Growth rate mediates hidden developmental plasticity of female yellow dung fly reproductive morphology in response to environmental stressors

Understanding how environmental variation influences even cryptic traits is important to clarify the roles of selection and developmental constraints in past evolutionary divergence and to predict future adaptation under environmental change. Female yellow dung flies (Scathophaga stercoraria) typically have three sperm storage compartments (3S), but occasionally four (4S). More spermathecae are thought to be a female adaptation facilitating sperm sorting after mating, but the phenotype is very rare in nature. We manipulated the flies' developmental environment by food restriction, pesticides, and hot temperatures to investigate the nature and extent of developmental plasticity of this trait, and whether spermatheca expression correlates with measures of performance and developmental stability, as would be expected if 4S expression is a developmental aberration. The spermathecal polymorphism of yellow dung fly females is heritable, but also highly developmentally plastic, varying strongly with rearing conditions. 4S expression is tightly linked to growth rate, and weakly positively correlated with fluctuating asymmetry of wings and legs, suggesting that the production of a fourth spermatheca could be a nonadaptive developmental aberration. However, spermathecal plasticity is opposite in the closely related and ecologically similar Scathophaga suilla, demonstrating that overexpression of spermathecae under developmental stress is not universal. At the same time, we found overall mortality costs as well as benefits of 4S pheno‐ and genotypes (also affecting male siblings), suggesting that a life history trade‐off may potentially moderate 4S expression. We conclude that the release of cryptic genetic variation in spermatheca number in the face of strong environmental variation may expose hidden traits (here reproductive morphology) to natural selection (here under climate warming or food augmentation). Once exposed, hidden traits can potentially undergo rapid genetic assimilation, even in cases when trait changes are first triggered by random errors that destabilize developmental processes.

Female yellow dung flies naturally vary in number of sperm storage compartments (3S or 4S).

This spermathecal polymorphism is strongly heritable but also developmentally plastic.

4S expression is linked to growth rate and weakly correlated with fluctuating asymmetry, so potentially a developmental aberration.

There are mortality costs as well as benefits for 4S phenotypes, suggesting adaptive life‐history trade‐offs.

Spermathecal plasticity differs in the closely related and ecologically similar Scathophaga suilla.

Environmental changes can expose hidden traits with initially no function to natural selection.

Comprehensive thermal performance curves for yellow dung fly life history traits and the temperature-size-rule

Ambient temperature strongly determines the behaviour, physiology, and life history of all organisms. The technical assessment of organismal thermal niches in form of now so-called thermal performance curves (TPC) thus has a long tradition in biological research. Nevertheless, several traits do not display the idealized, intuitive dome-shaped TPC, and in practice assessments often do not cover the entire realistic or natural temperature range of an organism. We here illustrate this by presenting comprehensive sex-specific TPCs for the major (juvenile) life history traits of yellow dung flies (Scathophaga stercoraria; Diptera: Scathophagidae). This concerns estimation of prominent biogeographic rules, such as the temperature-size-rule (TSR), the common phenomenon in ectothermic organisms that body size decreases as temperature increases. S. stercoraria shows an untypical asymptotic TPC of continuous body size increase with decreasing temperature without a peak (optimum), thus following the TSR throughout their entire thermal range (unlike several other insects presented here). Egg-to-adult mortality (our best fitness estimator) also shows no intermediate maximum. Both may relate to this fly entering pupal winter diapause below 12 °C. While development time presents a negative exponential relationship with temperature, development rate and growth rate typify the classic TPC form for this fly. The hitherto largely unexplored close relative S. suilla with an even more arctic distribution showed very similar responses, demonstrating large overlap among two ecologically similar, coexisting dung fly species, thus implying limited utility of even complete TPCs for predicting species distribution and coexistence.

Is qualitative and quantitative metabarcoding of dung fauna biodiversity feasible?

In biodiversity assessments, especially of small-bodied organisms for which taxonomic expertise is lacking, identification by genetic barcoding may be a cost-effective and efficient alternative to traditional identification of species by morphology, ecology, and behavior. The authors tested the feasibility and accuracy of such an approach using dung insects of practical relevance in ecotoxicological assessments of veterinary pharmaceutical residues in the environment. They produced 8 known mixtures that varied in absolute and relative composition of small-bodied and large-bodied species to see whether mitochondrial cytochrome c oxidase subunit 1 barcoding picks up all species qualitatively and quantitatively. As demonstrated before in other contexts, such metabarcoding of large numbers of dung insect specimens is principally possible using next-generation sequencing. The authors recovered most species in a sample (low type I error), at minimum permitting analysis of species richness. They obtained even quantitative responses reflecting the body size of the species, although the number of specimens was not well detected. The latter is problematic when calculating diversity indices. Nevertheless, the method yielded too many closely related false positives (type II error), thus generally overestimating species diversity and richness. These errors can be reduced by refining methods and data filtering, although this requires bioinformatics expertise often unavailable where such research is carried out. Identification by barcoding foremost hinges on a good reference database, which does not yet exist for dung organisms but would be worth developing for practical applications. Environ Toxicol Chem 2016;35:1970-1977. © 2015 SETAC.

Infections with Wolbachia and Spiroplasma in the Scathophagidae and other Muscoidea

The microbes Wolbachia and Spiroplasma are common reproductive parasites of arthropods and may strongly influence reproduction of infected hosts and also impact on reproductive isolation. Such infections could hence influence results of many studies assessing reproductive behaviour and fitness of possible hosts, as well as reproductive isolation. Previous work indicates that infections with the microbes Wolbachia and Spiroplasma are common in the Drosophilidae. However, extensive and targeted surveys of other Dipteran families are lacking. Here we survey the yellow dung fly Scathophaga stercoraria and a range of other species from the Muscoidea (families Scathophagidae, Anthomyiidae, Fanniidae and Muscidae) collected in the field or obtained from museum collections for infection with the widespread reproductive parasites Wolbachia and Spiroplasma. Both have been shown to be heritable symbionts and affect reproduction in other Diptera. S. stercoraria is a very important model for the study of sexual selection, and in particular of postcopulatory processes, as it has played a major role in the history of research on sperm competition and cryptic female choice. Infections with Wolbachia were found to be widespread across the Muscoidea, whereas infections with Spiroplasma were rarer. We discuss the consequences of these findings and directions for future research on the impact of reproductive parasites on host reproduction in the Scathophagidae.