Molecular evolution of the TAC1 gene from rice (Oryza sativa L.)

Tiller angle is a key feature of the architecture of cultivated rice (Oryza sativa), since it determines planting density and influences rice yield. Our previous work identified Tiller Angle Control 1 (TAC1) as a major quantitative trait locus that controls rice tiller angle. To further clarify the evolutionary characterization of the TAC1 gene, we compared a TAC1-containing 3164-bp genomic region among 113 cultivated varieties and 48 accessions of wild rice, including 43 accessions of O. rufipogon and five accessions of O. nivara. Only one single nucleotide polymorphism (SNP), a synonymous substitution, was detected in TAC1 coding regions of the cultivated rice varieties, whereas one synonymous and one nonsynonymous SNP were detected among the TAC1 coding regions of wild rice accessions. These data indicate that little natural mutation and modification in the TAC1 coding region occurred within the cultivated rice and its progenitor during evolution. Nucleotide diversities in the TAC1 gene regions of O. sativa and O. rufipogon of 0.00116 and 0.00112, respectively, further indicate that TAC1 has been highly conserved during the course of rice domestication. A functional nucleotide polymorphism (FNP) of TAC1 was only found in the japonica rice group. A neutrality test revealed strong selection, especially in the 3'-flanking region of the TAC1 coding region containing the FNP in the japonica rice group. However, no selection occurred in the indica and wild-rice groups. A phylogenetic tree derived from TAC1 sequence analysis suggests that the indica and japonica subspecies arose independently during the domestication of wild rice.