The evolution of morph-biased genes in spadefoot toads

Polyphenisms allow for the expression of two or more discrete phenotypes from a single genotype in an environment dependent manner. These phenotypes are associated with extensive differences in levels of gene expression and recent work suggests that genes with differential expression between phenotypes (biased genes) evolve at elevated rates relative to genes showing no difference between phenotypes (unbiased genes). The causes of this difference in evolutionary rate remain relatively unexplored. Using a combination of molecular and genomic approaches I identified genes with biased expression between the environmentally induced alternative morphs of spadefoot toad tadpoles (genus Spea). I further characterized these morph-biased genes in closely related spadefoot toad species that do not express alternative tadpole morphologies (genus Scaphiopus). Phylogenetic estimates of protein evolutionary rates in Spea reveal that morph-biased genes are evolving more quickly than unbiased genes, but this was dependent on rate estimation method. Surprisingly, in Scaphiopus morph-biased orthologs are evolving more quickly than unbiased orthologs, regardless of estimation method. Finally, morph-biased genes in Spea are evolving no more quickly than their orthologs in Scaphiopus. In total, our work suggests that genes with higher rates of evolution are more likely to evolve differential expression, but differential expression has no direct influence on a gene's evolutionary rate