The cross-breeding of different species is, with a few
exceptions — think ligers and mules — generally a fruitless
exercise. For worms though, it can prove deadly.
Zoologists from the Universities of Maryland and Toronto have
discovered that when they mated worms of different species, the
lifespan of the female worms and their offspring was dramatically
reduced. Even those that survived the cross-species mating were
usually found to be sterile if they then mated with a male specimen
of the same species.
When the researchers examined the dying female worms under
microscopes they saw that the sperm from the foreign males could
break through the sphincter of the worm’s uterus and into the
ovaries themselves. Once they were inside the ovaries they would
fertilise the female worm’s eggs prematurely, leaving them unable
to develop. The destruction of the ovaries left the worms
But the vicious sperm would not stop there. Once the ovaries
were done for, they would often start to invade the rest of the
female worm’s body, resulting in tissue damage and then death.
So how does normal worm sperm turn into killer worm sperm? The
researchers believe that the answer lies in the divergence of
sexual organ development in different worm species. Worm sperm is
feisty at the best of times as it often has to compete with the
sperm of multiple worms — depending on how many males a female has
mated with — in order to gain access to the eggs. The females are
usually able to cope physically with the sperm fight happening
inside them, in order to go on to produce offspring. But this
tolerance, as well as the aggressiveness of the male worms’ sperm,
seems to differ from species to species.
This means that a female from one species is only equipped with
the aggressive nature of the sperm from her species, so if she
mates with a worm from a different species with more aggressive
sperm, she is pretty doomed.
To make things even more complicated, several species of worm
are hermaphrodites and fertilise their own eggs with their own
sperm, and these worms were far more susceptible to death and
sterility by killer sperm if they mated with a male of a different
species. Hermaphrodite worms generally try and avoid males of other
species as a result.
“The findings may be worth investigating in other species as
well, because similar coordination problems may be relevant to
infertility in other organisms,” said Eric Haag, associate professor of biology at the
University of Maryland. “Punishing cross-species mating by
sterility or death would be a powerful evolutionary way to maintain
a species barrier.”
The research was published today in the journal PLOS Biology and can be read in full here. In a
followup study, the researchers will test whether the occasional
offspring that is produced as a result of this cross-breeding is
capable of breeding with other species itself.