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Parasite hybrid zones resulting from host secondary contact have never been described in nature although parasite hybridization is well known and secondary contact should affect them similarly to free-living organisms. When host populations are isolated, diverge and recontact, intimate parasites (host specific, direct life cycle) carried during isolation will also meet and so may form parasite hybrid zones. If so, we hypothesize these should be narrower than the host's hybrid zone as shorter parasite generation time allows potentially higher divergence. We investigate multilocus genetics of two parasites across the European house mouse hybrid zone. We find each host taxon harbours its own parasite taxa. These also hybridize: Parasite hybrid zones are significantly narrower than the host's. Here, we show a host hybrid zone is a suture zone for a subset of its parasite community and highlight the potential of such systems as windows on the evolutionary processes of host-parasite interactions and recombinant pathogen emergence., (© 2018 John Wiley & Sons Ltd.)

Background: Molecular genetic studies are revealing an increasing number of cryptic lineages or species, which are highly genetically divergent but apparently cannot be distinguished morphologically. This observation gives rise to three important questions: 1) have these cryptic lineages diverged in phenotypic traits that may not be obvious to humans; 2) when cryptic lineages come into secondary contact, what are the evolutionary consequences: stable co-existence, replacement, admixture or differentiation and 3) what processes influence the evolutionary dynamics of these secondary contact zones?, Methodology/principal Findings: To address these questions, we first tested whether males of the Iberian lizard Lacerta schreiberi from two highly genetically divergent, yet morphologically cryptic lineages on either side of an east-west secondary contact could be differentiated based on detailed analysis of morphology, coloration and parasite load. Next, we tested whether these differences could be driven by pre-copulatory intra-sexual selection (male-male competition). Compared to eastern males, western males had fewer parasites, were in better body condition and were more intensely coloured. Although subtle environmental variation across the hybrid zone could explain the differences in parasite load and body condition, these were uncorrelated with colour expression, suggesting that the differences in coloration reflect heritable divergence. The lineages did not differ in their aggressive behaviour or competitive ability. However, body size, which predicted male aggressiveness, was positively correlated with the colour traits that differed between genetic backgrounds., Conclusions/significance: Our study confirms that these cryptic lineages differ in several aspects that are likely to influence fitness. Although there were no clear differences in male competitive ability, our results suggest a potential indirect role for intra-sexual selection. Specifically, if lizards use the colour traits that differ between genetic backgrounds to assess the size of potential rivals or mates, the resulting fitness differential favouring western males could result in net male-mediated gene flow from west to east across the current hybrid zone.

Parasite-mediated selection on major histocompatibility complex (MHC) genes has mainly been explored at the intraspecific level, although many molecular studies have revealed trans-species polymorphism. Interspecific patterns of MHC diversity might reveal factors responsible for the long-term evolution of MHC polymorphism. We hypothesize that host taxa harbouring high parasite diversity should exhibit high levels of MHC genetic diversity. We test this assumption using data on rodent species and their helminth parasites compiled from the literature. Controlling for similarity due to common descent, we present evidence indicating that high helminth species richness in rodent species is associated with increased MHC class II polymorphism. Our results are consistent with the idea that parasites sharing a long-term coevolutionary history with their hosts are the agents of selection explaining MHC polymorphism. [ABSTRACT FROM AUTHOR]

1. Immunocompetence is the general capacity of an organism to mount an immune response against pathogens and parasites. We studied the relationship between the immunocompetence of a rodent host, Meriones crassus (Sundevall's Jird) and parasitism by the flea Xenopsylla ramesis. We hypothesized that flea parasitism affects physiological and immunological variables of the host, and that the host's level of immunocompetence affects fitness components in the flea parasite. We wanted to find out (a) the effect of flea parasitism on the haematocrit (Hct), leucocyte concentration (LC) and response to phytohaemagglutinin (PHA) of the host; (b) if and how the level of immunocompetence of the host is related to the fitness of fleas exploiting it; and (c) the relationship between the immunological and behavioural defences of the host. 2. We measured Hct and LC and response to PHA before and after 16 days of flea infestation in Sundevall's Jird and compared these variables between jirds subjected to flea parasitism and non-parasitized (control) animals. We evaluated egg production and hatching success in fleas exploiting hosts with different levels of immunocompetence. 3. The effect of flea parasitism on Hct and LC of rodents was manifested by an increase in among-host variation in the temporal changes of these variables. Response to PHA injection was significantly lower in parasitized than in control animals. 4. Flea egg production and hatching success were not related to either LC, prior to experimental treatments, or to the PHA response of rodents. However, both of these flea fitness traits were negatively correlated with changes in LC between the 1st and the 16th days of infestation. 5. At the end of the experiment, blood consumption of fleas was measured in jirds of both treatment groups. Fleas consumed significantly more blood when they fed on previously parasitized than on non-parasitized animals. Among fleas that fed on previously parasitized animals, blood consumption was positively correlated with the initial LC of the hosts, and negatively correlated with differences in LC between the 1st and the 16th days of flea infestation. We found no correlation between blood consumption in fleas that fed on control animals and either immunological variable of the hosts. 6. No trade-off was found between behavioural and immune defences of the rodent hosts. [ABSTRACT FROM AUTHOR]