What Does the Molecular Genetics of Different Types of Genes Imply?

Cytoplasmic male sterility (CMS) is a widely used trait for hybrid seed production. Although male sterility is caused by S cytoplasm (male-sterility inducing mitochondria), the action of S cytoplasm is suppressed by (), a nuclear gene. Hence, the genetics of has attained particular interest among plant breeders. The genetic model posits diversity in which an specifically suppresses the cognate S cytoplasm. Molecular analysis of loci in plants has identified various genes; however, pentatricopeptide repeat (PPR) protein (a specific type of RNA-binding protein) is so prominent as the -gene product that s have been categorized into two classes, PPR and non-PPR. In contrast, several shared features between PPR- and some non-PPR s are apparent, suggesting the possibility of another grouping. Our present focus is to group s by molecular genetic classes other than the presence of PPRs. We propose three categories that define partially overlapping groups of s: association with post-transcriptional regulation of mitochondrial gene expression, resistance gene-like copy number variation at the locus, and lack of a direct link to (a mitochondrial ORF associated with CMS). These groups appear to reflect their own evolutionary background and their mechanism of conferring S cytoplasm specificity.