Meeting Abstract
Now considered to be the earliest diverging animal lineage, ctenophores (comb jellies) have become a valuable model for understanding the evolution of conserved metazoan features (e.g., neurons, photoreceptors). Conversely, ctenophores are unified by their possession of a truly unique, phylum-specific cell type (the colloblast), making these animals also an important model for the evolution of novelty. Colloblasts are secretory cells that line the tentacles of ctenophores; they release a sticky glue upon contact to facilitate capture of their planktonic prey. To understand the origin of colloblasts, we examined tentacle development in the model ctenophore Mnemiopsis leidyi. Tentacle development is complex in M. leidyi, with multiple lineages of embryonic micromeres contributing cells to the tentacles during early development. In the larval (cydippid) stage, the cells comprising the tentacles are generated constantly from a population of stem cells found in the base of the tentacle (the tentacle bulb). Treatment of cydippid larvae with a reversible inhibitor of cell proliferation (hydroxyurea) resulted in loss of tentacles in M. leidyi. One lineage of ctenophores (Beroidae) lacks tentacles completely, feeding exclusively on other ctenophores. The phylogenetic position of Beroidae suggests that tentacles (and therefore, colloblasts) were secondarily lost in this lineage. We compared cell dynamics and gene expression in Beroe ovata with wildtype and hydroxyurea-treated (tentacle-free) M. leidyi. Using this framework, we generate hypotheses about evolutionary gain and loss of cell types and the impact of these processes on speciation.
