FIG. 28.7

How do bacteria influence the life cycles of choanoflagellates?

BACKGROUND Choanoflagellates are the closest protistan relatives of animals. Cell adhesion, cell differentiation, and multicellularity in these organisms are being studied to increase our understanding of early events in the evolution of animal multicellularity. Nicole King and her colleagues cultured the choanoflagellate species Salpingoeca rosetta in the laboratory. High-speed videos revealed that these choanoflagellates differentiate into several distinct cell types in the course of their life cycle. Actively swimming cells can be fast swimmers or slow swimmers that differ in shape and ornamentation (Fig. 28.7a). When swimming cells make contact with a solid surface, they can differentiate to form a stalk that tethers them to the substrate (Fig. 28.7b). Simple multicellular colonies in the shape of rosettes (Fig. 28.7c) were also seen when the choanoflagellates were cultured in the presence of many bacteria, including Algoriphagus machipongonesis.

HYPOTHESIS The bacterium Algoriphagus machipongonesis induces the choanoflagellate to form rosette-like colonies.

EXPERIMENT The scientists grew the choanoflagellates in a medium containing only A. machipongonesis, as well as in media without this prey bacterium.

RESULTS The multicellular state formed only when A. machipongonesis was present (Fig. 28.7c).

CONCLUSION Choanoflagellates differentiate several cell types upon cues from the environment. In particular, molecular signals from prey bacteria can induce a simple form of multicellularity. The direct ancestors of animals may have gained simple multicellularity by a similar route.

FOLLOW-UP WORK Research is continuing to investigate the molecular mechanisms of cell differentiation, cell adhesion, and signal reception in these distant relatives of ours.

SOURCE Dayel, M., et al. 2011. “Cell Differentiation and Morphogenesis in the Colony-Forming Choanoflagellate Salpingoeca rosetta.” Developmental Biology 357:73–82.