Genomic Porosity between Invasive Chondrostoma nasus and Endangered Endemic Parachondrostoma toxostoma (Cyprinidae): The Evolution of MHC IIB Genes

Monogeneans in intergeneric hybrids of leuciscid fish: Is parasite infection driven by hybrid heterosis, genetic incompatibilities, or host-parasite coevolutionary interactions?

BACKGROUND

Several hypotheses have been proposed to explain parasite infection in parental species and their hybrids. Hybrid heterosis is generally applied to explain the advantage for F1 generations of hybrids exhibiting a lower level of parasite infection when compared to parental species. Post-F1 generations often suffer from genetic incompatibilities potentially reflected in the higher level of parasite infection when compared to parental species. However, the presence of specific parasites in an associated host is also limited by close coevolutionary genetic host-parasite associations. This study focused on monogenean parasites closely associated with two leuciscid fish species-common bream and roach-with the aim of comparing the level of monogenean infection between parental species and hybrids representing two F1 generations with different mtDNA and two backcross generations with different cyto-nuclear compositions.

RESULTS

Monogenean infection in F1 generations of hybrids was lower when compared to parental species, in line with the hybrid heterosis hypothesis. Monogenean infection in backcross generations exhibited similarities with the parental species whose genes contributed more to the backcross genotype. The distribution of monogeneans associated with one or the other parental species showed the same asymmetry with a higher proportion of roach-associated monogeneans in both F1 generations and backcross generation with roach in the paternal position. A higher proportion of common bream-associated monogeneans was found in backcross generation with common bream in the paternal position.

CONCLUSIONS

Our study indicated that cyto-nuclear incompatibilities in hybrids do not induce higher monogenean infection in backcross generations when compared to parental species. However, as backcross hybrids with a higher proportion of the genes of one parental taxon also exhibited high level of this parental taxon-associated parasites, host-parasite coevolutionary interactions seem to play an obvious role in determining the level of infection of host-specific monogeneans in hybrids.

Diversity of MHC IIB genes and parasitism in hybrids of evolutionarily divergent cyprinoid species indicate heterosis advantage

The genes of the major histocompatibility complex (MHC) are an essential component of the vertebrate immune system and MHC genotypes may determine individual susceptibility to parasite infection. In the wild, selection that favors MHC variability can create situations in which interspecies hybrids experience a survival advantage. In a wild system of two naturally hybridizing leuciscid fish, we assessed MHC IIB genetic variability and its potential relationships to hosts' ectoparasite communities. High proportions of MHC alleles and parasites were species-specific. Strong positive selection at specific MHC codons was detected in both species and hybrids. MHC allele expression in hybrids was slightly biased towards the maternal species. Controlling for a strong seasonal effect on parasite communities, we found no clear associations between host-specific parasites and MHC alleles or MHC supertypes. Hybrids shared more MHC alleles with the more MHC-diverse parental species, but expressed intermediate numbers of MHC alleles and positively selected sites. Hybrids carried significantly fewer ectoparasites than either parent species, suggesting a hybrid advantage via potential heterosis.

Vigour-related traits and immunity in hybrids of evolutionary divergent cyprinoid species: advantages of hybrid heterosis?

Hybrid advantage, described as the superiority of hybrids in some traits over their parents and termed the "heterosis effect," is widely documented in the case of reciprocal crosses of parental species (i.e., hybrids representing the F1 generation). In fish, high survival, fast growth and better health status have been widely documented in F1 hybrids. Nonetheless, the effects of interspecific hybridization on vigour, physiology and immunity-related traits in fish are largely unknown, especially concerning native systems of coexisting parental and hybrid genomes in the same habitat. The present study examined the potential physiological and immune aspects of hybrid heterosis by comparing condition status (measured especially by indexes), haematological profile, glucose concentration and selected parameters of non-specific and specific immunity between the evolutionarily divergent non-congeneric cyprinoid species Abramis brama and Rutilus rutilus and their hybrids representing the F1 generation, all of them caught in nature. Clear differences were documented for vigour-related, physiological and immune parameters between the two divergent species. Hybrids generally tended to express intermediate characters of the measured traits, likely generated by the evolutionary divergence of the hybridizing species; nonetheless, for some traits, hybrids exhibited a character that was more similar to one parental species than to the other. This was interpreted as the heterozygote advantage for F1 hybrids. It is suggested that a maternally inherited genetic background may potentially influence the expression of some branches of non-specific immunity or other aspects related to the fish health status.

Microbiota Diversity Within and Between the Tissues of Two Wild Interbreeding Species

Understanding the role of microbiota as reproductive barriers or sources of adaptive novelty in the fundamental biological phenomenon of speciation is an exciting new challenge necessitating exploration of microbiota variation in wild interbreeding species. We focused on two interbreeding cyprinid species, Chondrostoma nasus and Parachondrostoma toxostoma, which have geographic distributions characterized by a mosaic of hybrid zones. We described microbiota diversity and composition in the three main teleost mucosal tissues, the skin, gills and gut, in the parental parapatric populations. We found that tissue type was the principal determinant of bacterial community composition. In particular, there was strong microbiota differentiation between external and internal tissues, with secondary discrimination between the two species. These findings suggest that specific environmental and genetic filters associated with each species have shaped the bacterial communities, potentially reflecting deterministic assemblages of bacteria. We defined the core microbiota common to both Chondrostoma species for each tissue, highlighting the occurrence of microbe-host genome interactions at this critical level for studies of the functional consequences of hybridization. Further investigations will explore to what extend these specific tissue-associated microbiota signatures could be profoundly altered in hybrids, with functional consequences for post-mating reproductive isolation in relation to environmental constraints.

The evolution of the major histocompatibility complex in upstream versus downstream river populations of the longnose dace

Populations in upstream versus downstream river locations can be exposed to vastly different environmental and ecological conditions and can thus harbor different genetic resources due to selection and neutral processes. An interesting question is how upstream–downstream directionality in rivers affects the evolution of immune response genes. We used next‐generation amplicon sequencing to identify eight alleles of the major histocompatibility complex (MHC) class II β exon 2 in the cyprinid longnose dace (Rhinichthys cataractae) from three rivers in Alberta, upstream and downstream of municipal and agricultural areas along contaminant gradients. We used these data to test for directional and balancing selection on the MHC. We also genotyped microsatellite loci to examine neutral population processes in this system. We found evidence for balancing selection on the MHC in the form of increased nonsynonymous variation relative to neutral expectations, and selection occurred at more amino acid residues upstream than downstream in two rivers. We found this pattern despite no population structure or isolation by distance, based on microsatellite data, at these sites. Overall, our results suggest that MHC evolution is driven by upstream–downstream directionality in fish inhabiting this system.

When Anthropogenic River Disturbance Decreases Hybridisation between Non-Native and Endemic Cyprinids and Drives an Ecomorphological Displacement towards Juvenile State in Both Species

Understanding the impact of non-native species on native species is a major challenge in molecular ecology, particularly for genetically compatible fish species. Invasions are generally difficult to study because their effects may be confused with those of environmental or human disturbances. Colonized ecosystems are differently impacted by human activities, resulting in diverse responses and interactions between native and non-native species. We studied the dynamics between two Cyprinids species (invasive Chondrostoma nasus and endemic Parachondrostoma toxostoma) and their hybrids in 16 populations (from allopatric to sympatric situations and from little to highly fragmented areas) corresponding to 2,256 specimens. Each specimen was assigned to a particular species or to a hybrid pool using molecular identification (cytochrome b and 41 microsatellites). We carried out an ecomorphological analysis based on size, age, body shape, and diet (gut vacuity and molecular fecal contents). Our results contradicted our initial assumptions on the pattern of invasion and the rate of introgression. There was no sign of underperformance for the endemic species in areas where hybridisation occurred. In the unfragmented zone, the introduced species was found mostly downstream, with body shapes similar to those in allopatric populations while both species were found to be more insectivorous than the reference populations. However, high level of hybridisation was detected, suggesting interactions between the two species during spawning and/or the existence of hybrid swarm. In the disturbed zone, introgression was less frequent and slender body shape was associated with diatomivorous behaviour, smaller size (juvenile characteristics) and greater gut vacuity. Results suggested that habitat degradation induced similar ecomorphological trait changes in the two species and their hybrids (i.e. a transition towards a pedomorphic state) where the invasive species is more affected than the native species. Therefore, this study reveals a diversity of relationships between two genetically compatible species and emphasizes constraints on the invasion process in disturbed areas.

Trans-Species Polymorphism in Immune Genes: General Pattern or MHC-Restricted Phenomenon?

Immunity exhibits extraordinarily high levels of variation. Evolution of the immune system in response to host-pathogen interactions in particular ecological contexts appears to be frequently associated with diversifying selection increasing the genetic variability. Many studies have documented that immunologically relevant polymorphism observed today may be tens of millions years old and may predate the emergence of present species. This pattern can be explained by the concept of trans-species polymorphism (TSP) predicting the maintenance and sharing of favourable functionally important alleles of immune-related genes between species due to ongoing balancing selection. Despite the generality of this concept explaining the long-lasting adaptive variation inherited from ancestors, current research in TSP has vastly focused only on major histocompatibility complex (MHC). In this review we summarise the evidence available on TSP in human and animal immune genes to reveal that TSP is not a MHC-specific evolutionary pattern. Further research should clearly pay more attention to the investigation of TSP in innate immune genes and especially pattern recognition receptors which are promising candidates for this type of evolution. More effort should also be made to distinguish TSP from convergent evolution and adaptive introgression. Identification of balanced TSP variants may represent an accurate approach in evolutionary medicine to recognise disease-resistance alleles.