High-energy lifestyles led to evolution of the sexes

December 19, 2011 By Peter Hurrell
High-energy lifestyles led to evolution of the sexes

Scientists are a step closer to explaining one of the most enduring mysteries of modern biology; why are there males and females?

Researchers at University College London have shown that by inheriting a certain set of genes from just one parent, the cells of sexually reproducing species could more efficiently produce the energy they need to survive.

"[Two] is not the evolutionary optimal number of sexes, or mating types, for a population," says Zena Hadjivasiliou, the who led the research. She adds: "You end up only being able to mate with fifty per cent of the population, so there must be some reason why it has spread."

To make their discovery, the scientists looked at one of the key differences between the sexes; during reproduction, males pass on only the genes from the of their cells. In contrast, females pass on all of the the offspring requires, including the both nuclear genes and a small extra set of genes in the tiny power-generating structures called mitochondria.

Mitochondria, which supply all of the energy the cell needs, contain a handful of genes that must work alongside the nuclear genes to control part of the process.

Using a mathematical model, the UCL team found that if one parent was prevented from contributing their to their offspring, it was easier for the remaining mitochondrial genes and the to coordinate their activities as they evolved over several generations. The result was that the cell functioned more efficiently. Ultimately, this would favour the evolution of two sexes, only one of which, the female, would pass on its mitochondria.

The researchers' model also showed that, although the advantage of having two sexes was fairly small in simple single-celled organisms, it was more pronounced when individual cells contained lots of mitochondria. This is particularly true of organisms that have high and are made up of many trillions of cells, like us.

In many sexually reproducing species, can be distinguished by their sex cells, eggs and sperm. The female produces a few large, immobile eggs while males produce many small, mobile sperm. But even in species with no obvious differences between the sex cells, researchers can still tell them apart. The key is that only the females pass their mitochondria, and the genes they contain, to their offspring.

It was this tiny difference that Hadjivasiliou and colleagues wanted to study, to see if it was enough to overcome the disadvantage of only being able to mate with half the population. Their idea was that if an organism inherited two different sets of mitochondria – one from each parent - the nuclear genome would not be able to evolve to work efficiently with both. As a result, the organism would be less healthy than others of its species that inherited just one set of mitochondria.

Their allowed the team to compare the effects on the fitness of a population of single-celled organisms of inheriting mitochondria from one parent with the effects of inheriting mitochondria from both.

The results clearly showed that populations in which mitochondria were inherited from just one parent were generally fitter.

Fitter individuals tend to reproduce more successfully, so they would eventually become the majority, allowing the evolution of two to spread. Through billions of years of evolution, these advantages persisted, resulting in the sexual division we're familiar with today.

The research appears in Proceedings of the Royal Society B.

Explore further: Mitochondria and the great gender divide

More information: Hadjivasiliou, Z, et al. Selection for mitonuclear co-adaptation could favour the evolution of two sexes. Proceedings of the Royal Society B. Published online: December 7, 2011, doi: 10.1098/rspb.2011.1871

Related Stories

Mitochondria and the great gender divide

December 9, 2011

(Medical Xpress) -- Why are there two sexes? It’s a question that has long perplexed generations of scientists, but researchers from UCL have come up with a radical new answer: mitochondria.

Research breakthrough on male infertility

May 13, 2011

(Medical Xpress) -- Around one in 20 men is infertile, but despite the best efforts of scientists, in many cases the underlying causes of infertility have remained a mystery. New findings by a team of Australian and Swedish ...

Mothers curse linked to male infertility

May 16, 2011

(Medical Xpress) -- Researchers have discovered the first real evidence of the 'mother's curse' and its connection to male infertility due to genetic mutations in mitochondria. Led by Dr. Damian Dowling from Monash University ...

The cost of keeping eggs fresh for mother cockroaches

February 26, 2007

One of the defining differences between the sexes is in the size of their gametes. Males make many tiny sperm while females make only a few large eggs. This suggests that sperm are cheap while eggs are expensive.

Worms can evolve to survive intersex populations

December 6, 2011

(PhysOrg.com) -- Sexually reproducing species need at least two sexes in order to produce offspring, but there are many ways that nature produces different sexes. Many animals (including humans and other mammals) use a chromosomal ...

The sexual tug-of-war -- a genomic view

March 15, 2010

The genes that are most beneficial to males are the most disadvantageous for females, and vice versa. However, this genetic conflict between the sexes is important in maintaining genetic variation within a species, researchers ...

Recommended for you

Tasmanian tiger doomed long before humans came along

December 12, 2017

The Tasmanian tiger was doomed long before humans began hunting the enigmatic marsupial, scientists said Tuesday, with DNA sequencing showing it was in poor genetic health for thousands of years before its extinction.

Searching for the CRISPR Swiss-army knife

December 12, 2017

Scientists at the University of Copenhagen, led by the Spanish Professor Guillermo Montoya, are investigating the molecular features of different molecular scissors of the CRISPR-Cas system to shed light on the so-called ...


Adjust slider to filter visible comments by rank

Display comments: newest first

5 / 5 (2) Dec 19, 2011

Before differentiation of the sexes, organisms were hermaphrodites, these organisms produce both sperm and eggs, so they actually dont mix their mytohondria as well, so why didnt they evolved more than we do?(having the advantage to have sex with everybody not mixing the mytohondria)
not rated yet Dec 19, 2011

Before differentiation of the sexes, organisms were hermaphrodites, these organisms produce both sperm and eggs, so they actually dont mix their mytohondria as well, so why didnt they evolved more than we do?(having the advantage to have sex with everybody not mixing the mytohondria)

Your question will be answered in the next paper.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.