Genetic diversity and population structure of cotton (Gossypium spp.) of the New World assessed by SSR markers

A global analysis of cotton (Gossypium spp.) genetic diversity is the first step to understanding its geographical distribution, dissemination, genetic relatedness, and population structure. To assess the genetic diversity and population structure in Gossypium species, 111 cotton accessions representing five allotetraploids (AD(1)-AD(5) genomes), 23 Asiatic diploids of the Old World (A(1) and A(2) genomes), and 82 diploids of the New World subgenus Houzingenia (D-1-D-11 genomes) species were assessed using simple sequence repeats (SSR) markers with wide genome coverage. The mean genetic distance (GD) between the two most important New World tetraploid cottons (Upland (Gossypium hirsutum L.) and Pima (Gossypium barbadense L.)) was 0.39. Among the three shrub type sections (Houzingenia, Integrifolia, and Caducibracteolata) and three arborescent sections (Erioxylum, Selera, and Austroamericana), the GD ranged between 0.19 and 0.41. Phylogenetic analyses clustered all species into distinct phylogenetic groups, which were consistent with genomic origin, evolutionary history, and geographic distribution or ecotypes of these accessions, suggesting the existence of clear structured strata. With all of the genomes, the highest statistical analysis of Structure test through measurements of ad hoc (Delta K) occurred at K = 2, with group Q1 with the Old World diploid A genomes and with group Q2 with all the New World diploids of the D genome. AD genome accessions shared nearly equal alleles from both Q1 and Q2 groups. With all of the diploids of the New World D genomes, the highest value of Delta K occurred at K = 5. These results are consistent with the fundamental knowledge of tetraploid AD-genome formation and the rapid radiation of the American diploid cotton linage that took place somewhere in southwestern Mexico, followed by a differentiation-speciation during angiosperm evolution. In addition, SSR markers provide an alternative solution for distinguishing phylogenetic relationships between accessions of different ecotypes and for elucidating population structure of cottons of the New World.