Figure 1.

(a) Disruptive selection on a trait with an additive genetic basis generates epistasis for fitness. The trait is controlled by four genes, each gene having two alleles (denoted here as plus and minus signs). For simplicity, we represented the situation for a haploid organism, but the same property would emerge for diploid organisms. The pluses and minuses represent the relative effects of alleles on the trait (i.e. plus alleles increase the trait value and minus alleles decrease it). The fitness effect of a particular allele depends on the alleles present at other genes (i.e. epistasis for fitness). For example, a plus allele is detrimental when accompanied by three minus alleles because it will move the mean phenotype towards a larger trait value, away from the extreme and associated with lower fitness. By contrast, the same plus allele will be favoured when accompanied by at least two more plus alleles. Figure modified and redrawn from [25]. (b) An inversion located on linkage group 8 is associated with cryptic body colouration in Timema cristinae (i.e. the Mel-Stripe locus depicted here). Body colouration loci are deleted in T. cristinae green haplotypes (indel locus). The deletion is located in the vicinity of a breakpoint of the inversion. The existence of other loci associated with divergently selected traits within Mel-Stripe, but away from the indel locus, is yet to be tested and is the core objective of the current study. Mel-Stripe is estimated to have a length of at least 13.4 megabase pairs, the indel locus approximately 1.4 megabase pairs. (Online version in colour.)