Male wing shape differs between condition-dependent alternative reproductive tactics in territorial damselflies

Mixed support for an alignment between phenotypic plasticity and genetic differentiation in damselfly wing shape

The relationship between genetic differentiation and phenotypic plasticity can provide information on whether plasticity generally facilitates or hinders adaptation to environmental change. Here, we studied wing shape variation in a damselfly (Lestes sponsa) across a latitudinal gradient in Europe that differed in time constraints mediated by photoperiod and temperature. We reared damselflies from northern and southern populations in the laboratory using a reciprocal transplant experiment that simulated time-constrained (i.e. northern) and unconstrained (southern) photoperiods and temperatures. After emergence, adult wing shape was analysed using geometric morphometrics. Wings from individuals in the northern and southern populations differed significantly in shape when animals were reared in their respective native environment. Comparing wing shape across environments, we found evidence for phenotypic plasticity in wing shape, and this response differed across populations (i.e. G × E interactions). This interaction was driven by a stronger plastic response by individuals from the northern population and differences in the direction of plastic wing shape changes among populations. The alignment between genetic and plastic responses depended on the specific combination of population and rearing environment. For example, there was an alignment between plasticity and genetic differentiation under time-constrained, but not under non-time-constrained conditions for forewings. We thus find mixed support for the hypothesis that environmental plasticity and genetic population differentiation are aligned. Furthermore, although our laboratory treatments mimicked the natural climatic conditions at northern and southern latitudes, the effects of population differences on wing shape were two to four times stronger than plastic effects. We discuss our results in terms of time constraints and the possibility that natural and sexual selection is acting differently on fore- and hindwings.

Territorial damselflies are larger and show negative allometry in their genitalia

The ‘functional allometry’ hypothesis proposes that the variation in allometric patterns of sexually selected traits is related to their function. We hypothesize that the allometric patterns for genitalia of aggressively territorial organisms are different from those in non-territorial organisms and predict that in aggressively territorial species, where body size is related directly to reproductive success, males must allocate more resources to body size than to genitalia. We studied 59 species of damselflies in 51 genera. Species were divided into three categories: highly territorial and aggressive; low aggressive; and not aggressive. We measured the length of the genital ligula, the width at the basis and its maximum width, and we used body length and wing length as descriptors of body size. The slope of allometric relationships was estimated using ordinary least squares and reduced major axis regressions. Our results indicated first, that territorial damselflies are larger and that body length and wing length are not equivalent as estimators of body size in odonates. Second, ordinary least squares and reduced major axis regressions provided different results in some of the analyses. Third, we found that aggressive species have less steep allometric slopes than non-aggressive species, both for the length of the ligula and for the width at its basis.

Antagonistic natural and sexual selection on wing shape in a scrambling damselfly

Wings are a key trait underlying the evolutionary success of birds, bats, and insects. For over a century, researchers have studied the form and function of wings to understand the determinants of flight performance. However, to understand the evolution of flight, we must comprehend not only how morphology affects performance, but also how morphology and performance affect fitness. Natural and sexual selection can either reinforce or oppose each other, but their role in flight evolution remains poorly understood. Here, we show that wing shape is under antagonistic selection with regard to sexual and natural selection in a scrambling damselfly. In a field setting, natural selection (survival) favored individuals with long and slender forewings and short and broad hindwings. In contrast, sexual selection (mating success) favored individuals with short and broad forewings and narrow-based hindwings. Both types of selection favored individuals of intermediate size. These results suggest that individuals face a trade-off between flight energetics and maneuverability and demonstrate how natural and sexual selection can operate in similar directions for some wing traits, that is, wing size, but antagonistically for others, that is, wing shape. Furthermore, they highlight the need to study flight evolution within the context of species' mating systems and mating behaviors.