In modern society, females are often stereotyped as being calm, demure, and passive. This archetype seems to have entered every facet of modern living, but I never assumed biology would be one of them.
When asked about sexual reproduction, most of us are familiar with the imagery of a billion frenzied sperm racing towards a calm, ominous egg.
It turns out that the female egg may actually have more power in the reproductive process than previously thought— having the ability to selectively discriminate against certain kinds of sperm.
“Female reproductive anatomy is more cryptic and difficult to study, but there’s growing recognition of the female role of fertilization,” evolutionary biologist Mollie Manier explained.
As can be imagined, this discovery is evening out the fertilization playing field and is making feminists everywhere rejoice.
Mind-blown? Let’s go over the basics.
Common Notion: Active Sperm “Competes” for Passive Egg
Since grade school, we’ve been taught it’s the sperm that has the “power” to achieve fertilization— with absolutely zero input from the female egg. The ‘fastest swimmer wins the race’ has been the general consensus on reproduction for as long as most can remember.
“The egg is seen as large and passive,” anthropologist Emily Martin explained.
“It does not move or journey but passively ‘is transported’… along the fallopian tube.”
“In utter contrast, sperm are small, ‘streamlined’ and invariably active.”
Later on, as the new millennium approached, scientists like Martin found themselves wondering if they had focused too much attention on male aspects of biology while simply assuming the female egg had no role to play.
That’s when they turned their attention towards the egg.
In cases where fertilization happens outside the body, it’s long been known that the females coat the eggs in a protein-rich ovarian fluid. In 2013, Matthew Gage, from the University of East Anglia, decided to take a closer look.
Gage conducted studies using female fish eggs, including salmon and trout. What he discovered was that the ovarian fluid seemed to contain chemical signals which attracted the correct species of fish.
During Gage’s experiments, where he placed specific eggs near groups of both species of fish, the male sperm managed to fertilize its own species’ eggs more than 70% of the time. These odds are significantly greater than what is expected by chance.
Gage demonstrated that female eggs can selectively choose which individual they reproduce with. But when it came down to the actual sperm race, it seemed the egg was still a passive player while the male sperm held fate in their hands. Reproduction was, in effect, totally random
Cue Dr. Joseph H. Nadeau from the Pacific Northwest Research Institute in Seattle!
New Notion: The Egg is an Equal and Active Player in Reproduction, Adding Layers of Evolutionary Control
As mentioned, the idea of reproduction being entirely random is one of the fundamental tenants of Mendelian genetics. It is a basis of Mendel’s law of segregation, which has been used in genetics for over a hundred years.
Mendel’s law of segregation helps scientist predict the probability of certain genetic combinations in offspring. “It’s one of the most broadly applicable rules in biology,” Nadeau said, in an interview with Quanta Magazine. However, Mendel’s law of segregation only applies if reproduction is random.
Despite the wide acceptance of Mendel’s law of segregation, as Nadeau conducted reproductive experiments, his results just weren’t adding up. That’s when he started to wonder if reproduction may not be as random as we’ve always assumed…
Nadeau set out to conduct two experiments, attempting to yield results similar to Mendelian predictions:
- In the first experiment, Nadeau gave female mice one mutant gene associated with testicular cancer. All the males were normal, and the two groups were then bred.
- In the second experiment, Nadeau reversed the process, breeding healthy mice with males who carried the testicular cancer gene.
In Nadeau’s first experiment, where the females had a mutant gene, the offspring reflected the predicted Mendelian ratios. However, in the second experiment, where the male mice (and sperm) carried the mutant gene, only 27% of the offspring inherited it— a shocking difference from the predicted 75%.
“We really thought it was just a weird pattern of inheritance,” one of Nadeau’s partners explained.
“We hadn’t thought about non-random fertilization.”
The scientist went over all the possible reasons for these results— but in the end, none of them seemed to fit.
It appeared as if Nadeau had discovered “genetically biased fertilization” and simultaneously shed doubt on modern biology’s understanding of the male and female roles in fertilization.
“If you’ve eliminated the impossible, then what remains, however, unlikely, must be the truth,” he quipped.
Scientists still aren’t sure how the egg manages to screen for the best potential sperm, but Nadeau believes it involves folic acid signaling between the two of them. Because the egg develops while in the reproductive tract, another possibility is that it can divide its cells in a way that makes it more penetrable to the strongest and healthiest sperm.
“Females were seen as passive objects with no choice, but females are going to have a vested interest in the outcome of fertilization,” said Renee Firman, evolutionary biologist.
“We still have a long way to go to understand this process, but I don’t think we really appreciate how common this is and how often it happens.”
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