We are interested in understanding how differences in cell-cell adhesion influence patterns of cell differentiation in echinoderms. In sea urchins--the best-understood model system for echinoderm embryonic development--cells are very compact: there is virtually no intercellular space between early blastomeres (left image). In sea urchins, specification of endomesoderm cell fates depends on Wnt/b-catenin signaling. Similarly, in sea star embryos, endomesoderm specification also depends on Wnt/b-catenin; however, their embryonic cells are very loose: there is quite a bit of intercellular space between early blastomeres (right image). These blastomeres are so loosely adhered, in fact, that they will spontaneously detach from each other if not forced together by the fertilization envelope. Why is this interesting? Because b-catenin is a protein with at least two functions: when it is in the nucleus (as a consequence of active Wnt signaling), it acts as a transcription factor and initiates cell differentiation; when it is at the membrane it binds cadherins, facilitating and regulating cell-cell adhesion. Vanessa Barone is testing whether the formation of more or less strong cell-cell contacts influences Wnt signaling, and in turn endomesoderm specification. Furthermore, is it possible that sea urchin and sea star embryos exhibit different Wnt signaling dynamics, because they have cells with different adhesive properties? These questions are being tackled by: 1) observing how cell-cell contacts and Wnt/b-catenin signaling correlate in vivo in real time, with high resolution live imaging; 2) measuring how changing cell-cell adhesion alters Wnt/b-catenin signaling, with genetic and mechanical manipulations; and 3) testing how we can explain patterns of cell differentiation based on patterns of cell-cell adhesion, with theoretical modelling.
Most marine organisms have evolved the ability to produce calcified structures for housing, support, or defense—think about the biominerals that are associated with the successful diversification of corals, molluscs, tube worms, echinoderms and vertebrates. But are these biominerals homologous (i.e. their last common ancestor made a shell/test/bone that was modified in the lineages as they diverged)?, or are they the product of convergent evolution (i.e. independently evolved after these lineages diverged)?. If we look just at the genes responsible for producing biomineralized structures, it becomes apparent that extensive gene evolution has occurred. For example, many genes associated with biomineralization have intrinsically disordered domains, extensive use of acidic and negatively charged amino acids, and novelty in sequence structure. At the same time, they also incorporate ancient domains that have been exapted into new proteins involved in biomineralization. Because independent evolution seems to be a defining characteristic of shell matrix proteins, one of the research interests in the lab is to understand the evolutionary origins of gene regulatory networks that go into producing calcified structures. Are gene regulatory network kernels homologous between animals that have diverged nearly 500 million years ago such as echinoderms and molluscs? Grant Batzel is using the slipper-snail, Crepidula fornicata, as a model for understanding shell formation in the gastropods. We are interested in understanding: 1) What genes are responsible for specifying the fate of shell-forming cells involved in biomineralization and 2) How those genes are regulated (i.e. the signaling genes and transcription factors). Crepidula fornicata is an excellent model for asking these questions because their embryos are amenable to functional studies.
Nudibranchs are shell-less, uncoiled, gastropod molluscs that exhibit many interesting biological and behavioral attributes, but are not well understood from a developmental or functional genomics perspective. We are using the species Berghia stephanieae, which is ideal for studying both pre- and post-metamorphic development. Following early development via the sterotyped spiral cleavage program, B. stephanieae embryos make a lecithitrophic veliger larva (including a shell), and eventually hatch out of their egg cases as juveniles, hunting for their first meal, the sea anemone Aiptasia. Juveniles hatch around 19 days post fertilization, and become gravid in under 60 days. This species is particularly well suited for studying post-metamorphic developmental processes. These include: 1) the loss of shell production, 2) robust anterior and posterior regenerative capabilities, 3) the ability to sequester dinoflagellates, and cnidarian nematocysts, 4) complex social behaviors such as hunting, and 5) stereotyped organization of its central nervous system with large and experimentally accessible neurons. This last feature, its highly stereotyped nervous system, is the subject on our current collaboration with the Katz Lab at UMass Amherst. We are currently looking for a post doc to work on developing transgenic methods for this project (see ad here). Park Masterson and Carl Whitesel are perfecting a protocol for rearing Berghia on the bench top and techniques for working with their embryos
Gastrulation is a critical embryonic event during which presumptive endodermal and mesodermal cells are internalized. Gastrulation is closely tied to the development of key axial properties, and to the patterning of certain organ systems, such as the digestive tract; the openings of the mouth and/or anus often arise at or close to the site of gastrulation. Gastrulation also holds a pivotal role in evolutionary theories about the emergence and divergence of bilaterian body-plans. Thus, understanding the phylogenetic history of this event in different metazoan lineages remains an important question for evolutionary and developmental biology. The highly conserved spiral cleavage program allows comparison of homologous cells and tissues at single-cell resolution, across hundreds of millions of years of evolution. The unique body plan of each taxa begins to emerge during gastrulation stages, and so gastrulation is a critical process to study for understanding how different morphologies arise in development. We have used the slipper snail Crepidula fornicata to carry out the first detailed study of gastrulation and morphogenic processes among molluscs. Using detailed fate mapping, in vivo imaging, and gene expression analysis we investigate how the early cleavage stage embryo transforms into the molluscan body plan. Current projects focus on the functional analysis of the molecular pathways controlling these processes. See our previous work with the Henry Lab in the following publications: Lyons et al 2012, Lyons and Henry, 2014, Lyons et al 2015, Perry et al 2015, Lyons et al 2017.
Unlike terrestrial and aquatic developmental model systems, which have established transgenic lines, there are only a few marine animals for which transgenic lines are available, and none are echinoderms. In collaboration with my SIO colleague Amro Hamdoun, my lab is developing the local painted urchin, Lytechinus pictus, for making the first transgenic lines in this phylum. We have chosen this species because it has a relatively fast generation time (~4 months), because its embryos are optically clear and are easy to work with, and because it has been used for environmental and population genetics studies on the West Coast of California. We have completed a full genome sequencing project to support these efforts. A custom marine transgenics facility in Hubbs Hall at SIO will facilitate maintaining lines of these animals. Our labs have recently described updated culturing methods, and a developmental transcriptome, for this species.