Evolutionary consequence of a change in life cycle complexity: A link between precocious development and evolution toward female-biased sex allocation in a hermaphroditic parasite

Comparing the hermaphroditic mating system of a parasitic flatworm between populations with an ancestral, three-host life cycle and a derived, facultative precocious life cycle

Evolutionary changes in development and/or host number of parasite life cycles can have subsequent ecological and evolutionary consequences for parasites. One theoretical model based on the mating systems of hermaphroditic parasites assumes a life cycle with fewer hosts will result in more inbreeding, and predicts a truncated life cycle most likely evolves in the absence of inbreeding depression. Many populations of the hermaphroditic trematode Alloglossidium progeneticum maintain an ancestral obligate three-host life cycle where obligate sexual reproduction occurs among adults in catfish third hosts. However, some populations have evolved a facultative precocious life cycle, where sexual development can occur while encysted within crayfish second hosts, likely leading to high inbreeding as individuals are forced to self-mate while encysted. Whether selfing represents a derived state remains untested. We compared selfing rates of 5 precocious populations to that of 4 populations with an ancestral obligate three-host life cycle. We also compared demographic estimates to genetic estimates of selfing to test the prediction of no inbreeding depression in precocious populations. Results showed that while the ancestral obligate three-host life cycle is associated with high outcrossing rates, the facultative precocious populations are highly selfing and show little evidence for inbreeding depression.

Testing the Mating System Model of Parasite Complex Life Cycle Evolution Reveals Demographically Driven Mixed Mating

AbstractMany parasite species use multiple host species to complete development; however, empirical tests of models that seek to understand factors impacting evolutionary changes or maintenance of host number in parasite life cycles are scarce. Specifically, one model incorporating parasite mating systems that posits that multihost life cycles are an adaptation to prevent inbreeding in hermaphroditic parasites and thus preclude inbreeding depression remains untested. The model assumes that loss of a host results in parasite inbreeding and predicts that host loss can evolve only if there is no parasite inbreeding depression. We provide the first empirical tests of this model using a novel approach we developed for assessing inbreeding depression from field-collected parasite samples. The method compares genetically based selfing rate estimates to a demographic-based selfing rate, which was derived from the closed mating system experienced by endoparasites. Results from the hermaphroditic trematode Alloglossidium renale, which has a derived two-host life cycle, supported both the assumption and the prediction of the mating system model, as this highly inbred species had no indication of inbreeding depression. Additionally, comparisons of genetic and demographic selfing rates revealed a mixed mating system that could be explained completely by the parasite's demography (i.e., its infection intensities).

Phylogeny, genetics, and the partial life cycle of Oncomegas wageneri in the Gulf of Mexico

Despite the diversity and ecological importance of cestodes, there is a paucity of studies on their life stages (i.e., complete lists of intermediate, paratenic, and definitive hosts) and genetic variation. For example, in the Gulf of Mexico (GoM) 98 species of cestodes have been reported to date; however, data on their intraspecific genetic variation and population genetic studies are lacking. The trypanorhynch cestode, Oncomegas wageneri, is found (among other places) off the American Western Atlantic Coast, including the GoM, and has been reported as an adult from stingrays and from several teleost species in its larval form (as plerocerci). This study represents the first report of 2 previously unregistered definitive hosts for O. wageneri, namely the Atlantic sharpnose shark Rhizoprionodon terraenovae and the southern stingray Hypanus americanus. In this work, partial sequences of the 28S (region D1-D2) ribosomal DNA were analyzed to include O. wageneri within an eutetrarhynchoid phylogenetic framework. All O. wageneri individuals (which included plerocerci and adults) were recovered as monophyletic and Oncomegas celatus was identified as the sister species of O. wageneri. Furthermore, population genetic analyses of O. wageneri from the southern GoM were carried out using DNA sequences of the mitochondrial cytochrome c oxidase subunit 1 (COI) gene, which reflected high genetic variation and a lack of genetic structure among the 9 oceanographic sampling sites. Based on these results, O. wageneri is panmictic in the southern GoM. More extensive sampling along the species entire distribution is necessary to make more accurate inferences of population genetics of O. wageneri.

The interaction between sex-specific selection and local adaptation in species without separate sexes

Local adaptation in hermaphrodite species can be based on a variety of fitness components, including survival, as well as both female and male sex-functions within individuals. When selection via female and male fitness components varies spatially (e.g. due to environmental heterogeneity), local adaptation will depend, in part, on variation in selection through each fitness component, and the extent to which genetic trade-offs between sex-functions maintain genetic variation necessary for adaptation. Local adaptation will also depend on the hermaphrodite mating system because self-fertilization alters several key factors influencing selection and the maintenance of genetic variance underlying trade-offs between the sex-functions (sexually antagonistic polymorphism). As a first step to guide intuition regarding sex-specific adaptation in hermaphrodites, we develop a simple theoretical model incorporating the essential features of hermaphrodite mating and adaptation in a spatially heterogeneous environment, and explore the interaction between sex-specific selection, self-fertilization and local adaptation. Our results suggest that opportunities for sex-specific local adaptation in hermaphrodites depend strongly on the extent of self-fertilization and inbreeding depression. Using our model as a conceptual framework, we provide a broad overview of the literature on sex-specific selection and local adaptation in hermaphroditic plants and animals, emphasizing promising future directions in light of our theoretical predictions.This article is part of the theme issue 'Linking local adaptation with the evolution of sex differences'.

Autonomy and integration in complex parasite life cycles

Complex life cycles are common in free-living and parasitic organisms alike. The adaptive decoupling hypothesis postulates that separate life cycle stages have a degree of developmental and genetic autonomy, allowing them to be independently optimized for dissimilar, competing tasks. That is, complex life cycles evolved to facilitate functional specialization. Here, I review the connections between the different stages in parasite life cycles. I first examine evolutionary connections between life stages, such as the genetic coupling of parasite performance in consecutive hosts, the interspecific correlations between traits expressed in different hosts, and the developmental and functional obstacles to stage loss. Then, I evaluate how environmental factors link life stages through carryover effects, where stressful larval conditions impact parasites even after transmission to a new host. There is evidence for both autonomy and integration across stages, so the relevant question becomes how integrated are parasite life cycles and through what mechanisms? By highlighting how genetics, development, selection and the environment can lead to interdependencies among successive life stages, I wish to promote a holistic approach to studying complex life cycle parasites and emphasize that what happens in one stage is potentially highly relevant for later stages.