A preliminary analysis of genome structure and composition in Gossypium hirsutum.

Results: 142 BAC sequences from Gossypium hirsutum cv. Maxxa were downloaded http://www.ncbi.nlm.nih.gov webcite and confirmed. These BAC sequence analysis revealed that the tetraploid cotton genome contains over 70,000 candidate genes with duplicated gene copies in homoeologous A- and D-subgenome regions. Gene distribution is uneven, with gene-rich and gene-free regions of the genome. Twenty-one percent of the 142 BACs lacked genes. BAC gene density ranged from 0 to 33.2 per 100 kb, whereas most gene islands contained only one gene with an average of 1.5 genes per island. Retro-elements were found to be a major component, first an enriched LTR/gypsy and second LTR/copia. Most LTR retrotransposons were truncated and in nested structures. In addition, 166 polymorphic loci amplified with SSRs developed from 70 BAC clones were tagged on our backbone genetic map. Seventy-five percent (125/166) of the polymorphic loci were tagged on the D-subgenome. By comprehensively analyzing the molecular size of amplified products among tetraploid G. hirsutum cv. Maxxa, acc. TM-1, and G. barbadense cv. Hai7124, and diploid G. herbaceum var. africanum and G. raimondii, 37 BACs, 12 from the A- and 25 from the D-subgenome, were further anchored to their corresponding subgenome chromosomes. After a large amount of genes sequence comparison from different subgenome BACs, the result showed that introns might have no contribution to different subgenome size in Gossypium.