Media Releases

“Killer sperm” prevents mating between worm species

July 30, 2014

TORONTO, ON — The clas­sic def­i­n­i­tion of a bio­log­i­cal species is the abil­i­ty to breed with­in its group, and the inabil­i­ty to breed out­side it. A study pub­lished today in the jour­nal PLOS Biol­o­gy offers some impor­tant clues about the evo­lu­tion of bar­ri­ers to breed­ing.

The vast major­i­ty of the time, mat­ing across species is mere­ly unsuc­cess­ful in pro­duc­ing off­spring, though there are excep­tions. Breed­ing a horse and a don­key, for exam­ple, may result in a live mule off­spring, but mules are near­ly always ster­ile due to genom­ic incom­pat­i­bil­i­ty between the two species.

How­ev­er, when researchers and lead co-authors Jan­ice Ting at the Uni­ver­si­ty of Toron­to (U of T) and Gavin Woodruff at the Uni­ver­si­ty of Mary­land mat­ed Caenorhab­di­tis worms of dif­fer­ent species, they found fur­ther vari­a­tions on bar­ri­ers. The lifes­pan of the female worms and their num­ber of prog­e­ny were dras­ti­cal­ly reduced com­pared with females that mat­ed with the same species. In addi­tion, as with mules, the females that sur­vived cross-species mat­ing were often ster­ile, even if they sub­se­quent­ly mat­ed with their own species.

“We observed the mat­ed females under a micro­scope and by using a flu­o­res­cent stain to visu­al­ize sperm in live worms, we dis­cov­ered that the for­eign sperm had bro­ken through the sphinc­ter of the worm’s uterus and invad­ed the ovaries,” said Ting, a PhD can­di­date in the Depart­ment of Ecol­o­gy & Evo­lu­tion­ary Biol­o­gy at U of T. There, the sperm pre­ma­ture­ly fer­til­ized the eggs, which were then unable to devel­op into viable off­spring. The sperm even­tu­al­ly destroyed the ovaries, result­ing in steril­i­ty, and then trav­eled far­ther through­out the worm’s body, result­ing in tis­sue dam­age and death.

“Our find­ings were quite sur­pris­ing because females typ­i­cal­ly just select sperm from males of their own species dur­ing fer­til­iza­tion, an action that does not lead to long-term con­se­quences because there is no gene flow between the species,” said Ash­er Cut­ter, asso­ciate pro­fes­sor of ecol­o­gy and evo­lu­tion­ary biol­o­gy at U of T and co-inves­ti­ga­tor of the study.

TORONTO, ON — The results sug­gest the inter­ac­tion between sperm and the female repro­duc­tive tract as a nov­el rea­son for failed mat­ing in worms, not­ed Uni­ver­si­ty of Mary­land co-inves­ti­ga­tor and asso­ciate pro­fes­sor of biol­o­gy Eric Haag. “The find­ings may be worth inves­ti­gat­ing in oth­er species as well, because sim­i­lar coor­di­na­tion prob­lems may be rel­e­vant to infer­til­i­ty in oth­er organ­isms,” he added.

The researchers believe the “killer sperm” may be the result of a diver­gence in the evo­lu­tion of worm species’ sex­u­al organs—in par­tic­u­lar, the abil­i­ty of sperm to phys­i­cal­ly com­pete with one anoth­er. When a female worm mates with mul­ti­ple males, the sperm jos­tle each oth­er, com­pet­ing for access to the eggs. Female worms’ bod­ies must be able to with­stand this com­pe­ti­tion to sur­vive and pro­duce off­spring. The researchers hypoth­e­size that the aggres­sive­ness of the sperm and the abil­i­ty of the uterus to tol­er­ate the sperm are the same with­in a sin­gle species, but not across mul­ti­ple species. Thus, a female from a species with less active sperm may not be able to tol­er­ate the aggres­sive sperm from a dif­fer­ent species.

There is evi­dence for this the­o­ry. In the cur­rent study, three species of her­maph­ro­dite worms—which pro­duce their own sperm and fer­til­ize their own eggs to reproduce—were espe­cial­ly sus­cep­ti­ble to steril­i­ty and death when mat­ed with males of oth­er species. The her­maph­ro­dite uterus may have evolved to tol­er­ate “gen­tler” sperm, but not the larg­er, more active sperm of non-her­maph­ro­dite species, accord­ing to the researchers.

“We found that her­maph­ro­dites can sense, and try to avoid, males of species that can harm them,” added Haag.

This instance of lethal cross-species mat­ing is of spe­cial inter­est to evo­lu­tion­ary biol­o­gists, Haag notes, because it’s unclear how the many species on earth—8.7 mil­lion, not count­ing bac­te­ria, accord­ing to an esti­mate pub­lished in Nature—remain dis­tinct from each oth­er.

“Pun­ish­ing cross-species mat­ing by steril­i­ty or death would be a pow­er­ful evo­lu­tion­ary way to main­tain a species bar­ri­er,” Haag said.

The research is described in a study titled “Intense Sperm-Medi­at­ed Sex­u­al Con­flict Pro­motes Repro­duc­tive Iso­la­tion in Caenorhab­di­tis Nema­todes“, pub­lished July 29 in PLOS Biol­o­gy. It was sup­port­ed by fund­ing from the Nat­ur­al Sci­ences and Engi­neer­ing Research Coun­cil of Cana­da and the Nation­al Insti­tutes of Health.

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Note to media: Vis­it and for images and an ani­ma­tion illus­trat­ing the research described here.


Jan­ice Ting
Depart­ment of Ecol­o­gy and Evo­lu­tion­ary Biol­o­gy
Uni­ver­si­ty of Toron­to

Ash­er Cut­ter
Depart­ment of Ecol­o­gy and Evo­lu­tion­ary Biol­o­gy
Uni­ver­si­ty of Toron­to

Sean Bet­tam
Com­mu­ni­ca­tions, Fac­ul­ty of Arts & Sci­ence
Uni­ver­si­ty of Toron­to