Alternative models for the evolution of MHM1 region in chicken. Reduced male-to-female expression ratios near MHM1 in chicken could be due to female up-regulation (A) or male down-regulation (B). In scenario A, low expression in females is the ancestral state, and up-regulation in females has evolved recently in chicken. An expression pattern similar to that shown in A, in other words, higher expression in female chickens than in female tit and ostrich, would suggest that the reduced male-to-female expression ratios seen in chicken evolved due to up-regulation in females. In scenario B, high expression in males is ancestral and the depicted pattern of expression would suggest recently evolved down-regulation in males. For A and B, the time for the split of avian lineages is from Jarvis et al. (2014) and Zhou et al. (2014). (C) Pairwise species differences in expression of the MHM1-neighboring genes (adult brain). Expression was averaged across samples per species, and the pairwise expression difference (the ratio of expression of species 1 to the expression of species 2) was log2-scaled. Only orthologs present in all species were used (numbers of genes compared are in parentheses). The statistical significance was evaluated using paired Mann–Whitney U tests. (**) P < 0.01, (*) P < 0.05, (NS) not significant.
