Comparative karyotype analysis of Indian Dipcadi (Asparagaceae): insights on chromosome evolution and biogeography

Karyotypes of 23 accessions of Indian Dipcadi Medik. (Asparagaceae) revealed three counts: 2n = 12, 20 and 22. Chromosomes had median (m), submedian (sm), subterminal (st) and terminal (t) region centromeres. It was inferred that bimodality (asymmetric karyotype) has given rise to trimodality (symmetric karyotype). Multivariate analysis based on base number (x), 2n, total chromosome length (TCL), coefficient of variation of chromosome length, coefficient of variation of centromeric index and mean centromeric asymmetry grouped the accessions in four clusters. Cluster I and II included accessions with 2n = 12 chromosomes. Accessions in Cluster III and IV had 2n = 20 and 22 chromosomes, respectively. Reconstruction of ancestral haploid chromosome number using ChromEvol 2.0 revealed n = 6. The main events that led to the evolution of karyotype were base number transitions, chromosome losses, gain and demi-duplication, with base number transition being the most frequent in the internal branches of phylogeny. Molecular dating and biogeographic analysis inferred that Dipcadi originated in Africa and diversified between 18.14 and 34.18 MYA, with a median age of 25.33 MYA. The age of the Indian Dipcadi was between 9.53 and 18.48 MYA, with a median age of 13.59 MYA. From an African origin, there have been two radiations within Dipcadi, one in a region encompassing East Africa and Arabia and another in India. We recommend Dipcadi as a model system for understanding the evolution of highly specialized bimodal karyotypes and differences in the genome of small and large chromosome both at structural and functional level.