Biology subjects

Havrilak, J. A.

Publications and source records attributed to Havrilak, J. A..

4 recordsLinked to original sources

bioRxiv2025

NvashA function reveals temporal differences in neural subtype generation in cnidarians

Understanding how cnidarians pattern their nervous systems can provide insight into the ancestral mechanisms of neurogenesis that are shared with bilaterians, shedding light on the evolution of nervous systems. While previous studies have revealed deeply conserved mechanisms for neural induction and progenitor selection between cnidarians and bilaterians, less is known about how distinct neuronal subtypes are specified over time in cnidarians. We utilized single-cell mRNA sequencing to profile NvashA-expressing cells across embryonic and planula-larva stages of Nematostella neurogenesis, and functional experiments identified a dynamic role for NvashA over time. Our analysis revealed that unique neuronal subtypes emerge at different developmental stages, providing evidence for temporal patterning in developing cnidarian nerve nets. This can provide a foundation to better our understanding of neurogenic gene regulatory networks, and to compare neurogenesis across cnidarians, and with bilaterians, to improve our knowledge of nervous system evolution.

developmental biology↗

bioRxiv2025

Conserved and lineage-restricted gene regulatory programs modulate developmental cnidocyte specification in Nematostella vectensis.

Cnidocytes are a synapomorphy of cnidarians that have evolved a range of morphologies and functions within and across extant species, which makes them an excellent model to investigate how novel cell types emerge and radiate in evolution. One way to gain insight into how cell types evolve is to investigate the gene regulatory networks (GRNs) that pattern them, leading to the identification of shared and divergent programs that likely represent components of the ancestral and evolved phenotypes. Efforts to identify early acting transcription factors in the sea anemone Nematostella vectensis revealed that NvfoxE-like is a lineage-restricted cnidocyte transcription factor. Expression and functional studies confirmed that NvfoxE-like and its targets are found exclusively in cnidocytes. By investigating interactions with regulators of the known cnidogenesis GRN and comparing the targets of known cnidocyte regulators, we identified at least four regulatory programs contributing to the cnidogenic GRN in Nematostella. The previously identified cnidocyte regulators Nvznf845 and NvpaxA represent two core pan-cnidarian regulatory programs, while the NvfoxE-like dependent program appears to be restricted to Hexacorallia or Actinaria, where we hypothesize it functions to pattern the unique capsule features present in those lineages. Here, we confirm the hypothesized core conserved transcriptional program, as well as demonstrate how a novel transcription factor was co-opted into cnidogenesis in a subset of cnidarians, thus contributing to the diversification and continued evolution of cnidocytes.

evolutionary biology↗

bioRxiv2021

Brain regulatory program predates central nervous system evolution

Understanding if bilaterian centralized nervous systems (CNS) evolved once or multiple times has been debated for over a century. Recent efforts determined that the nerve chords found in bilaterian CNSs likely evolved independently, but the origin(s) of brains remains debatable. Developing brains are regionalized by stripes of gene expression along the anteroposterior axis. Gene homologs are expressed in the same relative order in disparate species, which has been interpreted as evidence for homology. However, regionalization programs resemble anteroposterior axial patterning programs, which supports an alternative model by which conserved expression in brains arose convergently through the independent co-option of deeply conserved axial patterning programs. To begin resolving these hypotheses, we sought to determine when the neurogenic role for axial programs evolved. Here we show that the nerve net in the cnidarian Nematostella vectensis and bilaterian brain are regionalized by the same molecular programs, which indicates nervous system regionalization predates the emergence of bilaterians and CNSs altogether. This argues that shared regionalization mechanisms are insufficient to support the homology of brains and supports the notion that axial programs were able to be co-opted multiple times during evolution of brains.

evolutionary biology↗

bioRxiv2019

Dynamics and variability in regenerative potential of neuronal subtypes in the Nematostella nerve net

BackgroundWe are interested in nervous system dynamics in adult and regenerating animals. Preliminary studies suggest that some species alter neuronal number to scale with changes in body size. Similarly, in some species regenerates resulting from wholebody axis regeneration are smaller than their pre-amputated parent, but they maintain the correct proportionality, suggesting that tissue and neuronal scaling also occurs in regenerates. The cell dynamics and responses of neuronal subtypes during nervous system regeneration, scaling, and whole-body axis regeneration are not well understood in any system. The cnidarian sea anemone Nematostella vectensis is capable of wholebody axis regeneration, and its transparent, "simple" body plan and the availability of fluorescent reporter transgenic lines allow neuronal subtypes to be tracked in vivo in adult and regenerating animals. A number of observations suggest this anemone is able to alter its size in responses to changes in feeding. We utilized the NvLWamide-like::mCherry neuronal subtype transgenic reporter line to determine the in vivo response of neuronal subtypes during growth, degrowth, and regeneration. ResultsNematostella alters its size in response to caloric intake, and the nervous system responds by altering neuronal number to scale as the animal changes in size. Neuronal numbers in both the endodermal and ectodermal nerve nets decreased as animals shrunk, increased as they grew, and the changes were reversible. Whole-body axis regeneration resulted in regenerates that were smaller than their pre-amputated size, and the regenerated nerve nets were reduced in neuronal number. Different neuronal subtypes had several distinct responses during regeneration that included consistent, no, and conditional increases in number. Conditional responses were regulated, in part, by the size of the remnant fragment and the position of the amputation site. Regenerates and adults with reduced nerve nets displayed normal behaviors, indicating that the nerve net retains functionality as it scales. ConclusionThese data suggest that the Nematostella nerve net is dynamic, capable of scaling with changes in body size, and that neuronal subtypes display differential regenerative responses, which we propose may be linked to the scale state of the regenerating animals.

neuroscience↗