Epistatic interactions are widespread, and many of these interactions involve combinations of alleles at different loci that are deleterious when present in the same individual. The average genetic environment of sex-linked genes differs from that of autosomal genes, suggesting that the population genetics of interacting X-linked and autosomal alleles may be complex. Using both analytical theory and computer simulations, we analyzed the evolutionary trajectories and mutation–selection balance conditions for X–autosome synthetic lethals and steriles. Allele frequencies follow a set of fundamental trajectories, and incompatible alleles are able to segregate at much higher frequencies than single-locus expectations. Equilibria exist, and they can involve fixation of either autosomal or X-linked alleles. The exact equilibrium depends on whether synthetic alleles are dominant or recessive and whether fitness effects are seen in males, females, or both sexes. When single-locus fitness effects and synthetic incompatibilities are both present, population dynamics depend on the dominance of alleles and historical contingency (i.e., whether X-linked or autosomal mutations occur first). Recessive synthetic lethality can result in high-frequency X-linked alleles, and dominant synthetic lethality can result in high-frequency autosomal alleles. Many X–autosome incompatibilities in natural populations may be cryptic, appearing to be single-locus effects because one locus is fixed. We also discuss the implications of these findings with respect to standing genetic variation and the origins of Haldane’s rule.

The similarities between Alfred Wallace and Charles Darwin, the pioneering thinkers who separately formulated theories of evolution by natural selection, are striking. Both were ardent students of physical and biological nature whose extensive studies in remote locales left indelible marks on them. Independently influenced by the writings of the economist Thomas Malthus, Wallace and Darwin each recognized that animals and plants were engaged in a “struggle for existence,” an insight both men viewed as critical for the formulation of their theories of evolution (Browne 1995, 2002; Shermer 2002; Smith 2003–).

This intriguing study finds that social organization, genome size, and transposable element abundance are correlated in snapping shrimp, with eusocial species having larger genomes and more transposable elements. Possible explanations for this pattern are discussed. Eusocial organisms are characterized by a division of reproduction output, with only one or a few individuals doing the vast majority of the reproductive output. Although common in the insect order Hymenoptera (bees, wasps, and ants), eusociality is rare outside that order. Snapping shrimp of the genus Synalpheus are one of these exceptions; this genus has many eusocial species as well as many others with different levels of social organization, making it an attractive taxon to examine correlates of social organization. Across the phylogeny, transitions to eusociality are associated with increases in genome size and transposable element abundance. Different subclasses of transposable elements accumulate in different lineages of eusocial shrimp, suggesting that the overall number - and not the identity - of the transposable elements is the key feature. One likely explanation for the association is that eusocial species tend to have lower effective population sizes. Accordingly, these species are more prone to accumulate slightly deleterious genetic material, such as most transposable elements. The authors also consider other hypotheses.