Patrick T. Rohner

Sexual selection on male size drives the evolution of male‐biased sexual size dimorphism via the prolongation of male development

Sexual size dimorphism (SSD) arises when the net effects of natural and sexual selection on body size differ between the sexes. Quantitative SSD variation between taxa is common, but directional intraspecific SSD reversals are rare. We combined micro‐ and macroevolutionary approaches to study geographic SSD variation in closely related black scavenger flies. Common garden experiments revealed stark intra‐ and interspecific variation: Sepsis biflexuosa is monomorphic across the Holarctic, while S. cynipsea (only in Europe) consistently exhibits female‐biased SSD. Interestingly, S. neocynipsea displays contrasting SSD in Europe (females larger) and North America (males larger), a pattern opposite to the geographic reversal in SSD of S. punctum documented in a previous study. In accordance with the differential equilibrium model for the evolution of SSD, the intensity of sexual selection on male size varied between continents (weaker in Europe), whereas fecundity selection on female body size did not. Subsequent comparative analyses of 49 taxa documented at least six independent origins of male‐biased SSD in Sepsidae, which is likely caused by sexual selection on male size and mediated by bimaturism. Therefore, reversals in SSD and the associated changes in larval development might be much more common and rapid and less constrained than currently assumed.

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.

Distribution, diversity gradients and Rapoport's elevational rule in the black scavenger flies of the Swiss Alps (Diptera: Sepsidae)

Elevational gradients influence the distribution and abundance of species drastically and can lead to variation in community composition. Although coprophagous flies are of ecological and economic importance, their biodiversity and distribution are largely neglected. We studied the impact of steep elevational gradients and geography on the distribution of sepsid flies in the Swiss Alps. Sepsidae are a family of acalyptrate flies strongly associated with decaying organic matter and vertebrate dung, and characterised by a high extent of sympatry in their breeding substrates. Historical, haphazardly sampled specimens from 116 locations covering an elevational range from 200 to 2000 m were available in ethanol collections of various Swiss museums. Nineteen species encompassing all native genera (Meroplius, Nemopoda, Saltella, Sepsis and Themira) were recorded. Local species richness increased linearly with elevation, while area‐corrected regional species richness of elevational belts increased asymptotically. Species occurring at higher altitudes had lower wing loadings and greater elevational ranges than lowland species, supporting Rapoports elevational rule. Despite compositional similarities, the sepsid communities of the northern lowlands differed significantly from the alpine sepsid fauna. The southern lowlands were particularly differentiated in community composition due to a number of presumably thermophilic species that predominantly occur south of the Alps. Relative abundances of several species were thus strongly affected by elevation and climatic variables. We illustrate the impact of elevational gradients and geography on a community of closely related, often sympatric species, and discuss potential mechanisms of niche partitioning via temporal succession, thermal adaptation and differential resource use.

Critical weight mediates sex‐specific body size plasticity and sexual dimorphism in the yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae)

Ultimate factors driving insect body size are rather well understood, while—apart from a few model species—the underlying physiological and developmental mechanisms received less attention. We investigate the physiological basis of adaptive size variation in the yellow dung fly Scathophaga stercoraria, which shows pronounced male‐biased sexual size dimorphism and strong body size plasticity. We estimate variation of a major physiological threshold, the critical weight, which is the mass at which a larva initiates pupariation. Critical weight was associated with sexual size dimorphism and sex‐specific plasticity, and is thus a likely target of selection on adult size. Detailed larval growth trajectories derived from individuals raised at two food and temperature treatments further reveal that sex‐specific size plasticity is mediated by faster initial growth of males that later becomes reduced by higher male weight loss during the wandering stage. We further demonstrate that integral growth rates, which are typically calculated as simple ratios of egg‐to‐adult development time and adult weight, do not necessarily well reflect variation in instantaneous growth rates. We illustrate the importance of detailed assessments of ontogenetic growth trajectories for the understanding of adaptive size variation and discuss the mechanistic basis of size determination in shaping sex‐specific phenotypic plasticity.

Genetic data confirm the species status of Sepsis nigripes Meigen (Diptera : Sepsidae) and adds one species to the Alpine fauna while questioning the synonymy of Sepsis helvetica Munari

Abstract. Due to their interesting biology, conspicuous sexual dimorphism and the ability to conduct experiments on species that breed under laboratory condition, sepsid flies (Diptera : Sepsidae) are becoming increasingly important model organisms in evolutionary biology. Accurate species boundaries and well supported phylogenetic hypotheses are thus of interest to many biologists. Here we resolve the conflict surrounding the taxonomic status of the European Sepsis nigripes Meigen, 1826, which is shown to be a valid species using morphological and molecular data applied to multiple species concepts. The species is also placed onto a phylogenetic tree for the genus Sepsis that includes most European and North American species. In addition, we assess the genetic variability between two populations of the Holarctic Sepsis luteipes Melander & Spuler, 1917 from Europe and North America and find conflicting evidence between morphology and DNA sequences. Different species concepts here yield different inferences, and if two species were to be accepted based on molecular data, Sepsis helvetica Munari, 1985 from Europe would have to be resurrected from synonymy. We provide high-resolution images for all species in order to aid in accurate identification. Both species are also added to Sepsidnet, the digital reference collection for Sepsidae (http://sepsidnet-rmbr.nus.edu.sg). Lastly, we discuss a field site in the Swiss Alps where 12 species of Sepsis occur sympatrically on the same pasture.

The evolution of male‐biased sexual size dimorphism is associated with increased body size plasticity in males

1. Sexual size dimorphism (SSD) can vary drastically across environments, demonstrating pronounced sex-specific plasticity. In insects, females are usually the larger and more plastic sex. However, the shortage of taxa with male-biased SSD hampers the assessment of whether the greater plasticity in females is driven by selection on size or represents an effect of the female reproductive role. Here, we specifically address the role of sex-specific plasticity of body size in the evolution of SSD reversals to disentangle sex and size effects. 2. We first investigate sex-specific body size plasticity in Sepsis punctum and Sepsis neocynipsea as two independent cases of intraspecific SSD reversals in sepsid flies. In both species, directional variation in SSD between populations is driven by stronger sexual selection on male size. Using controlled laboratory breeding, we find evidence for sex-specific plasticity and increased condition dependence of male size in populations with male-biased SSD, but not of female size in populations with female-biased SSD. 3. To extend the comparative scope, we next estimate sex-specific body size plasticity in eight additional fly species that differ in the direction of SSD under laboratory conditions. In all species with male-biased SSD we find males to be the more plastic sex, while this was only rarely the case in species with female-biased SSD, thus suggesting a more general trend in Diptera. 4. To examine the generality of this pattern in holometabolous insects, we combine our data with data from the literature in a meta-analysis. Again, male body size tends to be more plastic than female size when males are the larger sex, though female size is now also generally more plastic when females are larger. 5. Our findings indicate that primarily selection on size, rather than the reproductive role per se, drives the evolution of sex-specific body size plasticity. However, sepsid flies, and possibly Diptera in general, show a clear sexual asymmetry with greater male than female plasticity related to SSD, likely driven by strong sexual selection on males. Although further research controlling for phylogenetic and ecological confounding effects is needed, our findings are congruent with theory in suggesting that condition dependence plays a pivotal role in the evolution of sexual size dimorphism.

Interrelations of global macroecological patterns in wing and thorax size, sexual size dimorphism, and range size of the Drosophilidae

Support for macroecological rules in insects is mixed, with potential confounding interrelations between patterns rarely studied. We here investigate global patterns in body and wing size, sexual size dimorphism and range size in common fruit flies (Diptera: Drosophilidae) and explore potential interrelations and the predictive power of Allens, Bergmanns, Renschs and Rapoports rules. We found that thorax length (r2 = 0.05) and wing size (r2 = 0.09) increased with latitude, supporting Bergmanns rule. Contrary to patterns often found in endothermic vertebrates, relative wing size increased towards the poles (r2 = 0.12), a pattern against Allens rule, which we attribute to selection for increased flight capacity in the cold. Sexual size dimorphism decreased with size, evincing Renschs rule across the family (r2 = 0.14). Yet, this pattern was largely driven by the virilis – repleta radiation and only weakly in other lineages. Finally, range size did not correlate with latitude, although a positive relationship with latitude was present in a subset of the species investigated, providing no convincing evidence for Rapoports rule. We further found little support for confounding interrelations between body size, wing loading and range size in this taxon. Nevertheless, we demonstrate that studying several traits simultaneously at minimum permits better interpretation in case of multiple, potentially conflicting trends or hypotheses concerning the macroecology of insects.

Evolution of male costs of copulation in sepsid flies (Diptera: Sepsidae)

Reproduction is well known to be costly for females, but longevity costs of copulations in males are still poorly understood. In particular, the effect of the number of copulations on male longevity is rarely considered. Work on black scavenger flies (Diptera: Sepsidae) showed contrasting results: in Saltella sphondylii the number of copulations is strongly negatively correlated with male longevity, whereas in Sepsis cynipsea mated males did not suffer from reduced longevity. Here we summarize the findings of several studies covering four additional species of sepsid flies from across the phylogenetic tree of sepsids to better understand the evolution of male reproductive costs in this clade. After accounting for the mating system differences between species, we find no evidence for longevity costs in Allosepsis sp., Sepsis fulgens and Themira superba, while in Saltella nigripes multiple copulations drastically reduced longevity. Mapping this trait onto the most current phylogenetic hypothesis for Sepsidae suggests that male cost in Saltella is derived while there is an absence of longevity costs for all other sepsids. We discuss the origin of this novel longevity cost in Saltella in the context of a change in their reproductive strategy, namely the evolution of high polygynandry coupled with unusually brief copulations.

A Comparative Study of the Role of Sex-Specific Condition Dependence in the Evolution of Sexually Dimorphic Traits

Sexual selection can displace traits acting as ornaments or armaments from their viability optimum in one sex, ultimately giving rise to sexual dimorphism. The degree of dimorphism should not only mirror the strength of sexual selection but also the net viability costs of trait maintenance at equilibrium. As the ability of organisms to bear exaggerated traits will depend on their condition, more sexually dimorphic traits should also exhibit greater sex differences in condition dependence. While this has been demonstrated among traits within species, similar patterns are expected across the phylogeny. We investigated this prediction within and across 11 (sub)species of sepsid flies with varying mating systems. When estimating condition dependence for seven sexual and nonsexual traits that vary in their sexual dimorphism, we not only found a positive relationship between the sex difference in allometric slopes (our measure of condition dependence) and relative trait exaggeration within species but also across species for those traits expected to be under sexual selection. Species with more pronounced male aggression further had relatively larger and more condition-dependent male fore- and midlegs. Our comparative study suggests a common genetic/developmental basis of sexual dimorphism and sex-specific plasticity that evolves across the phylogeny—and that the evolution of size consistently alters scaling relationships and thus contributes to the allometric variation of sexual armaments or ornaments in animals.

Geographic clines in wing morphology relate to colonization history in New World but not Old World populations of yellow dung flies: THE FORMATION OF WING SHAPE CLINES

Geographic clines offer insights about putative targets and agents of natural selection as well as tempo and mode of adaptation. However, demographic processes can lead to clines that are indistinguishable from adaptive divergence. Using the widespread yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae), we examine quantitative genetic differentiation (QST) of wing shape across North America, Europe, and Japan, and compare this differentiation with that of ten microsatellites (FST). Morphometric analyses of 28 populations reared at three temperatures revealed significant thermal plasticity, sexual dimorphism, and geographic differentiation in wing shape. In North America morphological differentiation followed the decline in microsatellite variability along the presumed route of recent colonization from the southeast to the northwest. Across Europe, where S. stercoraria presumably existed for much longer time and where no molecular pattern of isolation by distance was evident, clinal variation was less pronounced despite significant morphological differentiation (QST>FST). Shape vector comparisons further indicate that thermal plasticity (hot‐to‐cold) does not mirror patterns of latitudinal divergence (south‐to‐north), as might have been expected under a scenario with temperature as the major agent of selection. Our findings illustrate the importance of detailed phylogeographic information when interpreting geographic clines of dispersal traits in an adaptive evolutionary framework.