'Can people choose a partner that's better than random sex?'

‘Test tube orgy’: Israeli scientists use yeast to assess humans’ mating preferences

Weizmann Institute researchers find that yeast born to preferred partners are fitter than those from one-night stands, showing how we choose mates who will produce healthiest offspring

Inspired by tests that asked women to choose mates solely by smelling their sweaty T-shirts, scientists from the Weizmann Institute are using yeast to probe how we and other living creatures choose the sexual partners most likely to give us healthy offspring.

More than 30 percent of yeast’s genetic makeup shares ancestry with humans, and like humans, this microscopic fungus needs two to produce offspring.

“You can put billions of yeast cells into a single test tube and allow them to mate like in a huge orgy,” explained Prof. Yitzhak Pilpel, who runs a laboratory in the molecular genetics department of the Weizmann Institute of Science in central Israel. “Within a day, you will have more mating events than it would take the world’s human population to achieve over decades.”

Pilpel’s inspiration came from several small experiments undertaken elsewhere over the years, which asked groups of women to rank different men’s sweaty T-shirts for sexual attractiveness. The results showed that different women preferred different men. DNA sequencing revealed that they tended to choose males who were not too genetically similar.

“Each had the ‘wisdom’ to highly rank a male at an intermediate genetic distance, as if this is an optimal choice,” said Pilpel. “But is it optimal?”

Without knowing the chemical composition of the sweat, he added that he could not say whether the magic ingredient guiding the women was pheromones.

Back in the Pilpel lab, the researchers took some 10 million cells from roughly 100 different strains of common bread yeast (Saccharomyces cerevisiae) and put them in test tubes both with and without oxygen, where they could mingle freely and reproduce over several generations.

As yeast cells look identical, the team used genetic engineering techniques to implant a genetic barcode into each cell via a chemical reaction. The barcodes distinguished between gender (yeast doesn’t have males or females) and strain. Barcodes for the offspring carried both parents’ details, much as couples often combine their surnames after marriage.

Once the mating-tracking program was converted into a computerized DNA sequencing program, the team was able to analyze mating preferences and to see which couplings produced the healthiest offspring.

“Each ‘female’ [cell from a single strain] has many copies,” said Pilpel. “While ‘she’ might have preferred a ‘male’ from strain one, she might also occasionally have had sex with strains two, three, and four, for example.”

This enabled the scientists to assess whether the offspring she had with her usual partner were fitter than those resulting from one-night stands.

The team found that in the test tubes without oxygen — oxygen creates stress for yeast, Pilpel explained — the health of the offspring reflected that of the parents, regardless of the genetic distance between the latter. Furthermore, the offspring born with the most preferred partners were slightly fitter than those conceived through casual sex.

“Without oxygen, they seem to make the right choices in terms of ensuring fit offspring,” Pilpel said.

In the oxygenated test tubes, by contrast, it was the parents’ genetic dissimilarity that determined the offspring’s health.

“There’s an optimal genetic distance, not too close, that optimizes the health of the offspring,” Pilpel said.

The next step in the research will be to identify the genetic mechanism that enables yeast cells to make these choices.

“How do they know the genetic distance of each partner, the relative fitness of each one?” said Pilpel. “We’ve knocked out some genes thought to allow them to make this choice. They can still mate. Imagine a human male or female were to choose a mate at random. Is there a gene, the absence of which they can no longer select for the fitness of offspring? If we find this, we would like to understand how it evolved and contributed to their sexual life.”

Pilpel added that the main goal of the research — part of an international collaboration — was to better understand the evolution of biparental sexual reproduction, which began over a billion years ago.

“We wanted to understand whether, in addition to the rudimentary ability to have sex, [living things] can choose a partner and the choice is better than random sex. A major question in sex research is what it is good for. Why did biology make it fun and pleasant? What are the evolutionary advantages? We believe it’s a way to mix and match between different genetic profiles in the population.”

He added that the research, published in Cell Reports, could also help farmers choose the best breeds or varieties in livestock and plants.