The mite Varroa destructor, an ectoparasite of bees and a great problem in apiculture, has a life cycle that includes a phase on adult bees, where the parasite spreads, and a phase on the developing host individuals inside the brood cells, where it reproduces [90]. In its original host, the Eastern honey bee Apis cerana, http://www.selleckchem.com/products/BAY-73-4506.html the mite reproduces exclusively in the presumptive drone (male bee) cells [76],[77],[91]. Mites carried into the brood cells by the adult nursing workers will stay in the brood cell if the larva within that cell is a presumptive drone, but not if it is a developing worker or queen (being repelled by a substance in the royal jelly fed to these larvae [92]). Brood cells with worker larvae are typically much less frequently visited by nursing adults [93], and this might have been the original trigger of the sex bias in parasite infection.

In the more recent host Apis mellifera, where the parasite can reproduce in both drone and worker larvae, the difference in nurse care can partly explain that drone cells are around 10-fold more infected than worker cells [93],[94]. Manipulating the Sex of the Host When a parasite is highly specialized on the characteristics of one host sex, infection of the ��wrong�� host type can carry high fitness costs; for example, if one sex-specific aspect of host anatomy is necessary for parasite growth or transmission. For sex-specialized parasites exposed to both host sexes, the cost of infecting the less suitable host type might be overcome by either a plastic response (i.e.

, the parasite will express different traits in different host types) or the manipulation of the host (i.e., the parasite will manipulate the traits of the host of the ��wrong�� sex). Host-sex manipulation has been described, for example, for parasitic barnacles of the genus Sacculina, which infect and sterilize crabs [95]. The parasite grows in the place where the host eggs are incubated (i.e., underside of the rear thorax), and spreads when female hosts perform egg-laying behavior. When these parasites infect male crabs, they induce the feminization of both morphology and behavior of infected males and, as a consequence, the parasites can be transmitted. The mechanism by which this feminization is induced is not well understood, but presumably involves the secretion of hormones by parasites [96].

If this secretion occurs inside male hosts but not inside female hosts, one can talk about plasticity in parasite traits relative to host sex. If, on the other hand, the secretion occurs in both infected females and males, one can talk about single-sex specialization of the parasite in the sense that this parasitic trait is adaptive only in males. A typical example of phenotypically plastic response to host sex is that of bacteria from the large group of the Rickettsia (e.g., Wolbachia [97]) and sex ratio�Cdistorting Microsporidia Drug_discovery [98].