(April 15th, 2014) When it comes to sex, people living in the southern states of the US are more conservative than their northern fellow citizens. Funnily, the same is true for a certain fruit fly species. What’s behind this opposing fly behaviour?
Scientists at the University of Liverpool, in collaboration with research groups from Exeter and Madrid, have uncovered a peculiar mating pattern in fruit flies (Drosophila pseudoobscura) in the United States: while female promiscuity is an accepted behaviour in the northern populations, females from southern locations prefer to mate with a single male. Mating with several partners may be key to avoid extinction as it eliminates the occurrence of selfish genes present in the X chromosome that lead to the production of only female offspring.
In these flies, gender is defined the same way as in humans - males have one X and one Y chromosome, while females have two X chromosomes. In a male, if a particular selfish genetic element – known as a meiotic drive element - is present in the X chromosome, it tries to “preserve” itself by eliminating all the competition, i.e. sperm containing the Y chromosome. As a consequence, these males only produce daughters.
This generates quite a gloomy prediction, with the number of females carrying this “corrupted” X chromosome doubling in frequency in every generation, quickly reaching 100% by which time the species would go extinct. However, “that's not what we see in nature at all,” explains evolutionary biologist Tom Price, from the University of Liverpool and lead author in the study.
“What we see is really stable frequencies and it's very strange: in the southern populations it gets to a frequency of about 30% - so about 30% of X chromosomes carry this meiotic driver - while in the north, you can't find it at all. And there's quite a nice decline as you go north from Arizona up to Utah, which has been stable for the last 80 or so years.” Previously, the team showed that these males produce less sperm and perform very poorly, and as a consequence, have a limited effect on this predicted sex distortion. However, this still cannot explain the north-south-divide.
To reveal what other factors may be at play, the team returned to the States to assess how mating behaviour differs across the country. It turns out that northern females have a much higher sex drive than their southern cousins: not only do they mate with more males but they’re also prepared to mate again sooner. Researchers suggested mating with multiple partners increases chances of finding a male without this selfish gene and with a normal sperm count. “If a female also mates with a normal male, the normal male would father almost all of the offspring,” says Price. It seems females have to engage in this strategy as they can’t identify which males carry the selfish gene. Otherwise, “if the females could just avoid mating with males who carry the selfish gene then they could simply avoid it that way.”
Researchers are still puzzled as to how this mechanism evolved. There are many reasons why multiple mating develops, but why some populations in the north consistently opt for this promiscuous behaviour while others in the south prefer monogamy remains a mystery. “I would be really surprised if the latitude didn’t have some kind of causal effect, because it's so strong,” says Price. “I work on another fruit fly in which a similar meiotic driver is only found in North Africa, but the flies are found right the way up into Sweden.” Tentatively, the author suggested that as females in the north are larger - a known effect caused by temperature – they may need more sperm supplies because they can lay more eggs than females from the south.
Not surprisingly, when asked about future work, Price has an easy answer: find out why the females in the north opt for multiple mating and the mechanism behind it. According to the researcher, the other big question is whether it may actually cause extinctions. “If this turned up in a species that didn’t have this multiple mating would it wipe the species out?” he asks.
The obvious potential is for pest control: If it was possible to insert this genetic element into a key pest species, for example onto the Y chromosome of a mosquito, it should spread quickly through the population, and potentially wipe it out due to a lack of females. “One of the reasons for my research,” says Price, “is, if this is ever going to happen, we need to understand how real selfish genes spread in natural populations.” With this in mind, the collaboration with the other research groups is set to continue. After all, “there are so few people working on meiotic drive, that we need to stick together!”
Photo: Markus Riedl/Vetmeduni Vienna