Mysterious genes may explain species’ origin

During the course of evolution, a kind of barrier quickly forms between different species. This means that the species can no longer cross with each other. In this way, new forms of life that differ from each other can emerge.

This is called reproductive isolation. Sometimes the offspring of two different species are also infertile, such as the horse and the donkey, says Deniz Ozata, a research assistant in the Department of Molecular Biosciences at the Wiener-Green Institute at Stockholm University.

Protecting the genetic mass of animals

Scientists have long not known exactly how this type of barrier occurs. But Deniz Ozata can be an interpretation of the tracks.

He studied a newly discovered group of RNA molecules that are found only in animal cells, the so-called PIWI-interacting RNA (piRNA).

In general, piRNA can be described as a guardian of the animal genetic mass. They protect the genetic material from so-called transposons. These are moving genetic elements — often leftovers from ancient viruses — that can move from one part of the genome to another and cause damage. But with the help of piRNA, the cell can defend itself against the transposons.

Together with the PIWI protein, piRNA forms a kind of molecular scissors that separates them.

Protection of the cell from transposons is thus the original function of piRNA. However, in mammals, a subset of piRNAs that have lost the ability to silence transposons have been found to form in testes.

Discover a genetic paradox

This is the second group of piRNAs that Deniz Ozata is turning his attention to. For some reason, they are produced in huge amounts when cells divide to form sperm. It is found mainly during the stage of cell division called pachytene and hence called pachytene – piRNA.

In the human genome, 90 distinct pachytene piRNA genes have been identified. When Deniz Ozata made these maps, he discovered a strange phenomenon: they were changing at a very fast rate.

We compared the genomes of nearly 2,500 different individuals and the pachytene-piRNA genes differ greatly. They belong to the least conserved genes in the entire human genome, says Deniz Ozata.

Vital genes that mutate rapidly

It was a paradoxical discovery. Because when Deniz Ozata and other researchers disabled some of these genes, it caused the mice to become infertile. Sperm began to swim poorly and developed several defects.

So how can genes essential to our fertility change so quickly? Otherwise, vital genes are usually very well preserved, as mutations can easily cause catastrophic damage.

– There seems to be a strong evolutionary pressure on these genes to change, says Deniz Ozata.

It builds the barrier between species

His qualified speculation is that rapid genetic changes are somehow building up the barrier between species. But how this happens is not clear, and pachytene-piRNA has even more enigmatic properties. For example, only a few of the 90 genes appear to be essential for sperm survival.

Most of the genes seem to have the main function of forcing piRNA production during spermatogenesis.

They are like selfish genetics. Deniz Ozata says he seems to only be there to boost his production.

Do some genes need to be silenced?

Massive piRNA production costs energy to the cell, and energy-consuming processes do not usually happen by chance during development. But why such large amounts of piRNA are needed is currently a mystery.

Genes are silenced

However, Deniz Ozata and colleagues have a main hypothesis: that pachytene-piRNA forms an epigenetic scissors with PIWI proteins that silence specific genes in sperm.

Because during sperm development, essentially all genes are active and their sequences are depicted in mRNA. The researchers note that some pachytene piRNAs can recognize mRNAs and ensure that PIWI proteins cleave them.

– Imagine that you have all these mRNAs, but only a part of them determines the compatibility of the sperm with the egg. Other mRNAs should be silenced, says Deniz Ozata.

More clues to mysterious molecules

His idea is that pachytene piRNA adjusts the presence of mRNA molecules in sperm so that a species-specific pattern is formed. This pattern can then build up the species barrier one way or another.

To see if the hypothesis holds true, his research group is currently working on mapping pachytene piRNA genes in two different subspecies of mice that are reproductively isolated from normal lab mice.

By investigating the type of pachytene-piRNA it carries, he hopes to get more clues about how the mystery molecules work.