The ancestral SIR2 gene may have duplicated and subfunctionalized prior to loss of one paralog.

(A) A model is shown depicting the hypothesized evolutionary history of the SIR2 gene within the CTG clade. An early duplication followed by subfunctionalization partitioned the ancestral functions. The product of one gene (HST1) acted at the rDNA locus, whereas the other (SIR2) possessed different functions, perhaps including a role at telomeres. In some lineages, including C. lusitaniae, only the HST1 gene was retained. (B) A species tree indicates with an asterisk (*) organisms whose genomes encode the additional SIR2 gene. The maximum likelihood tree is based on [43].