XIX.2.1 Parasite-induced pressure can lead to speciation in the host species
In the previous section, we discussed the hypothetical model of adaptation of the peptide vocabulary of a parasite to the peptide vocabulary of the host as an example of molecular mimicry.This model could also explain the process of speciation of a host species through development of the host vocabulary and its divergence in various subpopulations.If, for example, a certain subpopulation of hosts achieved a change in its peptide vocabulary, for example, if it removed a certain peptide from its vocabulary through substitution mutations, it would, to a certain degree, escape from the reach of a parasite whose vocabulary is adapted to the original host population.The members of this population would begin to identify the relevant peptide in the parasite proteins as a foreign element. It is apparent that vocabulary differentiation can fulfill a protective function only if crossing does not occur between individuals of the two host subpopulations.The advantageousness of genetic isolation can thus lead to the creation of selection pressure on the formation of reproduction barriers.Thus, pressure from parasites could indirectly lead to the formation of isolated species.
The increased susceptibility of crossed individuals occurring in the hybrid zones of some species could be related to differentiation of the peptide vocabularies of two related species.Such a greater susceptibility has been observed, for example, in mice caught in the hybrid zone of the species Mus musculus and Mus domesticus (Moulia et al. 1991; Sage et al. 1986; Moulia et al. 1995) (Fig.XIX.4).The hybrid mice necessarily have a more extensive peptide vocabulary, so they are unable to identify a great many more peptides as foreign than either of the parent species.
Fig. XIX.4 Reduced resistance of interspecific hybrids against infection. The histogram depicts the average intensity of infection by coccidia (number of parasites x 103/mg of host tissue) for two species of mice, i.e. Mus domesticus and Mus musculus, and their hybrids. The studied species M. musculus was not at all susceptible to the infection, while only more susceptible males of M. domesticus were infected. Interspecific crosses were far more susceptible to the infection. According to Derothe et al. (2001).