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Monday, Nov. 6, 2000

NATURAL SELECTIONS

Sexual wounding, kicking and early death


Sex can sometimes be awkward in humans, and sometimes painful, but rarely do human females have to put up with what females of the bean weevil endure. The male's penis carries a formidable array of sharp spines which lacerate the female reproductive tract during copulation.

These spines, say Helen Crudgington and Mike Siva-Jothy from the University of Sheffield, are the painful manifestation of reproductive conflict: For bean weevils, this is quite literally the battle of the sexes.

The bean weevil Callosobruchus maculatus is a small black beetle which infests stored grain, beans and pulses. Females lay their eggs on the beans, and the larvae hatch and eat their way inside. The fattened larvae pupate and emerge from the bean as adult beetles. From this time on they don't eat: Their only function is to reproduce.

For males, this means they should mate with as many females as possible. For females the best thing to do is different. Because each male provides them with tens of thousands of sperm, they don't need to mate with males as much as males need to mate with them. This is the root cause of sexual conflict.

The evolutionary outcomes of this conflict are as varied as the animal kingdom itself. In some animals the best thing for a male to do is to defend a harem of females, and try to prevent weaker males from mating. Red deer and elephant seals do this, and so, historically, did some rich human males.

In animals where females mate several times with different males, in order to ensure genetic representation in the next generation, a male must try to circumvent the sperm already stored in the female. This leads to the selective pressure known as sperm competition. In the competition to fertilize a female's eggs any trick which makes the female more likely to use your sperm will be favored by selection.

In a paper in Nature Oct. 19, Crudgington and Siva-Jothy suggest that the prodigious spines on the penis of the bean weevil are adaptations to make the female use more of that male's sperm. After the organ is inserted into the female, the spines, made from the same hard material that the shells of beetles are made from, are turned out in the middle region of the female's reproductive tract -- and start cutting into it.

Toward the end of copulation, the female kicks vigorously at the male with her back legs. The researchers prevented females from kicking by removing their back legs. To control for the possible traumatic effect of removing the legs, in another group of beetles they removed a pair of non-kicking legs. The copulation duration of females prevented from kicking the male was significantly longer than in the control groups.

It seems that females want to reduce copulation duration, and do so by kicking the male, but why? One look at the male's penis immediately suggests an answer: to reduce the damage caused by those spines.

The researchers looked at the reproductive tract of virgin and nonvirgin females. Nonvirgins had regions of scar tissue, but virgins had no wounds. When they looked at the genitals during copulation, they found that sure enough, the spines on the penis penetrated the lining of the female's reproductive tract.

By wounding the female, say the scientists, males might make a female wait longer before she mates again. "If females postpone remating to delay receiving additional genital damage," said Mike Siva-Jothy, "this might increase the number of eggs fertilized by the last male to mate."

Helen Crudgington, a post-graduate student of Siva-Jothy's, added: "Genital damage may increase immediate egg-laying rates because females perceive genital damage as a threat to survival and invest more in current reproduction."

In bean weevils genital wounding caused by copulation is costly, and therefore copulation frequency is likely to be the basis of sexual conflict. Genital wounding caused during copulation reduces female lifespan. The researchers measured the longevity of once- and twice-mated females that were kept in isolation. Twice-mated females died significantly faster than single-mated females.

In many species male traits are more obvious to human observers, and hence better understood than female traits. In red deer, for example, the benefit that a strong male gets from keeping a harem of females is obvious, but it is less obvious how female deer have reacted, over evolutionary time, to this domination by males.

In the bean weevil the female counter-adaptation seems more obvious: during copulation they aim, and repeatedly deliver, thumping kicks to the male.

One cell, two fates

Japanese scientists have discovered that the two types of cell that make up blood vessels develop from the same embryonic stem cell.

The discovery, by Jun Yamashita and colleagues of Kyoto University Graduate School of Medicine, opens the way to treating blood vessel disorders through "tissue engineering." Blood vessel disorders can lead to heart disease, blindness and cancer.

As we grow, our cells become specialized to carry out particular functions in different organs and tissues. But some cells -- stem cells -- retain the potential to develop into any type of cell. It is this potential which medical researchers seek to harness.

Human stem cells are already widely used in cell-replacement therapy, for example, in bone-marrow grafts which are used to treat leukemia patients. But it was thought that the two types of cells which make up blood vessels came from two types of precursor cells.

Yamashita's team describe a single stem cell precursor to the endothelial (vessel-lining) cells and smooth muscle cells which make up the blood vessels of the vascular system. The precursor cells develop, in the culture dish and in mice, into either cell type according to the growth factor they are exposed to.

The findings, reported Nov. 2 in the science journal Nature, "Offer potential for tissue engineering of the vascular system," Yamashita's team said.



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