Porcelain crab research sheds light on water flea genome

Porcelain crabs and tiny water fleas don't look at all alike but they are both crustaceans, and are thought to be close cousins that are genetically more similar to each other than to other arthropods, such as insects.

A photo of porcelain crabs

Petrolisthes cinctipes (left) and Petrolisthes manimaculis (right), two porcelain crab species studied in the Stillman laboratory at the Romberg Tiburon Center. Credit: Jonathon Stillman, SF State

Associate Professor of Biology Jonathon Stillman, a scientist at the Romberg Tiburon Center for Environmental Studies, contributed his findings on the genetics of porcelain crabs to an international research team that recently sequenced the genome of an important pond-dwelling water flea.

Daphnia pulex, a one-eyed water flea about the size of an equal sign on a keyboard, is the first crustacean to have its genome sequenced. The newly decoded genome is a welcome breakthrough in the environmental biology community because these minute creatures are highly sensitive to toxins and pollutants and serve as a "canary in the coal mine" for freshwater ecosystems. But before its promising genome could be fully understood, researchers needed the genetic blueprint of another crustacean for comparison. 

"Once a genome is sequenced, scientists need something to compare it to in order to determine which features are specific to the species and which are shared by their class," Stillman said. "Because Daphnia is the first crustacean genome to be decoded, our findings on the DNA of the porcelain crab provided the closest set of comparison data."

A photo of a water flea

The water flea Daphnia pulex with a brood of future offspring. Credit: Paul D.N. Hebert, University of Guelph

Stillman's laboratory hosts the Porcelain Crab Array Database, an extensive database of completed sequences for all segments of DNA that encode genes in the porcelain crab. This resource contains the largest amount of genetic data held on any crab species and represents the third largest sequencing effort for any crustacean.

"Comparing what we know about the genetics of the porcelain crab and Daphnia allows us to explore evolutionary questions about whether arthropods evolved from a common ancestor or multiple ancestors," Stillman said. Arthropods are a classification of invertebrates comprised of insects, spiders (and their kin), and crustaceans.

Within the crustacean lineage, water fleas are believed to be close relatives of a sub-group of crustaceans that includes crabs, shrimp and lobster, but Stillman's research contributions to the Daphnia Genomics Consortium suggest that the water flea shares a greater number of genes with six-legged insects, such as the fruit fly and honeybee, than with its fellow crustacean, the porcelain crab.

As a member of the Daphnia Genomics Consortium, Stillman co-authored two companion papers to the water flea genome study, which was recently published in the journal Science.

Stillman's contributions to the research effort on the Daphnia genome include "Recent advances in crustacean genomics" published in Integrative and Comparative Biology; "D- and L-lactate dehydrogenases during invertebrate evolution" published in BioMed Central Evolutionary Biology; and "The Porcelain Crab Transcriptome and PCAD the Porcelain Crab Microarray and Sequence Database" published in PLoS ONE. Copies of the papers can be found at:http://userwww.sfsu.edu/~stillmaj/publication.html

-- Elaine Bible