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

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.

Life in the Midwater: The Ecology of Deep Pelagic Animals

The water column of the deep ocean is dark, cold, low in food, and under crushing pressures, yet it is full of diverse life. Due to its enormous volume, this mesopelagic zone is home to some of the most abundant animals on the planet. Rather than struggling to survive, they thrive-owing to a broad set of adaptations for feeding, behavior, and physiology. Our understanding of these adaptations is constrained by the tools available for exploring the deep sea, but this tool kit is expanding along with technological advances. Each time we apply a new method to the depths, we gain surprising insights about genetics, ecology, behavior, physiology, diversity, and the dynamics of change. These discoveries show structure within the seemingly uniform habitat, limits to the seemingly inexhaustible resources, and vulnerability in the seemingly impervious environment. To understand midwater ecology, we need to reimagine the rules that govern terrestrial ecosystems. By spending more time at depth-with whatever tools are available-we can fill the knowledge gaps and better link ecology to the environment throughout the water column.

Recording cilia activity in ctenophores: effects of nitric oxide and low molecular weight transmitters

Cilia are the major effectors in Ctenophores, but very little is known about their transmitter control and integration. Here, we present a simple protocol to monitor and quantify cilia activity and provide evidence for polysynaptic control of cilia coordination in ctenophores. We also screened the effects of several classical bilaterian neurotransmitters (acetylcholine, dopamine, L-DOPA, serotonin, octopamine, histamine, gamma-aminobutyric acid (GABA), L-aspartate, L-glutamate, glycine), neuropeptide (FMRFamide), and nitric oxide (NO) on cilia beating in Pleurobrachia bachei and Bolinopsis infundibulum. NO and FMRFamide produced noticeable inhibitory effects on cilia activity, whereas other tested transmitters were ineffective. These findings further suggest that ctenophore-specific neuropeptides could be major candidates for signal molecules controlling cilia activity in representatives of this early-branching metazoan lineage.

Ancient gene linkages support ctenophores as sister to other animals

A central question in evolutionary biology is whether sponges or ctenophores (comb jellies) are the sister group to all other animals. These alternative phylogenetic hypotheses imply different scenarios for the evolution of complex neural systems and other animal-specific traits1-6. Conventional phylogenetic approaches based on morphological characters and increasingly extensive gene sequence collections have not been able to definitively answer this question7-11. Here we develop chromosome-scale gene linkage, also known as synteny, as a phylogenetic character for resolving this question12. We report new chromosome-scale genomes for a ctenophore and two marine sponges, and for three unicellular relatives of animals (a choanoflagellate, a filasterean amoeba and an ichthyosporean) that serve as outgroups for phylogenetic analysis. We find ancient syntenies that are conserved between animals and their close unicellular relatives. Ctenophores and unicellular eukaryotes share ancestral metazoan patterns, whereas sponges, bilaterians, and cnidarians share derived chromosomal rearrangements. Conserved syntenic characters unite sponges with bilaterians, cnidarians, and placozoans in a monophyletic clade to the exclusion of ctenophores, placing ctenophores as the sister group to all other animals. The patterns of synteny shared by sponges, bilaterians, and cnidarians are the result of rare and irreversible chromosome fusion-and-mixing events that provide robust and unambiguous phylogenetic support for the ctenophore-sister hypothesis. These findings provide a new framework for resolving deep, recalcitrant phylogenetic problems and have implications for our understanding of animal evolution.