Natural selection promotes the conservation of linkage of co-expressed genes.

Although there is increasing evidence that eukaryotic gene order is not always random, there is no evidence that putatively favourable gene arrangements are preserved by selection more than expected by chance. In yeast (Saccharomyces cerevisiae), for example, co-expressed genes tend to be linked, but whether such gene pairs tend to remain linked more often than expected under null neutral expectations is not known. We show using gene pairs in the S. cerevisiae-Candida albicans comparison that highly co-expressed gene pairs are conserved as pairs at about twice the average rate. However, co-expressed genes also tend to be in close physical proximity and, as expected from a null neutral model, genes (be they co-expressed or not) that are physically close together tend to be retained more often. This physical proximity, however, only accounts for a small proportion of the enhanced degree of conservation of co-expressed gene pairs. These results demonstrate that purely neutralist models of gene order evolution are not realistic.