Parasite diversity and ecology in a model species, the guppy (Poecilia reticulata) in Trinidad

Parasitism in viviparous vertebrates: an overview

The reproductive mode of viviparity has independently evolved in various animal taxa. It refers to the condition in which the embryos or young develop inside the female's body during gestation, providing advantages such as protection, nutrition, and improved survival chances. However, parasites and diseases can be an evolutionary force that limit the host's resources, leading to physiological, morphological, and behavioral changes that impose additional costs on both the pregnant female and her offspring. This review integrates the primary literature published between 1980 and 2021 on the parasitism of viviparous hosts. We describe aspects such as reproductive investment in females, offspring sex ratios, lactation investment in mammals, alterations in birth intervals, current reproductive investment, variations between environments, immune system activity in response to immunological challenges, and other factors that can influence the interaction between viviparous females and parasites. Maintaining pregnancy incurs costs in managing the mother's resources and regulating the immune system's responses to the offspring, while simultaneously maintaining an adequate defense against parasites and pathogens. Parasites can significantly influence this reproductive mode: parasitized females adjust their investment in survival and reproduction based on their life history, environmental factors, and the diversity of encountered parasites.

First integrative study of the diversity and specificity of metacercariae of Posthodiplostomum Dubois, 1936 from native and introduced fishes in the Caribbean

Metacercariae of the genus Posthodiplostomum are often recorded in freshwater fish hosts. While the diversity and taxonomy of this genus are receiving increasing attention in molecular phylogenetic studies, available data remain geographically biased. Most molecular studies of Posthodiplostomum and morphologically similar (neascus) worms originate in North America and Europe and Asia (more than 60% of DNA sequences are from USA and Canada), with few data currently available from the Neotropics, where high host diversity suggests high and under-sampled parasite diversity. In this study, we report molecular and morphological data from metacercariae of Posthodiplostomum in fish in Puerto Rico, where only a single species has been previously reported. Partial sequences of cytochrome c oxidase subunit 1 from metacercariae from Dajaus monticola (native to Puerto Rico) and the introduced fishes Poecilia reticulata, Parachromis managuensis, Lepomis macrochirus and Micropterus salmoides revealed 7 genetically distinct species-level lineages, of which 4 were novel. We report novel molecular life-cycle linkages in Posthodiplostomum macrocotyle (metacercariae in muscle of the cichlid Pa. managuensis), a species previously known only from adults in birds from South America; and in Posthodiplostomum sp. 23 (metacercariae in poeciliids), which has recently been found in Ardea herodias in Georgia, USA. We also report the first molecular data from Posthodiplostomum sp. 8 in M. salmoides in the Caribbean. Metacercariae of most species were morphologically distinguished and all displayed narrow specificity for fish hosts, with no indication of parasite sharing among introduced and native fishes.

Shoaling guppies evade predation but have deadlier parasites

Parasites exploit hosts to replicate and transmit, but overexploitation kills both host and parasite. Predators may shift this cost-benefit balance by consuming infected hosts or changing host behaviour, but the strength of these effects remains unclear. Here we use field and lab data on Trinidadian guppies and their Gyrodactylus spp. parasites to show how differential predation pressure influences parasite virulence and transmission. We use an experimentally demonstrated virulence-transmission trade-off to parametrize a mathematical model in which host shoaling (as a means of anti-predator defence), increases contact rates and selects for higher virulence. Then we validate model predictions by collecting parasites from wild, Trinidadian populations; parasites from high-predation populations were more virulent in common gardens than those from low-predation populations. Broadly, our results indicate that reduced social contact selects against parasite virulence.

Transmission dynamics of ectoparasitic gyrodactylids (Platyhelminthes, Monogenea): An integrative review

Parasite transmission is the ability of pathogens to move between hosts. As a key component of the interaction between hosts and parasites, it has crucial implications for the fitness of both. Here, we review the transmission dynamics of Gyrodactylus species, which are monogenean ectoparasites of teleost fishes and a prominent model for studies of parasite transmission. Particularly, we focus on the most studied host–parasite system within this genus: guppies, Poecilia reticulata, and G. turnbulli/G. bullatarudis. Through an integrative literature examination, we identify the main variables affecting Gyrodactylus spread between hosts, and the potential factors that enhance their transmission. Previous research indicates that Gyrodactylids spread when their current conditions are unsuitable. Transmission depends on abiotic factors like temperature, and biotic variables such as gyrodactylid biology, host heterogeneity, and their interaction. Variation in the degree of social contact between hosts and sexes might also result in distinct dynamics. Our review highlights a lack of mathematical models that could help predict the dynamics of gyrodactylids, and there is also a bias to study only a few species. Future research may usefully focus on how gyrodactylid reproductive traits and host heterogeneity promote transmission and should incorporate the feedbacks between host behaviour and parasite transmission.

Functional immunogenetic variation, rather than local adaptation, predicts ectoparasite infection intensity in a model fish species

Natural host populations differ in their susceptibility to infection by parasites, and these intrapopulation differences are still an incompletely understood component of host‐parasite dynamics. In this study, we used controlled infection experiments with wild‐caught guppies (Poecilia reticulata) and their ectoparasite Gyrodactylus turnbulli to investigate the roles of local adaptation and host genetic composition (immunogenetic and neutral) in explaining differences in susceptibility to infection. We found differences between our four study host populations that were consistent between two parasite source populations, with no indication of local adaptation by either host or parasite at two tested spatial scales. Greater values of host population genetic variability metrics broadly aligned with lower population mean infection intensity, with the best alignments associated with major histocompatibility complex (MHC) “supertypes”. Controlling for intrapopulation differences and potential inbreeding variance, we found a significant negative relationship between individual‐level functional MHC variability and infection: fish carrying more MHC supertypes experienced infections of lower severity, with limited evidence for supertype‐specific effects. We conclude that population‐level differences in host infection susceptibility probably reflect variation in parasite selective pressure and/or host evolutionary potential, underpinned by functional immunogenetic variation.

Depletion of MHC supertype during domestication can compromise immunocompetence

The major histocompatibility complex (MHC) plays an important role in infectious disease resistance. The presence of certain MHC alleles and functionally similar groups of MHC alleles (i.e., supertypes) has been associated with resistance to particular parasite species. Farmed and domesticated fish stocks are often depleted in their MHC alleles and supertype diversity, possibly as a consequence of artificial selection for desirable traits, inbreeding (loss of heterozygosity), genetic drift (loss of allelic diversity) and/or reduced parasite biodiversity. Here we quantify the effects of depletion of MHC class II genotype and supertype variation on resistance to the parasite Gyrodactylus turnbulli in guppies (Poecilia reticulata). Compared to the descendants of wild‐caught guppies, ornamental fish had a significantly reduced MHC variation (i.e., the numbers of MHC alleles and supertypes per individual, and per population). In addition, ornamental fish were significantly more susceptible to G. turnbulli infections, accumulating peak intensity 10 times higher than that of their wildtype counterparts. Four out of 13 supertypes were associated with a significantly reduced parasite load, and the presence of some supertypes had a dramatic effect on the intensity of infection. Remarkably, the ornamental and wildtype fish differed in the supertypes that were associated with parasite resistance. Analysis with a genetic algorithm showed that resistance‐conferring supertypes of the wildtype and ornamental fish shared two unique amino acids in the peptide‐binding region of the MHC that were not found in any other alleles. These data show that the supertype demarcation captures some, but not all, of the variation in the immune function of the alleles. This study highlights the importance of managing functional MHC diversity in livestock, and suggests there might be some immunological redundancy among MHC supertypes.

Distribution of parasites of slimy sculpin Cottus cognatus Richardson, 1836 (Scorpaeniformes: Cottidae) in the Athabasca drainage, Alberta, Canada, and their relation to water quality

The composition and diversity of parasite communities are useful tools to characterise ecosystem health and integrity. Environmental disturbances may affect parasite infection in fish directly, by their effects on the free-living stages, or indirectly, on the intermediate hosts. Slimy sculpins, Cottus cognatus, a small fish inhabiting cold waters of North America, have been considered as sentinels due to their limited mobility, often occupying relatively small areas throughout their lives and thus reflecting the local environment. Ninety-six specimens of C. cognatus were sampled from four tributaries of the Athabasca River to assess patterns of helminth parasite community structure in this fish and to study the composition and diversity of its parasite communities in relation to water quality. The localities included single samples from High Hills, Horse and Dunkirk rivers, and two from the Steepbank River. Twelve metazoan parasite species were found, most of them being larval forms. Significant differences occurred in the structure and composition of parasite assemblages of sculpins from the tributaries, although similarities were observed in connected and nearby sites. Parasite communities were influenced mainly by a combination of local environmental conditions, distance and connectivity, and were separated based on the distribution and abundance of autogenic and allogenic parasites. Water quality appeared to influence the distribution of trematode species that use gastropods as intermediate hosts, while proximity and connectivity of sites led to sharing allogenic parasite species in slimy sculpin.