A transcriptomic investigation of handicap models in sexual selection
Handicap models link the evolution of secondary sexual ornaments to physiological costs and thus provide a mechanistic explanation for signal honesty in sexual selection. Two commonly invoked models, the immunocompetence handicap hypothesis (ICHH) and the oxidative stress handicap hypothesis (OSHH), propose suppression of immunocompetence or increase of oxidative stress by testosterone, but empirical evidence for both models is controversial and based on morphological and physiological assays. Here, we investigated these two models on the gene transcription level using microarrays to quantify the transcriptomic response of red grouse (Lagopus lagopus scoticus) caecal, spleen and liver tissues to experimental manipulation of testosterone levels. We used a geneontology framework to identify genes related to immune function and response to reactive oxygen species and examined how transcription levels changed under experimentally increased testosterone levels in birds with parasites present or absent. Contrary to our expectations, testosterone had virtually no effect on gene transcription in spleen and liver. A small number of genes were significantly differentially regulated in caecum, and while their functions and transcription changes are consistent with the ICHH, we found little support for the OSHH. More genes responded to testosterone in the presence rather than absence of parasites, suggesting that handicap mechanisms may be context dependent and more pronounced in the presence of adverse environmental conditions. These findings illustrate the utility of transcriptomics to investigating handicap models, suggest that classic models may not underlie the handicap mechanism, and indicate that novel emerging models involving different mediators and physiological systems should be examined.