Ctenophores are direct developers that reproduce continuously beginning very early after hatching

Reproductive success of inbred strain MV31 of the ctenophore Mnemiopsis leidyi in a self-sustaining inland laboratory culture system

Ctenophores are an attractive phylogenetic lineage for studying animal evolution due to their early divergence from other metazoans. Among Ctenophora, Mnemiopsis leidyi is a model system for developmental, cellular, molecular genetic, and evolutionary studies. Until recently, many of these studies were conducted on wild-caught animals, limiting access to researchers on the coast. Here we present significant advancements towards culturing M. leidyi in laboratories without coastal access, enabling its wider use as an experimental and genetic model system. We detail updated feeding regimens that take advantage of co-culturing Brachionus rotifers with Apocyclops copepods, and quantify the reproductive output of our M. leidyi lab strain on this diet. Our updated feeding regimen maintains reproductive fitness comparable to wild-caught individuals. Importantly, we have eliminated the logistical complexities and costs of regularly feeding live larval fish to M. leidyi. Our updated protocols make it feasible to maintain continuous ctenophore cultures independent of access to both coastal populations of wild M. leidyi and larval fish culturing facilities.

Evolution of Maternal Provisioning and Development in the Ophiuroidea: Egg Size, Larval Form, and Parental Care

The Ophiuroidea is the most speciose class of echinoderms and has the greatest diversity of larval forms, but we know less about the evolution of development (evo-devo) in this group than for the other echinoderm classes. As is typical of echinoderms, evo-devo in the Ophiuroidea resulted in the switch from production of small eggs and feeding (planktotrophic) larvae to large eggs and non-feeding (lecithotrophic) larvae. Parental care (ovoviviparity or viviparity/matrotrophy) is the most derived life history. Analysis of egg data for 140 species (excluding viviparity and facultative planktotrophy) indicated a bimodal distribution in egg volume corresponding to planktotrophy and lecithotrophy + ovoviviparity, with three significant egg size groups due to the very large eggs of the ovoviviparous species. The marked reduction in fecundity in species with extremely large eggs is exemplified by the ovoviviparous species. Egg size in the two species with facultative planktotrophy was intermediate with respect to the two modes. Identifying the ancestral larval life history pattern and the pathways in the switch from feeding to non-feeding larvae is complicated by the two patterns of metamorphosis seen in species with planktotrophic development: Type I (ophiopluteus only) and Type II (ophiopluteus + vitellaria larva). The variability in arm resorption at metamorphosis across ophiuroid families indicates that the Type I and II patterns may be two ends of a morphological continuum. This variability indicates ancestral morphological plasticity at metamorphosis, followed by canalization in some taxa to the vitellaria as the metamorphic larva. Vestigial ophiopluteal traits in lecithotrophic ophioplutei and vitellaria indicate evolution from the ancestral (feeding larva) state. Parental care has evolved many times from an ancestor that had a planktonic ophiopluteus or vitellaria and is often associated with hermaphroditism and paedomorphosis. A secondary reduction in egg size occurred in the viviparous species.

Rapid speciation in the holopelagic ctenophore Mnemiopsis following glacial recession

Understanding how populations diverge is one of the oldest and most compelling questions in evolutionary biology. An in depth understanding of how this process operates in planktonic marine animals, where barriers for gene flow are seemingly absent, is critical to understanding the past, present, and future of ocean life. Mnemiopsis plays an important ecological role in its native habitat along the Atlantic coast of the Americas and is highly destructive in its non-native habitats in European waters. Although historical literature described three species of Mnemiopsis, the lack of stable morphological characters has led to the collapse of this group into a single species, Mnemiopsis leidyi. We generate high-quality reference genomes and use a whole-genome sequencing approach to reveal that there are two species of Mnemiopsis along its native range and show that historical divergence between the two species coincides with historical glacial melting. We define a hybridization zone between species and highlight that environmental sensing genes likely contribute to the invasive success of Mnemiopsis. Overall, this study provides insights into the fundamental question of how holopelagic species arise without clear barriers to gene flow and sheds light on the genomic mechanisms important for invasion success in a highly invasive species.

Reverse development in the ctenophore Mnemiopsis leidyi

Reverse development, or the ability to rejuvenate by morphological reorganization into the preceding life cycle stage is thought to be restricted to a few species within Cnidaria. To date, Turritopsis dohrnii is the only known species capable of undergoing reverse development after the onset of sexual reproduction. Here, we demonstrate that the ctenophore Mnemiopsis leidyi is capable of reversal from mature lobate to early cydippid when fed following a period of stress. Our findings illuminate central aspects of ctenophore development, ecology, and evolution and show the high potential of M. leidyi as a unique model system to study reverse development and rejuvenation. Besides shedding light on the plasticity of developmental programs, these results raise fundamental questions about early animal development, body plans, and life cycles.

Evolutionary origin of the nervous system from Ctenophora prospective

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.

The ctenophore Mnemiopsis leidyi deploys a rapid injury response dating back to the last common animal ancestor

Regenerative potential is widespread but unevenly distributed across animals. However, our understanding of the molecular mechanisms underlying regenerative processes is limited to a handful of model organisms, restricting robust comparative analyses. Here, we conduct a time course of RNA-seq during whole body regeneration in Mnemiopsis leidyi (Ctenophora) to uncover gene expression changes that correspond with key events during the regenerative timeline of this species. We identified several genes highly enriched in this dataset beginning as early as 10 minutes after surgical bisection including transcription factors in the early timepoints, peptidases in the middle timepoints, and cytoskeletal genes in the later timepoints. We validated the expression of early response transcription factors by whole mount in situ hybridization, showing that these genes exhibited high expression in tissues surrounding the wound site. These genes exhibit a pattern of transient upregulation as seen in a variety of other organisms, suggesting that they may be initiators of an ancient gene regulatory network linking wound healing to the initiation of a regenerative response.

Brief History of Ctenophora

Ctenophores are the descendants of the earliest surviving lineage of ancestral metazoans, predating the branch leading to sponges (Ctenophore-first phylogeny). Emerging genomic, ultrastructural, cellular, and systemic data indicate that virtually every aspect of ctenophore biology as well as ctenophore development are remarkably different from what is described in representatives of other 32 animal phyla. The outcome of this reconstruction is that most system-level components associated with the ctenophore organization result from convergent evolution. In other words, the ctenophore lineage independently evolved as high animal complexities with the astonishing diversity of cell types and structures as bilaterians and cnidarians. Specifically, neurons, synapses, muscles, mesoderm, through gut, sensory, and integrative systems evolved independently in Ctenophora. Rapid parallel evolution of complex traits is associated with a broad spectrum of unique ctenophore-specific molecular innovations, including alternative toolkits for making an animal. However, the systematic studies of ctenophores are in their infancy, and deciphering their remarkable morphological and functional diversity is one of the hot topics in biological research, with many anticipated surprises.

Integrating Complex Life Cycles in Comparative Developmental Biology

The goal of comparative developmental biology is identifying mechanistic differences in embryonic development between different taxa and how these evolutionary changes have led to morphological and organizational differences in adult body plans. Much of this work has focused on direct-developing species in which the adult forms straight from the embryo and embryonic modifications have direct effects on the adult. However, most animal lineages are defined by indirect development, in which the embryo gives rise to a larval body plan and the adult forms by transformation of the larva. Historically, much of our understanding of complex life cycles is viewed through the lenses of ecology and zoology. In this review, we discuss the importance of establishing developmental rather than morphological or ecological criteria for defining developmental mode and explicitly considering the evolutionary implications of incorporating complex life cycles into broad developmental comparisons of embryos across metazoans.

Syncytial nerve net in a ctenophore adds insights on the evolution of nervous systems

A fundamental breakthrough in neurobiology has been the formulation of the neuron doctrine by Santiago Ramón y Cajal, which stated that the nervous system is composed of discrete cells. Electron microscopy later confirmed the doctrine and allowed the identification of synaptic connections. In this work, we used volume electron microscopy and three-dimensional reconstructions to characterize the nerve net of a ctenophore, a marine invertebrate that belongs to one of the earliest-branching animal lineages. We found that neurons in the subepithelial nerve net have a continuous plasma membrane that forms a syncytium. Our findings suggest fundamental differences of nerve net architectures between ctenophores and cnidarians or bilaterians and offer an alternative perspective on neural network organization and neurotransmission.

Speciation of pelagic zooplankton: Invisible boundaries can drive isolation of oceanic ctenophores

The study of evolution and speciation in non-model systems provides us with an opportunity to expand our understanding of biodiversity in nature. Connectivity studies generally focus on species with obvious boundaries to gene flow, but in open-ocean environments, such boundaries are difficult to identify. Due to the lack of obvious boundaries, speciation and population subdivision in the pelagic environment remain largely unexplained. Comb jellies (Phylum Ctenophora) are mostly planktonic gelatinous invertebrates, many of which are considered to have freely interbreeding distributions worldwide. It is thought that the lobate ctenophore Bolinopsis infundibulum is distributed throughout cooler northern latitudes and B. vitrea warmer. Here, we examined the global population structure for species of Bolinopsis with genetic and morphological data. We found distinct evolutionary patterns within the genus, where B. infundibulum had a broad distribution from northern Pacific to Atlantic waters despite many physical barriers, while other species were geographically segregated despite few barriers. Divergent patterns of speciation within the genus suggest that oceanic currents, sea-level, and geological changes over time can act as either barriers or aids to dispersal in the pelagic environment. Further, we used population genomic data to examine evolution in the open ocean of a distinct lineage of Bolinopsis ctenophores from the North Eastern Pacific. Genetic information and morphological observations validated this as a separate species, Bolinopsis microptera, which was previously described but has recently been called B. infundibulum. We found that populations of B. microptera from California were in cytonuclear discordance, which indicates a secondary contact zone for previously isolated populations. Discordance at this scale is rare, especially in a continuous setting.

Molecular and cellular architecture of the larval sensory organ in the cnidarian Nematostella vectensis

Cnidarians are the only non-bilaterian group to evolve ciliated larvae with an apical sensory organ, which is possibly homologous to the apical organs of bilaterian primary larvae. Here, we generated transcriptomes of the apical tissue in the sea anemone Nematostella vectensis and showed that it has a unique neuronal signature. By integrating previously published larval single-cell data with our apical transcriptomes, we discovered that the apical domain comprises a minimum of six distinct cell types. We show that the apical organ is compartmentalised into apical tuft cells (spot) and larval-specific neurons (ring). Finally, we identify ISX-like (NVE14554), a PRD class homeobox gene specifically expressed in apical tuft cells, as an FGF signalling-dependent transcription factor responsible for the formation of the apical tuft domain via repression of the neural ring fate in apical cells. With this study, we contribute a comparison of the molecular anatomy of apical organs, which must be carried out across phyla to determine whether this crucial larval structure evolved once or multiple times.

Multigenerational laboratory culture of pelagic ctenophores and CRISPR-Cas9 genome editing in the lobate Mnemiopsis leidyi

Despite long-standing experimental interest in ctenophores due to their unique biology, ecological influence and evolutionary status, previous work has largely been constrained by the periodic seasonal availability of wild-caught animals and difficulty in reliably closing the life cycle. To address this problem, we have developed straightforward protocols that can be easily implemented to establish long-term multigenerational cultures for biological experimentation in the laboratory. In this protocol, we describe the continuous culture of the Atlantic lobate ctenophore Mnemiopsis leidyi. A rapid 3-week egg-to-egg generation time makes Mnemiopsis suitable for a wide range of experimental genetic, cellular, embryological, physiological, developmental, ecological and evolutionary studies. We provide recommendations for general husbandry to close the life cycle of Mnemiopsis in the laboratory, including feeding requirements, light-induced spawning, collection of embryos and rearing of juveniles to adults. These protocols have been successfully applied to maintain long-term multigenerational cultures of several species of pelagic ctenophores, and can be utilized by laboratories lacking easy access to the ocean. We also provide protocols for targeted genome editing via microinjection with CRISPR-Cas9 that can be completed within ~2 weeks, including single-guide RNA synthesis, early embryo microinjection, phenotype assessment and sequence validation of genome edits. These protocols provide a foundation for using Mnemiopsis as a model organism for functional genomic analyses in ctenophores.