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Sex-free shark birth startles scientists, and worries them

May 22, 2007
Courtesy Queen's University Belfast
and World Science staff

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.

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

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.

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 un­der­pop­u­lated zones, he said. “This could has­ten the ero­sion of popula­t­ion ge­net­ic di­vers­ity and per­pet­u­ate the pro­duc­tion of ge­net­ic­ally dis­ad­van­taged off­spring.”

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Scientists say a female hammerhead shark gave birth without having sex—the first scientific report that this ancient lineage of fish can reproduce asexually, or sexlessly. The development startled scientists, and worried them. Asexual reproduction, especially of the type involved in this case, leaves babies “genetically disadvantaged” because they lack 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—a few reptiles, birds and amphibians and a more modern lineage of fish known as teleosts. The discovery 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. It also suggests asexual reproduction evolved in vertebrates much longer ago than previously thought, Mahmood Shivji of Nova Southeastern University in Fort Lauderdale, Florida, a member of the research team. That opens up the possibility that a wider range of modern species, possibly even mammals, retain the capability, he added. “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 are, rays and skates are members of the lineage 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 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 shark’s DNA only matched hers—no paternal DNA was found. “Parthenogenesis is the likely explanation behind the anecdotal but increasing observations of other species of female sharks reproducing successfully in captivity despite not having contact with males,” said Shivji. So it seems at least some female sharks can switch from sexual to asexual reproduction in the absence of males—which isn’t good, he added, since the offspring lack helpful genetic variation that would come from a father’s DNA contribution. Worse, the researchers found that the most likely form of asexual reproduction involved was a specific type called “automictic parthenogenesis,” in which half of 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 published May 23 in the research journal Biology Letters. “During 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 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 Nova Southeastern. “Female sharks might reproduce like this more often when they have difficulty finding mates” in underpopulated zones, he said. “This could hasten the erosion of population genetic diversity and perpetuate the production of genetically disadvantaged offspring.”