A spiky classic: 125 years of discovery in the sea urchin

Barbed and globular, the sea urchin seems an unlikely research subject, yet it is one of the oldest and best established marine model organisms in biology. As students in the MBL Embryology course recently discovered, the ongoing role of the sea urchin in revealing how an embryo develops is entwined with the career of David McClay of Duke University, a former co-director of the course.

McClay led the students through a two-day boot camp in sea-urchin developmental biology and genetics, from embryo to spiny sphere. Before kicking off his introductory lecture in Speck Auditorium, McClay recalled his first visit to the MBL in the 1980s to attend the first conference on sea urchin development. "In this very room," he said, he described a new method for identifying (or “staining”) various sea urchin cell types, eliciting excitement in the audience that he remembers to this day.

This year, McClay had two main goals for his lecture, one being to describe sea urchin early development before encouraging students to get creative in the lab. As is customary in the MBL’s Discovery Courses, students receive guidance and advanced tools to design experiments and address a research question of their choosing. “Sea urchins are particularly useful in [embryological] research since they are relatively simple and quick to develop,” McClay said.

But first, McClay emphasized the field’s history, reminding students, “It’s important to know where you’ve come from to understand where you’re going.”

McClay’s historical overview began in 1891 at the Stazione Zoologica in Naples. There, Theodor Boveri deduced from sea urchin experiments that chromosomes are the objects of heredity, a theory demonstrated in fruit flies by Thomas Hunt Morgan in the early 1900s. McClay recapped the major mid-century experiments in his field, which were essentially limited to “throwing dyes and poisons” at the urchin embryo and observing what happened next. With the advent of dramatically improved microscopes in the 1960s, it became clear that embryonic development requires a complex game of telephone between molecules in the cell. But, at the time, there was no way to manipulate those molecules to determine how and in what order they communicate.

The subsequent shift toward genetic analysis to address this issue was spearheaded by the late Eric Davidson of Caltech, who had an enduring connection with the MBL. “In the early 1970s, at the dawn of molecular biology, Davidson realized these new tools could help uncover what exactly in DNA drives development,” McClay said. Davidson’s lab began cloning sea urchin genes during the 1980s, leading to a “golden age” in the 1990s, aided by the emergence of molecular probes capable of targeting certain stretches of DNA. “Every time we turned around, we found a new molecule that led to a new understanding of the sea urchin embryo,” McClay said.

In the 1990s, Davidson and McClay collaborated to expose the first gene regulatory networks (GRNs) in the sea urchin embryo—that is, the lines of cellular communication that dictate when, where, how long, and to what extent certain genes are expressed as the organism takes shape. In 2008, Davidson and McClay co-founded the MBL’s Gene Regulatory Networks course, still taught by McClay today.

The genome of the purple sea urchin (S. purpuratus) was sequenced in 2006, which has helped draw a sizeable research community to the organism. And while the GRNs are works-in-progress, McClay noted that, at this point, “Sea urchins have become transparent, at least metaphorically. We now have the tools to access virtually all the molecular underpinnings of this system.”

With that, off went the Embryology course students, eager to try those tools for themselves.