Cross section of genetic diversity in mainland and insular populations of Costus speciosus (Koen ex. Retz.) Sm. using SPAR markers reveal patterns linked to allopolyploidy and biogeography

Costus speciosus (Koen ex. Retz.) Sm. is a major source of diosgenin, used for the commercial synthesis of cortisone, sex hormones and contraceptives. The genetic diversity analysis in wild populations of C. speciosus from 3 biogeographic regions viz., Western Ghats (WG), Eastern Ghats (EG) and Andaman and Nicobar Islands (AN) were done using 2 different Single Primer Amplification Reaction (SPAR) methods. A total of 70 accessions spanning these regions were used in the present study. The assay yielded a total of 314 amplicons of which 268 were polymorphic, exhibiting 85.35% of polymorphism. The prevalence of high rate of genetic differentiation (mean Gst = 0.90) and low gene flow (mean Nm= 0.06) are the main attributes of the observed low diversity in these populations. The accessions clustered broadly under 2 major groups corresponding to the three biogeographic zones with insular populations diverse from the mainland. This was further resolved by AMOVA analysis. C. speciosus is found to exist in different cytotypes exhibiting allopolyploidy. The differences in distribution and genetic fitness of the population from EG and WG may be attributed to the allopolyploid nature of the taxa. In the present study, Island populations comprise very low heterozygosity (Ht = 0.10) suggesting that the rate of fixation is more in these populations. Similarly, the rate of gene flow was almost absent (Nm = 0.02). The higher levels of genetic similarity (0.99) may be due to an increase in fixation of the genes resulting from allopolyploidy. This is the first study on comparative genetic diversity of C. speciosus using SPAR markers