Parasitic Males Go Spermless in Absence of Females

by VR Sreeraman on Nov 13 2008 7:00 PM

A species of parasitic nematode worm doesn't make any sperm when females are not around, according to a new study.

Earlier, scientist believed that sperm production is quite similar to the production of eggs in females, but recently the notion has been challenged in animals that need to produce a lot of sperm or particularly large sperm.

For example, males of some other species of rodents, fish, and insects reduce the sperm count depending on the social circumstances. But they generally don't go without.

"This is very unusual for a male to need a female to be present before he produces sperm. We searched the literature but could find no report of this in any other animal. Animals have evolved all sorts of strategies, some quite bizarre, to increase their lifetime reproductive success, but this particular one does not appear to be common," said Christine Griffin of the National University of Ireland, Maynooth.

Lifetime reproductive success refers to the number of offspring produced over the course of a lifetime.

However, till date, researchers haven't shown how this behaviour in the parasitic nematodes, called Steinernema longicaudum benefits the males of the species.

In fact, they said that the newly discovered behaviour makes some sense in light of the worms' unusual life history.

Most animals can move around in search of a mate, and so should be ready to make the most of any opportunities that present themselves. Like many parasites, Steinernema enter their host insect when they are still juvenile and develop inside. A male that finds himself alone cannot leave the insect to search for a mate," explained Griffin.

As it is only juvenile worms invade insects, the single males just have to wait until a young mating prospect joins him and grows up. Thus, in such a scenario a mating partner can't appear suddenly, leaving the male no reason to be sexually mature.

The researchers made the discovery while studying the reproductive behaviour of the nematode and were surprised to find that it took some time before successful mating occurred between a pair of worms placed together.

"Initially, we thought that perhaps the female was unreceptive and/or immature, as this would be the more normal situation in animals. However, further experiments showed that it was the male that was unready. We then took a closer look and discovered to our great surprise that the reproductive tract of naive males was smaller than that of males that had been with a female for some time, and it contained no sperm," said Griffin.

The investigators hypothesised that the signal from females to mature may be chemical in nature, as lone males matured despite the fact that females were placed on the other side of a permeable barrier.

However, the researchers still don't know if the pheromones that stimulate male maturity are the same as those that sexually attract him to females.

The study is published in the latest issue of Current Biology, a Cell Press publication.