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How one microscopic creature juggles seven sexes

March 27, 2013
Courtesy of Public Library of Science
and World Science staff

It’s been more than 50 years since sci­en­tists found out that a one-celled or­gan­ism called Tetrahy­mena ther­mo­phila has sev­en sexes. But they’ve nev­er known how na­ture de­ter­mines each cell’s sex, or “mat­ing type.”

Now they do, ac­cord­ing to a re­search re­port.
Bi­ol­o­gists said they iden­ti­fied both the genes that de­cide the mat­ing type, and an un­usu­al pro­cess of DNA re­ar­range­ments needed to de­ter­mine it. The find­ing could have med­i­cal im­plica­t­ions, they added.

Tetrahy­mena ther­mophila, with col­ors added to br­ing out fea­tures more clear­ly. One of the two nu­clei is shown in blue. (Cour­te­sy of Eduar­do Orias)


A se­ries of “cut and paste” events in which DNA is moved around, called re­com­bina­t­ion, ran­domly de­ter­mines the sex of each mi­cro­sco­p­ic, fuzzy-looking Te­tra­hy­me­na cell, the study found. The pro­cess re­sults in the as­sembly of one com­plete gene pair and the de­le­tion of oth­ers.

“We found a pair of genes that have a spe­cif­ic se­quence which is dif­fer­ent for each mat­ing type,” ex­plained Ed­uar­do Orias of the Uni­vers­ity of Cal­i­for­nia San­ta Bar­ba­ra, a mem­ber of the re­search team. “They are very si­m­i­lar genes—clearly re­lat­ed to one an­oth­er, go­ing back probably to a com­mon an­ces­tor—but they have be­come dif­fer­ent. And each is dif­fer­ent in a spe­cif­ic way that de­ter­mines the mat­ing type.”

Each Te­tra­hy­me­na, a rel­a­tive of the bet­ter-known Par­a­me­ci­um, has two DNA-con­tain­ing nu­clei, called the “germline nu­cle­us” and the “so­matic nu­cle­us.” The first serves to store ge­net­ic in­forma­t­ion for prog­e­ny cells, like ovaries. The sec­ond con­tains the code that serves for the or­gan­ism’s day to day use—the “work­ing copy” of the ge­nome.

When the crea­tures mate, fer­til­iz­a­tion re­sults in two new nu­clei built us­ing pieces from the germline nu­cle­us of each par­ent. No, it does­n’t take sev­en cells to mate—just two, but they must be of dif­fer­ent mat­ing types. Al­so like peo­ple, Te­tra­hy­me­na has two cop­ies of each gene.

The re­search­ers found that the germ­line nu­cle­us con­tains an array of in­com­plete gene pairs—one for each mat­ing type. In the new so­mat­ic nu­cle­us, a com­plete gene pair is built when DNA seg­ments from each end of this ar­ray are fused to one of the six incom­plete pairs, and the re­main­ing five are de­let­ed. This pre­cise but ran­dom re­ar­range­ment leaves the new cell with one gene pair and one mat­ing type.

The study was pub­lished March 26 on­line in the jour­nal PLoS Bi­ol­o­gy.

“The mat­ing type of the ‘par­ents’ has no in­flu­ence what­so­ev­er on the sex of the prog­e­ny,” Orias said. It’s “as if they had rou­lette wheel with six num­bers and wher­ev­er the mar­ble ends up is what they get. By chance they may have same mat­ing type as the par­ents—but it’s only by chance.”

“By un­der­stand­ing this pro­cess bet­ter in Te­tra­hy­me­na, what we learn ul­ti­mately may be of use in medicine,” he added. “Te­tra­hy­me­na has about as many genes as the hu­man ge­nome. For thou­sands of those genes you can rec­og­nize the se­quence si­m­i­lar­ity to cor­re­spond­ing genes in the hu­man ge­nome with the same bi­o­log­i­cal func­tion.”


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It’s been more than 50 years since scientists found out that a one-celled organism called Tetrahymena thermophila has seven sexes. But they’ve never known how each cell’s sex, or “mating type,” is determined. Now they do, according to a new research report. Biologists said they identified both the genes that decide the mating type, and an unusual process of DNA rearrangements needed to determine each new cell’s mating type. The finding could have medical implications, they reported. A series of “cut and paste” events in which DNA is moved around, called recombination, randomly determines the sex of each fuzzy-looking Tetrahymena cell, the study found. The process results in the assembly of one complete gene pair and the deletion of others. “We found a pair of genes that have a specific sequence which is different for each mating type,” explained Eduardo Orias of the University of California Santa Barbara, part of the research team. “They are very similar genes—clearly related to one another, going back probably to a common ancestor—but they have become different. And each is different in a specific way that determines the mating type of the cell.” Each one-celled Tetrahymena, a relative of the better-known Paramecium, boasts two nuclei, called the “germline nucleus” and the “somatic nucleus.” The first serves to store genetic information for progeny cells, like ovaries. The second contains the code that serves for the organism’s day to day use—the “working copy” of the genome. When the creatures mate, fertilization results in two new nuclei built using pieces from the germline nucleus of each parent. No, it doesn’t take seven cells to mate—just two, but they must be of different mating types. Also like people, Tetrahymena has two copies of each gene. The researchers found that the germline nucleus contains a group of similarly organized but incomplete gene pairs—one for each mating type (although Tetrahymena have seven sexes, the particular cell line used in this study has just six). In the new somatic nucleus, they found, one complete gene pair is built when DNA segments from each end of this array are fused to one of the six incomplete pairs, and the remaining five are deleted. This precise but random rearrangement leaves the new cell with one gene pair and one mating type. The study was published March 26 online in the journal PLoS Biology. “The mating type of the ‘parents’ has no influence whatsoever on the sex of the progeny,” Orias said. “It’s completely random, as if they had roulette wheel with six numbers and wherever the marble ends up is what they get. By chance they may have same mating type as the parents—but it’s only by chance. It’s a fascinating system.” “By understanding this process better in Tetrahymena, what we learn ultimately may be of use in medicine,” he added. “Tetrahymena has about as many genes as the human genome. For thousands of those genes you can recognize the sequence similarity to corresponding genes in the human genome with the same biological function.”