Rapid evolutionary divergence of Gossypium barbadense and G. hirsutum mitochondrial genomes

Abstract

Background: The mitochondrial genome from upland cotton, G. hirsutum, was previously sequenced. To elucidate the evolution of mitochondrial genomic diversity within a single genus, we sequenced the mitochondrial genome from Sea Island cotton (Gossypium barbadense L.). Methods: Mitochondrial DNA from week-old etiolated seedlings was extracted from isolated organelles using discontinuous sucrose density gradient method. Mitochondrial genome was sequenced with Solexa using paired-end, 90 bp read. The clean reads were assembled into contigs using ABySS and finished via additional fosmid and BAC sequencing. Finally, the genome was annotated and analyzed using different softwares. Results: The G. barbadense (Sea Island cotton) mitochondrial genome was fully sequenced (677,434-bp) and compared to the mitogenome of upland cotton. The G. barbadense mitochondrial DNA contains seven more genes than that of upland cotton, with a total of 40 protein coding genes (excluding possible pseudogenes), 6 rRNA genes, and 29 tRNA genes. Of these 75 genes, atp1, mttB, nad4, nad9, rrn5, rrn18, and trnD(GTC)-cp were each represented by two identical copies. A single 64 kb repeat was largely responsible for the 9 % difference in genome size between the two mtDNAs. Comparison of genome structures between the two mitochondrial genomes revealed 8 rearranged syntenic regions and several large repeats. The largest repeat was missing from the master chromosome in G. hirsutum. Both mitochondrial genomes contain a duplicated copy of rps3 (rps3-2) in conjunction with a duplication of repeated sequences. Phylogenetic and divergence considerations suggest that a 544-bp fragment of rps3 was transferred to the nuclear genome shortly after divergence of the A- and D- genome diploid cottons. Conclusion: These results highlight the insights to the evolution of structural variation between Sea Island and upland cotton mitochondrial genomes.