An international team of paleontologists discovered in a piece of amber analyzed in Myanmar, Southeast Asia, some fossilized sperm from a tiny crustacean that was 100 million years old. They are the oldest found to date.
As explained by the team of researchers led by Dr. He Wang, of the Chinese Academy of Sciences, in an article published on Wednesday in the prestigious scientific journal Journal of Royal Society, the oldest fossilized sperm found was 17 million years old.
These sperm belong to an ostracode called by scientists “Myanmarcypris hui”, a species of 500 million years old that measures less than a millimeter and is currently present in oceans, lakes and rivers.
Its soft body is protected by a limestone shell of the bivalve type that generally does not exceed one millimeter.
During the Cretaceous period, which began 145 million years ago and ended 66 million years ago, the ostracodes studied probably lived on the shores of present-day Myanmar, where they were trapped in a cluster of tree resin.
That’s where the amber piece was found: first in the possession of a Chinese collector, it was given in 2017 to Dr. Wang for him to study.
The discovered sperm were found inside a female specimen, indicating that it was fertilized before being trapped in amber.
The other peculiarity is that these sperm are considered “giant” because they can be four times larger than the male parent.
“This would be equivalent to (a sperm) of 7.30 meters in a man of 1.70 meters, that is, it takes a lot of energy to produce them!”, Explained Renate Matzke-Karasz, from Ludwig-Maximilians University of Munich, who is co-author of the study.
“This piece allowed us to validate our hypotheses that these giant sperm cells had been around for 100 million years,” added the biologist.
Until then, these hypotheses were based on the discovery, in 2009, of large genital organs in these crustaceans, assuming the existence of disproportionate sperm.
It is, therefore, an exception, since all males, including man, produce tens of millions of tiny sperm.
However, these crustaceans, like certain fruit flies, would do the opposite: they produce a small number of sperm, but of high quality and size XXL, to increase your chances of winning the race to fertilize the eggs.
Several contradictory hypotheses clash about its evolutionary usefulness: in some animals, “a high degree of competition between males can lead to an extension of the life of the sperm, while (in others) a low degree of competition also leads to a lifespan. longer sperm, “says Matzke-Karasz.
The researcher is convinced that, in the case of these ostracodes, the size of the sperm is proof of physical fitness for males, a characteristic “preferred” by females who consequently, or originally, adopted appropriate genital characteristics to accommodate these giant gametes: ” This is a coevolution “.
“This is quite impressive for a trait that requires such an investment in males and females, especially considering that many ostracodes can reproduce parthenogenetically, without the need for males. Reproduction with giant sperm should have a definite advantage over asexual reproduction.” , argues Matzke-Karasz.
This finding shows that “reproduction with giant sperm is not an aberration of endangered evolution, but an important long-term advantage for the survival of the species,” he said.