20 SCIENTIFIC AMERICAN January 1995

recently isolated a netrin gene in thefruit ßy Drosophila. ÒThis shows astrong conservation of biological func-tions between species,Ó Tessier-Lavignenotes.

The researchers studying worms, ßiesand vertebrates plan to collaborate ex-tensively. They are now testing wheth-er netrins and unc-6 function equallywell in vertebrate and invertebrate sys-tems. ÒAside from axonal projections,unc-6 controls the circumferential mi-grations of cells in worms,Ó Tessier-La-vigne adds. ÒSo cell migrations in verte-brate embryos might use netrins asguidance cues.Ó

Such analogies may help the groupanswer other questions as well. The sci-entists have uncovered a slightly small-er protein, dubbed NSA for netrin-syn-ergizing activity, that seems to inßu-ence netrin potency. Perhaps NSA, likecertain proteins in other signaling path-ways, mediates how well the netrinsbind to their receptors. ÒWe really wantto know if NSA is an essential cofactoror not,Ó Tessier-Lavigne states. Also, thenetrins tend to adhere to cell surfacesfor reasons as yet unknown.

What is known is that these novel pro-teins are probably just two words in anentire language of chemical instruc-

tions that direct embryonic develop-ment. Neurobiologists hope to discoverchemicals that can ward oÝ outgrowingaxons and thus prevent them from mak-ing faulty connections. Perhaps otherkinds of cues exist as well. At any rate,Tessier-Lavigne predicts that progresswill be swift because similar chemicalwords seem to speak of the same bio-logical functions in diÝerent species.ÒNow we can go back and forth be-tween diÝerent systems and share ourinsights,Ó he says. For a while, it seemsthe netrins will bring scientists togeth-er as surely as they connect searchingaxons. ÑKristin Leutwyler

Socializing with Non-Naked Mole Rats

Big and hairy, the Damaraland mole rat is not as re-nowned as its hairless cousin. Nevertheless, this specieshas proved just as intriguing as the naked mole rat of zooand cartoon fame. Both forms of mole rat are eusocial—that is, they live in groups in which only a queen and sev-eral males reproduce, whereas the rest of the colony coop-erates to care for the young. This behavior—like that oftermites and ants—is found in very few mammals, and ithas remained a puzzle of natural selection.

By comparing Damaraland and naked mole rats, Jen-nifer U. M. Jarvis and Nigel C. Bennett of the University ofCape Town and others have begun to determine the char-acteristics that appear central to the evolution of eusocial-ity—and hair is clearly not one of them. “The Damaralandis important because it does not have many of the charac-teristics of the naked mole rat,” notes Paul W. Sherman ofCornell University. So it “tells us something that we did notknow.” The degree of genetic relatedness between mem-

bers of a colony, for instance, does not appear as crucialto eusociality as some had believed. In the case of nakedmole rats, siblings raise one another because the survivalof a sister or brother is virtual cloning. A Damaraland col-ony, however, appears much more genetically diverse.Once a queen dies, these mole rats wait to reproduce untilanother female is introduced from somewhere else—atleast in the laboratory.

Instead ecological determinants seem more significantto eusociality. Both Damaraland and naked mole rats livein arid regions where the food supply, underground tu-bers, is sparse and rainfall unpredictable. Cooperative liv-ing ensures finding these precious resources—solitary an-imals would be unable to tunnel extensively enough to lo-cate adequate sustenance. The other, noneusocial formsof mole rats live in regions where food is more readilyavailable. In other words, the more patchy the food, themore the cooperation. —Marguerite Holloway

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Copyright 1994 Scientific American, Inc.