Roxana R. Zenuto

Females prefer good genes: MHC-associated mate choice in wild and captive tuco-tucos

The major histocompatibility complex (MHC) genes are one of the most suitable candidates for elucidating the genetic bases of mate choice in vertebrates, given the potential benefits in terms of immunocompetence that can be passed to the offspring through MHC-associated mate choice. Female mate choice may favour males that possess particular MHC alleles or those with diverse MHC genotypes (good genes hypothesis), or males that possess MHC genotypes that differ from that of the female (compatibility hypothesis). Our goal was to evaluate mate choice in relation to MHC genotype in the subterranean rodent Ctenomys talarum. Using both laboratory and field analyses, we investigated whether the (1) number of shared MHC alleles between males and females, (2) number of amino acid differences between female and male MHC alleles, (3) MHC heterozygosity of males, (4) number of amino acid differences between male MHC alleles, and (5) frequency of MHC alleles of males differ between preferred and nonpreferred males in the laboratory and between potential sires and random males from the population in the field. In the laboratory, our results indicate that MHC alleles of preferred males differ in fewer amino acids compared to MHC alleles of nonpreferred males. Concomitantly, in the field, MHC alleles of possible sires differed in fewer amino acids than those of random males in the population. Plus, possible sires were more heterozygous and carried distinct MHC alleles compared with random males, thus providing more support to the ‘good genes’ hypothesis. We discuss the possible reasons why MHC-based mate choice was more evident in the field and the implications of such a female mating strategy in the subterranean environment.

PHA-induced inflammation is not energetically costly in the subterranean rodent Ctenomys talarum (tuco-tucos).

Immune activity has been proposed to be associated with substantial costs, due to trade-offs with other functions or activities that share common resources and contribute to an animals fitness. However, direct estimates of the cost of mounting an immune response are few and have been performed mainly in birds. Thus, further work is needed to clarify the relative costs of different components of the immune system and the role of environmental and life-history traits in modulating the costs of resistance. Within the components of immunity, inflammation is considered to be associated with a larger energetic expenditure. Here, we evaluated the energetic cost of the inflammatory response to phytohemagglutinin (PHA) in a wild population of a subterranean rodent, Ctenomys talarum, and the trade-offs between immune activity and reproduction. C. talarum develops an inflammatory response to PHA, but contrary to our predictions, this response was not associated with an increase in oxygen consumption regardless of reproductive status or sex. Our study shows that an immune challenge may not always result in a detectable energetic cost. We discuss the possibility that other currencies could be underlying the cost, such as micro-or macronutrients requirements, autoimmunity or oxidative stress.

Immune challenge but not dietary restriction affects spatial learning in the wild subterranean rodent Ctenomys talarum

Several lines of evidence suggest that learning and triggering an immune response are both metabolically expensive and thus likely to be subject to nutritional trade-offs between them and other competing demands. Therefore, we evaluated if an immune challenge with a novel antigen affects spatial learning in the subterranean rodent Ctenomys talarum under two different dietary conditions. The results showed that immune-challenged animals were affected in their spatial learning capabilities, increasing the number of errors and marginally the time required to reach the goal of a complex labyrinth. No effect of the dietary restriction nor interaction between factors were observed. This work provides support for the existence of a trade-off between the costs of the immune defense and learning abilities, indicating that when investment is required to fight infection, fewer resources are available for learning. The absence of effect of nutritional condition on this trade-off suggests that other physiological processes, besides cognition, may be limited by the energetic resources necessary to the more immediately critical immune response.

Parasite infection negatively affects PHA-triggered inflammation in the subterranean rodent Ctenomys talarum

Magnitude and effectiveness of immune responses vary greatly between and within species. Among factors reported to determine this variation, parasitism is a critical one, although controversial effects of parasites over immunological indices have been reported. Information regarding immune strategies in species with different life histories is crucial to better understand the role of immune defenses in an ecological and evolutionary context. Here, we examine the influence of the parasite community on immune responsiveness of a solitary subterranean rodent, Ctenomys talarum. To do this, we assessed the impact of the natural parasite community and the experimental infection with Eimeria sp. on the phytohemagglutinin (PHA)-response, as well as other immune, condition, nutrition, and stress parameters. PHA-triggered inflammation was similarly impaired by Eimeria sp. infection alone or co-occurring with a number of gastrointestinal nematodes. None of the other physiological parameters studied were affected by parasitism. This indicates that parasitism is a general key factor modulating immune responsiveness of the host, and in particular for C. talarum, it could explain the great inter-individual variation previously observed in the PHA-response. Thus, our results highlight the importance of taking the parasite community into account in ecoimmunological studies, particularly when using immunological indices.

Interpopulation differences in parasite load and variable selective pressures on MHC genes in Ctenomys talarum

Abstract We explore potential geographic variation in the pathogen-mediated selective pressures acting on major histocompatibility complex (MHC) loci in the subterranean rodent Ctenomys talarum, evaluating the impact of differences in parasite load between 2 populations (Mar de Cobo [MdC] and Necochea [NC]) on immunogenetic variation and selection on MHC genes in this species. Because Ctenomys from NC face lower parasite load and presumably weaker pathogen-mediated selection on MHC, we expected to find a weaker correlation between MHC variation and parasite load or immunocompetence, or both, in this population compared to that at MdC. MHC-associated cues are used in other species of rodents as kinship markers to avoid inbreeding, and because kinship structure is less pronounced in NC, we predicted that use of MHC-associated cues in mate choice would be less apparent in this population. We characterized MHC variation in NC as a function of parasite load and immunocompetence and compared our results with previous findings for MdC. The 2 populations were sampled across different, but consecutive, years. Using coinertia analyses, we found a significant positive association between a specific DRB allele and intensity of infection by fleas in NC. We explored the use of MHC-associated cues in mate choice in NC and found support for both the “good-genes” and the “genetic compatibility” hypotheses. As expected, associations between MHC and parasite load or immunocompetence were weaker in NC. Evidence indicated that females in NC selected for males with lower MHC diversity. This suggests that parasite-driven selection acting directly on MHC genes seems to be greater for the population facing higher parasite load. However, parasite-driven selection mediated by mate choice may not only be influenced by levels of parasite diversity in the population but also by characteristics of the mating system. Resumen Se exploró la variación geográfica potencial en las presiones selectivas mediadas por patógenos sobre los loci de Complejo Principal de Histocompatibilidad (MHC) en el roedor subterráneo Ctenomys talarum, evaluando las diferencias en cargas parasitarias entre dos poblaciones (Mar de Cobo [MdC] y Necochea [NC]) con el fin de determinar de qué forma la variación espacial en este atributo influye sobre la variación y selección en los genes de MHC en esta especie. Dado que los tuco-tucos de NC enfrentan menores cargas parasitarias, se esperaba encontrar una correlación más débil entre la variación de MHC y la carga parasitaria/inmunocompetencia en esta población comparada con MdC. También se predijo que el uso de pistas asociadas al MHC en el proceso de elección de pareja fuera menos evidente en NC. Caracterizamos la variación de MHC en NC en función de la carga parasitaria y de la inmunocompetencia y comparamos nuestros resultados con datos previos de MdC. Ambas poblaciones fueron muestreadas a través de años diferentes, pero consecutivos. Utilizando análisis de co-inercia, se encontró una asociación significativamente positive entre un alelo específico de DRB y la intensidad de infección por pulgas en NC. Analizamos el uso de pistas asociadas al MHC en la elección de pareja en NC y encontramos soporte para las hipótesis de “buenos genes” y “compatibilidad genética.” De acuerdo a lo esperado, las asociaciones entre MHC y carga parasitaria o inmunocompetencia fueron menos frecuentes en NC, la población en la cual los tuco-tucos enfrentan menores cargas parasitarias. La evidencia indicó que las hembras en NC escogieron a machos con baja diversidad de MHC. Esto sugiere que la selección mediada por parásitos que actúa directamente sobre los genes de MHC parece ser de mayor magnitud en la población que enfrenta mayores cargas parasitarias. Sin embargo, la selección mediada por parásitos a través de la elección de pareja puede no estarúnicamente influida por niveles de diversidad parasitaria en la población sino también por características del sistema de apareamiento.

Assessing the energetic costs and trade-offs of a PHA-induced inflammation in the subterranean rodent Ctenomys talarum: immune response in growing tuco-tucos.

A traditional approach used to assess whether immune defense is costly is to explore the existence of trade-offs between immunity and other functions; however, quantitative studies of the energetic costs associated with the activation of the immune system are scarce. We assessed the magnitude of a PHA-triggered immune response and the associated energetic costs in 60-day old Ctenomys talarum. We expected that the magnitude of the macroscopic inflammatory response to PHA is lower in young tuco-tucos compared with that of adults, given the allocation of substantial energy to growth, and that the magnitude of the inflammation is lower in male pups compared to females, due to the higher investment in growth of the larger sex. Concomitantly, we expected that the pups challenged with PHA show an increase in oxygen consumption compared to control animals and that a positive association exists between magnitude of the PHA-induced inflammation and oxygen consumption. Contrary to what was expected, young tuco-tucos mounted a higher inflammatory response compared with adults and there were no differences in the magnitude of this response between sexes. The inflammatory response induced by a PHA injection did not represent a significant energetic cost for young tuco-tucos. There were no differences in oxygen consumption between PHA-injected and control animals, and tuco-tucos that mounted a higher inflammatory response to PHA did not show higher oxygen consumption. Energy expenditure, however, is not the only physiological cost involved in trade-offs between immune response and various functions of the organism, and other currencies are discussed.

When allowed, females prefer novel males in the polygynous subterranean rodent Ctenomys talarum (tuco-tuco).

The ability to recognize familiar conspecifics plays an important role at the time of choosing a mating partner in rodents. A laboratory study using preference test was used in order to test the hypothesis that, in the polygynous subterranean rodent Ctenomys talarum, females prefer novel males when offered two odors, or individuals (one familiar and one novel) limited in their movements so that male aggression is prevented. Our findings show that females prefer novel tuco-tucos at three levels of male assessment: odor samples (consisting of shavings soiled with urine, feces, and presumably, other body secretions collected from the male home cage), confined males behind a wire mesh, and full contact with tethered males. Previous studies of this species demonstrated that in the wild, male-male competition and male coercion severely limit the possibility of females mating non-neighbors, i.e. novel males. Females mating neighbors to whom they are familiarized, obtain high quality mating since they are territorial, highly competitive males. Nonetheless, when females have the opportunity, as shown in the two-choice experiments, they choose novel males, probably benefiting their progeny from novel genetic combinations. Hence, combining evidence from laboratory and field studies in C. talarum, it is possible to better understand female preferences in modeling individual reproductive strategies.

Female choice, male dominance and condition-related traits in the polygynous subterranean rodent Ctenomys talarum

Ctenomys talarum is a solitary and highly territorial species in which dominant males aggressively deter other males and monopolize reproductive activity. Female preference for dominant males is not easy to assess due to coercive mating by males. Hence, we aimed to answer if behavioural dominance and several condition-related traits like testosterone and cortisol levels, endoparasite load, and hematocrit volume may affect female preference when they have the opportunity to exert it. We designed a laboratory experiment using wild-caught C. talarum and employed a combined approach involving behavioural observations and the measurements of parameters of physical condition. We staged dyadic encounters between males to determine dominance index and then analyzed female preference towards tethered males (n=15) or their odours (n=18). Male dominance did not affect female preferences when odours were presented. When two tethered males were offered, females preferred those with higher dominance index. Preference of females for dominant males would mainly represent indirect benefits. Females did not show preference for males in relation to any physiological trait evaluated. Dominance was found negatively related to cortisol levels, probably avoiding the glucocorticoids-related costs, and positively related to parasite diversity, since they could tolerate it without impairing their health.

Different regulation of cortisol and corticosterone in the subterranean rodent Ctenomys talarum: Responses to dexamethasone, angiotensin II, potassium, and diet

When harmful environmental stimuli occur, glucocorticoids (GCs), cortisol and corticosterone are currently used to evaluate stress status in vertebrates, since their secretions are primarily associated to an increased activity of the hypothalamic-pituitaryadrenal (HPA) axis. To advance in our comprehension about GCs regulation, we evaluated the subterranean rodent Ctenomys talarum to assess cortisol and corticosterone response to (1) the negative feedback of the HPA axis using the dexamethasone (DEX) suppression test, (2) angiotensin II (Ang II), (3) potassium (K+) intake, and (4) different diets (vegetables, grasses, acute fasting). Concomitantly, several indicators of individual condition (body mass, neutrophil to lymphocyte ratio, blood glucose, triglycerides and hematocrit) were measured for diet treatments. Results confirm the effect of DEX on cortisol and corticosterone in recently captured animals in the field but not on corticosterone in captive animals. Data suggest that Ang II is capable of stimulating corticosterone, but not cortisol, secretion. Neither cortisol nor corticosterone were responsive to K+ intake. Cortisol levels increased in animals fed with grasses in comparison to those fed with vegetables while corticosterone levels were unaffected by diet type. Moreover, only cortisol responded to fasting. Overall, these results confirm that cortisol and corticosterone are not interchangeable hormones in C. talarum.