Comparison of dN/dS ratios shows little evidence for faster-Z effect in Furcifer chameleons after controlling for gene-specific evolutionary rates.

Abstract

The faster-X/Z effect hypothesis states that genes linked to X/Z chromosomes should accumulate mutations faster than autosomal genes. Although faster evolution of X/Z-linked genes has been reported in several plant and animal lineages, conflicting results have been reported in others. We examined the faster-Z effect in chameleons of the genus Furcifer, a lineage with differentiated ZZ/ZW chromosomes for at least 20 million years. We sequenced the genomes of four species of Furcifer chameleons in the Illumina platform and compared the substitution rates of synonymous and non-synonymous mutations and their ratios among autosomal, Z-specific, and pseudoautosomal protein-coding genes. The inclusion of two chameleon outgroups lacking the differentiated ZZ/ZW sex chromosomes allowed us to control for gene-specific evolutionary rates that might confound the testing of the faster-X/Z effect. Significant differences in evolutionary rates were found between autosomal, Z-specific, and pseudoautosomal genes of Furcifer chameleons. However, the inclusion of the outgroups with different sex chromosomes suggests that these genes had different evolutionary rates prior to their incorporation into the differentiated ZZ/ZW sex chromosomes of the Furcifer genus. The results highlight the need to control for differences in the evolutionary rates of individual genes when testing for the faster X/Z effect.