If you thought that killing for love was just a phrase, then think again as the interbreeding of Hawaiian and Bristol strains of a nematode worm, Caenorhabditis elegans, might result in the death of their offspring.
This interbreeding may lead to a genetic incompatibility killing a quarter of their offspring. The genes responsible for this seem to be retained by natural selection.
The study led by Leonid Kruglyak, found that embryos of C. elegans that miss one particular gene, called zeel-1, die early in development if their male parent delivered a metaphorical bomb into their embryotic life.
This bomb is in the form of a compound produced from an incompatible version of another gene, peel-1, in his sperm, reports Nature magazine.
The research team led by Hannal Seidel at Princeton University in New Jersey, discovered this rare trait by mistake, when they crossed worms for a different experiment.
When they found that one in four embryos was dying, they suspected some genetic phenomenon. This led them to work on matching up different worms with each other to see what had happened to the offspring.
Usually the worm C. elegans is hermaphroditic, and reproduces on its own. But there are times when a true male offspring (never a female) is produced by one of these worms. This particular male can later have sex with another worm. This occurs once in 1,000 generations.
While the missing gene zeel-1 making the embryos sensitive to an unknown molecule that stops development comes from the Hawaiian strain, the Bristol strain releases the gene for the killer sperm comes.
Thus interbreeding between Bristol males and Hawaiian hermaphrodites might be fatal. Both types of worm are found together all over the world.
Kruglyak said that it is the rarity of sex that enables the lethal gene to perpetuate, instead of fading away.
According to the authors, some unknown benefit to the genes involved may cancel out the reproductive costs involved in carrying the gene.
"It would have to be a big advantage because of the lethality," the mag quoted Patrick Phillips, of the University of Oregon in Eugene, who also studies C. elegans evolution, as saying.