Wing morphological responses to latitude and colonisation in a range expanding butterfly

Populations undergoing rapid climate-driven range expansion experience distinct selection regimes dominated both by increased dispersal at the leading edges and steep environmental gradients. Characterisation of traits associated with such expansions provides insight into the selection pressures and evolutionary constraints that shape demographic and evolutionary responses. Here we investigate patterns in three components of wing morphology (size, shape, colour) often linked to dispersal ability and thermoregulation, along latitudinal gradients of range expansion in the Speckled Wood butterfly (Pararge aegeria) in Britain (two regions of expansion in England and Scotland). We measured 774 males from 54 sites spanning 799 km with a 10-year mean average temperature gradient of 4 °C. A geometric morphometric method was used to investigate variation in size and shape of forewings and hindwings; colour, pattern, and contrast of the wings were examined using a measure of lightness (inverse degree of melanism). Overall, wing size increased with latitude by ∼2% per 100 km, consistent with Bergmann’s rule. Forewings became more rounded and hindwings more elongated with history of colonisation, possibly reflecting selection for increased dispersal ability. Contrary to thermal melanism expectations, wing colour was lighter where larvae developed at cooler temperatures and unrelated to long-term temperature. Changes in wing spot pattern were also detected. High heterogeneity in variance among sites for all of the traits studied may reflect evolutionary time-lags and genetic drift due to colonisation of new habitats. Our study suggests that temperature-sensitive plastic responses for size and colour interact with selection for dispersal traits (wing size and shape). Whilst the plastic and evolutionary responses may in some cases act antagonistically, the rapid expansion of P. aegeria implies an overall reinforcing effect between these two mechanisms.

The temporal distribution of directional gradients under selection for an optimum

Temporal variation in phenotypic selection is often attributed to environmental change causing movements of the adaptive surface relating traits to fitness, but this connection is rarely established empirically. Fluctuating phenotypic selection can be measured by the variance and autocorrelation of directional selection gradients through time. However, the dynamics of these gradients depend not only on environmental changes altering the fitness surface, but also on evolution of the phenotypic distribution. Therefore, it is unclear to what extent variability in selection gradients can inform us about the underlying drivers of their fluctuations. To investigate this question, we derive the temporal distribution of directional gradients under selection for a phenotypic optimum that is either constant or fluctuates randomly in various ways in a finite population. Our analytical results, combined with population- and individual-based simulations, show that although some characteristic patterns can be distinguished, very different types of change in the optimum (including a constant optimum) can generate similar temporal distributions of selection gradients, making it difficult to infer the processes underlying apparent fluctuating selection. Analyzing changes in phenotype distributions together with changes in selection gradients should prove more useful for inferring the mechanisms underlying estimated fluctuating selection.

Accelerated evolution of morph-biased genes in pea aphids

Phenotypic plasticity, the production of alternative phenotypes (or morphs) from the same genotype due to environmental factors, results in some genes being expressed in a morph-biased manner. Theoretically, these morph-biased genes experience relaxed selection, the consequence of which is the buildup of slightly deleterious mutations at these genes. Over time, this is expected to result in increased protein divergence at these genes between species and a signature of relaxed purifying selection within species. Here we test these theoretical expectations using morph-biased genes in the pea aphid, a species that produces multiple morphs via polyphenism. We find that morph-biased genes exhibit faster rates of evolution (in terms of dN/dS) relative to unbiased genes and that divergence generally increases with increasing morph bias. Further, genes with expression biased toward rarer morphs (sexual females and males) show faster rates of evolution than genes expressed in the more common morph (asexual females), demonstrating that the amount of time a gene spends being expressed in a morph is associated with its rate of evolution. And finally, we show that genes expressed in the rarer morphs experience decreased purifying selection relative to unbiased genes, suggesting that it is a relaxation of purifying selection that contributes to their faster rates of evolution. Our results provide an important empirical look at the impact of phenotypic plasticity on gene evolution.

Spalt expression and the development of melanic color patterns in pierid butterflies

Background

Little is currently known about wing pattern development in the butterfly family Pieridae, which consists mostly of black melanized elements on white or yellow/orange backgrounds. A single transcription factor, Spalt (Sal), has been previously associated with the development of some pattern elements in Pieris rapae, but it is unclear to what extent Sal is associated with patterns in other pierid species.

Results

We use immunohistochemistry targeting Sal proteins across several pierids and show that Sal is associated with dense patches of melanization across species but is not associated with vein-melanization or diffuse melanization on the wing. In addition, Sal is expressed along cross-veins and wing compartment midlines that do not develop melanization. Male and female P. rapae spots are sexually dimorphic in size and this dimorphism is also present in the domains of Sal expression. Finally, by disrupting cells positioned in the center of the anterior black spots of P. rapae, before and during the time of Sal expression, spot size was reduced.

Conclusions

Our results suggest, but do not conclusively show, that pierid spots may develop in a manner similar to that of nymphalid eyespots, that is, containing a group of signaling cells at the center of the pattern responsible for the differentiation of the complete spot, and that spots and eyespots share at least one signal-response gene in common, the transcription factor Sal. We propose that focal differentiation and focal signaling mechanisms evolved prior to the split of the nymphalid and pierid lineages.

The redder the better: wing color predicts flight performance in monarch butterflies

The distinctive orange and black wings of monarchs (Danaus plexippus) have long been known to advertise their bitter taste and toxicity to potential predators. Recent work also showed that both the orange and black coloration of this species can vary in response to individual-level and environmental factors. Here we examine the relationship between wing color and flight performance in captive-reared monarchs using a tethered flight mill apparatus to quantify butterfly flight speed, duration and distance. In three different experiments (totaling 121 individuals) we used image analysis to measure body size and four wing traits among newly-emerged butterflies prior to flight trials: wing area, aspect ratio (length/width), melanism, and orange hue. Results showed that monarchs with darker orange (approaching red) wings flew longer distances than those with lighter orange wings in analyses that controlled for sex and other morphometric traits. This finding is consistent with past work showing that among wild monarchs, those sampled during the fall migration are darker in hue (redder) than non-migratory monarchs. Together, these results suggest that pigment deposition onto wing scales during metamorphosis could be linked with traits that influence flight, such as thorax muscle size, energy storage or metabolism. Our results reinforce an association between wing color and flight performance in insects that is suggested by past studies of wing melansim and seasonal polyphenism, and provide an important starting point for work focused on mechanistic links between insect movement and color.

Developmental plasticity in sexual roles of butterfly species drives mutual sexual ornamentation

Current explanations for why sexual ornaments are found in both sexes include genetic correlation, same sex competition, and mutual mate choice. In this study, we report developmental plasticity in mating behavior as induced by temperature during development in the butterfly Bicyclus anynana. Males and females reciprocally change their sexual roles depending on their larval rearing temperatures. This switch is correlated with a change in mating benefits to females and costs to males. The discrete seasonal environments, wet season and dry season, are known to produce the two developmental forms and as a consequence impose alternating, symmetrical patterns of sexual selection, one season on male ornaments, the following season on female ornaments. Thus, reciprocal selection through time may result in mutual sexual ornamentation.

Evolution of sexual dimorphism in the Lepidoptera

Among the animals, the Lepidoptera (moths and butterflies) are second only to beetles in number of described species and are known for their striking intra- and interspecific diversity. Within species, sexual dimorphism is a source of variation in life history (e.g., sexual size dimorphism and protandry), morphology (e.g., wing shape and color pattern), and behavior (e.g., chemical and visual signaling). Sexual selection and mating systems have been considered the primary forces driving the evolution of sexual dimorphism in the Lepidoptera, and alternative hypotheses have been neglected. Here, we examine opportunities for sexual selection, natural selection, and the interplay between the two forces in the evolution of sexual differences in the moths and butterflies. Our primary goal is to identify mechanisms that either facilitate or constrain the evolution of sexual dimorphism, rather than to resolve any perceived controversy between hypotheses that may not be mutually exclusive.

A simulation study of mutations in the genetic regulatory hierarchy for butterfly eyespot focus determination

The color patterns on the wings of butterflies have been an important model system in evolutionary developmental biology. A recent computational model tested genetic regulatory hierarchies hypothesized to underlie the formation of butterfly eyespot foci [Evans, T.M., Marcus, J.M., 2006. A simulation study of the genetic regulatory hierarchy for butterfly eyespot focus determination. Evol. Dev. 8, 273-283]. The computational model demonstrated that one proposed hierarchy was incapable of reproducing the known patterns of gene expression associated with eyespot focus determination in wild-type butterflies, but that two slightly modified alternative hierarchies were capable of reproducing all of the known gene expressions patterns. Here we extend the computational models previously implemented in Delphi 2.0 to two mutants derived from the squinting bush brown butterfly (Bicyclus anynana). These two mutants, comet and Cyclops, have aberrantly shaped eyespot foci that are produced by different mechanisms. The comet mutation appears to produce a modified interaction between the wing margin and the eyespot focus that results in a series of comet-shaped eyespot foci. The Cyclops mutation causes the failure of wing vein formation between two adjacent wing-cells and the fusion of two adjacent eyespot foci to form a single large elongated focus in their place. The computational approach to modeling pattern formation in these mutants allows us to make predictions about patterns of gene expression, which are largely unstudied in butterfly mutants. It also suggests a critical experiment that will allow us to distinguish between two hypothesized genetic regulatory hierarchies that may underlie all butterfly eyespot foci.

Relating environmental variation to selection on reaction norms: an experimental test

Theoretical models predict that selection on reaction norms should depend on the relative frequency of environmental states experienced by a population. We report a laboratory experimental test of this prediction for thermal performance curves of larval growth rate in Pieris rapae in relation to their thermal environment. We measured short-term relative growth rate (RGR) for each individual at a series of five temperatures, and then we assigned individuals randomly to warm or cool selection treatments, which differ in the frequency distributions of environmental temperatures. Selection gradient analyses of two independent experiments demonstrated significant positive selection for increasing RGR, primarily through its effects on survival to adulthood and on development rate. In both the warm and cool selection treatments, the magnitude of directional selection on RGR was consistently greater at lower (suboptimal) temperatures than at higher temperatures; differences in selection between the treatments did not match model predictions. The temporal order and duration of environmental conditions may affect patterns of selection on thermal performance curves and other continuous reaction norms, complicating the connections between variation in environment, phenotype, and fitness.

The evolution of wing color: male mate choice opposes adaptive wing color divergence in Colias butterflies

Correlated evolution of mate signals and mate preference may be constrained if selection pressures acting on mate preference differ from those acting on mate signals. In particular, opposing selection pressures may act on mate preference and signals when traits have sexual as well as nonsexual functions. In the butterfly Colias philodice eriphyle, divergent selection on wing color across an elevational gradient in response to the thermal environment has led to increasing wing melanization at higher elevations. Wing color is also a long-range signal used by males in mate searching. We conducted experiments to test whether sexual selection on wing melanization via male mate choice acts in the same direction as natural selection on mate signals due to the thermal environment. We performed controlled mate choice experiments in the field over an elevational range of 1500 meters using decoy butterflies with different melanization levels. Also, we obtained a more direct estimate of the relation between wing color and sexual selection by measuring mating success in wild-caught females. Both our experiments showed that wing melanization is an important determinant of female mating success in C. p. eriphyle. However, a lack of elevational variation in male mate preference prevents coevolution of mate signals and mate preference, as males at all elevations prefer less-melanized females. We suggest that this apparently maladaptive mate choice may be maintained by differences in detectability between the morphs or by preservation of species recognition.

A tadpole-induced polyphenism in the salamander Hynobius retardatus

Larvae of the salamander Hynobius retardatus have two distinct morphs: normal and broad-headed, cannibal morphs. We performed three experiments to differentiate among the following hypotheses: The broad-headed morph is induced to allow: (1) feeding on nutritious conspecifics; (2) exclusion of strong competitors for food or space; or (3) feeding on large, tough prey when smaller prey items are unavailable. When newly hatched larvae were reared with a heterospecific, Rana pirica (an anuran amphibian) tadpoles, the broad-headed morph was induced more frequently compared with those reared with conspecifics. The phenotype expressed depended on the size of the tadpoles: The broad-headed morph occurred more frequently with small and the normal morph with large tadpoles. Metamorphosis occurred sooner in larvae fed conspecifics compared with those fed heterospecific tadpoles, and the mean growth rate of larvae fed conspecifics was significantly faster than that of those fed tadpoles, suggesting that the heterospecific tadpoles were less nutritive than the conspecifics. These results do not support the hypotheses that the broad-headed morph evolved for consuming conspecifics because of their better balance of nutrients or for excluding strong competitors for food or space. We tentatively conclude that the morph evolved to eat large, tough prey, including both conspecifics and heterospecific tadpoles. Because H. retardatus usually spawns very early in the spring in small ponds partially covered with ice and snow, newly hatched larvae may starve from the lack of proper food owing to extremely low water temperatures. Thus, the broad-headed morph of H. retardatus may represent a cold-habitat adaptation to overcome the severe circumstance when the only food items available are relatively large conspecifics or heterospecific tadpoles.