In extreme cases, one host sex may be so rare (e.g., males in cyclically parthenogenetic species, such as aphids, are absent for large parts of the year) that the parasite rarely encounters them (Figure 2B). In this case, parasites sampled from the rare host would actually be adapted to the other sex (the common sex), and parasites from both origins would www.selleckchem.com/products/Abiraterone.html be fitter in the common host sex (Figure 1B). Alternatively, the parasite could adapt to a host trait that is found in only one host sex, such as primary or secondary sexual traits. The parasite populations may adapt only to this sex, even if the likelihood of encountering the other sex is high (Figure 2C). In this case, parasites sampled in the host sex to which they are not adapted (if this is possible), would perform better in the opposite host sex (Figure 1B).

Plastic Sex-Specific Disease Expression Phenotypic plasticity, a property whereby the same genotype translates into distinct phenotypes depending on the environment, is a common way for organisms to deal with fluctuating environments [15]. Parasites facing distinct male and female host environments might have evolved plasticity in relation to those environments and be able to express host sex�Cspecific traits accordingly. Following Scheiner [16], the plastic expression of a trait is favored when 1) variability among environments is high, 2) environments are equally abundant, 3) the strength of selection is equal in both environments, 4) the environmental cue determining the phenotype is highly correlated with the environment of selection, and 5) the cost of plasticity, which is the cost of maintaining the genetic and cellular machinery necessary to be plastic, is compensated by its advantage.

If these conditions are met, phenotypic plasticity is expected to evolve (Figure 2D); otherwise, a single generalist phenotype will be favored. If there is plasticity, then parasites originating from different host sexes will be equally fit when tested in the same sex environment (Figure 1A). Host Population Structure and Parasite Transmission The evolution of sex-specific parasite adaptation is affected by the likelihood of parasites being transmitted within or between host sexes (Figure 2). This depends strongly on the host species and the ecological circumstances (Table 2). Here, we focus mainly on cases where the likelihood of encountering a host of the opposite sex is low.

For example, males and females are not always equally abundant and, therefore, parasite transmission will occur among the most common sex. Biased sex ratios are often observed in natural populations [17]�C[20], and are even an intrinsic characteristic of certain species, for example, the abundance of females in cyclically parthenogenetic species (e.g., aphids, cladocera, rotifers), in sequential hermaphrodite species [21], and in many haplodiploid GSK-3 species such as ants, bees, wasps, and mites.