This effect of host sex, recorded even in humans, has mainly been attributed to sex-specific differences in immune response, hormones, and resource allocation Tofacitinib Citrate side effects [1],[6]�C[11]. For example, the male hormone negatively affects the efficiency of the immune system. Other sex-dependent characteristics, however, including morphological, physiological, behavioral, dietary, and life history traits, may also contribute to these observations. Parasite populations are expected to have adapted to the characteristics of their most common host type [12]. If a parasite population evolves mainly in one sex (e.g., those transmitted among extremely sex-biased host populations), sex-specific characteristics may impact how the parasite adapts to that host (Table 1).

Therefore, without considering the sex of the host in which the parasite primarily evolved, it is difficult to disentangle whether sex-biased parasitism is the result of differences among hosts only, or if adaptation of parasites contributed to these characteristics as well. Table 1 Examples of sexually dimorphic traits that might influence parasite evolution. Here, we argue that the sex of the host can impose selection on the parasite itself, which in turn will contribute to variation in disease prevalence and expression among male and female hosts. This hypothesis could be tested in systems where hosts and parasites can be used in experimental infections and where parasite isolates can be obtained from both host sexes.

But, to our knowledge, such experiments have never been done, probably because it is assumed that the prevalence and severity of disease found in different host sexes are caused by the characteristics of the host alone. We propose that parasite adaptation to specific host sexes can lead to three different evolutionary outcomes for the parasite: 1) parasites that adapt differently to each sex, leading to dimorphism in the parasite population, here called ��host sex�Cspecific dimorphism��, 2) parasites that specialize on only one sex: ��single sex specialization��, and 3) parasites with phenotypically plastic traits, whose expression is dependent on the sex of their host: ��plastic sex-specific disease expression��. We will begin by explaining in more detail these three evolutionary scenarios using a simple experimental design to help distinguish them (Box 1, Figure 1).

Batimastat The conditions under which these scenarios may evolve differ strongly. We also attempt to pinpoint those conditions that are likely to play a crucial role for the evolution of sex-specific parasite adaptation and lead either to monomorphic parasite populations or to dimorphic parasite populations (Figure 2). Then, we discuss how host demographic properties, notably host sex ratio and social structure, can influence the extent to which the parasite evolves.