Characterization and adaptive evolution of α-tubulin genes in the Miscanthus sinensis complex (Poaceae)

To investigate the organization of and mode of selection in the α-tubulin genes, full-length α-tubulin genes were cloned from four intraspecific taxa of Miscanthus sinensis and its close relatives M. floridulus and M. condensatus using standard polymerase chain reaction (PCR) and rapid amplification of genomic ends (RAGE)-PCR strategies. Genealogical analysis of angiosperms recovered a monophyletic group of Miscanthus α-tubulin genes, which is homologous to the tua5 locus of maize. Two clusters of nearly equal frequency revealed paraphyly within each Miscanthus taxon. Between-cluster recombination was frequent. Additional evidence for cooccurrence of two haplotypes within individuals and a large-scale crossover all suggested a likely allelic relationship between the Miscanthus clusters. Given a long between-species divergence time in Miscanthus, wide occurrence of the trans-species polymorphisms in α-tubulin genes and the approximately equal frequency of each allelic type make it extremely unlikely that α-tubulin diversity has been maintained under neutrality. Balancing selection may have contributed to such an apportioning of genetic variability as well as to high levels of genetic variation in α-tubulin and higher substitution rates at synonymous sites of exons than at intron bases of M. sinensis. In addition, certain effects of demographic oscillation may have distorted the scenario of a functional locus operating under balancing selection.