The oldest fossilized animal sperm, 100 million years old and measuring up to 4.6 times the size of their progenitor, were discovered in tiny crustaceans trapped in a piece of amber by an international team of paleontologists.
So far, the oldest known examples of fossilized sperm are 17 million years old, says the team of researchers led by Dr. He Wang of the Chinese Academy of Sciences in an article published Wednesday in the prestigious journal Journal of Royal Society.
The animal in question is an ostracode, called “Myanmarcypris hui” by these scientists, a crustacean that has existed for 500 million years and which is represented today by thousands of species living in oceans, lakes and rivers. . Their soft body is protected by a calcareous bivalve-type shell which generally does not exceed a millimeter.
During the Cretaceous period (approximately -145 to -66 million years ago), these ostracods probably lived in the coastal waters of present-day Burma, where they found themselves trapped in a mass of tree resin.
This is 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.
Quality rather than quantity
Scientists first confirmed that these were indeed the oldest fossilized spermatozoa identified to date. They were found in the body of a female specimen, indicating that she must have been fertilized shortly before being trapped in amber.
Another surprise was the confirmation of their hypothesis of the ancestral existence of “giant spermatozoa”, a particularity of these ostracods, which can measure up to 4.6 times the size of the body of the parent male.
“It’s as if a human being is producing a sperm of about 7.30 meters, so it takes a lot of energy to make them!” », Explains to AFP Renate Matzke-Karasz, of the Louis-et-Maximilien University of Munich (Germany) and co-author of the study.
“This piece allowed us to validate our hypotheses according to which these giant spermatozoa already existed for 100 million years”, continues the biologist. Until then, these hypotheses were based on the discovery, in 2009, of large genitals in these crustaceans, assuming the existence of spermatozoa therefore disproportionate.
This is sort of an exception in the animal kingdom, as most males (including humans) typically produce tens of millions of tiny sperm but in very large quantities.
However, these crustaceans, like certain fruit flies, would favor the opposite: they produce a small number of spermatozoa, but of high quality and XXL size, to increase their chances of winning the race to fertilize the eggs.
Several contradictory hypotheses collide as to their evolutionary usefulness: in some animals, “a high degree of competition between males can lead to an extension of sperm life, while (in others) a low degree of competition also leads to a longer sperm lifespan, ”says Matzke-Karasz.
The researcher is convinced that in the case of these ostracods, the size of the spermatozoa is proof of good physical shape for the males, a trait “favored” by the females who consequently, or originally, have adopted appropriate genital characteristics. to accommodate these giant gametes: “This is a co-evolution”.
“This is quite impressive for a trait that requires such a large investment from both males and females, especially considering that many ostracods can reproduce parthenogenetically, without the need for males. Sexual reproduction with giant spermatozoa must have a definite advantage over asexual reproduction, ”argues Ms. Matzke-Karasz.
This finding shows “that reproduction with giant sperm is not an extinct evolutionary extravaganza, but a serious long-term benefit for the survival of a species.”
And to be ironic about the many fossils of animals much larger than the ostracods found so far, but overwhelmed by this recent discovery: “Please never underestimate the small”.