How does breeding system variation modulate sexual antagonism?

Abstract

The study of sexually antagonistic (SA) traits remains largely limited to dioecious (separate sex), mobile animals. However, the occurrence of sexual conflict is restricted neither by breeding system (the mode of sexual reproduction, e.g. dioecy or hermaphroditism) nor by sessility. Here, we synthesize how variation in breeding system can affect the evolution and expression of intra- and inter-locus sexual conflicts in plants and animals. We predict that, in hermaphrodites, SA traits will (i) display lower levels of polymorphism; (ii) respond more quickly to selection; and (iii) involve unique forms of interlocus conflict over sex allocation, mating roles and selfing rates. Explicit modelling and empirical tests in a broader range of breeding systems are necessary to obtain a general understanding of the evolution of SA traits.

Figure 1.

Resolution and outcome of intralocus sexual conflict in dioecious and hermaphroditic species. The figure illustrates a sexually antagonistic mutation (SA, black square) increasing female fitness, but decreasing male fitness. (a) Comparison of the regulatory mechanisms that can occur in response to the presence of an SA allele and the implications for evolution and the intensity of selection. The hatched SA allele represents sexual masking of this allele owing to sex-specific regulation. Sexual masking occurs here in males because the mutation is beneficial to females but would occur in females in the reciprocal case. (b) Representation of the fitness curves (solid lines) and phenotype distributions (dashed lines) for a sexually antagonistic trait. Thick red and blue arrows show female and male selection pressures, respectively. Thin black arrows display the constraint owing to intersexual correlation.

Figure 2.

Scenarios for interlocus conflict in hermaphrodites. In all graphs, focal individuals (red curves) express their current trait optimum (dashed line). Their partners maximize fitness at different values than focal trait expression (blue double lines), and therefore aim at removing focals from their trait optimum (double arrow). (a) Hermaphrodites typically maximize fitness at roughly balanced sex allocation, whereas their partners gain most from inseminating focals with highly female-biased sex allocation (conflict over sex allocation). (b) Among prospective mates, the propensity to assume the (sometimes costly) female mating role may be low unless depleted sperm stores make sperm receipt necessary. This may conflict with the partners’ interest to achieve insemination (conflict over mating roles). (c) When inbreeding depression is low, focal sperm recipients may prefer to produce some selfed offspring, inevitably conflicting with the interests of a sperm donating partner (conflict over selfing rate).