Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

A change in the sex of the parasitized host is an interesting way of affecting the physiology of a host organism (Hurst, Hurst, & Majerus 1993).Parasitic bacteria, mostly of the  Wolbachia genus, which frequently spread vertically in the population, e.g. transovarially, are capable changing the sex of the individual on which they parasitize , for example terrestrial crustaceans of the Armadillidium, Chaetophiloscia or Porcellionides families (Juchault et al. 1994).In a great many populations of these crustaceans, the ratio of the sexes, which is determined by the sex chromosomes under normal circumstances, is shifted towards a substantial predominance of females.More detailed analysis indicates that some of the females are genetically males, whose sex has been changed by the action of the parasitic bacteria.The purpose of this manipulation of the sex of the host is obvious – the bacteria are capable of spreading transovarially, through the eggs of the host, but not through its sperm.A similar situation occurs for the freshwater shrimp Gammarus duebeni infected by microspora Octosporea effeminans (Bulnheim & Vavra 1968).Here it has even been observed (Dunn & Hatcher 1997)that the intensity of infection of oocytes and thus the degree of shift of the primary sex index towards an excess of females depends on external conditions, specifically on the salinity of the environment.Amongst other parasites, e.g. in some bacteria and viruses, the opposite situation occurs – they are capable of spreading only through sperm (Ndiaye & Mattei 1993).Here, some species can also change the sex ratio in their host species, however now in the opposite direction, i.e. in favor of males.