Origin of a function by tandem gene duplication limits the evolutionary capability of its sister copy.

The most remarkable outcome of a gene duplication event is the evolution of a novel function. Little information exists on how the rise of a novel function affects the evolution of its paralogous sister gene copy, however. We studied the evolution of the feminizer (fem) gene from which the gene complementary sex determiner (csd) recently derived by tandem duplication within the honey bee (Apis) lineage. Previous studies showed that fem retained its sex determination function, whereas the rise of csd established a new primary signal of sex determination. We observed a specific reduction of nonsynonymous to synonymous substitution ratios in Apis to non-Apis fem. We found a contrasting pattern at two other genetically linked genes, suggesting that hitchhiking effects to csd, the locus under balancing selection, is not the cause of this evolutionary pattern. We also excluded higher synonymous substitution rates by relative rate testing. These results imply that stronger purifying selection is operating at the fem gene in the presence of csd. We propose that csd's new function interferes with the function of Fem protein, resulting in molecular constraints and limited evolvability of fem in the Apis lineage. Elevated silent nucleotide polymorphism in fem relative to the genome-wide average suggests that genetic linkage to the csd gene maintained more nucleotide variation in today's population. Our findings provide evidence that csd functionally and genetically interferes with fem, suggesting that a newly evolved gene and its functions can limit the evolutionary capability of other genes in the genome.