Gene transfer and nucleotide sequence evolution by Gossypium cytoplasmic genomes indicates novel evolutionary characteristics.

The DNA fragments transferred among cotton cytoplasmic genomes are highly differentiated. The wild D group cotton species have undergone much greater evolution compared with cultivated AD group. Cotton (Gossypium spp.) is one of the most economically important fiber crops worldwide. Gene transfer, nucleotide evolution, and the codon usage preferences in cytoplasmic genomes are important evolutionary characteristics of high plants. In this study, we analyzed the nucleotide sequence evolution, codon usage, and transfer of cytoplasmic DNA fragments in Gossypium chloroplast (cp) and mitochondrial (mt) genomes, including the A genome group, wild D group, and cultivated AD group of cotton species. Our analyses indicated that the differences in the length of transferred cytoplasmic DNA fragments were not significant in mitochondrial and chloroplast sequences. Analysis of the transfer of tRNAs found that trnQ and nine other tRNA genes were commonly transferred between two different cytoplasmic genomes. The Codon Adaptation Index values showed that Gossypium cp genomes prefer A/T-ending codons. Codon preference selection was higher in the D group than the other two groups. Nucleotide sequence evolution analysis showed that intergenic spacer sequences were more variable than coding regions and nonsynonymous mutations were clearly more common in cp genomes than mt genomes. Evolutionary analysis showed that the substitution rate was much higher in cp genomes than mt genomes. Interestingly, the D group cotton species have undergone much faster evolution compared with cultivated AD groups, possibly due to the selection and domestication of diverse cotton species. Our results demonstrate that gene transfer and differential nucleotide sequence evolution have occurred frequently in cotton cytoplasmic genomes.