Human homologues in the humble hydra.

People may crown the evolutionary tree, but we may not be as far removed from our roots as we'd like to think. Humans share key genes not only with mice, fruit flies, and nematodes (SN: 10/15/94, p.244), but also with simple creatures such as the hydra.

The centimeter-long hydra basically consists of two layers of epithelial cells and a few other cells tucked in between that form nerves, stingers, and glands. The nerve cells connect loosely as a "nerve net," but the animal can survive without this primitive nervous system.

Of multicellular organisms with differentiated cells (metazoa), only sponges are simpler, says Ann Grens, a developmental biologist at the University of California, Irvine. Geneticists had shown that several vertebrates and insects have a group of genes that codes for gene-regulating proteins. At least some of the amino acids in these proteins curl to form what's called a basic helix-loop-helix motif.

Now, Grens has found that the hydra has the gene for one of these proteins.

This discovery shows that the gene "did show up early in evolution," Grens says. She doesn't know what this gene, called achaete-scute, does in hydra. But its counterpart in fruit flies leads to the development of touch-sensitive bristles and surface pits for detecting airborne chemicals. When she adds this hydra gene to fruit flies lacking their own version of it, the flies develop the missing pits and bristles, Grens reports.

Researchers have now found genes from about a half dozen vertebrate gene families in hydra, but the hydra tends to have just one family member, whereas higher organisms possess several. "It's looking like a general rule that most of the genes that are fairly widely distributed in metazoa do exist in Cnidaria [hydralike organisms]," Grens concludes.