Stockholm university

Mysterious genes may explain the origin of species

Why are the offspring of a horse and a donkey almost always infertile? This question has long puzzled researchers. Deniz Ozata, from Stockholm University, believes the explanation may lie in very mutable genes that are vital to the development of healthy sperm.

Deniz Ozata
Deniz Ozata's research group tries to explain how the barrier between different species arises, and thus how new forms of life can arise. Photo: Jens Olof Lasthein

As evolution takes place, a type of barrier rapidly develops between different species, which means they can no longer interbreed. This allows new forms of life to arise, different from one another.

“This is called reproductive isolation. Sometimes the offspring from two different species become infertile, such as from a horse and a donkey,” says Deniz Ozata, assistant professor at the Department of Molecular Biosciences, The Wenner-Gren Institute.

Exactly how this barrier between the species arises has long been a mystery to researchers, but Ozata believes he may partially have uncovered an explanation. He found the first clue in 2020, when he was a postdoctoral fellow in Prof. Phillip D. Zamore’s laboratory at UMass Chan Medical School in Worcester, US. There, he studied a recently discovered group of small RNA molecules that are only found in animal cells, called PIWI-interacting RNAs (piRNA).

 

Guardians of the genome also found in sperm

Sperms on a computer screen.
Pachytene piRNA is produced in huge amounts when cells divide to form sperm. Photo: Jens Olof Lasthein

In general, piRNAs can be described as the guardians of an animal’s genome, protecting it from transposons. These are moving genetic elements – often remnants of old viruses – that can jump from one part of the genome to another and cause damage. The cell can use piRNAs to protect its genome from the activity of transposons; along with a protein, PIWI, piRNAs form a type of molecular scissors that cut up these genetic elements.

“Protecting cells from transposons is the conserved function of piRNAs. However, the group of piRNAs, pachytene piRNAs, in the male germ cells of placental mammals has lost the ability to suppress transposons” says Ozata.

 

Discovered a genetic paradox

Ozata is focusing his attention on this second group of piRNA. For some reason, enormous amounts of these molecules are produced when male germ cells differentiate into mature sperm. They mainly exist during a phase of cell division called the pachytene, so are known as pachytene piRNA. Ninety distinct genes for pachytene piRNA have been identified in the human genome and, when Ozata mapped them, he discovered a strange phenomenon – they evolve very quickly.

“We compared the genomes of around 2,500 people, and the genes for pachytene piRNA show immense diversity. They are among the least preserved genes in the entire human genome,” says Ozata.

 

Vital genes that mutate quickly

This was a paradoxical discovery. When his colleagues and other researchers deactivated these genes in mice, the mice became infertile. Their sperm began to swim poorly and had defects. So how can genes that are crucial for our fertility change so rapidly? Usually, vital genes are very well-preserved, because mutations can easily cause catastrophic damage.
“There appears to be a strong evolutionary pressure on these genes to evolve,” says Ozata.

His educated guess is that these rapid genetic changes somehow build the barrier between species. However, he does not know how this happens – and pachytene piRNAs have other mysterious properties. For example, it seems as if only a few of its ninety genes are required for the sperm’s health. The function of most of the genes appears to be to force the production of piRNAs when sperm is formed.
“They are like selfish genetic elements. They only appear to exist to amplify their own production,” says Ozata.

 

All genes are active in sperm – but do some need to be suppressed?

Work in the lab
By investigating which pachytene-piRNA genes carry, the researchers hope to get more clues to how the enigmatic molecules work. Photo: Ingmarie Andersson

The enormous production of piRNAs takes a lot of energy for the cell, and energy-intensive processes do not usually evolve by chance. Why such large amounts of piRNAs are necessary is currently a mystery, but Ozata and his colleagues do have a main hypothesis: that pachytene piRNAs form a genetic scissor with PIWI proteins, suppressing specific genes as the sperm develop. During the development of sperm, in principle all genes are active, so they are transcribed into mRNA. Researchers have seen that some pachytene piRNAs can recognise mRNAs and ensure that PIWI proteins cut them into pieces.

“Imagine that you have all this mRNA, but that only some define the sperm’s compatibility with the egg. Then other mRNA needs to be suppressed,” says Ozata.

His idea is that pachytene piRNAs tweak the incidence of mRNA molecules in the sperm, creating a species-specific pattern. That pattern may, in some way, build the barrier between different species.

To see whether this hypothesis holds, his research group are now mapping genes for pachytene piRNAs in two mouse subspecies that are reproductively isolated from the standard laboratory mice. He hopes that investigating which pachytene piRNA they carry will allow him to discover more clues to how these enigmatic molecules work. The molecular mystery responsible for the species barrier remains, but clues are still being uncovered...

Read more on Deniz Ozata´s research.

Text: Ann Fernholm
English translation: Clare Barnes