Patterns in antipredator armature reduction and maintenance in isolated spring populations of an amphipod crustacean

Organisms colonizing new habitats can undergo adaptive change due to novel selective landscapes encountered in the new environment. Examples in nature where the development of the same traits has repeatedly occurred on multiple independent occasions upon colonizing a novel habitat represent instances of parallel evolution. Here we test whether the colonization of spring habitat by the principally lacustrine amphipod crustacean Pallaseopsis quadrispinosa has resulted in parallel evolution in armature traits using empirical data on morphology and mitochondrial DNA and through a breeding experiment. Analysis of mtDNA CO1 sequences shows that the spring populations share no common history and have evolved in isolation from each other and from their neighbouring lake populations since deglaciation approximately 12,000 years ago and are now fixed for different haplogroups. Dorsal spines and lateral projections were absent or less developed in all spring populations than in lake populations. Variation in armature development also could be explained by predator presence as populations with fish predators exhibited more developed spines than those without fish. In a laboratory breeding experiment, hybrid Spring × Lake F1 offspring had intermediate development of armature compared to offspring of Lake × Lake and Spring × Spring matings. The results support the hypothesis that armature reduction has independently evolved on multiple occasions in P. quadrispinosa. Recent research has questioned the degree to which parallel evolution actually explains variance in traits. Taking into account the predation regime, sexual dimorphism and mineral composition of the trait, a more precise understanding of the factors influencing parallel evolution emerges.

Size-dependent flight capacity and propensity in a range-expanding invasive insect

For capital-breeding insects, all resources available for adult metabolic needs are accumulated during larval feeding. Therefore, body size at adult eclosion represents the total energetic capacity of the individual. For female capital breeders, body size is strongly correlated with lifetime fecundity, while in males, body size, which correlates with fitness, is less understood. In capital-breeding species with wingless, flightless, or dispersal-limited females, flight potential for male Lepidoptera has important implications for mate-finding and may be correlated with body size. At low population densities, failure to mate has been identified as an important Allee effect and can drive the success or failure of invasive species at range edges and in species of conservation concern. Th capital-breeding European subspecies of Lymantria dispar (L.), was introduced to North America in 1869 and now ranges across much of eastern North America. In L. dispar, females are flightless and mate-finding is entirely performed by males. We quantified male L. dispar flight capacity and propensity relative to morphological and physiological characteristics using fixed-arm flight mills. A range of male body sizes was produced by varying the protein content of standard artificial diets while holding other dietary components constant. Wing length, a proxy for body size, relative thorax mass, and forewing aspect were all important predictors of total flight distance and maximum speed. These results have important implications for mate-finding and invasion dynamics in L. dispar and may apply broadly to other capital-breeding insects.

A comprehensive overview of the effects of urbanisation on sexual selection and sexual traits

Urbanisation can affect mating opportunities and thereby alter inter- and intra-sexual selection pressures on sexual traits. Biotic and abiotic urban conditions can influence an individual's success in pre- and post-copulatory mating, for example through impacts on mate attraction and mate preference, fertilisation success, resource competition or rival interactions. Divergent sexual selection pressures can lead to differences in behavioural, physiological, morphological or life-history traits between urban and non-urban populations, ultimately driving adaptation and speciation. Most studies on urban sexual selection and mating interactions report differences between urban and non-urban populations or correlations between sexual traits and factors associated with increased urbanisation, such as pollution, food availability and risk of predation and parasitism. Here we review the literature on sexual selection and sexual traits in relation to urbanisation or urban-associated conditions. We provide an extensive list of abiotic and biotic factors that can influence processes involved in mating interactions, such as signal production and transmission, mate choice and mating opportunities. We discuss all relevant data through the lens of two, non-mutually exclusive theories on sexual selection, namely indicator and sensory models. Where possible, we indicate whether these models provide the same or different predictions regarding urban-adapted sexual signals and describe different experimental designs that can be useful for the different models as well as to investigate the drivers of sexual selection. We argue that we lack a good understanding of: (i) the factors driving urban sexual selection; (ii) whether reported changes in traits result in adaptive benefits; and (iii) whether these changes reflect a short-term ecological, or long-term evolutionary response. We highlight that urbanisation provides a unique opportunity to study the process and outcomes of sexual selection, but that this requires a highly integrative approach combining experimental and observational work.

Natural variation in social conditions affects male mate choosiness in the amphipod Gammarus roeselii

The extent of male mate choosiness is driven by a trade-off between various environmental factors associated with the costs of mate acquisition, quality assessment and opportunity costs. Our knowledge about natural variation in male mate choosiness across different populations of the same species, however, remains limited. In this study, we compared male mate choosiness across 10 natural populations of the freshwater amphipod Gammarus roeselii (Gervais 1835), a species with overall high male mating investments, and evaluated the relative influence of population density and sex ratio (both affecting mate availability) on male mate choosiness. We investigated amplexus establishment after separating mating pairs and presenting focal males with a novel, size-matched female from the same population. Our analysis revealed considerable effects of sex ratio and (to a lesser extent) population density on time until amplexus establishment (choosiness). Male amphipods are able to perceive variable social conditions (e.g., sex ratio) and modify their mating strategy accordingly: We found choosiness to be reduced in increasingly male-biased populations, whereas selectivity increases when sex ratio becomes female biased. With this, our study expands our limited knowledge on natural variations in male mate choosiness and illustrates the importance of sex ratio (i.e., level of competition) for male mating decisions in natural environments. Accounting for variation in sex ratios, therefore, allows envisioning a distinctive variation of choosiness in natural populations and highlights the importance of considering social background information in future behavioral studies.

Much more than a clasp: evolutionary patterns of amplexus diversity in anurans

The evolution and diversification of animal reproductive modes have been pivotal questions in behavioural ecology. Amphibians present the highest diversity of reproductive modes among vertebrates, involving various behavioural, physiological and morphological traits. One such feature is the amplexus, which is the clasp or embrace of males on females during reproduction and is found almost universally in anurans. Hypotheses about the origin of amplexus are limited and have not been tested thoroughly, nor have they taken into account evolutionary relationships in most comparative studies. However, these considerations are crucial to an understanding of the evolution of reproductive modes. Here, using an evolutionary framework, we reconstruct the ancestral state of amplexus in 685 anuran species. We investigate whether the type of amplexus has a strong phylogenetic signal and test whether sexual size dimorphism could have influenced amplexus type or male performance while clasping females. Overall, we found evidence of ≥34 evolutionary transitions in amplexus type across anurans. We found that amplexus type exhibits a high phylogenetic signal and that amplexus type does not evolve in association with sexual size dimorphism. We discuss the implications of our findings for the diversity of amplexus types across anurans.

Genital coupling, morphology and evolution of male holding structures in Cicadinae (Hemiptera: Cicadidae)

Male and female genitalia include some of the most complex and morphologically diverse structures in Metazoa. Ornamentations in genitalia have been studied in several groups, and a variety of functional roles have been proposed. Although complex features of the genitalia have been observed in internal genitalia in cicadas, their functions have not yet been elucidated. These ornamentations, together with precopulatory sexual selection, make cicadas good models for evolutionary studies on genital coupling. We explore the structural interaction of male and female genitalia in Guyalna bonaerensis (Berg) (Cicadinae) and the morphology of male ornamentations in Cicadinae generally. We group these ornamentations into two traits according to their inferred function: anchoring or gripping. We analyse the theca and vesica of 24 species and perform ancestral trait reconstruction under maximum likelihood and stochastic mapping on a Bayesian tree. Ornamentations of the male vesica and the female seminal ampoule possibly ensure male attachment by working as an active lock to avoid the premature termination of intercourse. These ornamentations emerged independently in different lineages in Cicadinae, reinforcing the suggestion that they are important adaptations to achieve complete copulation. Our results foster questions for the field of sexual selection and associated mechanisms shaping the evolution of male and female genitalia.

Small-scale phenotypic differentiation along complex stream gradients in a non-native amphipod

Background

Selective landscapes in rivers are made up by an array of selective forces that vary from source to downstream regions or between seasons, and local/temporal variation in fitness maxima can result in gradual spatio-temporal variation of phenotypic traits. This study aimed at establishing freshwater amphipods as future model organisms to study adaptive phenotypic diversification (evolutionary divergence and/or adaptive plasticity) along stream gradients.

Methods

We collected Gammarus roeselii from 16 sampling sites in the Rhine catchment during two consecutive seasons (summer and winter). Altogether, we dissected n = 1648 individuals and quantified key parameters related to morphological and life-history diversification, including naturally selected (e.g., gill surface areas) as well as primarily sexually selected traits (e.g., male antennae). Acknowledging the complexity of selective regimes in streams and the interrelated nature of selection factors, we assessed several abiotic (e.g., temperature, flow velocity) and biotic ecological parameters (e.g., conspecific densities, sex ratios) and condensed them into four principal components (PCs).

Results

Generalized least squares models revealed pronounced phenotypic differentiation in most of the traits investigated herein, and components of the stream gradient (PCs) explained parts of the observed differences. Depending on the trait under investigation, phenotypic differentiation could be ascribed to variation in abiotic conditions, anthropogenic disturbance (influx of thermally polluted water), or population parameters. For example, female fecundity showed altitudinal variation and decreased with increasing conspecific densities, while sexual dimorphism in the length of male antennae—used for mate finding and assessment—increased with increasing population densities and towards female-biased sex ratios.

Conclusions

We provide a comprehensive protocol for comparative analyses of intraspecific variation in life history traits in amphipods. Whether the observed phenotypic differentiation over small geographical distances reflects evolutionary divergence or plasticity (or both) remains to be investigated in future studies. Independent of the mechanisms involved, variation in several traits is likely to have consequences for ecosystem functions. For example, leaf-shredding in G. roeselii strongly depends on body size, which varied in dependence of several ecological parameters.

Electronic supplementary material

The online version of this article (10.1186/s12983-019-0327-8) contains supplementary material, which is available to authorized users.