Although gender and ethnicity affect the expression of fewer genes than location, the plot of expression PC1 by PC2 for the most differentially expressed 1,500 genes in Figure 4 indicates that for many genes the interaction between these three factors is quite complex. This can also be seen in the expression profiles of characteristic individual genes (Supplementary Fig. 3 online). Boutroch and Ighrem villagers in general separate along PC1, while high values of PC2 are obtained for all Boutroch residents (cluster 1) and for Arab women in Ighrem (cluster 2). Amazigh women from Ighrem (cluster 3) and the Ighrem men (cluster 4) have lower values of PC2 similar to those observed for all Agadir residents. The simplest interpretation is that cultural or behavioral differences, likely including time spent outside the village, contribute strongly to the observed gender and ethnicity effects. Deeper sampling would be required to firmly establish whether intrinsic biological differences between the sexes and/or populations also make significant contributions to expression divergence in lymphocytes, as they appear to do for lymphoblast cell lines grown in culture16–19.
