Ryan S. Mohammed

BALANCING SELECTION, RANDOM GENETIC DRIFT, AND GENETIC VARIATION AT THE MAJOR HISTOCOMPATIBILITY COMPLEX IN TWO WILD POPULATIONS OF GUPPIES (POECILIA RETICULATA)

Abstract Our understanding of the evolution of genes of the major histocompatibility complex (MHC) is rapidly increasing, but there are still enigmatic questions remaining, particularly regarding the maintenance of high levels of MHC polymorphisms in small, isolated populations. Here, we analyze the genetic variation at eight microsatellite loci and sequence variation at exon 2 of the MHC class IIB (DAB) genes in two wild populations of the Trinidadian guppy, Poecilia reticulata. We compare the genetic variation of a small (Ne∼ 100) and relatively isolated upland population to that of its much larger (Ne∼ 2400) downstream counterpart. As predicted, microsatellite diversity in the upland population is significantly lower and highly differentiated from the population further downstream. Surprisingly, however, these guppy populations are not differentiated by MHC genetic variation and show very similar levels of allelic richness. Computer simulations indicate that the observed level of genetic variation can be maintained with overdominant selection acting at three DAB loci. The selection coefficients differ dramatically between the upland (s 0.2) and lowland (s≤ 0.01) populations. Parasitological analysis on wild‐caught fish shows that parasite load is significantly higher on upland than on lowland fish, which suggests that large differences in selection intensity may indeed exist between populations. Based on the infection intensity, a substantial proportion of the upland fish would have suffered direct or indirect fitness consequences as a result of their high parasite loads. Selection by parasites plays a particularly important role in the evolution of guppies in the upland habitat, which has resulted in high levels of MHC diversity being maintained in this population despite considerable genetic drift.

The Guppy as a Conservation Model: Implications of Parasitism and Inbreeding for Reintroduction Success

Ex situ conservation is of increasing importance to prevent the extinction of endangered animals in the wild. Despite low success rates of reintroduction programs few researchers have investigated empirically the efficacy of captive breeding regimes for the release of captive-bred vertebrates. We used guppies (Poecilia reticulata) from two populations in Trinidad to compare different conservation breeding regimes. The upper Aripo population was chosen for its small effective population size (N(e) approximately 100) and genetic isolation, which makes it representative of many endangered natural populations. By contrast, the lower Aripo population is a genetically diverse, much larger population (N(e) approximately 2400). We examined three captive-breeding regimes: (1) inbreeding fish crossed with their full siblings, (2) minimized inbreeding, no consanguineous matings, and (3) control fish crossed at random. We kept pedigree records for all regimes so that we could calculate inbreeding coefficients over four generations. The body size and fertility of guppies was significantly reduced due to inbreeding depression. The genetic load of sterile equivalents was particularly high for the lower Aripo population. Body size also declined due to breeding conditions in the captive environment. After four generations in captivity, the fish were released into a mesocosm in Trinidad. Captive-bred guppies were extremely susceptible to gyrodactylid parasites (58% survival rate) compared with their wild counterparts (96% survival). A reduced level of immunogenetic variation due to inbreeding and lack of exposure to natural parasites may have rendered captive-bred individuals more prone to infectious disease. The threat of disease outbreak is particularly high when naive captive-bred hosts are released in wild populations. Susceptible, captive-bred hosts could facilitate the transmission of parasites throughout the wild population, thus initiating an epidemic. This risk could potentially be reduced by prior exposure to parasites before release and gradual release of captive-bred individuals.

Parasites of Trinidadian guppies: evidence for sex- and age-specific trait-mediated indirect effects of predators.

Predation pressure can alter the morphology, physiology, life history, and behavior of prey; each of these in turn can change how surviving prey interact with parasites. These trait-mediated indirect effects may change in direction or intensity during growth or, in sexually dimorphic species, between the sexes. The Trinidadian guppy, Poecilia reticulata presents a unique opportunity to examine these interactions; its behavioral ecology has been intensively studied in wild populations with well-characterized predator faunas. Predation pressure is known to have driven the evolution of many guppy traits; for example, in high-predation sites, females (but not males) tend to shoal, and this anti-predator behavior facilitates parasite transmission. To test for evidence of predator-driven differences in infection in natural populations, we collected 4715 guppies from 62 sites across Trinidad between 2003 and 2009 and screened them for ectosymbionts, including Gyrodactylus. A novel model-averaging analysis revealed that females were more likely to be infected with Gyrodactylus parasites than males, but only in populations with both high predation pressure and high infection prevalence. We propose that the difference in shoaling tendency between the sexes could explain the observed difference in infection prevalence between males and females in high-predation sites. The infection rate of juveniles did not vary with predation regime, probably because juveniles face constant predation pressure from conspecific adults and therefore tend to shoal in both high- and low-predation sites. This represents the first evidence for age- and sex-specific trait-mediated indirect effects of predators on the probability of infection in their prey.

Ieredactylus rivuli gen. et sp. nov. (Monogenea, Gyrodactylidae) from Rivulus hartii (Cyprinodontiformes, Rivulidae) in Trinidad

A new genus and species of Gyrodactylidae, Ieredactylus rivuli gen. et sp. nov. (Platyhelminthes, Monogenea), is described from the skin of Hart’s Rivulus (Rivulus hartii Boulenger), a cyprinodontiform fish collected from streams of the Caroni and Oropouche drainages and the Pitch Lake in Trinidad (prevalence all localities: 16.7–94.6%; mean parasite intensity 1–9 parasites/fish; range 1–34) with the holotype originating from a tributary of the Aripo River. This viviparous monogenean is distinctive from other genera of Gyrodactylidae by its split ventral bar membrane, the shape of its male copulatory organ, the presence of two conical accessory pieces associated with the hamulus root and two differently shaped marginal hook sickles. Its unique rDNA sequence shows the closest ITS2 similarity (70%) to Gyrodactyloides andriaschewii Bychowsky et Poljansky, 1953. The presence of I. rivuli gen. et sp. nov. in the Pitch Lake indicates an adaptation to extreme environmental conditions such as high temperatures and hydrocarbons and adverse pH. Guppies may potentially serve as temporary hosts. The parasite displays distinct behaviours, including a characteristic ‘swimming-like’ movement. The ecology and phylogeny of I. rivuli gen. et sp. nov. is discussed in relation to the diversity of other gyrodactylids in Trinidad.

Can parasites use predators to spread between primary hosts

Parasites typically have low reproductive fitness on paratenic hosts. Such hosts offer other significant inclusive fitness benefits to parasites, however, such as increased mobility and migration potential. The parasite fauna of the guppy (Poecilia reticulata) is dominated by the directly transmitted ectoparasites Gyrodactylus bullatarudis and Gyrodactylus turnbulli. In the wild, close predatory and competitive interactions occur between the guppy and the killifish Rivulus hartii. Previous observations suggest that these fish can share gyrodactylids, so we tested experimentally whether these parasites can use R. hartii as an alternative host. In aquaria, G. bullatarudis was the only species able to transmit from prey to predator. Both parasite species transferred equally well to prey when the predator was experimentally infected. However, in semi-natural conditions, G. bullatarudis transmitted more successfully to the prey fish. Importantly, G. bullatarudis also survived significantly longer on R. hartii out of water. As R. hartii can migrate overland between isolated guppy populations, G. bullatarudis may have an enhanced ability to disperse and colonize new host populations, consistent with its wider distribution in the wild. To our knowledge, this is the first empirical study demonstrating a predator acting as a paratenic host for the parasites of its prey.

Immunogenetic novelty confers a selective advantage in host–pathogen coevolution

Significance The major histocompatibility complex (MHC) is one of the most polymorphic gene families in the vertebrate genome, with natural selection actively promoting and maintaining variability. The exact mechanism/mechanisms responsible for these characteristics remain unclear, but identifying them is fundamental to our understanding of host–pathogen dynamics. Using targeted crosses of the model Trinidadian guppy, a tractable parasite, and exposure-controlled infection trials, we show that novel MHC variants are associated with less severe infections. Uniquely, our experimental design separates novel variant advantage from other modes of selection and confounding variables, such as individual MHC variability and genomic background. We thus demonstrate a fundamental process driving evolution of the vertebrate immune system, which helps explain the unique features of MHC genes. The major histocompatibility complex (MHC) is crucial to the adaptive immune response of vertebrates and is among the most polymorphic gene families known. Its high diversity is usually attributed to selection imposed by fast-evolving pathogens. Pathogens are thought to evolve to escape recognition by common immune alleles, and, hence, novel MHC alleles, introduced through mutation, recombination, or gene flow, are predicted to give hosts superior resistance. Although this theoretical prediction underpins host–pathogen “Red Queen” coevolution, it has not been demonstrated in the context of natural MHC diversity. Here, we experimentally tested whether novel MHC variants (both alleles and functional “supertypes”) increased resistance of guppies (Poecilia reticulata) to a common ectoparasite (Gyrodactylus turnbulli). We used exposure-controlled infection trials with wild-sourced parasites, and Gyrodactylus-naïve host fish that were F2 descendants of crossed wild populations. Hosts carrying MHC variants (alleles or supertypes) that were new to a given parasite population experienced a 35–37% reduction in infection intensity, but the number of MHC variants carried by an individual, analogous to heterozygosity in single-locus systems, was not a significant predictor. Our results provide direct evidence of novel MHC variant advantage, confirming a fundamental mechanism underpinning the exceptional polymorphism of this gene family and highlighting the role of immunogenetic novelty in host–pathogen coevolution.

Evolutionary genetics of immunological supertypes reveals two faces of the Red Queen

Red Queen host–parasite co-evolution can drive adaptations of immune genes by positive selection that erodes genetic variation (Red Queen arms race) or results in a balanced polymorphism (Red Queen dynamics) and long-term preservation of genetic variation (trans-species polymorphism). These two Red Queen processes are opposite extremes of the co-evolutionary spectrum. Here we show that both Red Queen processes can operate simultaneously by analysing the major histocompatibility complex (MHC) in guppies (Poecilia reticulata and P. obscura) and swamp guppies (Micropoecilia picta). Sub-functionalisation of MHC alleles into ‘supertypes’ explains how polymorphisms persist during rapid host–parasite co-evolution. Simulations show the maintenance of supertypes as balanced polymorphisms, consistent with Red Queen dynamics, whereas alleles within supertypes are subject to positive selection in a Red Queen arms race. Building on the divergent allele advantage hypothesis, we show that functional aspects of allelic diversity help to elucidate the evolution of polymorphic genes involved in Red Queen co-evolution.Host-parasite coevolution can lead to arms races favouring novel immunogenetic alleles or the maintenance of diversity in a balanced polymorphism. Here, Lighten et al. combine data on MHC diversity across three guppy species and simulations to show that polymorphisms of immunogenetic supertypes may persist even as alleles within supertypes are involved in an arms race.

Upstream guppies (Poecilia reticulata, Peters, 1859) go against the flow

Guppies (Poecilia reticulata Peters 1859) in lakes and from captive-bred populations are predicted to show little rheotaxis compared to conspecifics in a stream environment that are regularly exposed to flash floods associated with involuntary downstream migration. Here we test this hypothesis using an artificial stream, examining guppies of two wild riverine populations, one lake population, and one ornamental strain. Guppies from the most upstream riverine habitat show the most pronounced rheotaxis and are less likely to be swept downstream during flooding events. However, there is no significant difference between guppies from the lowland riverine habitat, the Pitch Lake and ornamental strain. We propose that station-keeping behaviours are most strongly selected in the upstream population because large spatial differences exist in ecology and environment between up- and downstream habitats. Given that these sites are separated by barrier waterfalls that prevent compensatory upstream migration, natural selection operates particularly strong against upstream guppies that have been displaced downstream during flooding events.

Parasites pitched against nature: Pitch Lake water protects guppies ( Poecilia reticulata ) from microbial and gyrodactylid infections

SUMMARY The enemy release hypothesis proposes that in parasite depleted habitats, populations will experience relaxed selection and become more susceptible (or less tolerant) to pathogenic infections. Here, we focus on a population of guppies (Poecilia reticulata) that are found in an extreme environment (the Pitch Lake, Trinidad) and examine whether this habitat represents a refuge from parasites. We investigated the efficacy of pitch in preventing microbial infections in Pitch Lake guppies, by exposing them to dechlorinated water, and reducing gyrodactylid infections on non-Pitch Lake guppies by transferring them to Pitch Lake water. We show that (i) natural prevalence of ectoparasites in the Pitch Lake is low compared to reference populations, (ii) Pitch Lake guppies transferred into aquarium water develop microbial infections, and (iii) experimentally infected guppies are cured of their gyrodactylid infections both by natural Pitch Lake water and by dechlorinated water containing solid pitch. These results indicate a role for Pitch Lake water in the defence of guppies from their parasites and suggest that Pitch Lake guppies might have undergone enemy release in this extreme environment. The Pitch Lake provides an ideal ecosystem for studies on immune gene evolution in the absence of parasites and long-term evolutionary implications of hydrocarbon pollution for vertebrates.