Fluctuating Asymmetry in the Polymorphic Sand Cricket (Gryllus firmus): Are More Functionally Important Structures Always More Symmetric?

Simple Summary

Asymmetry in bilateral structures occurs when animals experience perturbations during development. This fluctuating asymmetry (FA) may serve as a reliable indicator of the functional importance of a structure. For example, locomotor structures often display lower levels of FA than other paired structures, highlighting that selection can maintain symmetry in traits important for survival or reproduction. Species that have multiple distinct morphs with unique behaviors and morphologies represent an attractive model for studying the relationship between symmetry and function. The sand field cricket (Gryllus firmus) has two separate morphs that allow us to directly test whether individuals maintain higher levels of symmetry in the structures most vital for maximizing fitness based on their specific life strategy. Longwing (LW) individuals can fly but postpone reproduction until after a dispersal event, whereas shortwing (SW) individuals cannot fly but begin reproducing in early adulthood. We quantified FA across a suite of key morphological structures indicative of investment in growth, reproduction, and flight capability for males and females across the morphs. Although we did not find significant differences in FA across traits, as predicted, locomotor compensation strategies may reduce selective pressures on symmetry or developmental patterns may limit the optimization between trait form and function.

Abstract

Fluctuating asymmetry (FA) may serve as a reliable indicator of the functional importance of structures within an organism. Primary locomotor structures often display lower levels of FA than other paired structures, highlighting that selection can maintain symmetry in fitness-enhancing traits. Polyphenic species represent an attractive model for studying the fine-scale relationship between trait form and function, because multiple morphs exhibit unique life history adaptations that rely on different traits to maximize fitness. Here, we investigated whether individuals of the wing polyphenic sand field cricket (Gryllus firmus) maintain higher levels of symmetry in the bilateral structures most vital for maximizing fitness based on their specific life history strategy. We quantified FA and directional asymmetry (DA) across a suite of key morphological structures indicative of investment in somatic growth, reproduction, and flight capability for males and females across the flight-capable longwing (LW) and flight-incapable shortwing (SW) morphs. Although we did not find significant differences in FA across traits, hindwings lacked DA that was found in all other structures. We predicted that functionally important traits should maintain a higher level of symmetry; however, locomotor compensation strategies may reduce the selective pressures on symmetry or developmental constraints may limit the optimization between trait form and function.

Polyphenism of visual and chemical secondary sexually-selected wing traits in the butterfly Bicyclus anynana: How different is the intermediate phenotype?

Polyphenism is a type of phenotypic plasticity supposedly adaptive to drastic and recurrent changes in the environment such as seasonal alternation in temperate and tropical regions. The butterfly Bicyclus anynana shows polyphenism with well-described wet and dry seasonal forms in sub-Saharan Africa, displaying striking morphological, physiological and behavioural differences in response to higher or lower developmental temperatures. During the seasonal transition in the wild, the intermediate phenotype co-occurs with wet and dry phenotypes. In this study, we aimed to characterize the secondary sexually-selected wing traits of the intermediate form to infer its potential fitness compared to wet and dry phenotypes. Among the previously described wing morphological traits, we first showed that the area of the fifth eyespot on the ventral hindwing is the most discriminant trait to identify wet, dry and intermediate phenotypes in both sexes. Second, we characterized the intermediate form for two secondary sexually-selected wing traits: the area and UV reflectance of the dorsal forewing pupil and the composition of the male sex pheromone. We showed that values of these two traits are often between those of the wet and dry phenotypes. Third, we observed increasing male sex pheromone production in ageing dry and wet phenotypes. Our results contrast with previous reports of values for sexually-selected traits in wet and dry seasonal forms, which might be explained by differences in rearing conditions or sample size effects among studies. Wet, dry and intermediate phenotypes display redundant sexually dimorphic traits, including sexually-selected traits that can inform about their developmental temperature in sexual interactions.

Nutritional Regulation of Phenotypic Plasticity in a Solitary Bee (Hymenoptera: Megachilidae)

Phenotypic plasticity involves adaptive responses to predictable environmental fluctuations and may promote evolutionary change. We studied the regulation of phenotypic plasticity in an important agricultural pollinator, the solitary alfalfa leafcutting bee (Megachile rotundata F.). Specifically, we investigated how larval nutrition affects M. rotundata diapause plasticity and how diapause plasticity affects adult female reproductive behavior. Field surveys and laboratory manipulations of aspects of larval diet demonstrated nutritional regulation of M. rotundata diapause plasticity. Manipulation of larval diet quality through the addition of royal jelly, the caste-determining substance of the honey bee Apis mellifera L., increased the probability of diapause in M. rotundata. We also found that larval nutrition and diapause status affected M. rotundata adult female reproductive behavior. Nutritional effects on larval diapause that also impact adult fitness have intriguing implications for the evolution of developmental plasticity in bees. In particular, as the solitary lifestyle of M. rotundata is considered to be the ancestral condition in bees, nutritionally regulated plasticity may have been an ancestral condition in all bees that facilitated the evolution of other forms of phenotypic plasticity, such as the castes of social bees.

Rapid Increase in Genome Size as a Consequence of Transposable Element Hyperactivity in Wood-White (Leptidea) Butterflies

Characterizing and quantifying genome size variation among organisms and understanding if genome size evolves as a consequence of adaptive or stochastic processes have been long-standing goals in evolutionary biology. Here, we investigate genome size variation and association with transposable elements (TEs) across lepidopteran lineages using a novel genome assembly of the common wood-white (Leptidea sinapis) and population re-sequencing data from both L. sinapis and the closely related L. reali and L. juvernica together with 12 previously available lepidopteran genome assemblies. A phylogenetic analysis confirms established relationships among species, but identifies previously unknown intraspecific structure within Leptidea lineages. The genome assembly of L. sinapis is one of the largest of any lepidopteran taxon so far (643 Mb) and genome size is correlated with abundance of TEs, both in Lepidoptera in general and within Leptidea where L. juvernica from Kazakhstan has considerably larger genome size than any other Leptidea population. Specific TE subclasses have been active in different Lepidoptera lineages with a pronounced expansion of predominantly LINEs, DNA elements, and unclassified TEs in the Leptidea lineage after the split from other Pieridae. The rate of genome expansion in Leptidea in general has been in the range of four Mb/Million year (My), with an increase in a particular L. juvernica population to 72 Mb/My. The considerable differences in accumulation rates of specific TE classes in different lineages indicate that TE activity plays a major role in genome size evolution in butterflies and moths.

A conceptual review of mate choice: stochastic demography, within-sex phenotypic plasticity, and individual flexibility

Mate choice hypotheses usually focus on trait variation of chosen individuals. Recently, mate choice studies have increasingly attended to the environmental circumstances affecting variation in choosers' behavior and choosers' traits. We reviewed the literature on phenotypic plasticity in mate choice with the goal of exploring whether phenotypic plasticity can be interpreted as individual flexibility in the context of the switch point theorem, SPT (Gowaty and Hubbell 2009). We found >3000 studies; 198 were empirical studies of within‐sex phenotypic plasticity, and sixteen showed no evidence of mate choice plasticity. Most studies reported changes from choosy to indiscriminate behavior of subjects. Investigators attributed changes to one or more causes including operational sex ratio, adult sex ratio, potential reproductive rate, predation risk, disease risk, chooser's mating experience, chooser's age, chooser's condition, or chooser's resources. The studies together indicate that “choosiness” of potential mates is environmentally and socially labile, that is, induced – not fixed – in “the choosy sex” with results consistent with choosers' intrinsic characteristics or their ecological circumstances mattering more to mate choice than the traits of potential mates. We show that plasticity‐associated variables factor into the simpler SPT variables. We propose that it is time to complete the move from questions about within‐sex plasticity in the choosy sex to between‐ and within‐individual flexibility in reproductive decision‐making of both sexes simultaneously. Currently, unanswered empirical questions are about the force of alternative constraints and opportunities as inducers of individual flexibility in reproductive decision‐making, and the ecological, social, and developmental sources of similarities and differences between individuals. To make progress, we need studies (1) of simultaneous and symmetric attention to individual mate preferences and subsequent behavior in both sexes, (2) controlled for within‐individual variation in choice behavior as demography changes, and which (3) report effects on fitness from movement of individual's switch points.

Host choice in a bivoltine bee: how sensory constraints shape innate foraging behaviors

Background

Many insects have multiple generations per year and cohorts emerging in different seasons may evolve their own phenotypes if they are subjected to different selection regimes. The bivoltine bee Andrena bicolor is reported to be polylectic and oligolectic (on Campanula) in the spring and summer generations, respectively. Neurological constraints are assumed to govern pollen diet in bees. However, evidence comes predominantly from studies with oligolectic bees. We have investigated how sensory constraints influence the innate foraging behavior of A. bicolor and have tested whether bees of different generations evolved behavioral and sensory polyphenism to cope better with the host flowers available in nature when they are active.

Results

Behavioral and sensory polyphenisms were tested in choice assays and electroantennographic analyses, respectively. In the bioassays, we found that females of both generations (1) displayed a similar innate relative reliance on visual and olfactory floral cues irrespective of the host plants tested; (2) did not prefer floral cues of Campanula to those of Taraxacum (or vice versa) and (3) did not display an innate preference for yellow and lilac colors. In the electroantennographic analyses, we found that bees of both generations responded to the same set of compounds.

Conclusion

Overall, we did not detect seasonal polyphenism in any trait examined. The finding that bees of both generations are not sensory constrained to visit a specific host flower, which is in strict contrast to results from studies with oligolectic bees, suggest that also bees of the second generation have a flexibility in innate foraging behavior and that this is an adaptive trait in A. bicolor. We discuss the significance of our findings in context of the natural history of A. bicolor and in the broader context of host-range evolution in bees.

Electronic supplementary material

The online version of this article (doi:10.1186/s12898-016-0074-z) contains supplementary material, which is available to authorized users.

Reproductive isolation and patterns of genetic differentiation in a cryptic butterfly species complex

Molecular studies of natural populations are often designed to detect and categorize hidden layers of cryptic diversity, and an emerging pattern suggests that cryptic species are more common and more widely distributed than previously thought. However, these studies are often decoupled from ecological and behavioural studies of species divergence. Thus, the mechanisms by which the cryptic diversity is distributed and maintained across large spatial scales are often unknown. In 1988, it was discovered that the common Eurasian Wood White butterfly consisted of two species (Leptidea sinapis and Leptidea reali), and the pair became an emerging model for the study of speciation and chromosomal evolution. In 2011, the existence of a third cryptic species (Leptidea juvernica) was proposed. This unexpected discovery raises questions about the mechanisms preventing gene flow and about the potential existence of additional species hidden in the complex. Here, we compare patterns of genetic divergence across western Eurasia in an extensive data set of mitochondrial and nuclear DNA sequences with behavioural data on inter- and intraspecific reproductive isolation in courtship experiments. We show that three species exist in accordance with both the phylogenetic and biological species concepts and that additional hidden diversity is unlikely to occur in Europe. The Leptidea species are now the best studied cryptic complex of butterflies in Europe and a promising model system for understanding the formation of cryptic species and the roles of local processes, colonization patterns and heterospecific interactions for ecological and evolutionary divergence.

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.

The diapause decision as a cascade switch for adaptive developmental plasticity in body mass in a butterfly

Switch-induced developmental plasticity, such as the diapause decision in insects, is a major form of adaptation to variable environments. As individuals that follow alternative developmental pathways will experience different selective environments the diapause decision may evolve to a cascade switch that induces additional adaptive developmental differences downstream of the diapause decision. Here, we show that individuals following alternative developmental pathways in a Swedish population of the butterfly, Pararge aegeria, display differential optimization of adult body mass as a likely response to predictable differences in thermal conditions during reproduction. In a more northern population where this type of selection is absent no similar difference in adult mass among pathways was found. We conclude that the diapause decision in the southern population appears to act as a cascade switch, coordinating development downstream of the diapause decision, to produce adult phenotypes adapted to the typical thermal conditions of their expected reproductive period.