By Sarah Lawhun
In her final year of graduate school, while studying the Antarctic clam Laternula elliptica for her thesis, Victoria A. Sleight, now a scientist at the University of Cambridge, noticed the clams had unexpectedly spawned in her lab. Suddenly, Sleight had the opportunity to observe their embryonic development, a phenomenon few scientists ever witness. The embryos sparked her curiosity and she began to ask questions that extended beyond her thesis. When do embryos begin to produce shells? What genes are involved? How does this differ between species? Do embryos build their early shells in the same way as adult seashells?
Sleight’s interest in the embryonic origin of shell production had been piqued and led her into the territory of evolutionary developmental biology (evo-devo) — comparing developmental stages between different organisms to draw evolutionary inferences. Her curiosity about evolution, especially that of shell production, brought her to the MBL this summer as a Whitman Center Fellow, studying the development, evolution, and repair of biomineralization, the process of shell production, in three species of snail.
To observe the early stages of shell formation, Sleight puts Crepidula fornicata embryos in special dyes that fluoresce when they’re incorporated into a cell’s calcium carbonate. She took these images using a fluorescence microscope. Pink areas show early shell formation and green areas show auto-fluorescence (perhaps due to the presence of auto-fluorescent algae and cell bodies). Credit: Victoria A. Sleight
Sleight’s habit of asking questions, some of which don’t have answers, began when she was a child. She loved to collect and study animals, from fish to frog spawn, showering her parents with a deluge of questions. But she always held a fondness for mollusks.
“My favorite feature of mollusks is their shells,” says Sleight. “Humans have had an intrinsic interest in shells throughout history. Shells were some of the earliest forms of currency and have been used to make jewelry and art for millennia. That’s part of why I love studying how they’re made.”
Mollusk shells are a splendid example of nature’s ability to produce seemingly perfect, organized structures. Remarkably, the molecular mechanisms behind this structure remain largely unknown and the genes that regulate them are only just beginning to be identified and understood. Sleight aims to fill in gaps in scientists’ knowledge.
This summer at MBL, Sleight is working with three native species of snail, Crepidula fornicata, Crepidula plana, and Crepidula convexa. After collecting the animals from around Woods Hole, she has begun the work necessary to compare the three species’ biomineralization during embryonic development, searching for a genetic “toolkit,” a set of shared genes used in all three for shell production.
She will begin by building a pooled developmental transcriptome for each species, which is the total sequences of genes expressed during the developmental window of interest — in this case, both before the shell has begun to form within the embryo and immediately after. This involves collecting and aligning all the RNA fragments from embryonic tissue samples and mapping them back to their gene locations within the transcriptome. By comparing fragments in pre and post- shell production, Sleight can determine which genes are upregulated, or expressed more often, during early biomineralization in each species of snail.
Sleight will also work with a fellow Whitman scientist, Mark Terasaki, to take advantage of scanning electron microscopy to examine the larval mantle, the organ that creates the shell, and the shell itself, observing and comparing the cellular machinery between the three species.
Finally, Sleight will remove small parts of adult and juvenile shells to observe their repair, determining whether adult shell repair occurs using the same pathways as normal embryonic shell formation.
Exploring the molecular and genetic pathways that regulate biomineralization is an important step to predict the future of these animals and their ecosystems, especially with acidifying and warming waters in many marine environments. Sleight emphasizes the importance of first understanding the biomineralization basics.
“If we can understand the mechanisms of biomineralization in not only mollusks, but also corals and other animals with calcium carbonate structures, then we might be in a better position to predict what’s going to happen to them under future climate change scenarios,” she says.