Genetic characterization of selected Macadamia germplasm and the implications on breeding and conservation in Kenya

Abstract

Macadamia is the most important nut crop in Kenya with an estimated annual production of about 13,510 metric tones with an estimated value of $ 4.8M. The cultivated macadamia is derived from two species; the rough-shelled M. tetraphylla and the smooth shelled M. integrifolia. The Kenyan macadamia industry relies on selection of superior trees from germplasm derived from open-pollinated seeds of the two species planted between 1946 and 1968. Twenty six macadamia accessions sampled from five populations; Bob Harries, Thika, Kirinyaga, Embu and Meru in central and eastern Kenya were analyzed using 110 polymorphic markers of six AFLP primer combinations. The highest percentage of polymorphic loci of 80.0% was observed in the Bob Harries population and the least, 67.3%, from Thika population. Expected heterozygosity was also highest in the Bob Harries population. Phylogenetic analysis based on UPGMA of genetic distances of the five populations revealed the Bob Harries population, the source of macadamia germplasm in Kenya to be closely related only to the Kirinyaga population and distantly related to the Thika population, a preservation block of selected superior germplasm. Cluster analysis based on the 26 accessions did not distinctly separate the species indicating a high level of heterozygosity and genetic mixture of the original gene pool used in Kenya. Analysis of molecular variance further indicated higher genetic variation within the populations, 96%, than among the populations, 4%, and high genetic differentiation among the accessions (theta=0.1909). The results of this study indicate high genetic diversity within the Kenyan macadamia germplasm. This information can immediately be used for enhanced breeding and more effective sampling of populations for conservation.