Foraging-induced craniofacial plasticity is associated with an early, robust and dynamic transcriptional response.

Phenotypic plasticity is the ability of a single genotype to vary its phenotype in response to the environment. Plasticity of the skeletal system in response to mechanical input is widely studied, but the timing of its transcriptional regulation is not well understood. Here, we used the cichlid feeding apparatus to examine the transcriptional dynamics of skeletal plasticity over time. Using three closely related species that vary in their ability to remodel bone and a panel of 11 genes, including well-studied skeletal differentiation markers and newly characterized environmentally sensitive genes, we examined plasticity at one, two, four and eight weeks following the onset of alternate foraging challenges. We found that the plastic species exhibited environment-specific bursts in gene expression beginning at one week, followed by a sharp decline in levels, while the species with more limited plasticity exhibited consistently low levels of gene expression. This trend held across nearly all genes, suggesting that it is a hallmark of the larger plasticity regulatory network. We conclude that plasticity of the cichlid feeding apparatus is not the result of slowly accumulating gene expression difference over time, but rather is stimulated by early bursts of environment-specific gene expression followed by a return to homeostatic levels.

Zebrafish crocc2 mutants exhibit divergent craniofacial shape, misregulated variability, and aberrant cartilage morphogenesis.

BACKGROUND: Phenotypic variation is of paramount importance in development, evolution, and human health; however, the molecular mechanisms that influence organ shape and shape variability are not well understood. During craniofacial development, the behavior of skeletal precursors is regulated by both biochemical and environmental inputs, and the primary cilia play critical roles in transducing both types of signals. Here, we examine a gene that encodes a key constituent of the ciliary rootlets, crocc2, and its role in cartilage morphogenesis in larval zebrafish. RESULTS: Geometric morphometric analysis of crocc2 mutants revealed altered craniofacial shapes and expanded variation. At the cellular level, we observed altered chondrocyte shapes and planar cell polarity across multiple stages in crocc2 mutants. Notably, cellular defects were specific to areas that experience direct mechanical input. Cartilage cell number, apoptosis, and bone patterning were not affected in crocc2 mutants. CONCLUSIONS: Whereas "regulatory" genes are widely implicated in patterning the craniofacial skeleton, genes that encode "structural" aspects of the cell are increasingly implicated in shaping the face. Our results add crocc2 to this list, and demonstrate that it affects craniofacial geometry and canalizes phenotypic variation. We propose that it does so via mechanosensing, possibly through the ciliary rootlet. If true, this would implicate a new organelle in skeletal development and evolution.

Breaking constraints: The development and evolution of extreme fin morphology in the Bramidae.

The developmental process establishes the foundation upon which natural selection may act. In that same sense, it is inundated with numerous constraints that work to limit the directions in which a phenotype may respond to selective pressures. Extreme phenotypes have been used in the past to identify tradeoffs and constraints and may aid in recognizing how alterations to the Baupläne can influence the trajectories of lineages. The Bramidae, a family of Scombriformes consisting of 20 extant species, are unique in that five species greatly deviate from the stout, ovaloid bodies that typify the bramids. The Ptericlinae, or fanfishes, are instead characterized by relatively elongated body plans and extreme modifications to their medial fins. Here, we explore the development of Bramidae morphologies and examine them through a phylogenetic lens to investigate the concepts of developmental and evolutionary constraints. Contrary to our predictions that the fanfishes had been constrained by inherited properties of an ancestral state, we find that the fanfishes exhibit both increased rates of trait evolution and differ substantially from the other bramids in their developmental trajectories. Conversely, the remaining bramid genera differ little, both among one another and in comparison, to the sister family Caristiidae. In all, our data suggest that the fanfishes have broken constraints, thereby allowing them to mitigate trade-offs on distinctive aspects of morphology.

Extreme Morphology, Functional Trade-offs, and Evolutionary Dynamics in a Clade of Open-Ocean Fishes (Perciformes: Bramidae).

When novel or extreme morphologies arise, they are oft met with the burden of functional trade-offs in other aspects of anatomy, which may limit phenotypic diversification and make particular adaptive peaks inaccessible. Bramids (Perciformes: Bramidae) comprise a small family of 20 extant species of fishes, which are distributed throughout pelagic waters worldwide. Within the Bramidae, the fanfishes (Pteraclis and Pterycombus) differ morphologically from the generally stout, laterally compressed species that typify the family. Instead, Pteraclis and Pterycombus exhibit extreme anterior positioning of the dorsal fin onto the craniofacial skeleton. Consequently, they possess fin and skull anatomies that are radically different from other bramid species. Here, we investigate the anatomy, development, and evolution of the Bramidae to test the hypothesis that morphological innovations come at functional (proximate) and evolutionary (ultimate) costs. Addressing proximate effects, we find that the development of an exaggerated dorsal fin is associated with neurocrania modified to accommodate an anterior expansion of the dorsal fin. This occurs via reduced development of the supraoccipital crest (SOC), providing a broad surface area on the skull for insertion of the dorsal fin musculature. While these anatomical shifts are presumably associated with enhanced maneuverability in fanfishes, they are also predicted to result in compromised suction feeding, possibly limiting the mechanisms of feeding in this group. Phylogenetic analyses suggest craniofacial and fin morphologies of fanfishes evolved rapidly and are evolutionarily correlated across bramids. Furthermore, fanfishes exhibit a similar rate of lineage diversification as the rest of the Bramidae, lending little support for the prediction that exaggerated medial fins are associated with phylogenetic constraint. Our phylogeny places fanfishes at the base of the Bramidae and suggests that nonfanfish bramids have reduced medial fins and re-evolved SOCs. These observations suggest that the evolution of novel fin morphologies in basal species has led to the phylogenetic coupling of head and fin shape, possibly predisposing the entire family to a limited range of feeding. Thus, the evolution of extreme morphologies may have carryover effects, even after the morphology is lost, limiting ecological diversification of lineages.

新規または極端な形態が発生すると、解剖学的構造の他の側面で機能的なトレードオフの負担に直面することが多く、表現型の多様化が制限され、特定の適応ピークにアクセスできなくなる可能性があります。ブラミド（スズキ目：シマガツオ科）は、20種の現存する魚の小さな家族で構成されており、世界中の遠洋水域に分布しています。ブラミダエ内では、シマガツオ（PteraclisとPterycombus）は、家族を代表する一般的に頑丈な横方向に圧縮された種と形態学的に異なります。代わりに、PteraclisとPterycombusは、頭蓋顔面骨格への背びれの極端な前方位置を示します。その結果、それらは他のブラミド種とは根本的に異なるひれと頭蓋骨の解剖学的構造を持っています。ここでは、形態学的革新が機能的（近接）および進化的（究極）コストでもたらされるという仮説をテストするために、ブラミダエの解剖学、発達、および進化を調査します。近接効果に対処すると、誇張された背びれの発達は、背びれの前方拡張に対応するように修正された脳頭蓋に関連していることがわかります。これは、後頭上頂（SOC）の発達の低下を介して発生し、背びれの筋肉組織を挿入するための頭蓋骨の広い表面積を提供します。これらの解剖学的変化はおそらくファンフィッシュの操作性の向上に関連していると思われますが、吸引餌の低下をもたらし、このグループの餌のメカニズムを制限する可能性もあると予測されています。系統発生分析は、シマガツオの頭蓋顔面およびヒレの形態が急速に進化し、ブラミド間で進化的に相関していることを示唆しています。さらに、ファンフィッシュは他のブラミダエと同様の系統多様化率を示し、誇張された内側のひれが系統発生の制約に関連しているという予測をほとんど支持していません。私たちの系統発生は、シマガツオをシマガツオ科の根元に配置し、シマガツオ以外のシマガツオが内側のひれを減らし、SOCを再進化させたことを示唆しています。これらの観察結果は、基底種における新しいヒレの形態の進化が、頭とヒレの形状の系統発生的結合をもたらし、おそらく家族全員が限られた範囲の摂食にかかりやすくなっていることを示唆しています。したがって、極端な形態の進化は、形態が失われた後でも持ち越し効果をもたらす可能性があり、系統の生態学的多様化を制限します。.

Cuando surgen morfologías nuevas o extremas, a menudo se encuentran con la carga de compensaciones funcionales en otros aspectos de la anatomía, lo que puede limitar la diversificación fenotípica y hacer inaccesibles los picos adaptativos particulares. Las bramidas (Perciformes: Bramidae) comprenden una pequeña familia de 20 especies de peces existentes, que se distribuyen en las aguas pelágicas de todo el mundo. Dentro de los Bramidae, los fanfishes (Pteraclis y Pterycombus) difieren morfológicamente de las especies generalmente robustas y comprimidas lateralmente que caracterizan a la familia. En cambio, Pteraclis y Pterycombus exhiben una posición anterior extrema de la aleta dorsal sobre el esqueleto craneofacial. En consecuencia, poseen anatomías de aletas y cráneo que son radicalmente diferentes de otras especies de bramidas. Aquí, investigamos la anatomía, el desarrollo y la evolución de Bramidae para probar la hipótesis de que las innovaciones morfológicas tienen un costo funcional (próximo) y evolutivo (último). Al abordar los efectos inmediatos, encontramos que el desarrollo de una aleta dorsal exagerada se asocia con neurocráneo modificado para adaptarse a una expansión anterior de la aleta dorsal. Esto ocurre a través del desarrollo reducido de la cresta supraoccipital (SOC), proporcionando una amplia área de superficie en el cráneo para la inserción de la musculatura de la aleta dorsal. Si bien estos cambios anatómicos presumiblemente están asociados con una mayor maniobrabilidad en los peces fanfishes, también se predice que darán como resultado una alimentación por succión comprometida, lo que posiblemente limite los mecanismos de alimentación en este grupo. Los análisis filogenéticos sugieren que las morfologías craneofaciales y de aletas de los fanfishes evolucionaron rápidamente y están correlacionadas evolutivamente entre las bramidas. Además, los fanfishes exhiben una tasa similar de diversificación de linajes que el resto de los Bramidae, lo que brinda poco apoyo a la predicción de que las aletas mediales exageradas están asociadas con restricciones filogenéticas. Nuestra filogenia coloca a los peces abanico en la base de las Bramidae y sugiere que las bramidas que no son peces abanico tienen aletas mediales reducidas y SOC reevolucionado. Estas observaciones sugieren que la evolución de nuevas morfologías de aletas en especies basales ha llevado al acoplamiento filogenético de la forma de la cabeza y la aleta, lo que posiblemente predisponga a toda la familia a un rango limitado de alimentación. Por lo tanto, la evolución de morfologías extremas puede tener efectos de arrastre, incluso después de que se pierde la morfología, lo que limita la diversificación ecológica de los linajes.

Quando surgem morfologias novas ou extremas, muitas vezes enfrentam o fardo de compensações funcionais em outros aspectos da anatomia, que podem limitar a diversificação fenotípica e tornar determinados picos adaptativos inacessíveis. Bramids (Perciformes: Bramidae) compreendem uma pequena família de 20 espécies existentes de peixes, que estão distribuídos em águas pelágicas em todo o mundo. Dentro dos Bramidae, os fanfishes (Pteraclis e Pterycombus) diferem morfologicamente das espécies geralmente robustas e comprimidas lateralmente que tipificam a família. Em vez disso, Pteraclis e Pterycombus exibem posicionamento anterior extremo da nadadeira dorsal no esqueleto craniofacial. Conseqüentemente, eles possuem anatomias de barbatana e crânio que são radicalmente diferentes de outras espécies de bramida. Aqui, investigamos a anatomia, o desenvolvimento e a evolução dos Bramidae para testar a hipótese de que as inovações morfológicas têm custos funcionais (proximais) e evolutivos (finais). Abordando os efeitos imediatos, descobrimos que o desenvolvimento de uma nadadeira dorsal exagerada está associado a neurocrania modificada para acomodar uma expansão anterior da nadadeira dorsal. Isso ocorre por meio do desenvolvimento reduzido da crista supraoccipital (SOC), proporcionando uma ampla área de superfície no crânio para a inserção da musculatura da nadadeira dorsal. Embora essas mudanças anatômicas estejam presumivelmente associadas a maior capacidade de manobra em peixes-leque, também se prevê que resultem em alimentação de sucção comprometida, possivelmente limitando os mecanismos de alimentação neste grupo. As análises filogenéticas sugerem que as morfologias craniofaciais e das nadadeiras de fanfishes evoluíram rapidamente e estão evolutivamente correlacionadas entre as bramidas. Além disso, fanfishes exibem uma taxa semelhante de diversificação de linhagem como o resto dos Bramidae, emprestando pouco suporte para a previsão de que nadadeiras mediais exageradas estão associadas a restrições filogenéticas. Nossa filogenia coloca fanfishes na base dos Bramidae e sugere que bramids não fanfish possuem nadadeiras mediais reduzidas e SOCs re-evoluídos. Essas observações sugerem que a evolução de novas morfologias de nadadeiras em espécies basais levou ao acoplamento filogenético da forma da cabeça e da nadadeira, possivelmente predispondo toda a família a uma faixa limitada de alimentação. Assim, a evolução de morfologias extremas pode ter efeitos de transporte, mesmo após a perda da morfologia, limitando a diversificação ecológica das linhagens.

Ciliary Rootlet Coiled-Coil 2 (crocc2) Is Associated with Evolutionary Divergence and Plasticity of Cichlid Jaw Shape.

Cichlid fishes exhibit rapid, extensive, and replicative adaptive radiation in feeding morphology. Plasticity of the cichlid jaw has also been well documented, and this combination of iterative evolution and developmental plasticity has led to the proposition that the cichlid feeding apparatus represents a morphological "flexible stem." Under this scenario, the fixation of environmentally sensitive genetic variation drives evolutionary divergence along a phenotypic axis established by the initial plastic response. Thus, if plasticity is predictable then so too should be the evolutionary response. We set out to explore these ideas at the molecular level by identifying genes that underlie both the evolution and plasticity of the cichlid jaw. As a first step, we fine-mapped an environment-specific quantitative trait loci for lower jaw shape in cichlids, and identified a nonsynonymous mutation in the ciliary rootlet coiled-coil 2 (crocc2), which encodes a major structural component of the primary cilium. Given that primary cilia play key roles in skeletal mechanosensing, we reasoned that this gene may confer its effects by regulating the sensitivity of bone to respond to mechanical input. Using both cichlids and zebrafish, we confirmed this prediction through a series of experiments targeting multiple levels of biological organization. Taken together, our results implicate crocc2 as a novel mediator of bone formation, plasticity, and evolution.

Rapid morphological change in multiple cichlid ecotypes following the damming of a major clearwater river in Brazil.

While anthropogenic disturbances can have damaging effects on biodiversity, they also offer an opportunity to understand how species adapt to new environments and may even provide insights into the earliest stages of evolutionary diversification. With these topics in mind, we explored the morphological changes that have occurred across several cichlid species following the damming of the Tocantins River, Brazil. The Tocantins was once a large (2,450 km), contiguous river system; however, upon closure of the Tucuruí Hydroelectric Dam in 1984, a large (~2,850 km2), permanent reservoir was established. We used geometric morphometrics to evaluate changes in native cichlids, comparing historical museum specimens collected from the Tocantins to contemporary specimens collected from the Tucuruí reservoir. Six species across five genera were included to represent distinct ecomorphs, from large piscivores to relatively small opportunistic omnivores. Notably, statistically significant changes in shape and morphological disparity were observed in all species. Moreover, the documented changes tended to be associated with functionally relevant aspects of anatomy, including head, fin, and body shape. Our data offer insights into the ways cichlids have responded, morphologically, to a novel lake environment and provide a robust foundation for exploring the mechanisms through which these changes have occurred.