Volume 1, Issue 3
The Evolutionary Secret of Body Segmentation
When UC Berkeley biologist Nipam Patel was searching for a new crustacean to study, one of his graduate students paid a visit to a large public aquarium. Rather than select an organism from one of the breeding tanks, the student sifted through the aquarium's filter system. There he found Parhyale hawaiensis, a sea scud. Because of its living conditions, Parhyale was already naturally selected to require minimal care, perfect for life in Patel's bustling laboratory. Patel, a professor of Integrative Biology and Molecular and Cell Biology, studies the development of arthropods to better understand evolutionary differences among a wide range of organisms.
A butterfly collector since he was a child, Nipam Patel also studies the pattern formation in butterfly wings.
"We focus on the development of crustaceans to better understand what characteristics are conserved between organisms and also the evolutionary differences," says Patel, also a researcher with the Howard Hughes Medical Institute.
Specifically, Patel's group studies how various animals segment their bodies during fetal development. Before delving into the evolutionary lineage of the scud, Patel made great strides several years ago studying genes that establish the body plan of several insects, including Drosophila melanogaster, the fruit fly. When the fly is still in an early embyronic state, a set of genes kick in that subdivide the fetus into increasingly smaller domains. Eventually those domains develop into the segments of the head, thorax, and abdomen of the adult fly.
Parhyale hawaiensis (male seen here) reach .5 to 1.0 cm in length with a generation time of seven weeks. (courtesy the researchers)
"One of the hallmarks in biology during the last couple of decades is the knowledge that the mechanisms and genes behind development seem to be quite universal in animals," Patel explains. "But of course, we don't look anything like flies. Why? How does evolution make different organisms with nearly the same set of genes?"
Recently, the researchers extended their comparative studies to include other arthropods such as grasshoppers, beetles, and lobsters. In one major breakthrough, Patel and his colleagues determined that evolutionary changes in the shape and function of certain crustacean appendages are tied to one particular gene family, known as the Hox genes.
Now he's hoping to tease out the evolutionary secrets behind Parhyale's segmentation. Patel's approach is two-pronged: determine how the animal segments and then compare its pattern-forming process to that of other arthropods. While it's clear that these animals' earliest shared ancestor was segmented, the mechanisms vary greatly between species. Patel would like to know which mechanisms and arthropod features are evolutionarily derived and which are common with the shared ancestor.
Already, the research group has made the remarkable determination that even when a Parhyale fetus is just eight cells old, those individual cells are already destined to become precursors of a particular type of tissue--muscle, for example.
At this stage in embryonic process of segmentation in Parhyale hawaiensis, the cells are organizing themselves into a precise pattern of rows and columns. The "engrailed" gene, expressed in cell rows stained red in this image, is instrumental in determining the pattern of each segment. (courtesy the researchers)
"There's no other animal where we've seen that level of restriction so early on," Patel says. "Now we'd like to know the level of that commitment."
The researchers are also analyzing the pattern of cell formation and its potential role in segmentation. The group is able to visually observe the amazingly orderly pattern in which the cells align by injecting a fetus, when it consists of just one cell, with RNA that expresses a protein that's fluorescently red. All of that single cell's progeny then glow red as well.
Another benefit of studying Parhyale as the "model system" for non-insect arthropods is that it appears well-suited to manipulation at the genetic level. By developing methods to alter gene expression during the fetal development, Patel hopes to gain insight into the similarities and, more importantly, the differences between Parhyale and its taxonomical brethren.
"Evolution is not a story of things staying the same, it's about how things change," he says.