The origin and maintenance of individual variation in burying beetles, Nicrophorus vespilloides
Lead Research Organisation:
University of Cambridge
Department Name: Zoology
Abstract
My project explores how social evolution drives variation. Social interactions are widely regarded as a major driver of evolutionary change, notably the social interactions between parents and offspring. My project focuses on variation within the family, analysing how differential patterns of parental care influence morphological variation within a brood. I have been using both wild populations, and experimentally evolving populations of the burying beetle, Nicrophorus vespilloides as a model organism to address this question.
Organisations
| Description | My project explores how social evolution drives variation. Social interactions are widely regarded as a major driver of evolutionary change, notably the social interactions between parents and offspring. My project focuses on variation within the family, analysing how differential patterns of parental care influence morphological variation within a brood. I have been using the burying beetle, Nicrophorus vespilloides as a model organism to address this question. Burying beetles exhibit highly advanced and intricate levels of biparental care. Both parents use carrion to provision their young, stripping it of its fur or feathers, before rolling it into a brood ball. The prepared brood ball then becomes the site for posthatching care. Parents actively feed their offspring until they complete development and migrate away to pupate. However, posthatching care is not obligate, offspring can survive without it too. The ability for larvae to survive without the presence of parents is adaptive because there is great variation and uncertainty with regard to the levels of posthatching care supplied by parents. As a result, the burying beetle's natural history makes it an excellent model to study social evolution and family dynamics in particular. The trait of interest for much of my work is larval mandible size. Mandibles are specialised appendages that are vital for self-feeding. In the absence of care, penetrating the carcass is the first major challenge that larvae face after hatching and if they fail in this task they face certain death. I found a large extent of variation in mandible size within families, which exceeded variation in mandible size between families. What is the cause of this sibling variation? A likely explanation is parental effects, since there is much variation expressed among the offspring of monogamous parents. Perhaps the most well-characterised parental effect in Nicrophorus is due to post-hatching provisioning of larvae by parents. The quantity and quality of resources that parents provide to larvae within a brood can have a major effect on offspring phenotype. To test how parent offspring interactions influence the evolution of larval mandible size, I have been analysing replicate experimental populations that have evolved for 46 generations to date under different regimes of parental care. Two populations have evolved in a Full Care environment, where parents are allowed to care for their offspring after hatching. Two other populations have evolved in a No Care environment, where both parents are removed after they have finished preparing the carcass, but before the larvae have hatched. No Care populations have rapidly adapted to a social environment without parental care, with traits evolving especially in the larvae. For example, in the absence of parents, larvae evolve to interact more cooperatively with each other. Larvae hatch more synchronously in the No Care environment now as well, perhaps because larger teams of first-hatched larvae can more effectively cooperate by working together to penetrate the carcass. Once the first-hatched group have gained access to the carcass, all later-hatching larvae are more likely to penetrate the carcass too and therefore more likely to survive. The relative size of larval mandibles at dispersal has diverged too, under exposure to contrasting yet predictable levels of parental care. In the Full Care populations, larval mandibles evolved to be smaller relative to larval mass, while larvae from the No Care populations evolved to have relatively larger mandibles. Together, these results suggest that larvae within the brood could pursue different tactics to gain access to the resources on the carcass. This might explain why I previously found so much variation in mandible size within broods. Perhaps individual larvae within the family each occupy their own special niche in the 'family ecosystem'? First-hatched larvae might be pioneers on the carcass, penetrating it for the benefit of all the brood. Later-hatched larvae might then more easily feed on the carcass by using the entry holes formed by their older siblings. Perhaps first-hatched larvae are selected to have larger mandibles for penetrating the carcass, while later-hatched larvae are able to have much smaller mandibles. In the first year of my project, I tested these ideas by exploring how mandible size in larval hatchlings varied with hatch sequence within broods, comparing families from both the No Care and Full Care evolving populations. In general, I found that first-hatched larvae possessed larger mandibles at hatching, whereas late-hatched larvae had smaller mandibles. However, this trend was evident not only in the No Care populations but in the Full Care populations too. These results suggest that a large mandibular phenotype in first-hatched larvae could be a highly conserved strategy in the burying beetle for enabling larvae to conquer the carcass and establish themselves upon it, remaining crucial even when parents are present to help larvae in this task. The work in my second year built on these initial findings. In my second year I repeated the previous experiment with some adjustments to the experimental protocol. This time I controlled for variation in larval mass by investigating the allometric relationship between mandible size and mass in early and late hatchlings. Mass is an important trait and is correlated with fitness. I explored how mass in larval hatchlings varied with hatch sequence within broods, comparing families from both the No Care and Full Care evolving populations. In Full Care I found that first-hatched larvae exhibited greater mass at hatching, whereas late-hatched larvae had smaller mass. However, in the No Care populations first-hatched larvae and late-hatched larvae had no significant difference in mass. These results suggest that in the No Care populations mothers have evolved to distribute resources equally across their offspring, whilst in Full Care mothers seem to promote asymmetries within the brood. I intend to explore this further and investigate how these symmetries and asymmetries develop and how they are adaptive. I also started to investigate adult mandible morphology to determine whether variation in larval mandibles persists into adulthood. Parental effects are not just restricted to larval phenotype, early life history can influence adult phenotype too. If there is an association between the size of larval mandibles and the size of mandibles in adulthood, then parental effects could influence the adult phenotype and evolutionary trajectory too. I explored how adult mandible size varied in both No Care and Full Care evolved selection lines. I also used this study as an opportunity to investigate variation in head size. Past studies suggest that head size is a proxy for feeding prowess. Larger heads appear to reflect a larger volume of the mandibular closer muscle. In other words, head size could be a valuable measurement for assessing chewing capability in the burying beetle. This is especially relevant in my research, as increased head size is associated with higher feeding rates. An equivalent change in head size could be induced by larvae self-feeding in the No Care populations. This is what I'm currently exploring at the moment. |
| Exploitation Route | Pinpointing key traits in developmental history that can be used for genomic analysis. |
| Sectors | Environment,Other |