Evolution & Development最新文献

Nervous system is one of the key adaptations underlying the evolutionary success of the majority of animal groups. Ctenophores (or comb jellies) are gelatinous marine invertebrates that were probably the first lineage to diverge from the rest of animals. Due to the key phylogenetic position and multiple unique adaptations, the noncentralized nervous system of comb jellies has been in the center of the debate around the origin of the nervous system in the animal kingdom and whether it happened only once or twice. Here, we discuss the latest findings in ctenophore neuroscience and multiple challenges on the way to build a clear evolutionary picture of the origin of the nervous system.

Disentangling the evolution of the molecular processes and genetic networks that facilitate the emergence of morphological novelties is one of the main objectives in evolutionary developmental biology. Here, we investigated the evolutionary history of a gene regulatory network controlling the development of novel tooth-like feeding structures in diplogastrid nematodes. Focusing on NHR-1 and NHR-40, the two transcription factors that regulate the morphogenesis of these feeding structures in Pristionchus pacificus, we sought to determine whether they have a similar function in Caenorhabditis elegans, an outgroup species to the Diplogastridae which has typical “rhabditid” flaps instead of teeth. Contrary to our initial expectations, we found that they do not have a similar function. While both receptors are co-expressed in the tissues that produce the feeding structures in the two nematodes, genetic inactivation of either receptor had no impact on feeding-structure morphogenesis in C. elegans. Transcriptomic experiments revealed that NHR-1 and NHR-40 have highly species-specific regulatory targets. These results suggest two possible evolutionary scenarios: either the genetic module responsible for feeding-structure morphogenesis in Diplogastridae already existed in the last common ancestor of C. elegans and P. pacificus, and subsequently disintegrated in the former as NHR-1 and NHR-40 acquired new targets, or it evolved in conjunction with teeth in Diplogastridae. These findings indicate that feeding-structure morphogenesis is regulated by different genetic programs in P. pacificus and C. elegans, hinting at developmental systems drift during the flap-to-tooth transformation. Further research in other “rhabditid” species is needed to fully reconstruct the developmental genetic changes which facilitated the evolution of novel feeding structures in Diplogastridae.

Cnidarians—the phylum including sea anemones, corals, jellyfish, and hydroids—are one of the oldest groups of predatory animals. Nearly all cnidarians are carnivores that use stinging cells called cnidocytes to ensnare and/or envenom their prey. However, there is considerable diversity in cnidocyte form and function. Tracing the evolutionary history of cnidocytes may therefore provide a proxy for early animal feeding strategies. In this study, we generated a time-calibrated molecular clock of cnidarians and performed ancestral state reconstruction on 12 cnidocyte types to test the hypothesis that the original cnidocyte was involved in prey capture. We conclude that the first cnidarians had only the simplest and least specialized cnidocyte type (the isorhiza) which was just as likely to be used for adhesion and/or defense as the capture of prey. A rapid diversification of specialized cnidocytes occurred through the Ediacaran (~654–574 million years ago), with major subgroups developing unique sets of cnidocytes to match their distinct feeding styles. These results are robust to changes in the molecular clock model, and are consistent with growing evidence for an Ediacaran diversification of animals. Our work also provides insight into the evolution of this complex cell type, suggesting that convergence of forms is rare, with the mastigophore being an interesting counterexample.

Recent advances in higher-level invertebrate phylogeny have leveraged shared features of genomic architecture to resolve contentious nodes across the tree of life. Yet, the interordinal relationships within Chelicerata have remained recalcitrant given competing topologies in recent molecular analyses. As such, relationships between topologically unstable orders remain supported primarily by morphological cladistic analyses. Solifugae, one such unstable chelicerate order, has long been thought to be the sister group of Pseudoscorpiones, forming the clade Haplocnemata, on the basis of eight putative morphological synapomorphies. The discovery, however, of a shared whole genome duplication placing Pseudoscorpiones in Arachnopulmonata provides the opportunity for a simple litmus test evaluating the validity of Haplocnemata. Here, we present the first developmental transcriptome of a solifuge (Titanopuga salinarum) and survey copy numbers of the homeobox genes for evidence of systemic duplication. We find that over 70% of the identified homeobox genes in T. salinarum are retained in a single copy, while representatives of the arachnopulmonates retain orthologs of those genes as two or more copies. Our results refute the placement of Solifugae in Haplocnemata. Subsequent reevaluation of putative interordinal morphological synapomorphies among chelicerates reveals a high incidence of homoplasy, reversals, and inaccurate coding within Haplocnemata and other small clades, as well as Arachnida more broadly, suggesting existing morphological character matrices are insufficient to resolve chelicerate phylogeny.

Larvae represent a distinct life history stage in which animal morphology and behavior contrast strongly to adult organisms. This life history stage is a ubiquitous aspect of animal life cycles, particularly in the marine environment. In many species, the structure and function of the nervous system differ significantly between metamorphosed juveniles and larvae. However, the distribution and diversity of neural cell types in larval nervous systems remains incompletely known. Here, the expression of neurotransmitter and neuropeptide synthesis and transport genes in the bat star Patiria miniata is examined throughout larval development. This characterization of nervous system structure reveals three main neural regions with distinct but overlapping territories. These regions include a densely innervated anterior region, an enteric neural plexus, and neurons associated with the ciliary band. In the ciliary band, cholinergic cells are pervasive while dopaminergic, noradrenergic, and GABAergic cells show regional differences in their localization patterns. Furthermore, the distribution of some neural subtypes changes throughout larval development, suggesting that changes in nervous system structure align with shifting ecological priorities during different larval stages, before the development of the adult nervous system. While past work has described aspects of P. miniata larval nervous system structure, largely focusing on early developmental timepoints, this work provides a comprehensive description of neural cell type localization throughout the extensive larval period.

Lissamphibians, represented today by frogs, salamanders, and caecilians, diverged deep in the tetrapod tree of life. Extensive morphological adaptations to disparate lifestyles have made linking extant lissamphibians to one another and to their extinct relatives difficult and controversial. However, the discovery of a feature on the atlas of the frog Xenopus laevis, may add to the small set of osteological traits that unite lissamphibians. In this study, we combine our observations of atlas development in X. laevis with a deep examination of atlantal interglenoid tubercle (TI) occurrence in fossil taxa. The TI is shown herein to occur transiently on the ossifying atlas of roughly one-third of X. laevis tadpoles but is absent in adults of this species. In ancestral character state estimations (ACSE), within the evolutionary context of lissamphibians as dissorophoid temnospondyls, this feature is found to be ancestrally shared among lissamphibians, its presence is uncertain in stem batrachians, and then the TI is lost in extant caecilians and frogs. However, our data suggests apparent TI loss around the origin of frogs may be explained by its ontogenetically transient nature. The only nonamphibian tetrapods with a TI are “microsaurs,” and this similarity is interpreted as one of many convergences that resulted from convergent evolutionary processes that occurred in the evolution of “microsaurs” and lissamphibians. The TI is thus interpreted to be ancestral to lissamphibians as it is found to be present in some form throughout each extant lissamphibian clade's history.

Static allometry is a major component of morphological variation. Much of the literature on the development of allometry investigates how functional perturbations of diverse pathways affect the relationship between trait size and body size. Often, this is done with the explicit objective to identify developmental mechanisms that enable the sensing of organ size and the regulation of relative growth. However, changes in relative trait size can also be brought about by a range of other distinctly different developmental processes, such as changes in patterning or tissue folding, yet standard univariate biometric approaches are usually unable to distinguish among alternative explanations. Here, we utilize geometric morphometrics to investigate the degree to which functional genetic manipulations known to affect the size of dung beetle horns also recapitulate the effect of horn shape allometry. We reasoned that the knockdown phenotypes of pathways governing relative growth should closely resemble shape variation induced by natural allometric variation. In contrast, we predicted that if genes primarily affect alternative developmental processes, knockdown effects should align poorly with shape allometry. We find that the knockdown effects of several genes (e.g., doublesex, Foxo) indeed closely aligned with shape allometry, indicating that their corresponding pathways may indeed function primarily in the regulation of relative trait growth. In contrast, other knockdown effects (e.g., Distal-less, dachs) failed to align with allometry, implicating these pathways in potentially scaling-independent processes. Our findings moderate the interpretation of studies focusing on trait length and highlight the usefulness of multivariate approaches to study allometry and phenotypic plasticity.

In many organisms, sensory abilities develop and evolve according to the changing demands of navigating, foraging, and communication across different environments and life stages. Teleost fish inhabit heterogeneous light environments and exhibit a large diversity in visual system properties among species. Cichlids are a classic example of this diversity; visual system variation is generated by different tuning mechanisms that involve both genetic factors and phenotypic plasticity. Here, we document the developmental progression of visual pigment gene expression in Lake Victoria cichlids and test if these patterns are influenced by variation in light conditions. We reared two sister species of Pundamilia to adulthood in two distinct visual conditions that resemble the light environments that they naturally inhabit in Lake Victoria. We also included interspecific first-generation hybrids. We focused on the four opsins that are expressed in Pundamilia adults (using real-time quantitative polymerase chain reaction (RT-qPCR)) (SWS2B, SWS2A, RH2A, and LWS) at 17 time points. We find that opsin expression profiles progress from shorter-wavelength sensitive opsins to longer-wavelength sensitive opsins with increasing age, in both species and their hybrids. The developmental trajectories of opsin expression also responded plastically to the visual conditions. Developmental and environmental plasticity in opsin expression may provide an important stepping stone in the evolution of cichlid visual system diversity.

Monarch butterflies (Danaus plexippus) are well studied for their annual long-distance migration from as far north as Canada to their overwintering grounds in Central Mexico. At the end of the cold season, monarchs start to repopulate North America through short-distance migration over the course of multiple generations. Interestingly, some populations in various tropical and subtropical islands do not migrate and exhibit heritable differences in wing shape and size, most likely an adaptation to island life. Less is known about forewing differences between long- and short-distance migrants in relation to island populations. Given their different migratory behaviors, we hypothesized that these differences would be reflected in wing morphology. To test this, we analyzed forewing shape and size of three different groups: nonmigratory, lesser migratory (migrate short-distances), and migratory (migrate long-distances) individuals. Significant differences in shape appear in all groups using geometric morphometrics. As variation found between migratory and lesser migrants has been shown to be caused by phenotypic plasticity, and lesser migrants develop intermediate forewing shapes between migratory and nonmigratory individuals, we suggest that genetic assimilation might be an important mechanism to explain the heritable variation found between migratory and nonmigratory populations. Additionally, our research confirms previous studies which show that forewing size is significantly smaller in nonmigratory populations when compared to both migratory phenotypes. Finally, we found sexual dimorphism in forewing shape in all three groups, but for size in nonmigratory populations only. This might have been caused by reduced constraints on forewing size in nonmigratory populations.