Phylogenetic and molecular evolutionary analyses of Ty1-copia group retrotransposons in cultivated Egyptian cotton, Gossypium barbadense L.

The Ty1-copia group retrotransposons have been characterized in cultivated Egyptian cotton, G.barbadense L., using degenerate PCR primers for their reverse transcriptase (RT) domains. Comparative nucleotide and amino acid sequences analyses showed that G. barbadense Ty1-copia RT sequences are heterogeneous and this heterogeneity is resolved into 11 distinct families. The high ratio of synonymous to nonsynonymous changes indicates that there is a strong selection for the RT domain of these families. Phylogenetic analysis revealed that two cultivated G. barbadense RT families are closely related to Ty1-copia group retrotransposons present in other plant species. In other words, these families mirror their own phylogenies rather than that of their host cultivars. On the other hand, the remaining G. barbadense RT families are closely related to their respective Gossypium species. These data show that Ty1-copia group retrotransposons tend to span species boundaries, suggesting that they existed early in plant evolution, and were diverged into heterogeneous sequences prior to modern plant species divergence.