By Amanda Rose Martinez

Students in this year’s Embryology course have just six weeks to wrap their heads around a, if not the, fundamental question of biology: How do you start with a single egg and get to a full embryo? As it turns out, explained Nicole King in Tuesday’s lecture on “Animal Origins,” this is also a fundamental question of evolutionary biology: How did multicellular animals evolve from their single-celled ancestors?

“Our goal,” said King, an associate professor of genetics, genomics, and development from the University of California, Berkeley, “is to try and identify genomic and cell biological innovations that might have contributed to this transition.” She then unveiled the two key players that may prove pivotal in helping scientists to uncover such innovations— choanoflagellates and sponges.

Choanoflagellate: evolution’s last stop on the way to animals. Photo by Mark Davel

Choanoflagellates have a spherical cell body, bordered at its crown by a collar of minute, hair-like projections, and a long, central appendage called a flagellum that resembles a whip and enables it to swim. King’s allowance that “it’s reasonable to think of these organisms as sperm cells with a collar,” sent a ripple of early-morning snickers through the auditorium. But what’s special about the unicellular choanoflagellates is that they’re the closest living relatives of animals. Just next to the choanoflagellates on an evolutionary chart lies the multicellular sponges, the earliest branching lineage of animals.

A sponge named Oscarella carmela. Photo by Scott Nichols

In this way, choanoflagellates and sponges “bracket the evolution of multicellularity,” said King. “If we can study these organisms at the molecular level, see what they share in common, see what’s different, hopefully someday, we’ll be able to reconstruct the molecular processes that contributed to the origin of animals.”

Later in the Lab…

Tuesday afternoon, the students got their first glimpse of choanoflagellate and sponge cells. Using a standard imaging technique, they added a fluorescent antibody to a structural protein (beta-tubulin) that is common to both organisms. When viewed under a microscope, the antibodies could be seen fluorescing bright green, which served to highlight the outline of each cell body and flagellum, but also emphasized the shared architecture between choanoflagellate and sponge cells.

“The hope is that these students have interests that are unique and eclectic, and that they’ll take these techniques and apply them in novel ways for their areas of interest,” said Stephen Fairclough, a graduate student in King’s lab at UC Berkeley and one of the teaching assistants for this year’s course.

Yi-Ju Chen (R) of Caltech gets advice on sample preparation from Embryology faculty member Nicole King (L). Photo by Amanda R. Martinez

Eclectic interests indeed. A quick canvas of the room revealed Yi-Ju Chen, a graduate student in physics from Caltech interested in evolutionary theory and pattern formation; Valerie Virta, a postdoc at the National Institute of Child Health and Human Development (NICHD), who is investigating the mechanism that causes some cells to stop moving and become the bones that form the face; and Joseph Campanale, a PhD student at the Scripps Institution of Oceanography, who studies how embryos eliminate or detoxify environmental chemicals during development.

Joseph Campanale of Scripps readies a piece of sponge for viewing under a microscope. Photo by Amanda R. Martinez

Valerie Virta of NICHD prepares humidification chambers for her samples. Photo by Amanda R. Martinez