Figure 6. The number of potential deleterious interactions among derived alleles only (left panels), or including ancestral alleles (right panels), that involve a specific derived fixation in lineage I, under a combinatorial epistatic model of hybrid incompatibility accumulation [15].

The number of potential interactions is evaluated at the most recent (top) time point. Loci that have undergone substitutions in lineage I and II are labeled with unique letters. Derived alleles, denoted with subscript ‘d’, are depicted at their time of origin along each lineage using open circles. For alleles that are derived in lineage I, ancestral alleles at the homologous locus in lineage II are depicted with closed circles (and subscript ‘a’) in the right panels. Potential interactions are depicted with arrows between alleles for each relevant pair (panels A and B) or trio (panels C and D) of alleles. A) Pairwise interactions involving an early mutation in lineage I (at locus C). B) Pairwise interactions involving a later mutation in lineage I (at locus I). In general, for pairwise interactions, at time t the possible number of derived-derived interactions (left panels) involving the ith derived allele in one lineage is simply to total number of derived alleles in the other lineage, regardless of when the ith allele arose during divergence (A and B, left panels). In contrast, the possible number of derived-ancestral interactions involving the ith derived allele in one lineage is [i-1] (A and B, right panels). C) Complex (3-locus) interactions involving a lineage I fixation that occurred early in the history of divergence between I and II (at locus C). D) Complex (3-locus) interactions involving a lineage I fixation that occurred late in the history of divergence between I and II (at locus I). For both C) and D), the second derived mutation in lineage I is at locus G. In general, for complex (3-locus) interactions, except for the first derived allele, at time t the possible number of complex derived interactions involving the ith and jth derived alleles in one lineage is simply the total number of derived alleles in the other lineage, regardless of when the ith allele arose during divergence (C and D, left panels). In contrast, except for the first derived allele, the possible number of derived-derived-ancestral interactions involving the ith and jth derived alleles in one lineage is [i-1] or [j-1], whichever is smaller (C and D, right panels).