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Scientists report flipping worms’ sexual preferences

Oct. 25, 2007
Courtesy University of Utah
and World Science staff

Bi­ol­o­gists have ge­net­ic­ally tweaked round­worms to make them at­tracted to worms of the same sex—part of a stu­dy, they say, that shows sex­u­al ori­enta­t­ion is ge­neti­cally wired in the crea­tures’ brains.

The nematode worm C. elegans. (Courtesy free.ed.gov)


Sex pref­er­ence seems to arise in “brain cir­cuits com­mon to both sexes of worms,” said Un­ivers­ity of Utah bi­ol­o­gist Er­ik Jor­gensen, one of the re­search­ers in the wide-rang­ing stu­dy. 

“We can­not say what this means for hu­man sex­u­al ori­enta­t­ion, but it raises the pos­si­bil­ity that sex­u­al pref­er­ence is wired in the brain… Hu­mans are sub­ject to ev­o­lu­tion­ary forc­es just like worms.”

The work ech­oes re­search with fruit flies in 2005, in which the flip of a ge­net­ic switch in­duced fe­males to make am­o­rous dis­plays to­ward oth­er fe­males. But there were sev­er­al dif­fer­ences.

The millimeter-long round­worm C. el­e­gans lives in soil and eats bac­te­ria. Like fruit flies, it’s of­ten used in bas­ic ge­net­ic re­search, as many of its genes are thought to be re­lat­ed to hu­man genes. The worm lacks eyes, so at­trac­tion is based on smell. There are no true fe­males and only one in 500 is ma­le. Most are her­maph­ro­dites, with both male and fe­male or­gans. Jor­gensen and White loosely re­fer to her­maph­ro­dites as fe­males be­cause they pro­duce off­spring.

“A her­maph­ro­dite makes both eggs and sper­m,” and can thus fer­ti­lize it­self, though this cre­ates few­er off­spring, Jor­gensen said. The abil­ity probably evolved be­cause round­worms are few and far be­tween in soil, so mates aren’t al­ways avail­a­ble, he added. Males, though, must find her­maph­ro­dites to repro­duce, and find them by smell.

“We took the her­maph­ro­dite brain and we ac­ti­vat­ed the genes that de­ter­mine male­ness,” but only in the brain, Jor­gensen said. The re­sult: “They look like girls, but act and think like boys,” said bi­ol­o­gist Ja­mie White of the un­ivers­ity. White is first au­thor of a pa­per on the find­ings to ap­pear in the Nov. 6, and the Oct. 25 ad­vance on­line, is­sues of the re­search jour­nal Cur­rent Bi­ol­o­gy.

The ar­ti­fi­cially ac­ti­vat­ed gene, called fem-3, nor­mally makes the body de­vel­op male struc­tures such as a tail, which male worms use to cop­u­late, White said. With fem-3 switched on only in the brain, her­maph­ro­dites kept their bod­ies un­changed; but they were at­tracted to oth­er her­maph­ro­dites, crawl­ing to­ward their sex­u­al chem­i­cal sig­nals, the in­ves­ti­ga­tors said.


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Biologists have genetically tweaked roundworms to make them attracted to worms of the same sex—part of a study, they say, that shows sexual orientation is wired in the creatures’ brains. The preference seems to arise in “brain circuits common to both sexes of worms,” said University of Utah biologist Erik Jorgensen, one of the researchers in the wide-ranging study. “We cannot say what this means for human sexual orientation, but it raises the possibility that sexual preference is wired in the brain… Humans are subject to evolutionary forces just like worms.” The work echoes research done with fruit flies in 2005, in which female flies were induced to make amorous displays toward other females with the flip of a genetic switch. But there were several differences. The millimeter-long roundworm C. elegans lives in soil and eats bacteria. Like fruit flies, it’s often used in basic genetic research, as many of its genes are thought to be related to human genes. The worm lacks eyes, so attraction is based on smell. There are no true females and only one in 500 is male. Most are hermaphrodites, with both male and female organs. Jorgensen and White loosely refer to hermaphrodites as females because they produce offspring. “A hermaphrodite makes both eggs and sperm,” and can thus mate with herself, though this results in fewer offspring, Jorgensen said. The ability probably evolved because roundworms are few and far between in soil, so mates aren’t always available, he added. Males, though, must find hermaphrodites to reproduce, and find them by smell. “We took the hermaphrodite brain and we activated the genes that determine maleness,” but only in the brain, Jorgensen said. The result: “They look like girls, but act and think like boys,” said biologist Jamie White of the university. White is first author of a paper on the findings to appear in the Nov. 6, and the Oct. 25 advance online, issues of the research journal Current Biology. The artificially activated gene, called fem-3, normally makes the body develop male structures such as a tail, which male worms use to copulate, White said. With fem-3 switched on only in the brain, hermaphrodites kept their bodies unchanged; but they were attracted to other hermaphrodites, crawling toward their sexual chemical signals, the investigators said.