Interlocus sexual conflict
Interlocus sexual conflict is a type of sexual conflict that occurs through the interaction of a set of antagonistic alleles at two or more different loci in males and females, resulting in the deviation of either or both sexes from the fitness optima for the traits.
Interlocus sexual conflict involves a co-evolutionary arms race between the two sexes in which either sex evolves a set of antagonistic adaptations that are detrimental to the fitness of the other sex. Interlocus sexual conflict can occur over aspects of male–female interactions such as mating frequency, fertilization, relative parental effort, female remating behavior, and female reproductive rate. The evolutionary pathways resulting from interlocus sexual conflict form part of interlocus contest evolution.
Versus intralocus sexual conflict
The genetic basis of the distinction between interlocus sexual conflict and intralocus sexual conflict is the location of the interacting antagonistic alleles. Conflict in which the antagonistic alleles are located at the same locus is termed intralocus sexual conflict, and conflict in which the antagonistic alleles are located at different loci in both sexes is termed interlocus sexual conflict. Intralocus sexual conflict occurs when males and females undergo different selective pressures at the same locus, resulting in either sex limiting the evolution of the other sex. In contrast, interlocus sexual conflict occurs when males and females undergo different selective pressures at different loci, potentially leading to agonistic coevolution.
Interlocus sexual conflict has been proposed as a cause of sexually agonistic coevolution. The model of sexual coevolution is as follows: When an allele at one locus in males increases their fitness while reducing the fitness of the females interacting with them, the females would evolve a counter-adaptation at a different locus to reduce the harm. In other words, females may exhibit resistance by developing favorable traits to reduce the direct costs implemented by males. For interlocus sexual conflict to be a valid cause of agonistic coevolution, the harm induced by the males across all loci has to outweigh the indirect benefits that the females gain by interacting with males.
Well-evidenced examples of interlocus sexual conflict come exclusively from the insect world; examples outside of these taxa are theoretical, though currently not well studied. Importantly, most examples of sexual conflict are not categorized into interlocus sexual conflict or intralocus sexual conflict as the genetic locations of the interacting alleles are not known or specified. Nonetheless, certain experimental results have provided evidence of interlocus sexual conflict as a putative cause of agonistic coevolution.
In the yellow dung fly Scathophaga stercoraria, females can be injured in the battles between males suitors. Males are selected to evolve traits for competitive ability that would increase their reproductive success, but females would evolve a set of antagonistic adaptations to reduce their chances of being injured in battles between the males. Therefore, there is an interlocus sexual conflict in males and females that could possibly lead to an "unresolvable evolutionary chase" when a novel trait in males increases their competitive ability while harming the females.
In a laboratory study of Drosophila melanogaster, a mutation that reduces the attractiveness of females is introduced into the genome of the experimental females. By reducing the attractiveness of the females expressing the trait, the mutation provides females with resistance to the direct costs of re-mating and male courtship. The results of this experiment show that the resistance allele significantly accumulated in the experimental group, suggesting that the direct costs of male-courtship are greater than the indirect benefits of male-courtship and thereby providing evidence for interlocus sexual conflict.
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