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Sex cells evolved to pass on quality mitochondria

According to new research, mammals immortalize their genes through eggs and sperm in order to ensure that future generations inherit the best mitochondria to power new cells.


In a new study funded by the Natural Environment Research Council, the Engineering & Physical Sciences Research Council and the Leverhulme Trust of University College London (UCL) in the United Kingdom, scientists developed a model of evolution to investigate how germ cell (egg and sperm) lines evolved. Their discovery is that germ cells in mammals were developed to select for the best mitochondria — the "batteries" of our cells. The work is published in PLOS Biology.

"There have been many theories about why mammals have a specialised germline when plants and other ancient animals don't," says Arunas Radzvilavicius PhD and first author and UCL PhD student. "Some suggest it is due to the complexity of tissues, or perhaps a selfish conflict between cells."

Surprisingly, the researchers didn't find this reason to be why germ cells evolved. Instead, it appears protecting future generations from mitochondrial mutations is the logic behind creating separate germ cell lines.
In plants, mitochondrial mutations happen slowly. Mutations are corrected through the process of natural selection, where only the individuals best suited to survive in that environment will survive.

But in mammals, genetic errors in mitochondria occur frequently due to our higher metabolic rate. Undergoing lots of division cycles is a liability. Therefore, mitochondria are only passed along to the next generation through dedicated female germ cells, large eggs. This sytem protects against most mutation errors passed through cell division as eggs undergo fewer division cycles than all other cells.

Having a protected germline ensures that the best quality mitochondria are transferred but restricts the genetic variation in the next generation of cells in the developing embryo. This is corrected for by mammals generating far too many egg cells which are removed during development. For example, humans are born with over 6 million egg-precursor cells, 90% of which are culled by the start of puberty in a mysterious process called atresia.

Senior author, Dr Nick Lane (UCL CoMPLEX and Genetics, Evolution & Environment): "We think the rise in mitochondrial mutation rate likely occurred in the Cambrian explosion 550 million years ago when oxygen levels rose. This was the first appearance of motile animals in the fossil record, things like trilobites that had eyes and armour plating - predators and prey. By moving around they used their mitochondria more and that increased the mutation rate. So to avoid these mutations accumulating they needed to have fewer rounds of cell division, and that meant sequestering a specialized germline."
"Scientists have long tried to explain the evolution of the germline in terms of complexity. Who would have thought it arose from selection of mitochondrial genes? We hope our discovery will transform the way researchers understand animal development, reproduction and aging."

Andrew PomiankowskiPhD, Professor, Genetics, Evolution & Environment, University College London, United Kingdom.

Abstract
The origin of the germline–soma distinction is a fundamental unsolved question. Plants and basal metazoans do not have a germline but generate gametes from pluripotent stem cells in somatic tissues (somatic gametogenesis). In contrast, most bilaterians sequester a dedicated germline early in development. We develop an evolutionary model which shows that selection for mitochondrial quality drives germline evolution. In organisms with low mitochondrial replication error rates, segregation of mutations over multiple cell divisions generates variation, allowing selection to optimize gamete quality through somatic gametogenesis. Higher mutation rates promote early germline sequestration. We also consider how oogamy (a large female gamete packed with mitochondria) alters selection on the germline. Oogamy is beneficial as it reduces mitochondrial segregation in early development, improving adult fitness by restricting variation between tissues. But it also limits variation between early-sequestered oocytes, undermining gamete quality. Oocyte variation is restored through proliferation of germline cells, producing more germ cells than strictly needed, explaining the random culling (atresia) of precursor cells in bilaterians. Unlike other models of germline evolution, selection for mitochondrial quality can explain the stability of somatic gametogenesis in plants and basal metazoans, the evolution of oogamy in all plants and animals with tissue differentiation, and the mutational forces driving early germline sequestration in active bilaterians. The origins of predation in motile bilaterians in the Cambrian explosion is likely to have increased rates of tissue turnover and mitochondrial replication errors, in turn driving germline evolution and the emergence of complex developmental processes.

Finnish research played a key role along with the Type 1 Diabetes Prediction and Prevention study (DIPP) which has significantly advanced research on a connection between viruses and diabetes. Additionally, collaboration with Vesa Hytönen PhD, Assistant Professor and other Tampere-based professionals, has been crucial in developing vaccines. Other noteworthy partners include the Karolinska Institutet in Stockholm, several universities and research institutes in Finland and abroad as well as Vactech Oy. Research has been funded by several different groups, such as the Academy of Finland, TEKES, the Sigrid Juselius Foundation, the Reino Lahtikari Foundation, the Diabetes Research Foundation, the European Union and the Juvenile Diabetes Research Foundation (JDRF).

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Jul 25, 2017   Fetal Timeline   Maternal Timeline   News   News Archive




Human egg surrounded in sperm.



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