Paired appendages—what we commonly think of as limbs—are so useful for moving, eating, fighting, and myriad other activities that they arose independently several times in evolution. The limbs of arthropods, like this crustacean, the amphipod Parhyale hawaiensis, are like a Swiss army knife that contains a variety of specialized tools. During development, specific genes function like the knife maker, to make sure that the right limb develops in the right part of the body. This tiny embryo, which is 500 micrometers in length, or about the size of a grain of salt, is transparent, except for two regions. The bright red and deep purple stains reveal the activation of two different genes: the gene identified by the red stain is responsible for jaw appendage development, and the other gene (purple) for leg appendage development. These two genes, and a few others that regulate embryonic development, work together to enable the formation of a diverse array of animal appendages.
A technique called in situ hybridization was used to detect genes responsible for appendage development. In situ hybridization involves specific, color-labeled pieces of RNA that bind to complementary sections on messenger RNA molecules that are produced when certain genes are transcribed. This technique specifically detects certain genes when they are active in an organism’s cells.
Image courtesy of Nipam Patel, PhD and Danielle Liubicich, PhD, University of California, Berkeley