Sex-free shark birth startles scientists, and worries them
May 25,2007 00:00 by Bend_Weekly_News_Sources

Sci­en­tists say a fe­male ham­mer­head shark gave birth with­out hav­ing sex—the first sci­en­tif­ic re­port that an an­cient line­age of ver­te­brates can re­pro­duce asex­u­al­ly, or with­out sex.

The de­vel­op­ment star­tled sci­en­tists, and wor­ried them. Asex­u­al re­pro­duc­tion, es­pe­cially of the type in­volved in this case, leaves ba­bies at a “ge­netic dis­ad­van­tage” due to lack of ge­net­ic di­vers­ity, one said. This would place ex­tra bur­dens on al­ready threat­ened shark popula­t­ions.

Among ver­te­brates, asex­u­al re­pro­duc­tion is known only in very few spe­cies: some rep­tiles, birds and am­phib­ians, and a few mem­bers of a re­la­tive­ly mod­ern line­age of fish known as te­le­osts.

A­sex­u­al re­pro­duc­tion was found in a type of ham­mer­head shark spe­cies known as the bon­net­head, or Sphyr­na ti­bu­ro. (Im­age cour­te­sy D. Chap­man)

The shark surprise leaves “mam­mals as the only ma­jor ver­te­brate group where this form of re­pro­duc­tion has not been seen,” said Pau­lo Pro­döhl of Queen’s Un­ivers­ity Bel­fast, one of the re­search­ers. 

Its occurrence in sharks al­so sug­gests asex­u­al re­pro­duc­tion evolved early in the ver­te­brate line­age, said co-re­search­er Mah­mood Shiv­ji, dir­ec­tor of the Guy Har­vey Re­search In­s­ti­tute in Da­nia Beach, Fla.

“As far as an­y­one knew, all sharks re­pro­duced only sex­u­ally by a male and fe­male mat­ing, re­quir­ing the em­bry­o to get DNA from both par­ents” as in mam­mals, Pro­döhl said. Sharks, rays and skates are mem­bers of the an­cient line of car­ti­lag­i­nous fish­es, de­scended al­most di­rectly from some of the first an­i­mals with jaws.

A sur­prise ham­mer­head birth in 2001 at an aquar­i­um at Hen­ry Doorly Zoo in Oma­ha, Neb. prompted the shark stu­dy. None of three pos­sible moth­er ham­mer­heads in the tank, of the spe­cies Sphyrna tiburo, had en­coun­tered any male ham­mer­heads since be­ing caught off Flor­i­da three years ear­li­er as ba­bies.

Sci­en­tists in­i­tially guessed a moth­er had mat­ed be­fore cap­ture, and then some­how stored the sperm; or pos­sibly mat­ed with a male shark of an­oth­er spe­cies in the tank. But af­ter iden­ti­fy­ing the moth­er through ge­net­ic tests, they found the ba­by’s DNA matched only hers; no pa­ter­nal DNA was found.

Asexual reproduction—or par­the­no­gen­e­sis, as it’s called in ver­te­brates—is “the likely ex­plana­t­ion be­hind the an­ec­do­tal but in­creas­ing ob­serva­t­ions of oth­er spe­cies of fe­male sharks re­pro­duc­ing suc­cess­fully in cap­ti­vity” with­out male con­tact, said Shivji. Re­search­ers don’t think par­th­e­n­o­gen­e­sis takes place in mam­mals due to a mech­an­ism called ge­no­mic im­print­ing that oc­curs in them, Shivji said.

In sharks, he added, it seems at least some fe­males can switch from sex­u­al to asex­u­al re­pro­duc­tion in the ab­sence of ma­les—which is­n’t good, since the off­spring lack help­ful ge­net­ic varia­t­ion that would come from a fa­ther’s DNA con­tri­bu­tion.

Worse, the re­search­ers found that the par­the­no­gen­e­sis in this case was pro­bab­ly of a spe­ci­fic type called “au­to­mic­tic,” in which half the moth­er’s ge­net­ic di­vers­ity al­so gets lost.

The ba­by “gets a double dose of ge­net­ic dis­ad­van­tage,” said Demian Chap­man, lead au­thor of a study on the find­ing to be pub­lished May 23 in the re­search jour­nal Bi­ol­o­gy Let­ters. In this pro­cess “the un­fer­ti­lized egg, which con­tains about half of the moth­er’s ge­net­ic di­vers­ity, is ac­ti­vat­ed to be­have as a nor­mal fer­ti­lized egg by a small, ge­net­ic­ally nearly-identical cell known as the sis­ter po­lar body.”

The find­ing raises con­cerns about the ge­net­ic and re­pro­duc­tive health of dwindling shark popula­t­ions, added Chap­man. He is now head of shark re­search at the New York-based Pew In­sti­tute for Ocean Sci­ence but took part in the study as a grad­u­ate stu­dent at Guy Har­vey. “Female sharks might re­pro­duce like this more of­ten when they have dif­fi­cul­ty find­ing mates” in under populated zones, he said. “This could has­ten the ero­sion of popula­t­ion ge­net­ic di­vers­ity and per­pet­u­ate the pro­duc­tion of ge­net­ic­ally dis­ad­van­taged off­spring.”

Courtesy Queen's University Belfast and World Science staff