Sex differences in gene expression
We then quantified how sex was associated with chest skin gene expression while including sample collection year in the model (year was significantly associated with both the first and second principal component of gene expression; PC1: year \(\beta\)=0.39, P =0.02; PC2: year \(\beta\)=-0.22, P =0.003, Table S1, Fig. S2 ). Males and females differed along the first principal component of gene expression while including year as a predictor variable (PC1: sex\(\beta\)=-0.39, P =0.01, Fig. S3 ; PC2: sex\(\beta\)=-0.13, P =0.049). The results of the principal component analysis did not change when we removed genes located on sex chromosomes and analyzed only those found on autosomes (Fig. S4 ).
At the level of the individual genes, we found that 10.5% of the 10,212 detectably expressed genes exhibited significant differential expression across males and females (n genes=1,068, FDR<20%), in a model controlling for the effects of RQN, RNA concentration, RNA extraction date, and year of sample collection. Of the 1,068 differentially expressed genes, 201 genes were female-biased and 867 genes were male-biased. The average standardized sex-bias gene expression level for each individual illustrates that subadult males exhibited an intermediate gene expression pattern that differed from adult males (\(\beta\)=-0.66, P =0.003) and adult females (\(\beta\)=0.45, P =0.04; Fig. 3B). The sample size for subadult females (n =2) was too small to draw conclusions about sex-bias gene expression trends in this age category.