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Gene study redraws arthropod family tree

ANN ARBOR—What do lobsters, fruit flies, water fleas, spiders, horseshoe crabs, centipedes and cockroaches have in common? Many things, according to a group of University of Michigan scientists, who believe the reason for these animals' similarities is that they are all descended from one ancestor-the mother of all arthropods-who lived roughly 1 billion years ago.

Arthropods are a group of animals with jointed legs and a hard exterior skeleton. By comparing the arrangement of genes encoded in arthropod mitochondrial DNA to those found in other kinds of animals (like snails and earthworms), U-M researchers have provided convincing evidence that all living arthropods stem from one common ancestor.

The results of the U-M study are reported in the current issue of Nature.

Mitochondria-microscopic structures that are part of all animal, plant and fungal cells-are the "power plants" responsible for converting most of the chemical energy in food molecules into a different energy form, called ATP, that can be used by the cell. In 1980, scientists discovered that the DNA present in animal mitochondria encodes 37 genes. Because the same 37 genes are present in the mitochondrial DNAs of all types of animals, they can be used as genetic markers. By analyzing similarities and differences in the mitochondrial gene arrangements found among different groups of animals, U-M researchers could infer in a clear-cut way the evolutionary relationships among them.

"With so many possible rearrangements of the 37 genes, it is very unlikely that unrelated groups would have evolved the same genetic structure simply by chance," said Wesley M. Brown, U-M professor of biology and a co-author of the Nature article.

In addition to providing strong qualitative evidence for the existence of a single ancient ancestor-an "arthropod Eve" -the U-M group's analysis of mitochondrial gene order also showed that members of the arthropod subgroups which chew their food with jaw-like structures called mandibles (like grasshoppers, lobsters and centipedes) are more closely related to one another than they are to subgroups which do not chew their food and simply use their pincer-equipped front legs to stuff food directly into their mouths (like scorpions and horseshoe crabs).

"Our data not only indicate that arthropods are all derived from a common ancestor, but also that a major split must have occurred early in arthropod evolution resulting in two major groups that came to have different types of mouth parts," said Brown.

Brown noted that the comparison of slowly changing features, such as the gene arrangements used in his U-M lab, promises to be valuable for further study of evolutionary relationships among animal groups, and could ultimately lead to an understanding of how all animals are related to one another. He added that an economically and medically important spinoff of such research will be the characterization of basic genetic differences among animal groups-differences that can potentially be exploited when such groups contain species that cause parasitic diseases or harm agricultural crops.

In addition to Brown, members of the U-M research team include Jeffrey L. Boore, a former U-M graduate student now at the University of Minnesota; David Stanton, a former U-M post-doctoral fellow now at Eastern Michigan University; and U-M researchers Timothy M. Collins and L. Lynne Daehler.

Funding for the research was provided by the National Science Foundation, the National Institutes of Health and the U-M's Rackham School of Graduate Studies.


Rackham School of Graduate Studies