Paul M. Brakefield

The dynamics of evolutionary stasis

Abstract The fossil record displays remarkable stasis in many species over long time periods, yet studies of extant populations often reveal rapid phenotypic evolution and genetic differentiation among populations. Recent advances in our understanding of the fossil record and in population genetics and evolutionary ecology point to the complex geographic structure of species being fundamental to resolution of how taxa can commonly exhibit both short-term evolutionary dynamics and long-term stasis.

The generation and diversification of butterfly eyespot color patterns

BACKGROUND A fundamental challenge of evolutionary and developmental biology is understanding how new characters arise and change. The recently derived eyespots on butterfly wings vary extensively in number and pattern between species and play important roles in predator avoidance. Eyespots form through the activity of inductive organizers (foci) at the center of developing eyespot fields. Foci are the proposed source of a morphogen, the levels of which determine the color of surrounding wing scale cells. However, it is unknown how reception of the focal signal translates into rings of different-colored scales, nor how different color schemes arise in different species. RESULTS We have identified several transcription factors, including butterfly homologs of the Drosophila Engrailed/Invected and Spalt proteins, that are deployed in concentric territories corresponding to the future rings of pigmented scales that compose the adult eyespot. We have isolated a new Bicyclus anynana wing pattern mutant, Goldeneye, in which the scales of one inner color ring become the color of a different ring. These changes correlate with shifts in transcription factor expression, suggesting that Goldeneye affects an early regulatory step in eyespot color patterning. In different butterfly species, the same transcription factors are expressed in eyespot fields, but in different relative spatial domains that correlate with divergent eyespot color schemes. CONCLUSIONS Our results suggest that signaling from the focus induces nested rings of regulatory gene expression that subsequently control the final color pattern. Furthermore, the remarkably plastic regulatory interactions downstream of focal signaling have facilitated the evolution of eyespot diversity.

Phenotypic plasticity, seasonal climate and the population biology of Bicyclus butterflies (Satyridae) in Malawi

Abstract. 1 Seasonal polyphenism is studied in a community of five African butterflies of the genus Bicyclus at the transition between a wet and a dry season from May to July. 2 Butterflies characterized by large eyespots and, especially in B.sufitza (Hewitson), a pale band (the wet season form) are replaced over this period by butterflies lacking conspicuous wing markings (the dry season form, dsf). The latter butterflies also tend to be larger, but more variable in size. Butterflies of an intermediate phenotype are recruited over a comparatively short interim period. 3 This turnover coincides with a period of declining temperature and drying of the habitat, including the grasses on which larvae feed. Butterflies are progressively more likely to rest on brown leaf litter rather than on green herbage. 4 A relationship with temperature is supported by laboratory experiments with B.saJitza and B.anynana (Butler) showing that increasingly extreme dsf butterflies develop with decreasing rearing temperature in the final larval instar. 5 Some differences in behaviour and activity were observed between the seasonal forms. Butterflies of the dsf develop ovarian dormancy and fat bodies. They can survive to reproduce at the beginning of the rains in November. 6 Capture‐recapture experiments showed that the adult butterflies have a comparatively long life expectancy and are quite sedentary. 7 The results are discussed in relation to a hypothesis linking the polyphenism to seasonal changes in resting background and selection for crypsis.

THE GENETICS AND EVO-DEVO OF BUTTERFLY WING PATTERNS

Understanding how the spectacular diversity of colour patterns on butterfly wings is shaped by natural selection, and how particular pattern elements are generated, has been the focus of both evolutionary and developmental biologists. The growing field of evolutionary developmental biology has now begun to provide a link between genetic variation and the phenotypes that are produced by developmental processes and that are sorted by natural selection. Butterfly wing patterns are set to become one of the few examples of morphological diversity to be studied successfully at many levels of biological organization, and thus to yield a more complete picture of adaptive morphological evolution.

Developmental constraints versus flexibility in morphological evolution.

Evolutionary developmental biology has encouraged a change of research emphasis from the sorting of phenotypic variation by natural selection to the production of that variation through development. Some morphologies are more readily generated than others, and developmental mechanisms can limit or channel evolutionary change. Such biases determine how readily populations are able to respond to selection, and have been postulated to explain stasis in morphological evolution and unexplored morphologies. There has been much discussion about evolutionary constraints but empirical data testing them directly are sparse. The spectacular diversity in butterfly wing patterns is suggestive of how little constrained morphological evolution can be. However, for wing patterns involving serial repeats of the same element, developmental properties suggest that some directions of evolutionary change might be restricted. Here we show that despite the developmental coupling between different eyespots in the butterfly Bicyclus anynana, there is great potential for independent changes. This flexibility is consistent with the diversity of wing patterns across species and argues for a dominant role of natural selection, rather than internal constraints, in shaping existing variation.

The Male Sex Pheromone of the Butterfly Bicyclus anynana: Towards an Evolutionary Analysis

Background Female sex pheromones attracting mating partners over long distances are a major determinant of reproductive isolation and speciation in Lepidoptera. Males can also produce sex pheromones but their study, particularly in butterflies, has received little attention. A detailed comparison of sex pheromones in male butterflies with those of female moths would reveal patterns of conservation versus novelty in the associated behaviours, biosynthetic pathways, compounds, scent-releasing structures and receiving systems. Here we assess whether the African butterfly Bicyclus anynana, for which genetic, genomic, phylogenetic, ecological and ethological tools are available, represents a relevant model to contribute to such comparative studies. Methodology/Principal Findings Using a multidisciplinary approach, we determined the chemical composition of the male sex pheromone (MSP) in the African butterfly B. anynana, and demonstrated its behavioural activity. First, we identified three compounds forming the presumptive MSP, namely (Z)-9-tetradecenol (Z9-14:OH), hexadecanal (16:Ald ) and 6,10,14-trimethylpentadecan-2-ol (6,10,14-trime-15-2-ol), and produced by the male secondary sexual structures, the androconia. Second, we described the male courtship sequence and found that males with artificially reduced amounts of MSP have a reduced mating success in semi-field conditions. Finally, we could restore the mating success of these males by perfuming them with the synthetic MSP. Conclusions/Significance This study provides one of the first integrative analyses of a MSP in butterflies. The toolkit it has developed will enable the investigation of the type of information about male quality that is conveyed by the MSP in intraspecific communication. Interestingly, the chemical structure of B. anynana MSP is similar to some sex pheromones of female moths making a direct comparison of pheromone biosynthesis between male butterflies and female moths relevant to future research. Such a comparison will in turn contribute to understanding the evolution of sex pheromone production and reception in butterflies.

Plasticity in butterfly egg size : why larger offspring at lower temperatures?

Dividing sister pairs of the butterfly Bicyclus anynana (reared in a common environment) between high and low temperature shows that oviposition temperature induces a plastic response in egg size. Females at a lower temperature laid significantly larger (but fewer) eggs than their sisters kept at a higher temperature, whereas total reproductive investment increased with temperature. Cross-transfer experiments demonstrated that this plastic response in egg size is reversible. Interestingly, this pattern parallels an almost universal temperature-induced developmental response in ectotherm body size. In both cases, however, we do not yet understand the underlying mechanisms or the potential adaptive significance. By cross-transferring the experimentally manipulated eggs between temperatures, we showed that the larger eggs produced at a lower temperature had a higher hatching success, and yielded larger hatchlings with a slightly higher probability of reaching maturity and shorter larval development time (at ...

Captivity masks inbreeding effects on male mating success in butterflies

Small isolated populations are frequently genetically less diverse than core populations, resulting in higher homozygosity that can hamper their long-term survival. The decrease in fitness of organisms owing to matings between relatives is well known from captive and laboratory animals. Such inbreeding can have strongly deleterious effects on life-history traits and survival, and can be critical to the success of population conservation. Because pedigrees are hard to follow in the wild, most field studies have used marker loci to establish that fitness declines with increasing homozygosity. Very few have experimentally explored the effects of inbreeding in the wild, or compared observations in the laboratory with field conditions. Here, using a technique involving the transfer of marker dusts during copulation, we show that a small decrease in mating success of captive inbred male butterflies in cages is greatly accentuated in conditions with unconstrained flight. Our results have important implications for conservation and for studies of sexual selection because they show that the behaviours underlying patterns of mating can be profoundly influenced by a history of inbreeding or by any restraining experimental conditions.

Does predation maintain eyespot plasticity in Bicyclus anynana

The butterfly Bicyclus anynana exhibits phenotypic plasticity involving the wet–season phenotype, which possesses marginal eyespots on the ventral surface of the wings, and the dry–season form, which lacks these eyespots. We examined the adaptive value of phenotypic plasticity of B. anynana in relation to the defence mechanisms of crypsis and deflection. We assessed the visibility differences between spotless and spotted butterflies against backgrounds of brown (dry season) or green (wet season) leaves. Spotless butterflies were highly cryptic and less predated by adult bird predators than were spotted ones when presented against brown leaf litter. However, the advantage of crypsis disappeared in the wet–season habitat as both forms were equally visible. In later experiments, naive birds presented with resting butterflies in the wet–season habitat tended to learn more rapidly to capture spotless butterflies, suggesting a slight selective advantage of possessing eyespots. Moreover, marginal eyespots increased significantly the escape probability of butterflies that were attacked by naive birds compared to those attacked by adult birds, although there were no differences in prey capture success within naive predators. Our results show that natural selection acts against eyespots in the dry season, favouring crypsis, whereas in the wet season it may favour eyespots as deflective patterns.

SEVERE INBREEDING DEPRESSION AND RAPID FITNESS REBOUND IN THE BUTTERFLY BICYCLUS ANYNANA (SATYRIDAE)

We established inbred laboratory lines of the satyrid Bicyclus anynana with one, three and 10 pairs of butterflies, which were subsequently allowed to increase freely to a maximum size of 300 butterflies. Minimally inbred control lines were established with 300 randomly selected virgin butterflies of equal sex ratio. We measured fecundity, egg weight, egg hatching, adult emergence, adult size, and the proportion of crippled adults in generations F2, F3, F5, and F7 (the latter two for the one pair bottleneck lines only). The most striking result was an unexpectedly large decrease in egg hatching with increase in inbreeding (25% per 10% increase in inbreeding). Such a level of inbreeding depression has not been reported previously for any insect. The distribution of egg hatching rate for individual clutches within inbred lines was markedly skewed, with a large fraction of clutches producing no eggs at all. This is interpreted as a relatively lower ratio of detrimental to lethal (or sterile) mutation loads than is found in Drosophila, the only insects for which mutation loads have been well characterized. Possible explanations for this severe inbreeding depression include a relatively high rate of mutation to recessive alleles with substantial damaging effects and infrequent episodes of inbreeding in nature. In the experiments, average egg hatching rate recovered rapidly between F2 and F7 in three of the six one‐pair lines. We discuss the implications of these results for survival of populations through extreme bottlenecks in nature and in captivity.