Female worms that mate with males originally from New York die more quickly than those that come from Ohio or Germany. Even when the females survive, they produced only half as many offspring.
Researchers wondered how the attributes of the New York males could still exist when they lead to so few successful offspring in the next generation.
In the animal world, differences in approaches to reproductive success can lead to sexual conflict.
Science has known that male fruit flies, for example, transfer proteins during mating that can alter the timing of a female’s egg laying and her tendency to later mate with other males. Some of these male-derived proteins also migrate from the female’s reproductive tract to her brain.
Now, scientists show that sexual conflicts can evolve rapidly in natural populations, driven by competition among males for mating success.
SPERM WARS
In the study, published in BMC Evolutionary Biology, researchers had just begun a set of experiments using the nematode Caenorhabditis remanei when they noticed the phenomenon with females and New York males. Even if the mating only occurred for a single day, the females still lived only half as long as they did spending their lives as virgins.
To find out why, researchers put the different males together in a competition. The sperm of the New York males easily outcompeted sperm from the Ohio and German males to successfully fertilize a female’s eggs.
“Despite their small size, nematode sperm is actually much larger than human sperm, and it is thought that the sperm from different males literally battle it out inside the female for access to her eggs,” says Patrick C. Phillips, professor of biology at the University of Oregon.
“So a reasonable evolutionary explanation would be that these males make bad mates but highly successful fathers.”
WIMPY WORMS
To test this hypothesis more rigorously, Phillips turned to Michael Palopoli, an evolutionary biologist at Bowdoin College.
Researchers used genetic tricks to transform the mating system of a closely related nematodeCaenorhabditis elegans.
These roundworms usually exist as hermaphrodites that reproduce using sperm and eggs produced inside the same individual. Males also exist in C. elegans, but they are rare and wimpy compared to males from male-female species like C. remanei.
The scientists used a genetic mutation that blocks sperm production in hermaphrodites, effectively turning them into females. They then mated these newly formed females with males and allowed them to evolve together for 60 generations.
MATING MYSTERY
Males in this highly competitive environment rapidly evolved larger sperm, as well as sperm that became much more competitive when compared to their female-free ancestors.
Most importantly for the evolutionary story, these males also evolved such that females died more frequently when mated with them than when they were mated with males that had never been subjected to the high male-competition treatment.
“Overall, we were able to rapidly recapitulate the evolutionary pattern that we see in the male-female species by converting a hermaphroditic species to be male-female and then allowing them to evolve under these new circumstances,” Palopoli says.
The mystery of why sexual conflict exists within these species is now replaced by the mystery of how the males are affecting the females, Ohillips says.
“It could be a change in the behavior of the males, or it could be something in the seminal fluid that they transfer during mating,” Phillips says. “We are following up on this work to figure that out.”
The National Science Foundation, National Center for Research Resources, National Institutes of Health, Howard Hughes Medical Institute’s Undergraduate Science Program and Bowdoin College supported the research.
Source: University of Oregon
