Genetic analysis and historical perspective of cashew (Anacardium occidentale L.) introduction into India.

Cashew (Anacardium occidentale L.), introduced into India about 400 years ago, is distributed widely in the coastal regions as spontaneous populations as well as in cultivation. Despite the plant's commercial exploitation, little is known about its actual introduction and diversification. We attempted to construct these events by investigating the level of genetic variation and genetic structure of cashew populations collected from different geographical regions of India. A total of 91 individuals from four populations were analysed using AFLP markers and morphometric data. AFLP analysis based on 354 polymorphic loci revealed Indian cashew to have low but relatively substantial genetic diversity for an introduced species (H(E) = 0.262 and I(S) = 0.404). Twenty-seven qualitative and quantitative traits also revealed the existence of considerable morphometric variation (24% around the mean values). Bayesian cluster analysis based on AFLP data did not indicate the existence of definite population differentiation. Morphometric analysis allocated 12% variation among all four populations, whereas AFLP variation observed in cashew individuals was entirely within populations. The results, supporting the possibility of cashew having been introduced into India repeatedly over a period of time but at a single location (west coast), are discussed and their implications for germplasm management are described.

Comparative assessment of DNA fingerprinting techniques (RAPD, ISSR and AFLP) for genetic analysis of cashew (Anacardium occidentale L.) accessions of India.

Nineteen cashew accessions were analysed with 50 random primers, 12 ISSR primers and 6 AFLP primer pairs to compare the efficiency and utility of these techniques for detecting variation in cashew germplasm. Each marker system could discriminate between all of the accessions, albeit with varied efficiency of polymorphism detection. AFLP exhibited maximum discrimination efficiency with a genotype index of 1. The utility of each molecular marker technique, expressed as marker index, was estimated as a function of average band informativeness and effective multiplex ratio. Marker index was calculated to be more than 10 times higher in AFLP than in RAPD and ISSR. Similarity matrices were determined based on the data generated by molecular and morphometric analyses, and compared for congruency. AFLP displayed no correspondence with RAPD and ISSR. Correlation between ISSR and RAPD similarity matrices was low but significant (r = 0.63; p < 0.005). The similarity matrix based on morphometric markers exhibited no correlation with any of the molecular markers. AFLP, with its superior marker utility, was concluded to be the marker of choice for cashew genetic analysis.