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Choanoflagellates and the Origin of Animals

Research Summary

Nicole King studies the closest living relatives of animals—the choanoflagellates—to reconstruct animal origins and elucidate core mechanisms underlying animal cell and developmental biology.

More than six hundred million years ago, an unusual group of microbial eukaryotes evolved the capacity for complex multicellularity and eventually spawned the full diversity of modern animals. Because modern animal development and physiology (including in humans) depend upon ancient mechanisms for cell adhesion and intercellular signaling, understanding animal origins promises to help illuminate modern animal development, homeostasis, and disease. To reconstruct the genetic, cell biological, and developmental foundations of animal origins, my lab studies the closest living relatives of animals, the choanoflagellates (Figure 1). We are particularly focused on understanding the biology of the choanoflagellate Salpingoeca rosetta, which is capable of alternating between single-celled and multicelled morphologies in response to a signaling molecule produced by environmental bacteria.

Although first described in 1847, choanoflagellates passed much of the following 150-plus years in obscurity. Thus, in addition to our goal of reconstructing animal origins, my lab has been developing choanoflagellates into experimentally tractable model organisms for studies of molecular mechanisms.

The Ancestry and Early Evolution of Animals Genes and Genomes
A key question in the origin of animals concerns how and when the "tool kit" of animal genes first evolved. To investigate whether genes required for animal development evolved before the origin of animal multicellularity, we sequenced the genomes of the choanoflagellates Monosiga brevicollis and S. rosetta (in collaboration with the Joint Genome Institute and the Broad Institute) and the transcriptomes of 19 additional choanoflagellate species. By comparing choanoflagellate genomes to genomes from diverse animals, we have begun to reconstruct the genome contents of the first animals and their ancestors. For example, the S. rosetta and M. brevicollis genomes unexpectedly revealed that choanoflagellates express homologs of diverse genes required for animal cell signaling and adhesion, including receptor tyrosine kinases, cadherins, and C-type lectins. Our goals are to build a searchable database of the ancestral animal genome and to investigate the premetazoan functions of diverse animal gene families.

Regulation of Cell Differentiation and Morphogenesis in the Choanoflagellate S. rosetta
The mechanisms by which choanoflagellates form colonies, establish cell polarity, and reproduce may provide crucial insights into the origin of animal multicellularity, but relatively little is known about their cell biology or natural history. We have found that S. rosetta can differentiate into at least three different single-celled morphs and at least two different multicelled morphs. S. rosetta colonies form through a process of incomplete cytokinesis (rather than cell aggregation) such that the mature colony is a clonal individual (Figure 2). In addition, neighboring cells in colonies are linked through fine intercellular bridges that may permit direct sharing of small molecules. We have also recently found that S. rosetta produces morphologically differentiated (i.e., "male" and "female") gametes and is capable of mating. To investigate the genetic determinants of cell differentiation and colony formation in S. rosetta, we plan to study patterns of gene expression combined with classical genetic screens.

Mechanisms and Evolution of Microbial Influences on Eukaryotic Biology and Morphogenesis
Perhaps the most unexpected breakthrough from my lab’s research has been the discovery that rosette colony development in S. rosetta is regulated by a secreted signal from the bacterium Algoriphagus. Notably, members of the Bacteroidetes phylum (of which Algoriphagus is one) regulate morphogenesis in at least three major groups of multicellular organisms: animals, red algae, and green algae. Although interactions between Bacteroidetes bacteria and their animal hosts play critical roles in animal development, metabolism, and evolution, relatively little is known about the molecular mechanisms involved. The close evolutionary relationship of choanoflagellates to animals and the experimental tractability of the S. rosetta-Algoriphagus interaction make this pairing useful for investigating the mechanisms by which environmental bacteria regulate eukaryotic cell biology and morphogenesis. 

In collaboration with the lab of Jon Clardy (Harvard Medical School), we have isolated the Algoriphagus signaling molecule and determined that it is a member of a novel class of sulfonolipids (Figure 3). These findings provide a prototypical example of bacterial sulfonolipids triggering eukaryotic development and suggest molecular mechanisms through which bacteria may have inadvertently contributed to the evolution of multicellularity in animals. We hypothesize that the Algoriphagus inducer binds to a surface-exposed receptor or that it has physicochemical properties that allow it to diffuse across the choanoflagellate cell membrane and interact with an intracellular receptor. One of our goals is to identify and characterize the S. rosetta receptor(s) of the inducing molecule.

Grants from the National Institutes of Health, the Gordon and Betty Moore Marine Microbiology Initiative, the American Cancer Society, and the National Science Foundation provided partial support for some of these projects.

As of February 19, 2016.

Scientist Profile

University of California, Berkeley
Evolutionary Biology, Developmental Biology