Gene regulatory evolution in cold-adapted fly populations neutralizes plasticity and may undermine genetic canalization

The relationships between adaptive evolution, phenotypic plasticity, and canalization remain incompletely understood. Theoretical and empirical studies have made conflicting arguments on whether adaptive evolution may enhance or oppose the plastic response. Gene regulatory traits offer excellent potential to study the relationship between plasticity and adaptation, and they can now be studied at the transcriptomic level. Here we take advantage of three closely-related pairs of natural populations of Drosophila melanogaster from contrasting thermal environments that reflect three separate instances of cold tolerance evolution. We measure the transcriptome-wide plasticity in gene expression levels and alternative splicing (intron usage) between warm and cold laboratory environments. We find that suspected adaptive changes in both gene expression and alternative splicing tend to neutralize the ancestral plastic response. Further, we investigate the hypothesis that adaptive evolution can lead to decanalization of selected gene regulatory traits. We find strong evidence that suspected adaptive gene expression (but not splicing) changes in cold-adapted populations are more vulnerable to the genetic perturbation of inbreeding than putatively neutral changes. We find some evidence that these patterns may reflect a loss of genetic canalization accompanying adaptation, although other processes including hitchhiking recessive deleterious variants may contribute as well. Our findings augment our understanding of genetic and environmental effects on gene regulation in the context of adaptive evolution.

Ant nest architecture is shaped by local adaptation and plastic response to temperature

Social insects are among the most abundant arthropods in terrestrial ecosystems, where they provide ecosystem services. The effect of subterranean activity of ants on soil is well-studied, yet little is known about nest architecture due to the difficulty of observing belowground patterns. Furthermore, many species’ ranges span environmental gradients, and their nest architecture is likely shaped by the climatic and landscape features of their specific habitats. We investigated the effects of two temperature treatments on the shape and size of nests built by Formica podzolica ants collected from high and low elevations in the Colorado Rocky Mountains in a full factorial experiment. Ants nested in experimental chambers with soil surface temperatures matching the local temperatures of sample sites. We observed a plastic response of nest architecture to conditions experienced during excavation; workers experiencing a high temperature excavated deeper nests than those experiencing a cooler temperature. Further, we found evidence of local adaptation to temperature, with a significant interaction effect of natal elevation and temperature treatment on nest size and complexity. Specifically, workers from high elevation sites built larger nests with more tunnels when placed in the cool surface temperature treatment, and workers from low elevation sites exhibited the opposite pattern. Our results suggest that subterranean ant nest architecture is shaped by a combination of plastic and locally adapted building behaviors; we suggest that the flexibility of this ‘extended phenotype’ likely contributes to the widespread success of ants.

Plasticity and flexibility in the anti-predator responses of treefrog tadpoles

Tadpoles can respond to perceived predation risk by adjusting their life history, morphology, and behavior in an adaptive way. Adaptive phenotypic plasticity can evolve by natural selection only if there is variation in reaction norms and if this variation is, at least in part, heritable. To provide insights into the evolution of adaptive phenotypic plasticity, we analyzed the environmental and parental components of variation in predator-induced life history (age and size at metamorphosis), morphology (tail depth), and behavior of Italian treefrog tadpoles (Hyla intermedia). Using an incomplete factorial design, we raised tadpoles either with or without caged predators (dragonfly larvae, gen. Aeshna) and, successively, we tested them in experimental arenas either with or without caged predators. Results provided strong evidence for an environmental effect on all three sets of characters. Tadpoles raised with caged predators (dragonfly larvae, gen. Aeshna) metamorphosed earlier (but at a similar body size) and developed deeper tails than their fullsib siblings raised without predators. In the experimental arenas, all tadpoles, independent of their experience, flexibly changed their activity and position, depending on whether the cage was empty or contained the predator. Tadpoles of the two experimental groups, however, showed different responses: those raised with predators were always less active than their predator-naive siblings and differences slightly increased in the presence of predators. Besides this strong environmental component of phenotypic variation, results provided evidence also for parental and parental-by-environment effects, which were strong on life-history, but weak on morphology and behavior. Interestingly, additive parental effects were explained mainly by dams. This supports the hypothesis that phenotypic plasticity might mainly depend on maternal effects and that it might be the expression of condition-dependent mechanisms. Significance statement Animals, by plastically adjusting their phenotypes to the local environments, can often sensibly improve their chances of survival, suggesting the hypothesis that phenotypic plasticity evolved by natural selection. We test this hypothesis in the Italian treefrog tadpoles, by investigating the heritable variation in the plastic response to predators (dragonfly larvae). Using an incomplete factorial common-garden experiment, we showed that tadpoles raised with predators metamorphosed earlier (but at similar body size), developed deeper tails, and were less active than their siblings raised without predators. The plastic response varied among families, but variation showed a stronger maternal than paternal component. This suggests that plasticity might largely depend on epigenetic factors and be the expression of condition-dependent mechanisms.

Male size mediates plastic response to winner-loser effects for some sexually selected traits

The outcomes of fights often affect the fitness of males by determining their access to mates. ‘Winner-loser’ effects, where winners often win their next contest, but losers tend to lose, can therefore influence how males allocate resources towards traits under pre- and post-copulatory sexual selection. We experimentally manipulated the winning/losing experiences of size-matched male Gambusia holbrooki for either a day, a week or three weeks to test whether prior winning/losing experiences differentially affect the plasticity of male investment into either mating effort (pre-copulatory) or ejaculates (post-copulatory). Winners had better pre-copulatory outcomes than losers for three of the four traits we measured: number of mating attempts, number of successful attempts, and time spent with the female. Winners also produced faster sperm than losers, but there was no difference in total sperm counts. Interestingly, absolute male size, an important predictor of fighting success, mediated the effect of winning or losing on how long males then spent near a female. Smaller winners spent more time with the female than did larger winners, suggesting that how males respond to prior social experiences is size-dependent. We discuss the general importance of controlling for inherent male condition when comparing male investment into condition-dependent traits.