Reviewer #2 (Public Review):
'Hairlessness' has convergently evolved numerous times in mammals. In this paper the authors look for patterns in the rate of DNA sequence evolution across the mammalian phylogeny to identify regions of the genome that are independently evolving at similar rates in hairless mammals. The authors find that signatures of convergent accelerated sequence evolution in hairless mammals is biased towards coding and gene regulatory regions known to be involved in hair biology, likely reflecting genetic drift following hair reduction. This bias toward hair-relevant genomic regions also highlights the utility of this approach to identify new candidate regions of the genome that haven't previously been implicated in hair biology and the authors describe several intriguing coding and non-coding candidates. Authors further find that genes and putative gene-regulatory regions have non-random patterns of drift, with mutations in coding regions biased toward proteins that compose physical aspects of the hair sheath.
The analysis in this paper is centered on the RERconverge tool. Importantly, the authors have taken numerous steps to address potential issues with such an approach. One issue with RERconverge is the need to include/exclude ancestral branches as having a trait, which introduces assumptions about ancestral states. The authors controlled for this by running multiple variations of RERconverge with and without ancestral states as being 'hairless' with no major impact on results. The authors also controlled for whether certain lineages are driving the correlation signal, and found that removal of any given lineage does not impact skin or hair follicle enrichments. Finally, the authors have adequately distinguished whether other common phenotypes in hairless mammals (e.g. marine lifestyle or body size) drive the convergent signals in the dataset and found the reported genetic signatures are best explained by hair loss compared to these other traits.
The paper should be of interest to a broad selection of biologists interested in evolution, development and phylogenomic methods. The candidate genes identified in this paper provide a compelling launching point for future experimental studies into the genetic basis of hair.