Sponges

reference taxon for studying metazoan origins. Like a living time capsule, they may allow us to infer the genome content and cell biology of the unicellular progenitors of Metazoa. For example, comparing choanoflagellate and metazoan genomes will reveal the minimal genetic toolkit of the first metazoan. Pilot studies of choanoflagellate expressed genes have already uncovered homologs of a remarkable diversity of signaling and adhesion genes characteristic of Metazoa, including tyrosine kinases, cadherins and C-type lectins, extending the history of these key gene families back to the common ancestor of choanoflagellates and Metazoa. Comparisons of full genome sequences from choanoflagellates, sponges and other metazoans will provide a more complete picture of when and how important gene families evolved, the extent and impact of gene duplication and divergence, and the modes of protein-and RNA-based gene regulation available before the origin of Metazoa. Moreover, choanoflagellates provide a living system in which to study the ancient functions of genes that first evolved in unicellular organisms and today are used to mediate metazoan multicellularity. Why are choanoflagellates important for our planet's health? Small as they are, choanoflagellates have a huge impact on the food chain, aiding the flow of carbon from primary producers to higher trophic levels (including ourselves!). They comprise 5–40% of heterotrophic nanoflagellates and may filter between 10% and 25% of coastal surface water each day. By grazing on bacteria and detritus, choanoflagellates in turn feed their larger predators and return dissolved organic material to the water column. Both in number and feeding efficiency, choanoflagellates form a critical link in the microbial food chain that supports the global ecology. What don't we know about choanoflagellates? Given the potential importance of choanoflagellates to our understanding of metazoan evolution and the environment, it is remarkable how little we understand about their biology. A full accounting of a choanoflagellate genome will allow new insights into the molecular biology, physiology, ecology and evolution of these fascinating organisms. Some pressing questions about choanoflagellate biology concern how they interact with each other and their environment. Do they have sex? How do they form colonies, and why? How do choanoflagellates find, distinguish between, and capture different species of bacterial food? How is the unusual cellular architecture of choanoflagellates transmitted to each successive generation? Is there a choanoflagellate genome project? Yes. Genome sequencing projects have been approved for two species of unicellular choanoflagellates, Monosiga brevicollis and Monosiga ovata. In addition, EST …