markers. The phylogenetic tree clearly distinguished clades of the two species. Within species, most of the accessions from the same population clustered together. Genetic and geographic distances among M. integrifolia populations were positively correlated (r = 0.26, p = 0.01), whereas the correlation was poor and not significant among M. tetraphylla populations (r = 0.08, p = 0.13). The relationship between genetic and geographic distance allows evaluating the relative influences of gene flow and drift on population structure within and between regions.