Development of molecular identification methods for Dryophytes suweonensis and D. japonicus, and their hybrids

Background

As hybridization can reduce biodiversity or cause extinction, it is important to identify both purebred parental species and their hybrids prior to conserving them. The Suwon tree frog, Dryophytes suweonensis, is an endangered wildlife species in Korea that shares its habitat and often hybridizes with the Japanese tree frog, D. japonicus. In particular, D. suweonensis, D. japonicus, and their hybrids often have abnormal ovaries and gonads, which are known causes that could threaten their existence.

Methods

We collected 57 individuals from six localities where D. suweonensis is known to be present. High-resolution melting curve (HRM) analysis of the mitochondrial 12S ribosomal RNA gene was performed to determine the maternal species. Thereafter, the DNA sequences of five nuclear genes (SIAH, TYR, POMC, RAG1, and C-MYC) were analyzed to determine their parental species and hybrid status.

Results

The HRM analysis showed that the melting temperature of D. suweonensis was in the range of 79.0-79.3 °C, and that of D. japonicus was 77.7-78.0 °C, which clearly distinguished the two tree frog species. DNA sequencing of the five nuclear genes revealed 37 single-nucleotide polymorphism (SNP) sites, and STRUCTURE analysis showed a two-group structure as the most likely grouping solution. No heterozygous position in the purebred parental sequences with Q values ≥ 0.995 were found, which clearly distinguished the two treefrog species from their hybrids; 11 individuals were found to be D. suweonensis, eight were found to be D. japonicus, and the remaining 38 individuals were found to be hybrids.

Conclusion

Thus, it was possible to unambiguously identify the parental species and their hybrids using HRM analysis and DNA sequencing methods. This study provided fundamental information for D. suweonensis conservation and restoration research.

PARASITISM IN HYBRID SUNFISH (LEPOMIS SPP.): PATTERNS OF INFECTION AT THE INDIVIDUAL AND COMMUNITY LEVEL

Sunfish (Lepomis spp.) are among the most common piscine inhabitants of freshwater lakes and ponds in North America. Lepomis spp. breed at the same time creating hybrid zones, where genetically distinct populations mate and produce mixed offspring that are sexually viable hybrids. One aspect of hybridization that may have important consequences is parasitism and its patterns of recruitment in the hybrid sunfish. This study investigated these patterns both at the level of the individual parasite species as well as in the parasite communities infecting the fish. Two sample sites possessing hybrid sunfish populations were investigated: 1 system had bluegill sunfish (Lepomis macrochirus), redear sunfish (Lepomis microlophus), and their hybrids, while the other system had bluegill sunfish (L. macrochirus), green sunfish (Lepomis cyanellus), and their hybrids. The hybrids were infected by mostly generalist parasites that commonly infect all Lepomis spp. Most of the individual parasite species followed a dominance pattern (59.1%) of infection, where parasite abundance in hybrids resembled at least one of the parental species, with the remainder exhibiting intermediate levels of parasitism, supporting an additive pattern of parasite recruitment (40.1%). At the community level, the patterns of parasite recruitment differed in L. macrochirus × L. microlophus hybrids, which showed a dominance pattern, and L. macrochirus × L. cyanellus hybrids, which showed an additive pattern of parasite recruitment. These differences in parasite recruitment between hybrid groups may be attributed to varying degrees of dietary and niche overlap between the parental species in the 2 study systems.

Prevalence and intensity of avian malaria in a quail hybrid zone

Hybrid zones have been described as natural laboratories by researchers who study speciation and the various mechanisms that may affect gene flow. The evolutionary consequences of hybridization depend not only on reproductive compatibility between sympatric species, but also on factors like vulnerability to each other's predators and parasites. We examined infection patterns of the blood parasite Haemoproteus lophortyx, a causative agent of avian malaria, at a site in the contact zone between California quail (Callipepla californica) and Gambel's quail (C. gambelii). Controlling for the potential influence of sex and year, we tested whether species identity predicted infection status and intensity. We found that infection prevalence was lower in California and hybrid quail compared with Gambel's quail. However, infected California and hybrid quail had higher infection intensities than Gambel's quail. California and hybrid quail exhibited no significant differences in prevalence or intensity of infection. These findings suggest that infection by H. lophortyx has the potential to influence species barrier dynamics in this system; however, more work is necessary to determine the exact evolutionary consequences of this blood parasite on hybridization.

Distribution of host-specific parasites in hybrids of phylogenetically related fish: the effects of genotype frequency and maternal ancestry?

Background

Host specificity is one of the outputs of the coevolution between parasites and their associated hosts. Several scenarios have been proposed to explain the pattern of parasite distribution in parental and hybrid genotypes ranging from hybrid resistance to hybrid susceptibility. We hypothesized that host-parasite co-adaptation limits the infection of host-specific parasites in hybrid genotypes even under the condition of the high frequency of hybrids. The experimental monogenean infection in pure breeds of Blicca bjoerkna and Abramis brama and cross-breeds (the F1 generation of hybrids) under the condition of similar frequencies of pure and hybrid genotypes was investigated. We also examined the potential effect of the maternal origin of hybrids (potential co-adaptation at the level of mitochondrial genes) on monogenean abundance.

Methods

Pure breeds of two cyprinids and two cross-breeds (one with B. bjoerkna, the next with A. brama in the maternal positions) were exposed to infection by monogeneans naturally occurring in B. bjoerkna and A. brama. The experiment was run under similar frequencies of the four breed lines.

Results

We showed similar levels of monogenean infection in B. bjoerkna and A. brama. However, each species harboured specific monogenean fauna. Hybrids harboured all monogenean species specifically infecting one or the other species. Monogenean infection levels, especially those of Dactylogyrus specific to A. brama, were lower in hybrids. For the majority of host-specific parasites, there was no effect of the maternal origin of hybrids on monogenean abundance. Asymmetry was found in the distribution of specific parasites in favour of specialists of B. bjoerkna in the monogenean communities of hybrids.

Conclusions

Our results indicate that the maternal mtDNA of hybrids is not an important predictor of host-specific monogenean infection, which may suggest that mitochondrial genes are not strongly involved in the coadaptation between monogeneans and their associated hosts. The asymmetry of species-specific parasites suggests similarity between the molecular components of the immune mechanisms in hybrids and B. bjoerkna. Our results revealed a difference between the degree of host-parasite coadaptation in specific parasites of A. brama and the degree of host-parasite coadaptation in specific parasites of B. bjoerkna and their associated hosts.

Parasites and Host Species Barriers in Animal Hybrid Zones

Species barriers are tested in hybrid zones when gene flow occurs between hybridizing species. Hybridization can erode species barriers, lead to the introgression of adaptive traits, or remain stable through time. Outcomes in hybrid zones are influenced by divergence between the hybridizing taxa, behavior, ecology, and geography. Parasites and pathogens play a major role in host fitness and appear to have varied impacts on species barriers in hybrid zones. We comprehensively reviewed the literature on parasitism in animal hybrid zones and present an evolutionary framework within which to consider parasite-hybrid interactions. Parasites most frequently show potential to contribute to species barrier breakdown in hybrid zones, but also frequently show potential to facilitate the maintenance of species barriers. Incorporating eco-immunology, parasite community theory, and spatiotemporal approaches will be important as genomic tools allow researchers to examine parasites and hybrid zones at greater resolution and in a diversity of natural habitats.

Prevalence and diversity of haemosporidian parasites in the yellow-rumped warbler hybrid zone

Parasites can play a role in speciation, by exerting different selection pressures on different host lineages, leading to reproductive barriers in regions of possible interbreeding. Hybrid zones therefore offer an ideal system to study the effect of parasites on speciation. Here, we study a hybrid zone in the foothills of the Rocky Mountains where two yellow-rumped warbler subspecies, Setophaga coronata coronata and S. c. auduboni, interbreed. There is partial reproductive isolation between them, but no evidence of strong assortative mating within the hybrid zone, suggesting the existence of a postzygotic selection against hybrids. Here, we test whether haemosporidian parasites might play a role in selecting against hybrids between S. c. coronata and S. c. auduboni. We screened birds from five transects across the hybrid zone for three phylogenetic groupings of avian haemosporidians Plasmodium, Haemoproteus and Leucocytozoon parasites and quantified intensity of infection. Contrary to our prediction, hybrids did not have higher haemosporidian parasite prevalence. Variation in Haemoproteus prevalence was best explained by an interaction between a birds' hybrid index and elevation, while the probability of infection with Leucocytozoon parasites was only influenced by elevation. We also found no significant difference in the diversity of haemosporidian lineages between the warbler subspecies and their hybrids. Finally, intensity of infection by Haemoproteus increased significantly with elevation, but was not significantly linked to birds' hybrid index. In conclusion, our data suggest that haemosporidian parasites do not seem to play a major role in selecting against hybrids in this system.

Helminths of wild hybrid marmosets (Callithrix sp.) living in an environment with high human activity

The objective of this study was to identify the helminth fauna in hybrid, non-native marmosets, through analysis of fecal samples. The study involved 51 marmosets (genus Callithrix) from five groups living in places with levels of human impact in Viçosa-MG. The marmosets were caught using a multiple-entrance trap and were anaesthetized. Feces were collected, refrigerated and analyzed by means of the sedimentation technique (Hoffmann-Pons-Janner). Eggs and parasites were identified, but not counted. Most of the marmosets (86%) were parasitized by at least one genus of helminths. Among the infected marmosets, 37% presented co-infection. The intestinal helminths comprised four different taxa: Primasubulura jacchi, Ancylostomatidae, Prosthenorchis sp. and Dilepididae. P. jacchi and Ancylostomatidae had higher prevalences (> 80% and > 40%, respectively) and were found in all marmoset groups. Dilepididae species were found in almost all the groups, but only accounted for around 30% of the marmosets. Prosthenorchis sp. showed a relatively low prevalence (< 10%) and was only found in one group. Although two parasites are commonly found in marmosets and other primates (P. jacchi and Prosthenorchis sp.), our study is the first record for Ancylostomatidae and Dilepididae. Factors like marmosets' feeding behavior and their contact with humans and other species of nonhuman primates seem to be determinants of infection among marmosets.

The maintenance of hybrids by parasitism in a freshwater snail

Hybrids have often been labelled evolutionary dead-ends due to their lower fertility and viability. However, there is growing awareness that hybridisation between different species may play a constructive role in animal evolution as a means to create variability. Thus, hybridisation and introgression may contribute to adaptive evolution, for example with regards to natural antagonists (parasites, predators, competitors) and adaptation to local environmental conditions. Here we investigated whether parasite intensity contributes to the continuous recreation of hybrids in 74 natural populations of Melanopsis, a complex of freshwater snails with three species. We also examined, under laboratory conditions, whether hybrids and their parental taxa differ in their tolerance of low and high temperatures and salinity levels. Infections were consistently less prevalent in males than in females, and lower in snails from deeper habitats. Infection prevalence in hybrids was significantly lower than in the parental taxa. Low hybrid infection rates could not be explained by sediment type, snail density or geographic distribution of the sampling sites. Interestingly, infected hybrid snails did not show signs of parasite-induced gigantism, whereas all parental taxa did. We found that hybrids mostly coped with extreme temperatures and salinity levels as well as their parental taxa did. Taken together, our results suggest that Melanopsis hybrids perform better in the presence of parasites and environmental stress. This may explain the widespread and long-term occurrence of Melanopsis hybrids as evidenced by paleontological and biogeographic data. Hybridisation may be an adaptive host strategy, reducing infection rates and resisting gigantism.

Climate-Driven Reshuffling of Species and Genes: Potential Conservation Roles for Species Translocations and Recombinant Hybrid Genotypes

Comprising 50%-75% of the world's fauna, insects are a prominent part of biodiversity in communities and ecosystems globally. Biodiversity across all levels of biological classifications is fundamentally based on genetic diversity. However, the integration of genomics and phylogenetics into conservation management may not be as rapid as climate change. The genetics of hybrid introgression as a source of novel variation for ecological divergence and evolutionary speciation (and resilience) may generate adaptive potential and diversity fast enough to respond to locally-altered environmental conditions. Major plant and herbivore hybrid zones with associated communities deserve conservation consideration. This review addresses functional genetics across multi-trophic-level interactions including "invasive species" in various ecosystems as they may become disrupted in different ways by rapid climate change. "Invasive genes" (into new species and populations) need to be recognized for their positive creative potential and addressed in conservation programs. "Genetic rescue" via hybrid translocations may provide needed adaptive flexibility for rapid adaptation to environmental change. While concerns persist for some conservationists, this review emphasizes the positive aspects of hybrids and hybridization. Specific implications of natural genetic introgression are addressed with a few examples from butterflies, including transgressive phenotypes and climate-driven homoploid recombinant hybrid speciation. Some specific examples illustrate these points using the swallowtail butterflies (Papilionidae) with their long-term historical data base (phylogeographical diversity changes) and recent (3-decade) climate-driven temporal and genetic divergence in recombinant homoploid hybrids and relatively recent hybrid speciation of Papilio appalachiensis in North America. Climate-induced "reshuffling" (recombinations) of species composition, genotypes, and genomes may become increasingly ecologically and evolutionarily predictable, but future conservation management programs are more likely to remain constrained by human behavior than by lack of academic knowledge.

Differential susceptibility to the Withering Syndrome agent and renal coccidia in juvenile Haliotis rufescens, Haliotis discus hannai and the interspecific hybrid

Withering Syndrome (WS) is a pathogenic chronic disease caused by the intracellular rickettsial-like bacterium "Candidatus Xenohaliotis californiensis" (WS-RLOs), which affects many abalone species. The renal coccidium (Margolisiella haliotis) has often been observed concurrent with WS infection. The red abalone Haliotis rufescens is a very susceptible species to WS and is also infected by the coccidium M. haliotis. In contrast, the Japanese abalone Haliotis discus hannai is not infected by these parasites. Interspecific hybridization is a method for improving important traits in animal husbandry. The objective of this study was to determine susceptibility to WS-RLO and M. haliotis infection in the hybrid generated from a cross between red and Japanese abalones. Juveniles from both species and the interspecific hybrid were challenged by exposure to effluent from red abalone adults infected with both parasites. The animals were analyzed by histology at 130days post-challenge. A 33% prevalence WS-RLOs was observed in the red abalone H. rufescens, whereas a 20% prevalence was observed in the hybrid. Infections were graded on a scale of 0-3. Of these red abalones infected, 53% presented grade 1 infection intensity, 10% had grade 2 infections, and 50% had grade 3 infections. However, the hybrids only presented intensities at the extremes of the scale; of those infected 33% showed grade 1 infections and 66% had grade 3 infections. The coccidium prevalence was 7% in red abalone individuals and 13% in the hybrid abalone. In contrast, the Japanese abalone did not present infections with either parasite. As with the prevalence, the infection intensities for the coccidium were higher in the hybrid abalone; of those infected 25% had grade 2 infections, and 75% had grade 3 infections, but the red abalone presented only grade 2 infection intensities. Therefore, the hybrid did not inherited non-susceptibility or resistance characteristics of the parental H. discus hannai and possessed biological conditions that could foster development of both parasites. Development of a culture based on this hybrid abalone should consider its susceptibility to infection by coccidian, WS-RLOs and the potential for developing the WS disease.

Where are the wormy mice? A reexamination of hybrid parasitism in the European house mouse hybrid zone

Wormy mice in a hybrid zone have been interpreted as evidence of low hybrid fitness, such that parasites contribute to species separation. However, because of its natural heterogeneity, observations of parasite load must be numerous with good field area coverage. We sampled 689 mice from 107 localities across the Bavaria-Bohemia region of the European house mouse hybrid zone and calculated their hybrid indices using 1401 diagnostic single nucleotide polymorphisms (SNPs). We tested whether hybrids have greater or lesser diversity and load of parasite helminths than additive expectations, performing load analyses on the four most common taxa. We found hybrids have significantly reduced diversity and load of each of the commonest helminths; rarer helminths further support reduced load. Although within-locality comparisons have little power, randomization tests show the repeated pattern is unlikely to be due to local parasite heterogeneity, and simulations show a patch of low parasite diversity is unlikely to fall by chance just so in the field area, such that it produces the observed effects. Our data therefore contradict the idea that helminths reduce hybrid fitness through increased load. We discuss a vicariant Red Queen model that implies immune genes tracking parasites will escape Dobzhansky-Muller incompatibilities, generating hybrid variants untargeted by parasites.

The Role of Parasitism in Adaptive Radiations—When Might Parasites Promote and When Might They Constrain Ecological Speciation?

Research on speciation and adaptive radiation has flourished during the past decades, yet factors underlying initiation of reproductive isolation often remain unknown. Parasites represent important selective agents and have received renewed attention in speciation research. We review the literature on parasite-mediated divergent selection in context of ecological speciation and present empirical evidence for three nonexclusive mechanisms by which parasites might facilitate speciation: reduced viability or fecundity of immigrants and hybrids, assortative mating as a pleiotropic by-product of host adaptation, and ecologically-based sexual selection. We emphasise the lack of research on speciation continuums, which is why no study has yet made a convincing case for parasite driven divergent evolution to initiate the emergence of reproductive isolation. We also point interest towards selection imposed by single versus multiple parasite species, conceptually linking this to strength and multifariousness of selection. Moreover, we discuss how parasites, by manipulating behaviour or impairing sensory abilities of hosts, may change the form of selection that underlies speciation. We conclude that future studies should consider host populations at variable stages of the speciation process, and explore recurrent patterns of parasitism and resistance that could pinpoint the role of parasites in imposing the divergent selection that initiates ecological speciation.

Avoidance by grazers facilitates spread of an invasive hybrid plant

Biological invasions greatly increase the potential for hybridization among native and non-native species. Hybridization may influence the palatability of novel hybrids to consumers potentially influencing invasion success; however, the palatability of non-native hybrids relative to the parent species is poorly known. In contrast, studies of native-only hybrids find they are nearly always more palatable to consumers than the parent species. Here, I experimentally demonstrate that an invasive hybrid cordgrass (Spartina) is dramatically less palatable to grazing geese than the native parent species. Using field and aviary experiments, I show that grazing geese ignore the hybrid cordgrass and preferentially consume native Spartina. I also experimentally demonstrate that reduced herbivory of the invasive hybrid may contribute to faster spread in a California estuary. These results suggest that biological invasions may increase future opportunities for creating novel hybrids that may pose a greater risk to natural systems than the parent species.

Tracking a heterosis effect in the field: tadpole resistance to parasites in the water frog hybridogenetic complex

Depending on the extent of evolutionary divergence among parent taxa, hybrids may suffer from a breakdown of co-adapted genes or may conversely exhibit vigour due to the heterosis effect, which confers advantages to increased genetic diversity. That last mechanism could explain the success of hybrids when hybridization zones are large and long lasting, such as in the water frog hybridization complex. In this hybridogenetic system, hybrid individuals exhibit full heterozygosity that makes it possible to investigatein situthe impact of hybridization. We have compared parasite intensity between hybridRana esculentaand parentalR. lessonaeindividuals at the tadpole stage in two populations inhabiting contrasted habitats. We estimated intensity ofGyrinicolasp. (Nematoda) in the gut, Echinostome metacercariae in the kidneys andHaplometra cylindraceain the body cavity (both species belong to Trematoda). Despite high sampling effort, no variation in parasite intensity was detected between taxa, except a possible higher tolerance toH. cylindraceain hybrid tadpoles. The low effect of hybridization suggests efficient gene co-adaptation between the two genomes that could result from hemiclonal selection. Variation in infection intensity among ponds could support the Red Queen hypothesis.

Variation in phenotype, parasite load and male competitive ability across a cryptic hybrid zone

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.

Infestation of the cupped oysters Crassostrea angulata, C. gigas and their first-generation hybrids by the copepod Myicola ostreae: differences in susceptibility and host response

We studied the prevalence and intensity of the parasitic copepod Myicola ostreae in 2 closely related oysters Crassostrea angulata and C. gigas and their F1 hybrids. The effects on host and host reaction were also analysed to better understand host-parasite relationships between copepods and bivalve molluscs. Full reciprocal crosses were carried out between C. angulata and C. gigas and the progenies were reared in the wild in Ria Formosa Lagoon (Portugal), allowing natural infestation by M. ostreae. Prevalence and intensity were significantly higher in C. angulata than in C. gigas. The parasite level of F1 hybrids was similar to C. angulata and significantly higher than in C. gigas. The results of our study support a hypothesis of dominantly inherited susceptibility to M. ostreae infestation. Moreover, copepods were observed on the gill surface of C. gigas engulfed by a capsule-like structure. Histological analyses revealed that the copepods were surrounded by a massive agglomerate of haemocyte-like cells encircled by a thin layer of fibroblast-like cells. This encapsulation response was not observed in C. angulata or in F1 hybrids. These results suggest that the differential susceptibility to M. ostreae between C. angulata and C. gigas may be ascribed to host defence factors.

Expanding the horizon: the Red Queen and potential alternatives

The Red Queen hypothesis (RQH) is one of the most widely accepted hypotheses explaining the persistence of sexual reproduction despite its costs. It posits that sexual species, compared with asexuals, are more adept at countering parasites, because their per-generation recombination rate is higher. Despite theoretical support, current empirical studies have failed to provide unanimous support. Here, we suggest that future tests of the RQH should more thoroughly elucidate its underlying assumptions and potential alternative hypotheses. While the RQH predicts that negative frequency-dependent selection shapes host–parasite interactions, differences between sexuals and asexuals are potentially important. Key assumptions about asexual species and their sexual close relatives include (i) ecological and behavioral traits are similar, (ii) among-individual genetic diversity is greater in sexuals than in asexuals, and (iii) within-individual genetic diversity is similar in asexuals and sexuals. We review current evidence for the RQH, highlight differences between asexual and sexual species and how those differences might translate into differential responses to parasite infections, and discuss how they can influence the results and interpretation of empirical studies. Considering differences between asexual and sexual species in future tests of the RQH will help to refine predictions and eliminate alternative hypotheses.

Heterozygosity and parasite intensity: lung parasites in the water frog hybridization complex

In hybridogenetic systems, hybrid individuals are fully heterozygous because one of the parental genomes is discarded from the germinal line before meiosis. Such systems offer the opportunity to investigate the influence of heterozygosity on susceptibility to parasites. We studied the intensity of lung parasites (the roundworm Rhabdias bufomis and the fluke Haplometra cylindracea) in 3 populations of water frogs of the Rana lessonae-esculenta complex in eastern France. In these mixed populations, hybrid frogs (R. esculenta) outnumbered parental ones (R. lessonae). Despite variation in parasite intensity and demographic variability among populations, the relationship between host age and intensity of parasitism suggests a higher susceptibility in parentals than in hybrids. Mortality is probably enhanced by lung parasites in parental frogs. On the other hand, while parental frogs harboured higher numbers of H. cylindracea than hybrid frogs, the latter had higher numbers of R. bufonis. Despite such discrepancies, these results support the hybrid resistance hypothesis, although other factors, such as differences in body size, age-related immunity, differential exposure risks and hemiclonal selection, could also contribute to the observed patterns of infection.

Co-invasion of a Red Sea fish and its ectoparasitic monogenean, Polylabris cf. mamaevi into the Mediterranean: observations on oncomiracidium behavior and infection levels in both seas

This study investigated aspects of the biology of the monogenean gill ectoparasite Polylabris cf. mamaevi (Polyopisthocotylea: Microcotyleae) infecting rabbitfish, Siganus rivulatus (Forskal) (Teleostei: Siganidae). Both host and parasite are Lessepsian immigrants that have co-invaded the Mediterranean Sea via the Suez Canal. The infection prevalence and mean intensity of the polyopisthocotylean was examined in both native and immigrant host populations and found to be three times greater in the new biogeographical region. In vitro observations on parasite eggs from both areas indicated that hatching occurred almost exclusively in the dark. The reaction of the larval oncomiracidia to water flow and secreted host chemicals indicated that neither Red Sea nor Mediterranean oncomiracidia exposed to waterborne host metabolites displayed any significant response or change in behavior; however, upon encountering flow, they ceased to swim and drifted passively downstream. Host specificity of P. cf. mamaevi may have co-evolved with temporal synchronization of the parasite with the host's diurnal activity. Hatching of P. cf. mamaevi eggs was rhythmical and the timing coincided with the known nocturnal resting behavior of the hosts, when their schools lie immobile on the sea bottom. After hatching, abrupt cessation of active swimming by the oncomiracidia upon sensing host inhalant gill-ventilating currents is likely to facilitate rapid, passive entry into the gill chamber of a suitable host. The greater abundance of P. cf. mamaevi in the invading (Mediterranean) populations is probably due to the changed, new environment, possibly impacting host resistance to the parasite and encouraging heavier infections.

The coexistence of hybrid and parental Daphnia: the role of parasites

Parasite driven time-lagged negative frequency-dependent selection of hosts has been studied in natural populations by following changes in host genotype frequencies over time. However, such dynamics have not been considered at higher taxonomic levels, for example, between parental species and their hybrids. In a field study on a Daphnia hybrid system, we observed that one Daphnia taxon first was relatively under-infected, but became over-infected after a strong increase in frequency. This finding is consistent with the idea of parasite evolution towards the most frequent host taxon. In two experiments, we investigated whether the assumptions made by a model of negative frequency-dependent selection apply to our host taxa system. First, we showed that the parasite can change the outcome of taxa competition and secondly, we confirmed that the over-infection of one host taxon observed in the field has a genetic basis. Our results indicate that the incorporation of host-parasite interactions at the species level may allow us to gain a more complete picture of forces driving dynamic taxa coexistence in Daphnia hybrid systems. More generally, we suggest that if hybrids coexist in sympatry with parental taxa, the infection patterns as observed under natural conditions may be rather temporal and unstable.

SPERM PARASITISM IN ANTS: SELECTION FOR INTERSPECIFIC MATING AND HYBRIDIZATION

Interspecific mating in eusocial Hymenoptera can be favored under certain conditions even if all hybrid offspring are completely infertile. This exploits two key features of the eusocial Hymenoptera: a haplodiploid genetic system and reproductive division of labor in females. Interspecifically mated queens can still produce viable sons that will mate intraspecifically. Apparent reduced fitness resulting from producing infertile daughter gynes can be also offset by advantages conferred by hybrid workers. An important advantage is likely to be superior ability at using marginal habitats. Interspecifically mated queens can nest in sites where intraspecific competition will be low. By mating interspecifically, a queen trades expected reproductive success through female offspring for a higher probability of achieving some reproductive success. Females that mate interspecifically can be considered "sperm parasites" on the males of the other species. I provide evidence that sperm parasitism is responsible for widespread hybridization in North America among two species of the ant subgenus Acanthomyops (genus Lasius), and review evidence for sperm parasitism in other hybridization phenomena in ants. Sperm parasitism in ants represents a novel form of social parasitism in ants and a dispersal polymorphism. It may also act as a precursor to the evolution of some other recently discovered phenomena, such as genetic caste determination.

Hybridization increases measures of innate and cell-mediated immunity in an endangered bird species

1. Given the increasing threat of 'emerging infectious diseases' (EIDs), and the vulnerability of small inbred populations to such novel challenges (at least partly through compromised immunity), a degree of interspecific hybridization may improve the viability of endangered species by enriching depauperate gene pools. 2. To investigate this hypothesis, we quantified indicators of the strength of both innate and cell-mediated immunity in wild populations of hybridizing parakeets on the Chatham Islands, New Zealand. 3. We show that both measures of immune function are markedly higher in the cosmopolitan red-crowned parakeet Cyanoramphus novaezelandiae than in the island endemic Forbes' parakeet C. forbesi, as is to be expected when comparing relatively outbred vs. relatively inbred species. 4. In addition, we show that both measures of immune function are higher in Forbes'x red-crowned parakeet hybrids than in the Forbes' parakeet. Even those individuals with few red-crowned characteristics, suggesting they are the products of matings between hybrids and Forbes' parakeets, have significantly higher measures of immune function. 5. As at least one of the immune indicators measured correlates directly with both survivorship and the viability of small bird populations, this study reveals a potential management option for hybridizing species of conservation concern, where an increase in resilience to novel disease challenges could outweigh some loss of genetic integrity.

Parasite infectivity to hybridising host species: a link between hybrid resistance and allopolyploid speciation?

Variation in host-specific infectivity was studied in monogenean polystome parasites (Protopolystoma spp.) of the interfertile, parapatric anurans Xenopus laevis laevis and Xenopus muelleri. Laboratory-raised host F1 hybrids were resistant to parasites respectively specific to each parent taxon in nature. This resistance occurred against parasite isolates from both inside and outside a host hybrid/sympatric zone (and no isolate was compatible with the foreign host species under experimental conditions). Geographical Protopolystoma xenopodis isolates showed variable infectivity to a single full-sib group of their usual host, X. l. laevis, and strains with high or low infectivity to these sibs co-occurred in spatially distant local areas (separated by 1,700 km). The host compatibility of P. xenopodis was also subject to host genotypexparasite genotype interactions. Refractoriness to some parasites or pathogens, as a consequence of hybridisation, may have conferred a selective advantage on the allopolyploid pathway by which most Xenopus spp. are believed to have evolved.

Susceptibility of natural hybrids between house mouse subspecies to Sarcocystis muris

A previous study showed that the susceptibility of hybrids between two Mus musculus musculus and Mus musculus domesticus did not apply to every parasite. The authors proposed that only parasites which exerted enough constraints would induce the selection of poorly compatible systems of resistance in the subspecies. This study completes the previous work. Experimental infections of mice of the two subspecies and their hybrids with the tissue-dwelling protozoan Sarcocystis muris show that hybrids are more susceptible to the coccidian than M. m. musculus and M. m. domesticus individuals. This result demonstrates that the hybrids are not only susceptible to intestinal helminths, and confirms the 'constraint hypothesis'.

Experimental trypanosomiasis of natural hybrids between house mouse subspecies

This study characterises the extent of the susceptibility to parasites (first demonstrated with helminths) of hybrids between Mus musculus domesticus and Mus musculus musculus. Experimental infections with Trypanosoma musculi of M. m. domesticus, M. m. musculus and their natural hybrids have been performed to compare their level of resistance/susceptibility. It appears that contrary to the results with helminths, hybrid mice present the same level of resistance/susceptibility to the trypanosome as M. m. musculus and M. m. domesticus individuals. This result is interpreted in the light of the modalities of host parasite interactions and leads us to hypothesise on the role of parasitism in the evolution of the house mouse hybrid zone.