For decades, scientists have portrayed sperm as active fighters battling their way towards a passive egg.
But new research suggest eggs aren’t as passive as one thought.
Researchers in Seattle claim a female’s eggs are able to select sperm with the best genes to ensure the healthiest offspring possible.
Semen does not appear to have the same ability to detect bad genes, researchers found.
The research defies Mendel’s Law that suggests fertilisation in random, and sheds light on how scientists have long projected traditional gender roles onto their work, leading to the portrayal of eggs as passive and sperm as active.
Researchers looking at mice found eggs were able to select semen with the healthiest genes to fertilise with. Pictured is a scanning electron micrograph shows the fertilization between a human egg and spermatozoa
In an in-depth feature Quanta Magazine, Joe Nadeau, principal scientist at the Pacific Northwest Research Institute outlined why he believes eggs are an active player in reproduction.
He has found examples that suggest fertilisation is not random and specific pairings between certain eggs and sperm are more common than others.
As part of his work, Professor Nadeau bred female mice carrying one normal and one mutant copy of a gene that increased the chance of getting testicular cancer.
The male mice all had normal genes.
The resulting offspring followed Mendel’s rules and there was a random dispersal of the mutated form among offspring.
But in the second experiment, Professor Nadeau reversed the breeding.
He gave males the mutant copy of the cancer gene while the females had the normal version.
Only 27 per cent of the offspring had the mutant variant, compared to the 75 per cent they expected to see.
Researchers found no evidence the mutated mice embryos were dying shortly after fertilisation, rather they were never fertilised.
William Eberhard, a behavioural ecologists from the Smithsonian Tropical Research Institute has dubbed this phenomenon ‘cryptic female choice’.
There is no evidence of how the eggs and sperm might do this but Dr Nadeau believes there are two possibilities.
One is that the rate of metabolism of B vitamin, such folic acid – which is an important signalling molecule – is different in sperm and eggs.
Research has revealed that these molecules play an important role in fertilisation.
Fussy eggs select sperm for their genes, according to research which suggests fertilisation in mammals – and potentially humans (stock image) – is not as random as previously thought
Changes in these signals may impact how much sperm and egg are attracted to one another.
Another hypothesis suggests sperm are in the female reproductive tracts before the egg is fully-formed.
The presence of the sperm influences how these cells divide so that its genes can be as well-suited to the sperm as possible.
‘We’ve been blinded by our preconceptions. It’s a different way to think about fertilization with very different implications about the process of fertilization’, said Dr Nadeau.
‘As soon as you have eggs and sperm, you have sexual selection. There are incredible things that eggs and seminal fluid can do,’ said Andrea Pilastro, an evolutionary biologist at the University of Padova in Italy, who is also working in a similar area.
This is more apparent with species that fertilise outside the body, such as with salmon and trout.
Females coat their eggs with ovarian fluid that contains chemicals which attract a sperm with the right sort of genes, research from the University of East Anglia showed in 2013.
Using sea urchin populations off the Pacific coast of Canada earlier this month researchers found eggs could choose their preferred mate from among the crowd.
In this study, Dr Levitan induced males to spawn and then collected tiny parcels of seawater – enough for a population of sperm to immediately interact with an egg – in syringes stocked with scores of unfertilized eggs.
‘While we expected to find that single eggs generally encounter sperm from a single male, we found that eggs are often simultaneously encountering sperm from more than one male in the brief interval between sperm contact and fertilization,’ Dr Levitan said.
‘This is the first evidence that sperm from different males compete for the same egg, which indicates an opportunity for eggs to have a choice.’
When sperm from multiple males arrive at an egg concurrently, eggs are able to choose the sperm whose recognition proteins are best suited for healthy fertilization.
This selection process can spur the evolution of new recognition proteins, eventually resulting in reproductive isolation and, in some cases, the creation of new species altogether.
‘These are the sort of sperm and egg cell interactions that drive divergence among populations, within populations and among species,’ Dr Levitan said.
‘What I’m hoping is to generate interest in these interactions so we can better understand how general they are for both internally and externally fertilizing species.’