By Victoria Gill
Science and nature reporter, BBC News
The worms, seen here under the microscope, are 1mm long
Experiments on tiny nematode worms have revealed why some creatures produce far more female than male offspring.
Male-producing sperm in this Rhabditis nematode worm are what the researchers described as "duds".
These sperm are paralysed - unable to move and therefore unable to to fertilise eggs.
Producing an "excess" of daughters enables these humble, very short-lived creatures to rapidly produce large numbers of offspring.
The findings are described in the journal Nature Communications.
The US-based researchers, Diane Shakes from the College of William & Mary in Williamsburg and Andre Pires da Silva from the University of Texas at Arlington, began studying the sperm of these strange little worms to find out how the species evolved three sexual forms - females, males, and self-fertile hermaphrodites.
By physically dissecting the sex organs of these 1mm-long creatures, the researchers were able to watch the sperm cells under the microscope as they divided, copying themselves to produce the millions of sperm cells required to ensure successful fertilisation.
In humans, sperm cells have either an X or a Y sex chromosome; if an X-sperm fertilises an egg, this produces a female (with two X chromosomes). But if a Y-sperm reaches the egg first, a male results.
The worms, by contrast, have X and O chromosomes - but the process is basically the same.
Watching the worm sperm cells as they divided, the researchers noticed that all of the materials needed to make functional, mobile sperm were "dragged into" to the X chromosome-bearing sperm.
"The non-X sperm - that would have been capable of yielding sons - are 'duds'", explained Professor Shakes.
Scientists were able to see the chromosomes (blue) dividing
"If they can't move, they can't fertilise eggs. Another way to think about it is that the non-X sperm are just thrown away."
She added: "We can now explain the cellular basis of why these males sire so few male offspring."
The precarious existence of these worms could explain why they have evolved such an unusual reproductive strategy.
Because of the discrete nature of the bacterial patches on which the worms feed, the worm populations fluctuate. The constant "boom and bust can lead to high levels of inbreeding," explained Professor Shakes.
Inbred offspring are usually less healthy and more likely to have genetic defects.
A mostly female population, or what the researchers described as a "uniparental solution" - such as self-fertilising hermaphrodites - help counterbalance this genetic disadvantage by "maximising the reproductive capacity of the individual worms", Professor Shakes said.