The evolution of flightlessness: Is history important?

Specialized diving traits in the generalist morphology of Fulica (Aves, Rallidae)

Foot-propelled diving comprises the primary locomotion-based feeding strategy for many birds, including families such as Phalacrocoracidae, Anhingidae, Podicipedidae, Gaviidae, and the diving ducks within Anatidae. While the morphology of specialized divers is well known, the corresponding morphology is less known for birds not as specialized but capable of diving, such as the coots (Rallidae, Fulica spp.). To compare the osteology of Fulica with other (non-diving) Rallidae, and with foot-propelled diving birds that are distantly related, we considered osteological characters, as well as the proportion of the hind limb bones and the femoral splay angle to construct a phylomorphospace, and to perform a comparative disparity analysis considering ecomorphologically relevant characters related to swimming and diving. Coots resulted to be significantly disparate from other Rallidae showing many traits of specialized foot-propelled divers, but only noticeable when compared with other rallids, as the degree of development of these traits is markedly less than in loons, grebes, or cormorants. This may correspond to a stabilizing selection of characteristics associated with a generalist morphology in Fulica. Studying adaptation in generalist taxa broadens our understanding of ecomorphologically significant features, thereby enabling us to generalize their evolutionary patterns.

Phylogeny and biogeography of the wingless orthopteran family Rhaphidophoridae

Cave crickets (Rhaphidophoridae) are insects of an ancient and wingless lineage within Orthoptera that are distributed worldwide except in Antarctica, and each subfamily has a high level of endemicity. Here, we show the comprehensive phylogeny of cave crickets using multi-gene datasets from mitochondrial and nuclear loci, including all extant subfamilies for the first time. We reveal phylogenetic relationships between subfamilies, including the sister relationship between Anoplophilinae and Gammarotettiginae, based on which we suggest new synapomorphies. Through biogeographic analyses based on divergence time estimations and ancestral range reconstruction, we propose novel hypotheses regarding the biogeographic history of cave crickets. We suggest that Gammarotettiginae in California originated from the Asian lineage when Asia and the Americas were connected by the Bering land bridge, and the opening of the western interior seaway affected the division of Ceuthophilinae from Tropidischiinae in North America. We estimate that Rhaphidophoridae originated at 138 Mya throughout Pangea. We further hypothesize that the loss of wings in Rhaphidophoridae could be the result of their adaptation to low temperatures in the Mesozoic era.

FoxO and rotund form a binding complex governing wing polyphenism in planthoppers

Wing polyphenism is found in a variety of insects and offers an attractive model system for studying the evolutionary significance of dispersal. The Forkhead box O (FoxO) transcription factor (TF) acts as a wing-morph switch that directs wing buds developing into long-winged (LW) or short-winged morphs in wing-dimorphic planthoppers, yet the regulatory mechanism of the FoxO module remains elusive. Here, we identified the zinc finger TF rotund as a potential wing-morph regulator via transcriptomic analysis and phenotypic screening in the brown plathopper, Nilaparvata lugens. RNA interference-mediated knockdown of rotund antagonized the LW development derived from in the context of FoxO depletion or the activation of the insulin/insulin-like growth factor signaling cascade, reversing long wings into intermediate wings. In vitro binding assays indicated that rotund physically binds to FoxO to form the FoxO combinatorial code. These findings broaden our understanding of the complexity of transcriptional regulation governing wing polyphenism in insects.

Phasmid species that inhabit colder environments are less likely to have the ability to fly

A vast majority of insects can fly, but some cannot. Flight generally increases how far an individual can travel to access mates, enables the exploitation of additional food resources, and aids in predator avoidance. Despite its functional significance, much remains unknown about the factors that influence the evolution of flight. Here, I use phylogenetic comparative methods to investigate whether average annual temperature or wind speed, two components of the flying environment, is correlated with the evolution of flight using data from 107 species of stick and leaf insects (Insecta: Phasmatodea). I find no association between wind speed and flying ability in this clade. However, I find that colder temperatures are associated with the lack of flying ability. This pattern may be explained by the additional metabolic costs required for insects to fly when it is cold. This finding contradicts previous patterns observed in other insect groups and supports the hypothesis that cold temperatures can influence the evolution of flight.

Complex plumages spur rapid color diversification in kingfishers (Aves: Alcedinidae)

Colorful signals in nature provide some of the most stunning examples of rapid phenotypic evolution. Yet, studying color pattern evolution has been historically difficult owing to differences in perceptual ability of humans and analytical challenges with studying how complex color patterns evolve. Island systems provide a natural laboratory for testing hypotheses about the direction and magnitude of phenotypic change. A recent study found that plumage colors of island species are darker and less complex than continental species. Whether such shifts in plumage complexity are associated with increased rates of color evolution remains unknown. Here, we use geometric morphometric techniques to test the hypothesis that plumage complexity and insularity interact to influence color diversity in a species-rich clade of colorful birds-kingfishers (Aves: Alcedinidae). In particular, we test three predictions: (1) plumage complexity enhances interspecific rates of color evolution, (2) plumage complexity is lower on islands, and (3) rates of plumage color evolution are higher on islands. Our results show that more complex plumages result in more diverse colors among species and that island species have higher rates of color evolution. Importantly, we found that island species did not have more complex plumages than their continental relatives. Thus, complexity may be a key innovation that facilitates evolutionary response of individual color patches to distinct selection pressures on islands, rather than being a direct target of selection itself. This study demonstrates how a truly multivariate treatment of color data can reveal evolutionary patterns that might otherwise go unnoticed.

Selective sweeps on different pigmentation genes mediate convergent evolution of island melanism in two incipient bird species

Insular organisms often evolve predictable phenotypes, like flightlessness, extreme body sizes, or increased melanin deposition. The evolutionary forces and molecular targets mediating these patterns remain mostly unknown. Here we study the Chestnut-bellied Monarch (Monarcha castaneiventris) from the Solomon Islands, a complex of closely related subspecies in the early stages of speciation. On the large island of Makira M. c. megarhynchus has a chestnut belly, whereas on the small satellite islands of Ugi, and Santa Ana and Santa Catalina (SA/SC) M. c. ugiensis is entirely iridescent blue-black (i.e., melanic). Melanism has likely evolved twice, as the Ugi and SA/SC populations were established independently. To investigate the genetic basis of melanism on each island we generated whole genome sequence data from all three populations. Non-synonymous mutations at the MC1R pigmentation gene are associated with melanism on SA/SC, while ASIP, an antagonistic ligand of MC1R, is associated with melanism on Ugi. Both genes show evidence of selective sweeps in traditional summary statistics and statistics derived from the ancestral recombination graph (ARG). Using the ARG in combination with machine learning, we inferred selection strength, timing of onset and allele frequency trajectories. MC1R shows evidence of a recent, strong, soft selective sweep. The region including ASIP shows more complex signatures; however, we find evidence for sweeps in mutations near ASIP, which are comparatively older than those on MC1R and have been under relatively strong selection. Overall, our study shows convergent melanism results from selective sweeps at independent molecular targets, evolving in taxa where coloration likely mediates reproductive isolation with the neighboring chestnut-bellied subspecies.

Chestnut-bellied Monarchs (Monarcha castaneiventris ugiensis) from two archipelagos in the Solomon Islands have evolved entirely black plumage from a chestnut ancestor (Monarcha castaneiventris megarhynchus), a phenomenon known as island melanism. We obtain and analyze whole genome sequences using traditional summary statistics and new methods that combine inference of the ancestral recombination graph with machine learning. We find multiple lines of evidence for independent selective sweeps on the MC1R and ASIP genes, a receptor/ligand pair which regulates the production of melanin. Melanism on each archipelago is mediated by mutations in one of these two genes. Mutations in and around MC1R underwent a recent soft sweep experiencing strong selection on the islands of Santa Ana and Santa Catalina, whereas selection was also strong but comparatively older for ASIP on the island of Ugi. We show how melanism originated under positive selection on independent molecular targets, evolving convergently in taxa where coloration mediates reproductive isolation.

The transcription factor Zfh1 acts as a wing-morph switch in planthoppers

Insect wing polyphenism is characterized by its ability to produce two or more distinct wing morphs from a single genotype in response to changing environments. However, the molecular basis of this phenomenon remains poorly understood. Here, we identified a zinc finger homeodomain transcription factor Zfh1 that acts as an upstream regulator for the development of long-winged (LW) or shorted-winged (SW) morphs in planthoppers. Knockdown of Zfh1 directs SW-destined nymphs to develop into LW morphs by down-regulating the transcriptional level of FoxO, a prominent downstream effector of the insulin/IGF signaling (IIS) pathway. The balance between transcriptional regulation via the Zfh1-FoxO cascade and post-translational regulation via the IIS-FoxO cascade provides a flexible regulatory mechanism for the development of alternative wing morphs. These findings help us understand how phenotypic diversity is generated by altering the activity of conserved proteins, and provide an extended framework for the evolution of wing morphological diversity in insects.

Macroevolutionary Analyses Provide New Evidence of Phasmid Wings Evolution as a Reversible Process

The concept that complex ancestral traits can never be recovered after their loss is still widely accepted, despite phylogenetic and molecular approaches suggest instances where phenotypes may have been lost throughout the evolutionary history of a clade and subsequently reverted back in derived lineages. One of the first and most notable examples of such a process is wing evolution in phasmids; this polyneopteran order of insects, which comprises stick and leaf insects, has played a central role in initiating a long-standing debate on the topic. In this study, a novel and comprehensive time tree including over 300 Phasmatodea species is used as a framework for investigating wing evolutionary patterns in the clade. Despite accounting for several possible biases and sources of uncertainty, macroevolutionary analyses consistently revealed multiple reversals to winged states taking place after their loss, and reversibility is coupled with higher species diversification rates. Our findings support a loss of or reduction in wings that occurred in the lineage leading to the extant phasmid most recent common ancestor, and brachyptery is inferred to be an unstable state unless co-opted for nonaerodynamic adaptations. We also explored how different assumptions of wing reversals probability could impact their inference: we found that until reversals are assumed to be over 30 times more unlikely than losses, they are consistently inferred despite uncertainty in tree and model parameters. Our findings demonstrate that wing evolution is a reversible and dynamic process in phasmids and contribute to our understanding of complex trait evolution.

Relationship between flightlessness and brain morphology among Rallidae

Studies have suggested that the brain morphology and flight ability of Aves are interrelated; however, such a relationship has not been thoroughly investigated. This study aimed to examine whether flight ability, volant or flightless, affects brain morphology (size and shape) in the Rallidae, which has independently evolved to adapt secondary flightlessness multiple times within a single taxonomic group. Brain endocasts were extracted from computed tomography images of the crania, measured by 3D geometric morphometrics, and were analyzed using principal component analysis. The results of phylogenetic ANCOVA showed that flightless rails have brain sizes and shapes that are significantly larger than and different from those of volant rails, even after considering the effects of body mass and brain size respectively. Flightless rails tended to have a wider telencephalon and more inferiorly positioned foramen magnum than volant rails. Although the brain is an organ that requires a large amount of metabolic energy, reduced selective pressure for a lower body weight may have allowed flightless rails to have larger brains. The evolution of flightlessness may have changed the position of the foramen magnum downward, which would have allowed the support of the heavier cranium. The larger brain may have facilitated the acquisition of cognitively advanced behavior, such as tool-using behavior, among rails.

A second view on the evolution of flight in stick and leaf insects (Phasmatodea)

BACKGROUND

The re-evolution of complex characters is generally considered impossible, yet, studies of recent years have provided several examples of phenotypic reversals shown to violate Dollo's law. Along these lines, the regain of wings in stick and leaf insects (Phasmatodea) was hypothesised to have occurred several times independently after an ancestral loss, a scenario controversially discussed among evolutionary biologists due to overestimation of the potential for trait reacquisition as well as to the lack of taxonomic data.

RESULTS

We revisited the recovery of wings by reconstructing a phylogeny based on a comprehensive taxon sample of over 500 representative phasmatodean species to infer the evolutionary history of wings. We additionally explored the presence of ocelli, the photoreceptive organs used for flight stabilisation in winged insects, which might provide further information for interpreting flight evolution. Our findings support an ancestral loss of wings and that the ancestors of most major lineages were wingless. While the evolution of ocelli was estimated to be dependent on the presence of (fully-developed) wings, ocelli are nevertheless absent in the majority of all examined winged species and only appear in the members of few subordinate clades, albeit winged and volant taxa are found in every euphasmatodean lineage.

CONCLUSION

In this study, we explored the evolutionary history of wings in Phasmatodea and demonstrate that the disjunct distribution of ocelli substantiates the hypothesis on their regain and thus on trait reacquisition in general. Evidence from the fossil record as well as future studies focussing on the underlying genetic mechanisms are needed to validate our findings and to further assess the evolutionary process of phenotypic reversals.

Reduced olfactory acuity in recently flightless insects suggests rapid regressive evolution

Background

Insects have exceptionally fast smelling capabilities, and some can track the temporal structure of odour plumes at rates above 100 Hz. It has been hypothesized that this fast smelling capability is an adaptation for flying. We test this hypothesis by comparing the olfactory acuity of sympatric flighted versus flightless lineages within a wing-polymorphic stonefly species.

Results

Our analyses of olfactory receptor neuron responses reveal that recently-evolved flightless lineages have reduced olfactory acuity. By comparing flighted versus flightless ecotypes with similar genetic backgrounds, we eliminate other confounding factors that might have affected the evolution of their olfactory reception mechanisms. Our detection of different patterns of reduced olfactory response strength and speed in independently wing-reduced lineages suggests parallel evolution of reduced olfactory acuity.

Conclusions

These reductions in olfactory acuity echo the rapid reduction of wings themselves, and represent an olfactory parallel to the convergent phenotypic shifts seen under selective gradients in other sensory systems (e.g. parallel loss of vision in cave fauna). Our study provides evidence for the hypothesis that flight poses a selective pressure on the speed and strength of olfactory receptor neuron responses and emphasizes the energetic costs of rapid olfaction.

Avian gut microbiomes taking flight

Birds harbor complex gut bacterial communities that may sustain their ecologies and facilitate their biological roles, distribution, and diversity. Research on gut microbiomes in wild birds is surging and it is clear that they are diverse and important - but strongly influenced by a series of environmental factors. To continue expanding our understanding of how the internal ecosystems of birds work in their natural settings, we believe the most pressing needs involve studies on the functional and evolutionary aspects of these symbioses. Here we summarize the state of the field and provide a roadmap for future studies on aspects that are pivotal to understanding the biology of avian gut microbiomes, emphasizing prospects for integrating gut microbiome work in avian conservation and host health monitoring.

Physical constraints lead to parallel evolution of micro- and nanostructures of animal adhesive pads: a review

Adhesive pads are functional systems with specific micro- and nanostructures which evolved as a response to specific environmental conditions and therefore exhibit convergent traits. The functional constraints that shape systems for the attachment to a surface are general requirements. Different strategies to solve similar problems often follow similar physical principles, hence, the morphology of attachment devices is affected by physical constraints. This resulted in two main types of attachment devices in animals: hairy and smooth. They differ in morphology and ultrastructure but achieve mechanical adaptation to substrates with different roughness and maximise the actual contact area with them. Species-specific environmental surface conditions resulted in different solutions for the specific ecological surroundings of different animals. As the conditions are similar in discrete environments unrelated to the group of animals, the micro- and nanostructural adaptations of the attachment systems of different animal groups reveal similar mechanisms. Consequently, similar attachment organs evolved in a convergent manner and different attachment solutions can occur within closely related lineages. In this review, we present a summary of the literature on structural and functional principles of attachment pads with a special focus on insects, describe micro- and nanostructures, surface patterns, origin of different pads and their evolution, discuss the material properties (elasticity, viscoelasticity, adhesion, friction) and basic physical forces contributing to adhesion, show the influence of different factors, such as substrate roughness and pad stiffness, on contact forces, and review the chemical composition of pad fluids, which is an important component of an adhesive function. Attachment systems are omnipresent in animals. We show parallel evolution of attachment structures on micro- and nanoscales at different phylogenetic levels, focus on insects as the largest animal group on earth, and subsequently zoom into the attachment pads of the stick and leaf insects (Phasmatodea) to explore convergent evolution of attachment pads at even smaller scales. Since convergent events might be potentially interesting for engineers as a kind of optimal solution by nature, the biomimetic implications of the discussed results are briefly presented.

High genetic diversity and strong genetic structure of Strongyllodes variegatus populations in oilseed rape production areas of China

Background

Strongyllodes variegatus (Fairmaire) is a major insect pest of oilseed rape in China. Despite its economic importance, the contribution of its population genetics in the development of any suitable protection control strategy for the management of oilseed rape crops is poorly studied. It is a much urgent need to prevent its spread to the rest of the world.

Results

Using the sequences of mitochondrial DNA cytochrome c oxidase subunit I (COI) and cytochrome b (Cytb) as genetic markers, we analyzed the population genetic diversity and structure of 437 individuals collected from 15 S. variegatus populations located in different oilseed rape production areas in China. In addition, we estimated the demographic history using neutrality test and mismatch distribution analysis. The high level of genetic diversity was detected among the COI and Cytb sequences of S. variegatus. The population structure analyses strongly suggested three distinct genetic and geographical regions in China with limited gene flow. The Mantel test showed that the genetic distance was greatly influenced by the geographical distance. The demographic analyses showed that S. variegatus had experienced population fluctuation during the Pleistocene Epoch, which was likely to be related to the climatic changes.

Conclusion

Overall, these results demonstrate that the strong genetic structure of S. variegatus populations in China, which is attributed by the isolation through the geographical distance among populations, their weak flight capacity and subsequent adaptation to the regional ecological conditions.

Wind plays a major but not exclusive role in the prevalence of insect flight loss on remote islands

Terrestrial species on islands often show reduced dispersal abilities. For insects, the generality of explanations for island flight loss remains contentious. Although habitat stability is considered the most plausible explanation, others are frequently highlighted. Adopting a strong inference approach, we examined the hypotheses proposed to account for the prevalence of flightlessness in island insect assemblages, for a region long suspected to be globally unusual in this regard-the Southern Ocean Islands (SOIs). Combining comprehensive faunal inventories, species' morphological information, and environmental variables from 28 SOIs, we provide the first quantitative evidence that flightlessness is exceptionally prevalent among indigenous SOI insect species (47%). Prevalence among species which have evolved elsewhere is much lower: Arctic island species (8%), species introduced to the SOIs (17%), and globally (estimated as approx. 5%). Variation in numbers of flightless species and genera across islands is best explained by variation in wind speed, although habitat stability (thermal seasonality proxy) may play a role. Variables associated with insularity, such as island size, are generally poor predictors of flightlessness. The outcomes redirect attention to Darwin's wind hypothesis. They suggest, however, that wind selects for flightlessness through an energy trade-off between flight and reproduction, instead of by displacement from suitable habitats.

Anthropogenic extinctions conceal widespread evolution of flightlessness in birds

Human-driven extinctions can affect our understanding of evolution, through the nonrandom loss of certain types of species. Here, we explore how knowledge of a major evolutionary transition-the evolution of flightlessness in birds-is biased by anthropogenic extinctions. Adding data on 581 known anthropogenic extinctions to the extant global avifauna increases the number of species by 5%, but quadruples the number of flightless species. The evolution of flightlessness in birds is a widespread phenomenon, occurring in more than half of bird orders and evolving independently at least 150 times. Thus, we estimate that this evolutionary transition occurred at a rate four times higher than it would appear based solely on extant species. Our analysis of preanthropogenic avian diversity shows how anthropogenic effects can conceal the frequency of major evolutionary transitions in life forms and highlights the fact that macroevolutionary studies with only small amounts of missing data can still be highly biased.

Evolutionary History of the Galápagos Rail Revealed by Ancient Mitogenomes and Modern Samples

The biotas of the Galápagos Islands are one of the best studied island systems and have provided a broad model for insular species’ origins and evolution. Nevertheless, some locally endemic taxa, such as the Galápagos Rail Laterallus spilonota, remain poorly characterized. Owing to its elusive behavior, cryptic plumage, and restricted distribution, the Galápagos Rail is one of the least studied endemic vertebrates of the Galapagos Islands. To date, there is no genetic data for this species, leaving its origins, relationships to other taxa, and levels of genetic diversity uncharacterized. This lack of information is critical given the adverse fate of island rail species around the world in the recent past. Here, we examine the genetics of Galápagos Rails using a combination of mitogenome de novo assembly with multilocus nuclear and mitochondrial sequencing from both modern and historical samples. We show that the Galápagos Rail is part of the “American black rail clade”, sister to the Black Rail L. jamaicensis, with a colonization of Galápagos dated to 1.2 million years ago. A separate analysis of one nuclear and two mitochondrial markers in the larger population samples demonstrates a shallow population structure across the islands, possibly due to elevated island connectivity. Additionally, birds from the island Pinta possessed the lowest levels of genetic diversity, possibly reflecting past population bottlenecks associated with overgrazing of their habitat by invasive goats. The modern and historical data presented here highlight the low genetic diversity in this endemic rail species and provide useful information to guide conservation efforts.

Convergent morphological responses to loss of flight in rails (Aves: Rallidae)

The physiological demands of flight exert strong selection pressure on avian morphology and so it is to be expected that the evolutionary loss of flight capacity would involve profound changes in traits. Here, we investigate morphological consequences of flightlessness in a bird family where the condition has evolved repeatedly. The Rallidae include more than 130 recognized species of which over 30 are flightless. Morphological and molecular phylogenetic data were used here to compare species with and without the ability to fly in order to determine major phenotypic effects of the transition from flighted to flightless. We find statistical support for similar morphological response among unrelated flightless lineages, characterized by a shift in energy allocation from the forelimbs to the hindlimbs. Indeed, flightless birds exhibit smaller sterna and wings than flighted taxa in the same family along with wider pelves and more robust femora. Phylogenetic signal tests demonstrate that those differences are independent of phylogeny and instead demonstrate convergent morphological adaptation associated with a walking ecology. We found too that morphological variation was greater among flightless rails than flighted ones, suggesting that relaxation of physiological demands during the transition to flightlessness frees morphological traits to evolve in response to more varied ecological opportunities.

Towards an evolutionary history of European-Alpine Trechus ground beetles: Species groups and wing reduction

The evolution of flight triggered the rise of pterygote insects, but secondary flightlessness has evolved numerous times and is often associated with reduced gene flow among populations and patterns of diversification. With 85 species most of which are wing reduced, the ground beetle genus Trechus in the European Alps may be one such example. Here, we reconstructed a molecular phylogeny using 72 of these species based on mitochondrial and nuclear DNA sequences as a basis for reconstructing their evolutionary history. We rearranged the species into 20 monophyletic species groups, of which five are novel and 15 were already established but with different species allocated. Wing measurements revealed a strong tendency for wing reduction but also variation within and among species, with the few fully-winged species distributed across multiple species groups containing also wing-reduced species. Using character mapping and phylogenetic independent contrasts, we found that neither distribution area, body size, pigmentation, elevational zone, nor hygrophily explained wing status in our sample. Assessing five completely sampled clades, we inferred that each of their ancestors had most likely already been wing reduced. We discuss putative scenarios explaining this pattern and the presence of wing polymorphism across the phylogeny. One plausible scenario would be an already wing-reduced last common ancestor of all Trechus species and multiple regains of full wing length via back mutation and/or hybridisation. Alternatively and possibly more likely, the ancestors were either fully winged, with subsequent rapid and repeated wing reduction explaining the current wing-status pattern, or polymorphic, with long-term polymorphism or reselection acting on standing genetic variation explaining the recent fully-winged species. Either way, Trechus ground beetles are a promising, taxonomically and ecologically diverse system for studying the evolution of flightlessness. Areas for future research include morphological assessment of flight muscles, functional analysis of flight capability, and exploration of the mechanistic and genetic bases of wing and flight evolution.

Flight Muscle and Wing Mechanical Properties are Involved in Flightlessness of the Domestic Silkmoth, Bombyx mori

Flight loss has occurred in many winged insect taxa. The flightless silkmoth Bombyx mori, is domesticated from the wild silkmoth, Bombyx mandarina, which can fly. In this paper, we studied morphological characteristics attributed to flightlessness in silkmoths. Three domestic flightless B. mori strains and one B. mandarina population were used to compare morphological components of the flight apparatus, including wing characteristics (shape, forewing area, loading, and stiffness), flight muscle (weight, ratio, and microscopic detail) and body mass. Compared with B. mandarina, B. mori strains have a larger body, greater wing loading, more flexible wings and a lower flight muscle ratio. The arrangement in microscopy of dorsal longitudinal flight muscles (DLFMs) of B. mori was irregular. Comparative analysis of the sexes suggests that degeneration of flight muscles and reduction of wing mechanical properties (stiffness) are associated with silkmoth flightlessness. The findings provide important clues for further research of the molecular mechanisms of B. mori flight loss.

Rapid loss of flight in the Aldabra white-throated rail

Flight loss has evolved independently in numerous island bird lineages worldwide, and particularly in rails (Rallidae). The Aldabra white-throated rail (Dryolimnas [cuvieri] aldabranus) is the last surviving flightless bird in the western Indian Ocean, and the only living flightless subspecies within Dryolimnas cuvieri, which is otherwise volant across its extant range. Such a difference in flight capacity among populations of a single species is unusual, and could be due to rapid evolution of flight loss, or greater evolutionary divergence than can readily be detected by traditional taxonomic approaches. Here we used genetic and morphological analyses to investigate evolutionary trajectories of living and extinct Dryolimnas cuvieri subspecies. Our data places D. [c.] aldabranus among the most rapid documented avian flight loss cases (within an estimated maximum of 80,000-130,000 years). However, the unusual intraspecific variability in flight capacity within D. cuvieri is best explained by levels of genetic divergence, which exceed those documented between other volant taxa versus flightless close relatives, all of which have full species status. Our results also support consideration of Dryolimnas [cuvieri] aldabranus as sufficiently evolutionary distinct from D. c. cuvieri to warrant management as an evolutionary significant unit. Trait variability among closely related lineages should be considered when assessing conservation status, particularly for traits known to influence vulnerability to extinction (e.g. flightlessness).

Fixation of genetic variation and optimization of gene expression: The speed of evolution in isolated lizard populations undergoing Reverse Island Syndrome

The ecological theory of island biogeography suggests that mainland populations should be more genetically divergent from those on large and distant islands rather than from those on small and close islets. Some island populations do not evolve in a linear way, but the process of divergence occurs more rapidly because they undergo a series of phenotypic changes, jointly known as the Island Syndrome. A special case is Reversed Island Syndrome (RIS), in which populations show drastic phenotypic changes both in body shape, skin colouration, age of sexual maturity, aggressiveness, and food intake rates. The populations showing the RIS were observed on islets nearby mainland and recently raised, and for this they are useful models to study the occurrence of rapid evolutionary change. We investigated the timing and mode of evolution of lizard populations adapted through selection on small islets. For our analyses, we used an ad hoc model system of three populations: wild-type lizards from the mainland and insular lizards from a big island (Capri, Italy), both Podarcis siculus siculus not affected by the syndrome, and a lizard population from islet (Scopolo) undergoing the RIS (called P. s. coerulea because of their melanism). The split time of the big (Capri) and small (Scopolo) islands was determined using geological events, like sea-level rises. To infer molecular evolution, we compared five complete mitochondrial genomes for each population to reconstruct the phylogeography and estimate the divergence time between island and mainland lizards. We found a lower mitochondrial mutation rate in Scopolo lizards despite the phenotypic changes achieved in approximately 8,000 years. Furthermore, transcriptome analyses showed significant differential gene expression between islet and mainland lizard populations, suggesting the key role of plasticity in these unpredictable environments.

Comparative transcriptomic analysis of a wing-dimorphic stonefly reveals candidate wing loss genes

Background

The genetic basis of wing development has been well characterised for model insect species, but remains poorly understood in phylogenetically divergent, non-model taxa. Wing-polymorphic insect species potentially provide ideal systems for unravelling the genetic basis of secondary wing reduction. Stoneflies (Plecoptera) represent an anciently derived insect assemblage for which the genetic basis of wing polymorphism remains unclear. We undertake quantitative RNA-seq of sympatric full-winged versus vestigial-winged nymphs of a widespread wing-dimorphic New Zealand stonefly, Zelandoperla fenestrata, to identify genes potentially involved in wing development and secondary wing loss.

Results

Our analysis reveals substantial differential expression of wing-development genes between full-winged versus vestigial-winged stonefly ecotypes. Specifically, of 23 clusters showing significant similarity to Drosophila wing development-related genes and their pea aphid orthologues, nine were significantly upregulated in full-winged stonefly ecotypes, whereas only one cluster (teashirt) was substantially upregulated in the vestigial-winged ecotype.

Conclusions

These findings suggest remarkable conservation of key wing-development pathways throughout 400 Ma of insect evolution. The finding that two Juvenile Hormone pathway clusters were significantly upregulated in vestigial-winged Zelandoperla supports the hypothesis that Juvenile Hormone may play a key role in modulating insect wing polymorphism, as has previously been suggested for other insect lineages.

Gradual evolution towards flightlessness in steamer ducks

Flightlessness in birds is the product of changes in suites of characters-including increased body size and reduced anterior limbs-that have evolved repeatedly and independently under similar ecological conditions (generally insularity). It remains unknown whether this phenotypic convergence extends to the genomic level, partially because many losses of flight occurred long ago (such as in penguins or ratites), thus complicating the study of the genetic pathways to flightlessness. Here, we use genome sequencing to study the evolution of flightlessness in a group of ducks that are current and dynamic exemplars of this major functional transition. These recently diverged Tachyeres steamer ducks differ in their ability to fly: one species is predominantly flighted and three are mainly flightless. Through a genome-wide association analysis, we identify two narrow candidate genomic regions implicated in the morphological changes that led to flightlessness, and reconstruct the number of times flightlessness has evolved in Tachyeres. The strongest association is with DYRK1A, a gene that when knocked out in mice leads to alterations in growth and bone morphogenesis. These findings, together with phylogenetic and demographic analyses, imply that the genomic changes leading to flightlessness in Tachyeres may have evolved once, and that this trait remains functionally polymorphic in two species.

Convergent genomic signatures of flight loss in birds suggest a switch of main fuel

Flight loss in birds is as characteristic of the class Aves as flight itself. Although morphological and physiological differences are recognized in flight-degenerate bird species, their contributions to recurrent flight degeneration events across modern birds and underlying genetic mechanisms remain unclear. Here, in an analysis of 295 million nucleotides from 48 bird genomes, we identify two convergent sites causing amino acid changes in ATGL^Ser321Gly and ACOT7^Ala197Val in flight-degenerate birds, which to our knowledge have not previously been implicated in loss of flight. Functional assays suggest that Ser321Gly reduces lipid hydrolytic ability of ATGL, and Ala197Val enhances acyl-CoA hydrolytic activity of ACOT7. Modeling simulations suggest a switch of main energy sources from lipids to carbohydrates in flight-degenerate birds. Our results thus suggest that physiological convergence plays an important role in flight degeneration, and anatomical convergence often invoked may not.

Flight loss has occurred numerous times in bird evolution. Here, the authors examine convergent sites in the exonic and intronic sequences of 48 bird genomes, finding amino-acid changes in two genes, ATGL and ACOT7, with potential implications for a change in metabolism rather than anatomy.

Effect of isolation on coat colour polymorphism of Polynesian rats in Island Southeast Asia and the Pacific

Populations of vertebrate species introduced onto islands regularly develop similar phenotypic changes, e.g., larger or smaller body size, shortened limbs, duller coats, as well as behavioural changes such as increased tameness and reduced flight-initiation distance. These changes overlap in part with those associated with the 'domestication syndrome', especially tameness and changes in coat patterns, and might indicate a similar neural crest involvement in the concurrent development of multiple phenotypic traits. Here I examine long-term data on free-living populations of wild Polynesian rats from seven mainland countries and 117 islands (n = 3,034), covering the species' native and introduced range. Mainland populations showed no aberrant coat patterns, with the exception of one albino, whereas aberrant coat patterns were found in 12 island populations. Observed coat colour polymorphisms consisted of leucistic (including singular white patches), melanistic (darkly pigmented) and piebald (mixed) coat patterns. After isolation for at least seven centuries, wild Polynesian rat populations on islands seem to exhibit a trend towards a higher incidence of aberrant coat patterns. These phenotypic changes are here explained as a neutral, non-adaptive process, likely part of the 'domestication syndrome' (via the commensal pathway of domestication), in combination with genetic drift, little or no gene flow between the islands and/or the mainland and a relaxed selection (as a result of the weakening or removal of competitor/predator pressure) under commensality.

Isolation on a remote island: genetic and morphological differentiation of a cosmopolitan odonate

Although low levels of genetic structure are expected in highly widespread species, geographical and/or ecological factors can limit species distributions and promote population structure and morphological differentiation. In order to determine the effects of geographical isolation on population genetic structure and wing morphology, 281 individuals of the cosmopolitan odonate Pantala flavescens were collected from four continental (Central and South America) and five insular sites (Polynesian islands and the Maldives). COI sequences and eight microsatellite loci were used to characterize genetic diversity and genetic structure between and within locations. Linear and geometric morphometry were used to evaluate differences in the size and shape of wings. Genetic analysis showed a global genetic difference between the continental and insular sites. American locations did not show genetic structure, even in locations separated by a distance of 5000 km. Easter Island showed the lowest values of genetic diversity (mainly mitochondrial diversity) and the highest values of genetic differences compared to other insular and continental sites. Individuals from Easter Island showed smaller forewings, a different abdomen length to thorax length ratio, and a different configuration of anal loop in the hindwings. Thus, the greater isolation, smaller area, and young geological age seem to have determined the genetic and morphological differences in P. flavescens of Easter Island, where selection could promote a loss of migratory behavior and may improve other life history traits, such as reproduction. This work provides new insight into how microevolutionary processes operate in isolated populations of cosmopolitan species.

Exploring mechanisms and origins of reduced dispersal in island Komodo dragons

Loss of dispersal typifies island biotas, but the selective processes driving this phenomenon remain contentious. This is because selection via, both indirect (e.g. relaxed selection or island syndromes) and direct (e.g. natural selection or spatial sorting) processes may be involved, and no study has yet convincingly distinguished between these alternatives. Here, we combined observational and experimental analyses of an island lizard, the Komodo dragon (Varanus komodoensis, the world's largest lizard), to provide evidence for the actions of multiple processes that could contribute to island dispersal loss. In the Komodo dragon, concordant results from telemetry, simulations, experimental translocations, mark-recapture, and gene flow studies indicated that despite impressive physical and sensory capabilities for long-distance movement, Komodo dragons exhibited near complete dispersal restriction: individuals rarely moved beyond the valleys they were born/captured in. Importantly, lizard site-fidelity was insensitive to common agents of dispersal evolution (i.e. indices of risk for inbreeding, kin and intraspecific competition, and low habitat quality) that consequently reduced survival of resident individuals. We suggest that direct selection restricts movement capacity (e.g. via benefits of spatial philopatry and increased costs of dispersal) alongside use of dispersal-compensating traits (e.g. intraspecific niche partitioning) to constrain dispersal in island species.

Genotyping-by-sequencing supports a genetic basis for wing reduction in an alpine New Zealand stonefly

Wing polymorphism is a prominent feature of numerous insect groups, but the genomic basis for this diversity remains poorly understood. Wing reduction is a commonly observed trait in many species of stoneflies, particularly in cold or alpine environments. The widespread New Zealand stonefly Zelandoperla fenestrata species group (Z. fenestrata, Z. tillyardi, Z. pennulata) contains populations ranging from fully winged (macropterous) to vestigial-winged (micropterous), with the latter phenotype typically associated with high altitudes. The presence of flightless forms on numerous mountain ranges, separated by lowland fully winged populations, suggests wing reduction has occurred multiple times. We use Genotyping by Sequencing (GBS) to test for genetic differentiation between fully winged (n = 62) and vestigial-winged (n = 34) individuals, sampled from a sympatric population of distinct wing morphotypes, to test for a genetic basis for wing morphology. While we found no population genetic differentiation between these two morphotypes across 6,843 SNP loci, we did detect several outlier loci that strongly differentiated morphotypes across independent tests. These findings indicate that small regions of the genome are likely to be highly differentiated between morphotypes, suggesting a genetic basis for wing reduction. Our results provide a clear basis for ongoing genomic analysis to elucidate critical regulatory pathways for wing development in Pterygota.

Clade-specific evolutionary diversification along ontogenetic major axes in avian limb skeleton

The evolutionary diversification of birds has been facilitated by specializations for various locomotor modes, with which the proportion of the limb skeleton is closely associated. However, recent studies have identified phylogenetic signals in this system, suggesting the presence of historical factors that have affected its evolutionary variability. In this study, to explore potential roles of ontogenetic integration in biasing the evolution in the avian limb skeleton, evolutionary diversification patterns in six avian families (Anatidae, Procellariidae, Ardeidae, Phalacrocoracidae, Laridae, and Alcidae) were examined and compared to the postnatal ontogenetic trajectories in those taxa, based on measurement of 2641 specimens and recently collected ontogenetic series, supplemented by published data. Morphometric analyses of lengths of six limb bones (humerus, ulna, carpometacarpus, femur, tibiotarsus, and tarsometatarsus) demonstrated that: (1) ontogenetic trajectories are diverse among families; (2) evolutionary diversification is significantly anisotropic; and, most importantly, (3) major axes of evolutionary diversification are correlated with clade-specific ontogenetic major axes in the shape space. These results imply that the evolutionary variability of the avian limbs has been biased along the clade-specific ontogenetic trajectories. It may explain peculiar diversification patterns characteristic to some avian groups, including the long-leggedness in Ardeidae and tendency for flightlessness in Anatidae.

High sequence variation in the exon 10 of TSHR gene is associated with flightless-domestic geese

The genetic and molecular mechanisms of the flightless birds without limb modification are rarely reported. To explore the possible reasons for losing flight ability without limb modification, we used the domestic geese as an ideal model to preliminarily study the possible mechanisms for this kind of flightlessness. We compared the sequence variations of the exon 10 of TSHR gene between three domesticated geese populations and two wild ancestor populations. The results showed that domestic geese had higher genetic diversity and more complex population structure than their wild ancestors. We did not detect any population expansion in domestic geese population. However, we detected clear relaxed selection signal and positive selection in domesticated geese groups. Furthermore, special phylogenetic relationship of the exon 10 of TSHR was observed in domesticated geese groups. Combined with its well-established function on metabolic regulation and photoperiod control, we speculate that relaxed selection of TSHR might have effects on flightlessness of domesticated geese.

A genetic signature of the evolution of loss of flight in the Galapagos cormorant

We have a limited understanding of the genetic and molecular basis of evolutionary changes in the size and proportion of limbs. We studied wing and pectoral skeleton reduction leading to flightlessness in the Galapagos cormorant (Phalacrocorax harrisi). We sequenced and de novo assembled the genomes of four cormorant species and applied a predictive and comparative genomics approach to find candidate variants that may have contributed to the evolution of flightlessness. These analyses and cross-species experiments in Caenorhabditis elegans and in chondrogenic cell lines implicated variants in genes necessary for transcriptional regulation and function of the primary cilium. Cilia are essential for Hedgehog signaling, and humans affected by skeletal ciliopathies suffer from premature bone growth arrest, mirroring skeletal features associated with loss of flight.

The thoracic morphology of the wingless dune cricket Comicus calcaris (Orthoptera: Schizodactylidae): Novel apomorphic characters for the group and adaptations to sand desert environments

Schizodactylidae, splay-footed or dune crickets, represents a distinct lineage among the highly diverse orthopteran subgroup Ensifera (crickets, katydids and allies). Only two extant genera belong to the Schizodactylidae: the winged Eurasian genus Schizodactylus, whose ecology and morphology is well documented, and the wingless South African Comicus, for which hardly any studies providing morphological descriptions have been conducted since its taxonomic description in 1888. Based on the first in-depth study of the skeletomuscular system of the thorax of Comicus calcaris Irish 1986, we provide information on some unique characteristics of this character complex in Schizodactylidae. They include a rigid connection of prospinasternite and mesosternum, a T-shaped mesospina, and a fused meso- and metasternum. Although Schizodactylidae is mainly characterized by group-specific anatomical traits of the thorax, its bifurcated profuca supports a closer relationship to the tettigonioid ensiferans, like katydids, wetas, and hump-winged crickets. Some specific features of the thoracic musculature of Comicus seem to be correlated to the skeletal morphology, e.g., due to the rigid connection of the tergites and pleurites in the pterothorax not a single direct flight muscle is developed. We show that many of the thoracic adaptations in these insects are directly related to their psammophilous way of life. These include a characteristic setation of thoracic sclerites that prevent sand grains from intrusion into vulnerable membranous areas, the striking decrease in size of the thoracic spiracles that reduces the respirational water loss, and a general trend towards a fusion of sclerites in the thorax.

THE GENETIC BASIS OF THE TRADE-OFF BETWEEN CALLING AND WING MORPH IN MALES OF THE CRICKET GRYLLUS FIRMUS

Wing dimorphisms exist in a wide range of insects. In wing-dimorphic species one morph is winged has functional flight muscles (LW), and is flight-capable, whereas the other has reduced wings (SW) and cannot fly The evolution and maintenance of wing dimorphisms is believed to be due to trade-offs between flight capability and fitness-related traits. Although there are well-established phenotypic trade-offs associated with wing dimorphism in female insects, there only exist two studies that have established a genetic basis to these trade-offs. The present study provides the first evidence for a genetically based trade-off in male insects, specifically in the sand cricket Gryllus firmus. Because they have to expend energy to maintain the flight apparatus (especially flight muscles), LW males are predicted to call less and therefore to attract fewer females. To be of evolutionary significance, call duration wing morph, and wing muscle condition (size and functionality) should all have measurable heritabilities and all be genetically correlated. Differences between morphs in male G. firmus in the likelihood of attracting a female were tested in the laboratory using a T-maze where females chose between a LW male and a SW male. Call duration for each male was recorded on the sixth day of adult life. A significant difference in call duration was found between SW and LW males (SW = 0.86 ± 0.01, LW = 0.64 ± 0.01 h). SW males attracted significantly more females than did LW males (63% vs. to 37%). All the traits involved in the trade-off had significant heritabilities (call = 0 75 ± 0 33; wing morph = 0.22 ± 007; muscle weight = 0.38 ± 0.09) and genetic correlations (call and wing morph = -0.46 ± 0.20 for SW, -0.68 ± 0.16 for LW; LW call and muscle weight = -0.80 ± 0.14). These results provide the first documented evidence that trade-offs between a dimorphic trait and a fitness-related character in males has a genetic basis and hence can be of evolutionary significance.

Evolution of central pattern generators and rhythmic behaviours

Comparisons of rhythmic movements and the central pattern generators (CPGs) that control them uncover principles about the evolution of behaviour and neural circuits. Over the course of evolutionary history, gradual evolution of behaviours and their neural circuitry within any lineage of animals has been a predominant occurrence. Small changes in gene regulation can lead to divergence of circuit organization and corresponding changes in behaviour. However, some behavioural divergence has resulted from large-scale rewiring of the neural network. Divergence of CPG circuits has also occurred without a corresponding change in behaviour. When analogous rhythmic behaviours have evolved independently, it has generally been with different neural mechanisms. Repeated evolution of particular rhythmic behaviours has occurred within some lineages due to parallel evolution or latent CPGs. Particular motor pattern generating mechanisms have also evolved independently in separate lineages. The evolution of CPGs and rhythmic behaviours shows that although most behaviours and neural circuits are highly conserved, the nature of the behaviour does not dictate the neural mechanism and that the presence of homologous neural components does not determine the behaviour. This suggests that although behaviour is generated by neural circuits, natural selection can act separately on these two levels of biological organization.

Genome-wide SNPs reveal fine-scale differentiation among wingless alpine stonefly populations and introgression between winged and wingless forms

Insect flight loss is a repeated phenomenon in alpine habitats, where wing reduction is thought to enhance local recruitment and increase fecundity. One predicted consequence of flight loss is reduced dispersal ability, which should lead to population genetic differentiation and perhaps ultimately to speciation. Using a dataset of 15,123 SNP loci, we present comparative analyses of fine-scale population structure in codistributed Zelandoperla stonefly species, across three parallel altitudinal transects in New Zealand's Rock and Pillar mountain range. We find that winged populations (altitude 200-500 m; Zelandoperla decorata) show no genetic structuring within or among streams, suggesting substantial dispersal mediated by flight. By contrast, wingless populations (Zelandoperla fenestrata; altitude 200-1100 m) exhibit distinct genetic clusters associated with each stream, and additional evidence of isolation by distance within streams. Our data support the hypothesis that wing-loss can initiate diversification in alpine insect populations over small spatial scales. The often deep phylogenetic placement of lowland Z. fenestrata within their stream-specific clades suggests the possibility of independent alpine colonization events for each stream. Additionally, the detection of winged, interspecific hybrid individuals raises the intriguing possibility that a previously flightless lineage could reacquire flight via introgression.

Butterfly survival on an isolated island by improved grip

On small isolated islands, natural selection is expected to reduce the dispersal capacity of organisms, as short distances do not require a high rate of dispersal, which might lead to accidental emigration from the population. In addition, individuals foregoing the high cost of maintaining flight capacity may instead allocate resources to other functions. However, in butterflies and many other insects, flight is necessary not only for dispersal but also for most other activities. A weakly flying individual would probably do worse and have an elevated rather than reduced probability of accidental emigration. Here, we report results consistent with the hypothesis that a butterfly population on an isolated island, instead of having lost its flight capacity, has evolved better grip to resist the force of wind and to avoid being blown off the island. Our study suggests that local adaptation has occurred in this population in spite of its very small size (Ne ∼ 100), complete isolation, low genetic variation and high genetic load.

Sexual dimorphism, mating systems, and nuptial gifts in two Asian fireflies (Coleoptera: Lampyridae)

Many insect taxa show conspicuous sexual dimorphism in which females lack functional wings and are therefore incapable of flight. In fireflies, this loss of female flight is correlated with male production of spermatophore nuptial gifts, with species with flightless females also lacking gift production. In this study we further examined the relationship between sexual dimorphism and nuptial gifts by examining the reproductive ecology of two Asian fireflies, the sexually monomorphic Aquatica ficta (Luciolinae) and dimorphic Pyrocoelia pectoralis (Lampyrinae). We describe the reproductive anatomy of P. pectoralis males and females, and the time course of A. ficta spermatophore transfer. Through a series of mating experiments, we determined the effect of the number of matings on female fecundity, egg hatching success and female lifespan. The spermatophore-producing A. ficta was found to be monandrous, and female lifespan doubled as a result of that single mating. P. pectoralis was found to lack spermatophores but females were polyandrous and fecundity and egg hatching success both increased with additional matings. These results contradict patterns known from North American firefly species and provide insight into the role of male-derived substances in mating systems.

Insular avian adaptations on two Neotropical continental islands

AIM: Most studies of avian insular adaptations have focused on oceanic islands, which may not allow characters that are insular adaptations to be teased apart from those that benefit dispersal and colonization. Using birds on continental islands, we investigated characters that evolved in situ in response to insular environments created by late Pleistocene sea level rise. LOCATION: Trinidad and Tobago, nearby Caribbean islands and continental South America. METHODS: We weighed fresh flight muscles and measured museum skeletal specimens of seven species of birds common to the continental islands of Trinidad and Tobago. RESULTS: When corrected for body size, study species exhibited significantly smaller flight muscles, sterna and sternal keels on Tobago than on larger Trinidad and continental South America. Tobago populations were more 'insular' in their morphologies than conspecifics on Trinidad or the continent in other ways as well, including having longer bills, longer wings, longer tails and longer legs. MAIN CONCLUSIONS: We hypothesize that the longer bills enhance foraging diversity, the longer wings and tails compensate for the smaller pectoral assemblage (allowing for retention of volancy, but with a probable reduction in flight power and speed), and the longer legs expand perching ability. Each of these differences is likely to be related to the lower diversity and fewer potential predators and competitors on Tobago compared with Trinidad. These patterns of smaller flight muscles and larger bills, legs, wings and tails in island birds are not the results of selection for island dispersal and colonization, but probably arose from selection pressures acting on populations already inhabiting these islands.

Macroscale evolutionary patterns of flight muscle dimorphism in the carrion beetle Necrophila japonica

Some insect species exhibit polymorphisms in flight muscles or wings, which provide opportunities for studying the factors that drive dispersal polymorphisms and the evolution of flightlessness in insects. We investigated the macroscale evolutionary pattern of flightlessness in the widespread Japanese beetle Necrophila japonica (Coleoptera: Silphidae), which exhibits flight muscle dimorphisms using phylogeographic approaches. N. japonica lives in both stable and unstable habitats, and the flight muscle dimorphisms may have been maintained through the use of these diverse habitats. We studied the distribution pattern of the proportion of individuals lacking flight muscles in relation to the genetic differentiation among geographic populations using an 842-base pair sequence of the COI-II gene. Both flight-capable and flightless individuals occurred over the distribution area, and the flight muscle condition showed no significant phylogeographic pattern. Several populations comprised flight-capable individuals only, whereas few comprised flightless ones only. Demographic expansion was suggested for major clades of COI-II haplotypes, and the genetic differentiation showed an isolation-by-distance pattern among the populations in Japan. The proportion of flightless individuals was higher in a population with a higher annual mean temperature and with higher genetic diversity among individuals. These results indicate that geographic expansion occurred recently while flight muscle dimorphisms have been maintained, that flight-capable individuals have colonized cooler (peripheral) habitats, and that flightlessness has increased in long-persisting populations as suggested by high genetic diversity.

Multiple losses of flight and recent speciation in steamer ducks

Steamer ducks (Tachyeres) comprise four species, three of which are flightless. The flightless species are believed to have diverged from a flying common ancestor during the Late Pleistocene; however, their taxonomy remains contentious. Of particular interest is the previously unstudied population of flying steamer ducks in the Falkland Islands. We present the first genetic data from this insular population, and illustrate that the flying and flightless steamer ducks on the Falkland Islands are genetically indistinguishable, in contrast to their traditional classification as separate species. The three species that reside in continental South America form a genetically distinct lineage from the Falkland Island ducks. The Falkland steamer ducks diverged from their continental relatives 2.2-0.6 million years ago, coincident with a probable land bridge connecting the Falkland Islands to the mainland. The three continental species share a common ancestor approximately 15 000 years ago, possibly owing to isolation during a recent glacial advance. The continental steamer duck species are not reciprocally monophyletic, but show some amount of genetic differentiation between them. Each lineage of Tachyeres represents a different stage between flight and flightlessness. Their phylogenetic relationships suggest multiple losses of flight and/or long-term persistence of mixed-flight capability. As such, steamer ducks may provide a model system to study the evolution of flightlessness.

Loss of flight promotes beetle diversification

The evolution of flight is a key innovation that may enable the extreme diversification of insects. Nonetheless, many species-rich, winged insect groups contain flightless lineages. The loss of flight may promote allopatric differentiation due to limited dispersal power and may result in a high speciation rate in the flightless lineage. Here we show that loss of flight accelerates allopatric speciation using carrion beetles (Coleoptera: Silphidae). We demonstrate that flightless species retain higher genetic differentiation among populations and comprise a higher number of genetically distinct lineages than flight-capable species, and that the speciation rate with the flightless state is twice that with the flight-capable state. Moreover, a meta-analysis of 51 beetle species from 15 families reveals higher genetic differentiation among populations in flightless compared with flight-capable species. In beetles, which represent almost one-fourth of all described species, repeated evolution of flightlessness may have contributed to their steady diversification since the Mesozoic era.

The loss of flight in some insect lineages may promote allopatric differentiation and result in a high speciation rate. Here, using the carrion beetle, loss of flight is shown to accelerate allopatric speciation with higher genetic differentiation than for flight-capable species.