Pharyngeal Jaw Research Articles

Ecological diversification associated with the pharyngeal jaw diversity of Neotropical cichlid fishes.

Innovations can facilitate bursts of diversification by increasing access to novel resources and the attainment of novel functional designs. Pharyngognathy, exhibited by highly diverse groups such as wrasses and cichlid fishes, is hypothesized to increase foraging capacity and efficiency. Here, I test the hypothesis that pharyngeal jaw shape and tooth morphology are adaptive in an ecologically diverse radiation of Neotropical cichlid fishes that spans North, Central and South America. I partitioned species into generalized trophic guilds using published stomach content analyses and quantified shape variation of the lower pharyngeal jaw (LPJ) using geometric morphometrics. Additionally, I tested for convergence in LPJ shape and trophic guild by mapping the phylogeny onto the principal components and testing for shifts towards similar evolutionary regimes. Major LPJ shape variation included the length and orientation (i.e. narrow or wide) of the lateral processes and length of the medial process, which varied based on the proportion of fishes and plants consumed. Pharyngeal tooth number, diversity and the frequency of tooth types were not evenly distributed among trophic guilds. There were seven distinct evolutionary regimes that converged upon four optima. Pharyngeal jaw diversification is associated with the exploitation of novel resources among Neotropical cichlids such that pharyngeal specialization has increased access to otherwise poorly accessible resources, such as resources that are difficult to crush (e.g. hard-shelled organisms) and assimilate (e.g. algae).

A pharyngeal jaw evolutionary innovation facilitated extinction in Lake Victoria cichlids.

Evolutionary innovations, traits that give species access to previously unoccupied niches, may promote speciation and adaptive radiation. Here, we show that such innovations can also result in competitive inferiority and extinction. We present evidence that the modified pharyngeal jaws of cichlid fishes and several marine fish lineages, a classic example of evolutionary innovation, are not universally beneficial. A large-scale analysis of dietary evolution across marine fish lineages reveals that the innovation compromises access to energy-rich predator niches. We show that this competitive inferiority shaped the adaptive radiation of cichlids in Lake Tanganyika and played a pivotal and previously unrecognized role in the mass extinction of cichlid fishes in Lake Victoria after Nile perch invasion.

Comparative ontogeny of the feeding apparatus of sympatric drums (Perciformes: Sciaenidae) in the Chesapeake Bay.

The anatomy of the feeding apparatus in fishes, including both oral and pharyngeal jaw elements, is closely related to the ecology of a species. During ontogeny, the oral and pharyngeal jaws undergo dramatic changes. To better understand how such ontogenetic changes occur and relate to the feeding ecology of a species, ontogenetic series of four closely related members of the family Sciaenidae (Cynoscion nebulosus, Cynoscion regalis, Micropogonias undulatus, and Leiostomus xanthurus) were examined. Sciaenids were selected because as adults they exhibit considerable specialization of the feeding apparatus correlated with differences in foraging habitats. However, it is not clear when during ontogeny the structural specializations of the feeding apparatus develop, and thereby enable early life history stage (ELHS) sciaenids to partition their foraging habitats. A regression tree was recovered from the analysis and three divergences were identified during ontogeny. There are no measurable differences in elements of the feeding apparatus until the first divergence at 8.4 mm head length (HL), which was attributed to differences in average gill filament length on the second ceratobranchial. The second divergence occurred at 14.1 mm HL and was associated with premaxilla length. The final divergence occurred at 19.8 mm HL and was associated with differences in the toothed area of the fifth certatobranchial. These morphological divergences suggest that ELHS sciaenids may be structurally able to partition their foraging habitats as early as 8.4 mm HL.

El primer registro de Pogonias cromis (Sciaenidae, Perciformes) en el Pleistoceno Tardío de México

Se describe la mandíbula faríngea inferior de <em>Pogonias cromis</em> colectada en sedimentos lacustres de edad del Pleistoceno tardío de la Laguna Madre, Tamaulipas, México. Este espécimen representa el primer registro fósil del pez corbina negra en el territorio mexicano, lo cual amplifica el área de distribución de la especie durante el Pleistoceno e incrementa el conocimiento de la familia Sciaenidae.<br /><br />

Sympatric ecological divergence associated with a color polymorphism.

BackgroundColor polymorphisms are a conspicuous feature of many species and a way to address broad ecological and evolutionary questions. Three potential major evolutionary fates of color polymorphisms are conceivable over time: maintenance, loss, or speciation. However, the understanding of color polymorphisms and their evolutionary implications is frequently impaired by sex-linkage of coloration, unknown inheritance patterns, difficulties in phenotypic characterization, and a lack of evolutionary replicates. Hence, the role of color polymorphisms in promoting ecological and evolutionary diversification remains poorly understood. In this context, we assessed the ecological and evolutionary consequences of a color polymorphic study system that is not hampered by these restrictions: the repeated adaptive radiations of the gold/dark Midas cichlid fishes (the Amphilophus citrinellus species complex) from the great lakes and crater lakes of Nicaragua, Central America.ResultsWe conducted multi-trait morphological and ecological analyses from ten populations of this young adaptive radiation (<6,000 years old), which revealed sympatric ecological differentiation associated with the conspicuous binary (gold/dark) color polymorphism. Varying degrees of intraspecific ecological divergence were observed across the ten color morph pairs, but most pairs exhibited a consistently parallel ecological and evolutionary trajectory across populations. Specifically, gold Midas cichlids are frequently deeper-bodied, have more robust pharyngeal jaws, and feed at a lower trophic level compared to conspecific, sympatric dark individuals. A common garden experiment suggests there is a genetic correlation of color and eco-morphological traits.ConclusionsWe demonstrate unprecedented ecological and evolutionary consequences of color polymorphism in this adaptive radiation. Across the species complex, sympatric conspecific individuals differed in eco-morphology depending on color morph (gold/dark) and the axis of differentiation tended to be consistent across replicates. The consistent divergence across wild populations and the common garden experiment suggests that color is genetically correlated to ecology. Because Midas cichlids are known to mate color assortatively, the putative genetic correlation of this color polymorphism with an eco-morphological divergence suggests an innate potential to promote ecological and evolutionary divergence across this species complex. However, there are to date no examples of speciation based on color in this radiation, suggesting long-term maintenance of this color polymorphism.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-015-0192-7) contains supplementary material, which is available to authorized users.

Functional morphology of durophagy in black carp, Mylopharyngodon piceus.

The black carp, Mylopharyngodon piceus (Osteichthyes: Cyprinidae), crushes its snail and other molluscan prey with robust pharyngeal jaws and strong bite forces. Using gross morphology, histological sectioning, and X-ray reconstruction of moving morphology (XROMM), we investigated structural, behavioral, and mechanical aspects of pharyngeal jaw function in black carp. Strut-like trabeculae in their pharyngeal jaws support large, molariform teeth. The teeth occlude with a hypertrophied basioccipital process that is also reinforced with stout trabeculae. A keratinous chewing pad is firmly connected to the basioccipital process by a series of small bony projections from the base of the pedestal. The pharyngeal jaws have no bony articulations with the skull, and their position is controlled by five paired muscles and one unpaired median muscle. Black carp can crush large molluscs, so we used XROMM to compare pharyngeal jaw postures as fish crushed ceramic tubes of increasing sizes. We found that black carp increase pharyngeal jaw gape primarily by ventral translation of the jaws, with ventral rotation and lateral flaring of the jaws also increasing the space available to accommodate large prey items. A stout, robust ligament connects left and right jaws together firmly, but allows some rotation of the jaws relative to each other. Contrasting with the pharyngeal jaw mechanism of durophagous perciforms with fused left and right lower pharyngeal jaws, we hypothesize that this ligamentous connection may serve to decouple tensile and compressive forces, with the tensile forces borne by the ligament and the compressive forces transferred to the prey.

Sexual dimorphism in a trophically polymorphic cichlid fish?

Sexual dimorphism in ecologically relevant traits is ubiquitous in animals. However, other types of intraspecific phenotypic divergence, such as trophic polymorphism, are less common. Because linkage to sex should often lead to balancing selection, understanding the association between sex and phenotypic divergence could help explain why particular species show high morphological variability. To determine if sexual dimorphism could be helping to maintain ecomorphological variation in a classic case of intraspecific trophic polymorphism, we examined the association between sex and morphological divergence in the cichlid Herichthys minckleyi. Although H. minckleyi with enlarged molariform teeth on their pharyngeal jaws have been reported to more commonly be male, we did not find an association between sex and pharyngeal morphotype. Sex was associated with divergence in body size (as measured through standard length). But, sex was not associated with any of the other trophic traits examined. However, pharyngeal morphotype did show an association with gut length, gape, and tooth number. Sexual dimorphism is not playing a central role in enhancing trophic diversity within H. minckleyi.

The sonic hedgehog signaling pathway and the development of pharyngeal arch Derivatives in Haplochromis piceatus, a Lake Victoria cichlid

ObjectivesPharyngeal arches develop in the head and neck regions, and give rise to teeth, oral jaws, the hyoid bone, operculum, gills, and pharyngeal jaws in teleosts. In this study, the expression patterns of genes in the sonic hedgehog (shh), wnt, ectodysplasin A (eda), and bone morphogenetic protein (bmp) pathways were investigated in the pharyngeal arches of Haplochromis piceatus, one of the Lake Victoria cichlids. Furthermore, the role of the shh pathway in pharyngeal arch development in H. piceatus larvae was investigated. MethodsThe expression patterns of lymphocyte enhancer binding factor 1 (lef1), ectodysplasin A receptor (edar), shh, patched 1 (ptch1), bmp4, sp5 transcription factor (sp5), sclerostin domain containing 1a (sostdc1a), and dickkopf 1 (dkk1) were investigated in H. piceatus larvae by in situ hybridization. The role of the shh pathway was investigated through morphological phenotypic characterization after its inhibition. ResultsWe found that lef1, edar, shh, ptch1, bmp4, dkk1, sostdc1a, and sp5 were expressed not only in the teeth, but also in the operculum and gill filaments of H piceatus larvae. After blocking the shh pathway using cyclopamine, we observed ectopic shh expression and the disappearance of ptch1 expression. After six weeks of cyclopamine treatment, an absence of teeth in the oral upper jaws and a poor outgrowth of premaxilla, operculum, and gill filaments in juvenile H. piceatus were observed. ConclusionsThese results suggest that the shh pathway is important for the development of pharyngeal arch derivatives such as teeth, premaxilla, operculum, and gill filaments in H. piceatus.

Rates of piscivory predict pharyngeal jaw morphology in a piscivorous lineage of cichlid fishes

AbstractThe pharyngeal jaw apparatus is a key innovation hypothesised to increase foraging efficiency and facilitate utilisation of novel resources among teleost fishes. Here, we tested whether dietary characteristics could predict pharyngeal jaw morphology among eight species of Neotropical Crenicichla. Additionally, we tested the hypothesis that pharyngeal jaws may impose a functional constraint on piscivory via pharyngeal gape. We quantified the shape of the lower pharyngeal jaw (LPJ) using linear and geometric morphometrics and quantified diet using the relative volumetric proportions of prey items. We used principal component analysis to describe major axes of variation in LPJ shape and dietary patterns. The major axis of dietary variation significantly predicted LPJ morphology, which was driven by a significant relationship between LPJ shape and rates of piscivory. We also found that rates of piscivory predicted size‐corrected LPJ depth. Size‐corrected pharyngeal gape also significantly predicted rates of piscivory such that pharyngeal jaws may constrain piscivory by limiting pharyngeal gape. Strong form‐to‐function linkage between pharyngeal morphology and trophic patterns suggests an adaptive quality of the pharyngeal apparatus but may also impose functional constraints when consumers must switch prey or when prey availability is temporally or spatially unpredictable.

The innervation of the zebrafish pharyngeal jaws and teeth.

Zebrafish (Danio rerio) teeth are increasingly used as a model to study odontogenesis in non-mammalians. Using serial semi-thin section histology and immunohistochemistry, the nerves innervating the pharyngeal jaws and teeth have been identified. The last pair of branchial arches, which are non-gill bearing but which carry the teeth, are innervated by an internal branch of a post-trematic ramus of the vagal nerve. Another, external, branch is probably responsible for the motor innervation of the branchiomeric musculature. Nerve fibres appear in the pulp cavity of the teeth only late during cytodifferentiation, and are therefore likely not involved in early steps of tooth formation. The precise role of the nervous system during continuous tooth replacement remains to be determined. Nonetheless, this study provides the necessary morphological background information to address this question.

Ecomorphological patterns linking morphology and diet across three populations of pumpkinseed sunfish (Lepomis gibbosus)

Relationships between morphological traits and their ecological function frequently result in patterns that are consistently observed within taxa. Across fishes, the field of ecomorphology has identified a number of morphological traits linked to foraging tactic. Here we examined the links between morphology and diet in pumpkinseed sunfish (Lepomis gibbosus (L., 1758)) from three temperate freshwater lakes. We focused on morphological variation in pharyngeal jaw and gill raker structures, both involved with the processing of prey after capture, in relation to diet. Using stomach contents and stable isotope analysis, we established mean resource use estimates for both populations and individual fish. Among populations and individuals within lakes, we observed that pharyngeal jaw size and gill raker spacing increased with the consumption of littoral prey (e.g., hard-shelled snails) relative to pelagic prey (e.g., zooplankton), but the morphological changes were greater for the pharyngeal jaws. Overall, the relationships that we observed between morphology and foraging tactic were consistent with patterns observed in pumpkinseed and across other fishes. Individual-level diet variation associated with morphology may result in phenotypic diversity within populations that has multiple ecological and evolutionary implications for these populations.

Unusual pharyngeal dentition in the African Chedrin fishes (Teleostei: Cyprinindae): Significance for phylogeny and character evolution

The dentition of cyprinid fishes is restricted to the right and left fifth ceratobranchials, which form the lower pharyngeal jaw (LPJ). The African members of the cyprinid subfamily Danioninae are nested within the tribe Chedrini. We compare the pharyngeal dentitions of African and of Asian Chedrini (Danioninae), including Danio rerio (Danionini), considering the following morphological characters: the pattern of dentition, the number of pharyngeal tooth rows, the curving and inclination of the tooth crowns, and the ratio of the tooth-bearing area to the length of the LPJ. Similar to a previous genetic based study a close relationship is proposed among the African genera Engraulicypris, Mesobola, and Rastrineobola, and among Leptocypris, Neobola, and Chelaethiops, all of which were formerly assigned as ‘Neobolines’. Species of Rastrineobola, Engraulicypris, Mesobola, and Neobola develop an additional (fourth) row of pharyngeal teeth. The dentition of Rastrineobola argentea (Pellegrin, 1904) and the shape of the LPJ of Engraulicypris sardella (Günther, 1868) characterize these two species as specialized within the Chedrini. An additional pharyngeal tooth row also occurs infrequently in the zebrafish D. rerio (Hamilton, 1822). The ‘fourth’ row of this fish species is possibly the result of a different process for acquiring four rows of pharyngeal teeth, likely by the splitting of one tooth row.

Preface: Advances in cichlid research: behavior, ecology, and evolutionary biology

With about 1,300validlydescribedspecies,butatleast2,200 species known, cichlid fishes (Cichlidae) repre-sent the second most species-rich family of freshwaterfishaftertheCyprinidae(2,700describedspecies),andaccount for about 13% of extant freshwater fishdiversity. They are naturally distributed from Centraland South America, across Africa to Madagascar, andsouthern India. Throughout this distributional range,they have repeatedly demonstrated their capacity offorming adaptive radiations—explosive speciationwith niche partitioning—generating an outstandingvariation of body shapes, color patterns, behavior, andecomorphological specializations (e.g., Schliewenet al., 1994; Danley & Kocher, 2001; Joyce et al.,2005; Barluenga et al., 2006; Muschick et al., 2012;Lo´pez-Ferna´ndez et al., 2013). This extraordinarydiversity and an often stunning pace of speciationmakes cichlid fishes particularly well suited forstudying the factors and mechanisms underlyingpopulation differentiation, diversification, and spe-ciation. They are now well established among theprime model systems in evolutionary biology research(Kocher, 2004; Seehausen, 2006). With the develop-ment of an increasing number of genetic and genomicresources, including the recent sequencing of severalcichlid genomes (Brawand et al. 2014), cichlids willreceive even greater attention in future evolutionaryand ecological research.This special issue comprises sixteen studies with aparticular focus on behavior, ecology, and/or evolu-tionary biology of cichlid fishes. Together, theyadvance our knowledge of the mechanisms generatingand maintaining the tremendous diversity of thisfreshwater fish family.Over the last few decades, a multitude of studieshave employed various approaches to shed light on thefactors responsible for the stunning diversity withinthis family. An elaborate reproductive behavior and ahighly developed pharyngeal jaw have long beenrecognized as important key factors for the cichlids’evolutionary success (Liem, 1973; Keenleyside,

The role of gill rakers in the adaptation and habitat partitioning of Nicaraguan crater lake cichlid fishes

Event Abstract Back to Event The role of gill rakers in the adaptation and habitat partitioning of Nicaraguan crater lake cichlid fishes Marta Barluenga1*, Gema Aguilera1 and Melinda J. Hofmann1 1 Museo Nacional de Ciencias Naturales, CSIC, Dept. Biodiversity & Evolutionary Biology, Spain The coexistence of species is facilitated by niche partitioning. Resource competition is a major cause of phenotypic differentiation.The major axes of differentiation in closely related species occur along the two ecological dimensions habitat and diet. Adaptive radiations typically follow these axes of variation. Some of the most important traits in the radiations of freshwater fish are body size, body shape, and a few trophic features such as mouth position, teeth type or gill rakers. In some of the most famous temperate freshwater radiations (i.e., sticklebacks, whitefish) gill rakers play a pivotal role in the adaptation to novel prey types. Cichlids are the most famous textbook examples of fast and divergent adaptive radiations. Often species differ in adaptation to alternative prey types associated to particular trophic morphologies. Pharyngeal jaw shape, oral and pharyngeal teeth and mouth position are the main traits typically studied in cichlid radiations. Gill rakers have however rarely been consider when trying to understand cichlid adaptation to the environment. Here we assess the variation of gill rakers in the cichlid assemblage inhabiting the Nicaraguan large and crater lakes. We found marked differences among species, and found interesting correlations between gill raker traits and environmental features demonstrating their role in adaptation and diversification. Keywords: adaptation, niche partitioning, cichlid fishes, Crater lakes, Gill rakers Conference: XV European Congress of Ichthyology, Porto, Portugal, 7 Sep - 11 Sep, 2015. Presentation Type: Oral Presentation Topic: Ecology, Conservation and Invasive Species Citation: Barluenga M, Aguilera G and Hofmann MJ (2015). The role of gill rakers in the adaptation and habitat partitioning of Nicaraguan crater lake cichlid fishes. Front. Mar. Sci. Conference Abstract: XV European Congress of Ichthyology. doi: 10.3389/conf.FMARS.2015.03.00234 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 17 Dec 2015; Published Online: 19 Dec 2015. * Correspondence: Dr. Marta Barluenga, Museo Nacional de Ciencias Naturales, CSIC, Dept. Biodiversity & Evolutionary Biology, Madrid, 28006, Spain, marta.barluenga@mncn.csic.es Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Marta Barluenga Gema Aguilera Melinda J Hofmann Google Marta Barluenga Gema Aguilera Melinda J Hofmann Google Scholar Marta Barluenga Gema Aguilera Melinda J Hofmann PubMed Marta Barluenga Gema Aguilera Melinda J Hofmann Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Testing the stages model in the adaptive radiation of cichlid fishes in East African Lake Tanganyika.

Adaptive radiation (AR) is a key process in the origin of organismal diversity. However, the evolution of trait disparity in connection with ecological specialization is still poorly understood. Available models for vertebrate ARs predict that diversification occurs in the form of temporal stages driven by different selective forces. Here, we investigate the AR of cichlid fishes in East African Lake Tanganyika and use macroevolutionary model fitting to evaluate whether diversification happened in temporal stages. Six trait complexes, for which we also provide evidence of their adaptiveness, are analysed with comparative methods: body shape, pharyngeal jaw shape, gill raker traits, gut length, brain weight and body coloration. Overall, we do not find strong evidence for the 'stages model' of AR. However, our results suggest that trophic traits diversify earlier than traits implicated in macrohabitat adaptation and that sexual communication traits (i.e. coloration) diversify late in the radiation.

French grunts grind teeth

![][1] Many fish are not as toothless as they may first appear. Far back in their throats they carry a second set of jaws – the pharyngeal jaws – equipped with tooth plates studded with teeth that are tailored to each species' own specific diet. Many species are also believed to

New insights into the role of the pharyngeal jaw apparatus in the sound-producing mechanism of Haemulon flavolineatum (Haemulidae).

Grunts are fish that are well known to vocalize, but how they produce their grunting sounds has not been clearly identified. In addition to characterizing acoustic signals and hearing in the French grunt Haemulon flavolineatum, the present study investigates the sound-production mechanism of this species by means of high-speed X-ray videos and scanning electron microscopy of the pharyngeal jaw apparatus. Vocalizations consist of a series of stridulatory sounds: grunts lasting ~47 ms with a mean period of 155 ms and a dominant frequency of ~700 Hz. Auditory capacity was determined to range from 100 to 600 Hz, with greatest sensitivity at 300 Hz (105.0±11.8 dB re. 1 μPa). This suggests that hearing is not tuned exclusively to detect the sounds of conspecifics. High-speed X-ray videos revealed how pharyngeal jaws move during sound production. Traces of erosion on teeth in the fourth ceratobranchial arch suggest that they are also involved in sound production. The similarity of motor patterns of the upper and lower pharyngeal jaws between food processing and sound production indicates that calling is an exaptation of the food-processing mechanism.

The phenomenology and natural history of idiopathic lower cranial dystonia.

BackgroundMany patients with lower cranial dystonia (LCrD) are misdiagnosed, and recognition of this condition by general practitioners and dental health professionals is limited.MethodsWe define the phenomenology and natural history of idiopathic LCrD, presenting in 41 patients with the disorder, the largest series of these patients reported to date.ResultsPhenomenology of dystonia included lower cranial and pharyngeal involvement, jaw opening and jaw closing dystonia, and tongue dystonia. Of 25 newly described patients, 72% (18) were female, average age at onset was 56 years, and delay before correct diagnosis was 3.8 years (0-25 years, median 2 years). Eleven patients (44%) reported a precipitating event, the most common of which was recent dental work. Geste antagonistes were found in 18 patients (72%). Response to treatment was mixed, indicating an unmet therapeutic need.ConclusionsIdiopathic LCrD is often missed and institution of effective therapy is often delayed. The clinical features and natural history of LCrD are similar to other forms of focal dystonia.Electronic supplementary materialThe online version of this article (doi:10.1186/2054-7072-1-3) contains supplementary material, which is available to authorized users.

Adaptive divergence between lake and stream populations of an East African cichlid fish.

Divergent natural selection acting in different habitats may build up barriers to gene flow and initiate speciation. This speciation continuum can range from weak or no divergence to strong genetic differentiation between populations. Here, we focus on the early phases of adaptive divergence in the East African cichlid fish Astatotilapia burtoni, which occurs in both Lake Tanganyika (LT) and inflowing rivers. We first assessed the population structure and morphological differences in A.burtoni from southern LT. We then focused on four lake-stream systems and quantified body shape, ecologically relevant traits (gill raker and lower pharyngeal jaw) as well as stomach contents. Our study revealed the presence of several divergent lake-stream populations that rest at different stages of the speciation continuum, but show the same morphological and ecological trajectories along the lake-stream gradient. Lake fish have higher bodies, a more superior mouth position, longer gill rakers and more slender pharyngeal jaws, and they show a plant/algae and zooplankton-biased diet, whereas stream fish feed more on snails, insects and plant seeds. A test for reproductive isolation between closely related lake and stream populations did not detect population-assortative mating. Analyses of F1 offspring reared under common garden conditions indicate that the detected differences in body shape and gill raker length do not constitute pure plastic responses to different environmental conditions, but also have a genetic basis. Taken together, the A.burtoni lake-stream system constitutes a new model to study the factors that enhance and constrain progress towards speciation in cichlid fishes.

Towards a gene regulatory network perspective on phenotypic plasticity, genetic accommodation and genetic assimilation.

Many organisms can produce alternative phenotypes in direct response to different environmental conditions, a phenomenon known as phenotypic plasticity. The environmentally sensitive gene regulatory networks (GRNs) that mediate such developmental flexibility are largely unknown. Yet, characterizing these GRNs is important not only for elucidating plasticity's molecular basis, but also for shedding light onto whether and how plasticity might impact evolution. In this issue of Molecular Ecology, Schneider et al.) describe one of the first efforts to determine the GRN underlying a plastic trait. They focus on diet-induced plasticity in the cichlid fish, Astatoreochromis alluaudi. Depending on whether soft food (e.g. insects) or hard food (e.g. molluscs) is consumed, this species forms a lower pharyngeal jaw (LPJ) with many fine teeth or with fewer molar-like teeth, respectively (Fig. 1). The authors previously identified genes that are differentially expressed between LPJ morphs during early development. In the present study, they examine the expression of 19 of these genes across development and diet. By analysing these transcriptional data in combination with information on putative transcription factor binding sites, they construct a GRN that explains observed gene expression patterns and is likely to control LPJ morphology. This work advances our understanding of how plasticity can arise as a consequence of environmentally sensitive GRNs and promises to help illuminate how changes in such GRNs could facilitate evolution.