Experimental evidence of early costs of reproduction in conspecific viviparous and oviparous lizards

Natural variation in the Caenorhabditis elegans egg-laying circuit modulates an intergenerational fitness trade-off

Evolutionary transitions from egg laying (oviparity) to live birth (viviparity) are common across various taxa. Many species also exhibit genetic variation in egg-laying mode or display an intermediate mode with laid eggs containing embryos at various stages of development. Understanding the mechanistic basis and fitness consequences of such variation remains experimentally challenging. Here, we report highly variable intra-uterine egg retention across 316 Caenorhabditis elegans wild strains, some exhibiting strong retention, followed by internal hatching. We identify multiple evolutionary origins of such phenotypic extremes and pinpoint underlying candidate loci. Behavioral analysis and genetic manipulation indicates that this variation arises from genetic differences in the neuromodulatory architecture of the egg-laying circuitry. We provide experimental evidence that while strong egg retention can decrease maternal fitness due to in utero hatching, it may enhance offspring protection and confer a competitive advantage. Therefore, natural variation in C. elegans egg-laying behaviour can alter an apparent trade-off between different fitness components across generations. Our findings highlight underappreciated diversity in C. elegans egg-laying behavior and shed light on its fitness consequences. This behavioral variation offers a promising model to elucidate the molecular changes in a simple neural circuit underlying evolutionary shifts between alternative egg-laying modes in invertebrates.

Extinction debt and functional traits mediate community saturation over large spatiotemporal scales

Determining if ecological communities are saturated (have a limit to the number of species they can support) has important implications for understanding community assembly, species invasions, and climate change. However, previous studies have generally been limited to short time frames that overlook extinction debt and have not explicitly considered how functional trait diversity may mediate patterns of community saturation. Here, we combine data from biodiversity surveys with functional and phylogenetic data to explore if the colonisation events after the Great American Biotic Interchange (closure of the Panamanian Isthmus) resulted in increases in species richness of communities of the snake family Dipsadidae. We determined the number and the direction of dispersal events between Central and South America by estimating ancestral areas based on a Bayesian time-calibrated phylogenetic analysis. We then evaluated whether variation in community saturation was mediated by the functional similarity of six traits for the resident and colonizing snakes and/or local environmental conditions. We found that colonised communities did not support more species than those that were not colonised. Moreover, we did not find an association between the functional diversity across sites and whether they were colonised by members from the lineages dispersing across the Isthmus or not. Instead, variation in species richness was predicted best by covariates such as time since colonisation and local environment. Taken together, our results suggest that snake communities of the Dipsadidae across the neotropics are saturated. Moreover, our research highlights two important factors to consider in studies of community saturation: extinction debt and the functional differences and similarities in species' ecological roles.

Climatic niche differences among Zootoca vivipara clades with different parity modes: implications for the evolution and maintenance of viviparity

Parity mode (oviparity/viviparity) importantly affects the ecology, morphology, physiology, biogeography and evolution of organisms. The main hypotheses explaining the evolution and maintenance of viviparity are based on bioclimatic predictions and also state that the benefits of viviparity arise during the reproductive period. We identify the main climatic variables discriminating between viviparous and oviparous Eurasian common lizard (Zootoca vivipara) occurrence records during the reproductive period and over the entire year.

Analyses based on the climates during the reproductive period show that viviparous clades inhabit sites with less variable temperature and precipitation. On the contrary, analyses based on the annual climates show that viviparous clades inhabit sites with more variable temperatures.

Results from models using climates during reproduction are in line with the “selfish-mother hypothesis”, which can explain the success of viviparity, the maintenance of the two reproductive modes, and why viviparous individuals cannot colonize sites inhabited by oviparous ones (and vice versa). They suggest that during the reproductive period viviparity has an adaptive advantage over oviparity in less risky habitats thanks to the selfish behaviour of the mothers. Moreover, the results from both analyses stress that hypotheses about the evolution and maintenance of viviparity need to be tested during the reproductive period.

Immune variation during pregnancy suggests immune component-specific costs of reproduction in a viviparous snake with disparate life-history strategies

Growing evidence suggests the existence of trade-offs between immune function and reproduction in diverse taxa. Among vertebrates, however, there is still a taxonomic bias toward studies in endotherms, particularly birds. We tested the hypothesis that reproduction entails immune-related costs in the viviparous garter snake, Thamnophis elegans, from populations that exhibit two life-history strategies, termed ecotypes, with contrasting paces of life. Between the two ecotypes, we predicted lower immune function in gravid than non-gravid females of both strategies, but with relatively larger immunity costs in the ecotype that generally invests more in current reproduction. Across individuals, we predicted greater immune costs for females investing more in the present specific reproductive event (i.e., higher fecundity) irrespective of their ecotype. We assessed leukocyte profiles and measured bactericidal capacity of plasma (innate immunity) and T- and B-lymphocyte proliferation (adaptive immunity) in gravid and non-gravid females in their natural habitats. We also collected data on reproductive output from these same gravid females brought into captivity. Gravid females of both ecotypes showed lower T-lymphocyte proliferation responses to concanavalin A than non-gravid females, but no differential costs were observed between ecotypes. The remaining immune variables did not vary between gravid and non-gravid females. Among gravid females within each ecotype, those with larger reproductive output showed lower total leukocyte counts, suggesting a fecundity-dependent trade-off. Our study contributes to the comparative ecoimmunology of vertebrates by highlighting the immune component-specificity of trade-offs between reproduction and immune function and showing that costs can be fecundity-dependent in some, but not all cases.

Local and systemic immune responses to different types of phytohemagglutinin in the green anole: Lessons for field ecoimmunologists

The phytohemagglutinin (PHA) skin test is commonly used by ecologists to assess cell-mediated immune function of wild animals. It can be performed quickly and easily in the field, involving injection of PHA and measurement of the resultant swelling. There are multiple formulations of PHA used in ecological studies, with potentially differing outcomes that could produce inconsistent results. We tested two common types of PHA in the green anole (Anolis carolinensis) to identify local and systemic immune responses underlying the resultant swelling at 6, 18, 24, and 48 hr post injection. There were differences in both local (injection site) and systemic (blood) leukocyte responses to PHA-L versus PHA-P. PHA-P injection produced a greater overall increase in local heterophil count at the injection site compared with PHA-L, and this response was greatest at 6 and 24 hr post injection. Systemically, heterophil percentage was higher in the blood of PHA-P- versus PHA-L-injected anoles at 24 hr post injection; the time point at which heterophil percentage peaked in PHA-P-injected anoles. These results indicate that although both PHA types are effective tests of immune function in green anoles, the PHA-P swelling response invokes a much stronger heterophilic response. PHA-L is a more specific test of lymphocyte function, particularly at 24 hr post injection, making it preferable for ecoimmunology studies.

The immunomodulatory role of the hypothalamus-pituitary-gonad axis: Proximate mechanism for reproduction-immune trade offs?

The present review discusses the communication between the hypothalamic-pituitary-gonad (HPG) axis and the immune system of vertebrates, attempting to situate the HPG-immune interaction into the context of life history trade-offs between reproductive and immune functions. More specifically, (i) we review molecular and cellular interactions between hormones of the HPG axis, and, as far as known, the involved mechanisms on immune functions, (ii) we evaluate whether the HPG-immune crosstalk serves as proximate mechanism mediating reproductive-immune trade-offs, and (iii) we ask whether the nature of the HPG-immune interaction is conserved throughout vertebrate evolution, despite the changes in immune functions, reproductive modes, and life histories. In all vertebrate classes studied so far, HPG hormones have immunomodulatory functions, and indications exist that they contribute to reproduction-immunity resource trade-offs, although the very limited information available for most non-mammalian vertebrates makes it difficult to judge how comparable or different the interactions are. There is good evidence that the HPG-immune crosstalk is part of the proximate mechanisms underlying the reproductive-immune trade-offs of vertebrates, but it is only one factor in a complex network of factors and processes. The fact that the HPG-immune interaction is flexible and can adapt to the functional and physiological requirements of specific life histories. Moreover, the assumption of a relatively fixed pattern of HPG influence on immune functions, with, for example, androgens always leading to immunosuppression and estrogens always being immunoprotective, is probably oversimplified, but the HPG-immune interaction can vary depending on the physiological and envoironmental context. Finally, the HPG-immune interaction is not only driven by resource trade-offs, but additional factors such as, for instance, the evolution of viviparity shape this neuroendocrine-immune relationship.

Terrestrial origin of viviparity in mesozoic marine reptiles indicated by early triassic embryonic fossils

Viviparity in Mesozoic marine reptiles has traditionally been considered an aquatic adaptation. We report a new fossil specimen that strongly contradicts this traditional interpretation. The new specimen contains the oldest fossil embryos of Mesozoic marine reptile that are about 10 million years older than previous such records. The fossil belongs to Chaohusaurus (Reptilia, Ichthyopterygia), which is the oldest of Mesozoic marine reptiles (ca. 248 million years ago, Early Triassic). This exceptional specimen captures an articulated embryo in birth position, with its skull just emerged from the maternal pelvis. Its headfirst birth posture, which is unlikely to be a breech condition, strongly indicates a terrestrial origin of viviparity, in contrast to the traditional view. The tail-first birth posture in derived ichthyopterygians, convergent with the conditions in whales and sea cows, therefore is a secondary feature. The unequivocally marine origin of viviparity is so far not known among amniotes, a subset of vertebrate animals comprising mammals and reptiles, including birds. Therefore, obligate marine amniotes appear to have evolved almost exclusively from viviparous land ancestors. Viviparous land reptiles most likely appeared much earlier than currently thought, at least as early as the recovery phase from the end-Permian mass extinction.

An experimental study of the gestation costs in a viviparous lizard: a hormonal manipulation

The trade-offs between reproduction and survival or future reproduction represent the costs of reproduction, which are central to the theory of life-history traits evolution. In particular, different stages of the reproductive cycle may be associated with different costs and thus explain the evolution of alternative reproductive strategies. Viviparity (live bearing) has evolved from oviparity (egg laying) several times independently in vertebrates. To better understand these transitions, we aimed to specifically investigate gestation costs in a squamate reptile with a new experimental procedure. We reduced litter size during gestation in the common lizard (Zootoca vivipara) with a hormonal injection of arginine vasotocin. This method is less invasive than a surgical method and does not reduce the number of offspring of future reproductive events. We monitored body mass change, immune response, endurance capacity, thermoregulatory behavior, offspring characteristic at birth, female and offspring survival, female body mass gain after parturition, and offspring growth rate after birth. Maternal treatment did not significantly change the offspring characteristics measured. Thus, litter size reduction did not change offspring development during gestation. For the females, there is evidence that endurance capacity during gestation is modified because of the physical burden of the litter and because of physiological changes. With respect to gestation costs, we did not observe a trade-off between the investment during gestation and females' resources postparturition (female body mass) or survival, but there was a facultative trade-off with the immune response. It will be interesting to replicate this study to increase the robustness of these results and to confirm the effects on the endurance capacity and the immune response. Gestation costs seem to be limited in this species, and they should be studied in more detail to evaluate their influence on the evolution of viviparity.