But in this case, the parasite is either specialized on one host sex (single-sex specialization), selleck compound and/or one host sex is a more suitable ��habitat�� than the other. To distinguish between no sex-specific adaptation and plastic sex-specific disease expression in the first case, or between single-sex specialization and a sex bias in host suitability in the second, you would need to investigate parasitic traits that you suspected might represent specific adaptations. It would be necessary to determine whether and how these also differ in relation to the test and the origin environment. Finally, when parasite performance depends on a combination of test and origin environments (i.e., when there is an interaction effect between the two factors; Figure 1C) we can conclude that the parasites sampled in male and female hosts have diverged and are sex-specifically adapted.

In addition to these quantitative analyses of parasite phenotypic traits, a population genetic approach can provide further information. Population genetic methods can be used to estimate the extent of genetic divergence between parasites collected from male versus female hosts [79] and to find candidate loci under selection. Population genetic methods have been used, for example, to establish differences between HIV viral populations sampled in humans reporting on distinct subtypes associated with male homosexual versus heterosexual transmission [80]. Three Paths to Adaptation Host Sex�CSpecific Dimorphism Male and female hosts may represent very different environments to which parasites adapt specifically.

This is analogous to local adaptation, where resident genotypes in a specific environment are, on average, fitter than genotypes originating from other environments [13]. Local adaptation implies antagonistic pleiotropy, whereby the selected alleles have opposite effects on fitness in different environments��in other words, there is a trade-off in performance between the environments [14]. One can equally view the two host sexes as two different environments. The trade-off is expected to result in parasite origin �� host sex interactions for parasite fitness (Figure 1C). In that context, the evolution of parasite divergence in a sexual host depends mainly on two parameters, the extent to which the host is sexually dimorphic (difference between environments) and the likelihood of a parasite encountering the opposite sex��the alternative environment��during transmission (Figure 2).

The latter is conceptually similar to gene flow between environments. If parasite populations Carfilzomib are structured by host sex, the parasite populations may have the opportunity to adapt to the conditions specific to the host sex they encounter most often. Thus, the parasite would evolve a host sex�Cspecific dimorphism (Figure 2A).