Is Picking a Mate Just Genetics?

April 15, 2009 — -- So after looking for years you finally found your perfect mate. Was it good judgment on your part, helped along by a lot of romance, or was it just a case of cold genetics?

It may well be that your genes, not your superior taste when it comes to the opposite sex, made the choice for you. But even your genes can get it wrong. At least if you are a fruit fly.

A team of scientists at Cornell University in Ithaca, N.Y., and the University of Queensland in Brisbane, Australia, have been trying for a number of years to figure out the role genes play in mate selection.

Of course, they would prefer to study the same thing in humans, but there seems to be an ethical problem with having people mate with strangers to produce children for scientific research.

So for now, fruit flies will have to do. These amazing creatures share many genes with higher life forms, including humans, and they go through so many generations in such a short period of time that they are very useful for research. And they're cheap, fortunately, because their lifespan is only a couple of weeks.

In their latest experiment, the researchers used two different strains of fruit flies that have lived in separate and controlled environments in their lab for 15 years.

Even Genes Can Choose Mr. Wrong

Would the flies from one strain prefer to mate with members of the other strain, instead of their closer kin, which can lead to all sorts of genetic problems? And would genes play a role in their decision?

"All the genes that the male had and all the genes that the female had somehow worked together in order to create the reproductive traits that ultimately happened during mating," said Lisa A. McGraw, the lead author of a study in the current issue of the journal Genetics.

She said it's clear that genes were the dominant factor, partly because "all of the females seemed to prefer one strain of males." Oddly enough, the strain they preferred turned out not to be the most successful males for procreation.

So their genes may have picked their mates, but the girls ended up with the wrong guys, reproductively speaking.

"It's not what you would predict," McGraw said.

Nice Guys Finish Last

For their experiment, the scientists picked a bunch of "virgin" flies from each strain, and each male was treated with a microscopic droplet of dye for later identification, blue for one strain and red for the other. Males and females from both strains were put in a "small mating chamber" made of clear plastic, affording the researchers a front-row view of the action.

All their genes must have been at work, because the girls overwhelmingly wanted the guys from one strain, and pretty much ignored the other strain. But the relatively few females that ended up with the rejected males "laid significantly more eggs" and produced more progeny, according to the study, so the best guys were chosen last.

"That's not a directionality that you would predict," McGraw said, because genes are supposed to facilitate finding the best mate for procreation of the species.

Females 'Poised' for Reproduction Even Before Meeting Males

Furthermore, the females had gone through genetic changes even before they mated that were expected to occur after mating. Sperm contains proteins that are supposed to trigger a number of changes in the female, including ovulation and the preparation of areas to store the sperm. The males, according to the study, had "negligible effect" on the genetic changes in the females.

The study concludes that the females were "poised" for reproduction even before they were turned over to the males.

There's one other thing that isn't clear. The scientists admit they aren't real sure exactly what happened in the "mating chamber."

It's "not possible to distinguish" whether the females really made the decision to yield to the male members of the desired strain, or whether the lucky males were more aggressive than the males in the other strain, they said.

Whatever, the geneticists are pretty sure they captured genes at work.

"Our research helps to shed light on the complex biochemistry (genetics) involved in mate selection and reproduction," said Mariana Wolfner, professor of developmental biology at Cornell and senior scientists on the project.

She added that the research may help scientists develop ways to control unwanted insect populations by activating or deactivating genes "that play a role in female mating decisions."

The fruit fly findings probably apply to other animals as well, but to humans? That's a "long stretch," McGraw said.

A human life is much more complex than that of a fruit fly, and even if fruit flies are unthinking robots when it comes to picking a mate, humans probably have more control over the process.

That doesn't necessarily mean we always get it right, of course.