Buried in amber during the Cretaceous, and hidden away in a collection for decades, the semen’s evolutionary grandfather waited patiently for scientists to discover it.
In 2020, that day finally arrived. In a new study, researchers report that they have discovered the oldest known sperm to date.
The cells, reported in a paper published Wednesday in Procedures from Royal Society B, was found in the reproductive tract of a hitherto undiscovered species of ostracod – a class of small crustaceans, approx. 1 millimeter in size.
The entire amber specimen dates back to the Cretaceous, meaning that for as long as 1
This creature, baptized Myanmarcypris hui, lived in the coastal and inland waters of Myanmar, which are lined with dense trees. M. hui, probably copulated with the owner of what is now the world’s oldest sperm shortly before they were encapsulated in a layer of amber that kept both its body and the soft sperm inside.
Renate Matzke-Karasz is the study’s second author and paleontologist at Ludwig Maximilian University in Munich. She Narrator Vise versa that these sperm are approx. 50 million years older than the previously identified oldest sperm. The former oldest sources that can be attributed to a source were found in the cocoon of an extinct species of Antarctic worm, which was reported in 2015.
Matzke-Karasz adds that her team encountered the sample in a private collection in China owned by a collector named Cheng Hu.
“He realized there were these little ostracodes in the amber,” she says Vise versa. “We named the new species after him. “
The age and condition of the find are both intriguing, but there was something extra special about the sperm contained in the Burmese amber: The sperm are far larger than one might expect for a creature that weighed less than a gram.
It is an example of semen gigantismand proof that a alternative sperm strategy was an effective way to copy millions of years ago.
“This is proof that giant semen is not an extravagant whim,” she explains.
What changes this discovery? – Semen typically conjures up the thought of small, approx. 50 micrometer swimmers found in men. Matzke-Karasz says this type of sperm represents a concept used by vertebrates and most other animals: producing as many sperm as possible and hoping that the strongest, fastest swimmers get to the woman’s eggs.
However, it is not the only sperm strategy that has been refined by nature. At the other end of the spectrum is sperm gigantism – the idea that a creature can produce a few, very long sperm cells compared to their body size.
That is the case too Drosophila bifurca, a fruit fly that produces semen that is 5.8 cm long when not wrapped (about 20 times its body length). Ostracods som M. hui is also known to have particularly long sperm, as long as 4.5 times their body length or around explains Matzke-Karasz.
For a human being 5 feet, 5 inches tall, that would translate to a sperm length of approx. 23 feet, she explains.
Semen is found in M. Hui is difficult to measure due to the “entangled mass” of the sample (not uncommon for ostracodes), the authors note. But they estimate that it is at least 200 micrometers long.
As impressive as these measurements are, it has been unclear, evolutionarily speaking, how useful this strategy was. Big semen costs, Matzke-Karasz explains.
Long semen, such as that found in M. hui, are immobile and must be drawn into the female’s reproductive tract, which means that the creature’s body must invest in additional organs and energy to reproduce. Large sperm must also be produced by larger reproductive organs, which create an extra drainage of energy.
“Reproductive organs are much larger than in other species, they take up a lot of space in the animal, and mating lasts a long time,” she says. “This is a lot of biological energy that needs to be allocated for reproduction.”
From an evolutionary point of view, “expensive” features without a clear return are believed to be counter-intuitive, although there are examples of creatures, or as extravagant features pay off. For example, the male peacock’s feathers help them attract mates.
Long semen from small creatures, however, poses no clear benefits. Some studies in mammals have suggested that greater sperm length may lead to faster, swimming more competitive sperm, but the evidence is mixed.
The fact that this is an example of sperm size exists at all is a signal that this copulation strategy continued for millions of years in ostracods. Matzke-Karasz sees this as proof that something about this strategy must have worked, even though we do not yet know what it is.
“In the past, we were not sure whether animals that ‘switched’ to use this giant sperm at some point in their evolutionary history are doomed to extinction very quickly,” she continues. “But in ostracods, it worked for more than 100 million years.”
Abstract: The bivalve crustaceans have the richest fossil record of any arthropod group and show complex reproductive strategies that contribute to their evolutionary success. Sexual reproduction involving giant sperm, shared by three superfamilies of live ostracod crustaceans, is among the most fascinating behaviors. However, the origin and evolution of this reproductive mechanism has been largely unexplored because fossil conservation of such traits is extremely rare. Here we report unusually preserved ostracodes with soft parts (appendages and reproductive organs) in a single piece of intermediate chalk Kachin amber (approx. 100 Mya). The Ostracod collection consists of 39 people. Thirty individuals belong to a new species and genus, the Myanmarcypris hui gen. et sp. nov., exhibits an ontogenetic sequence from adolescents to adults (male and female). Seven people have been assigned Thalassocypria sp. (Cypridoidea, Candonidae, Paracypridinae) and one to Sanyuania sp. (Cytheroidea, Loxoconchidae). Our micro-CT reconstruction provides direct evidence of the male clasper, sperm pumps (Zenker organs), hemispheres, eggs and female sperm containers with giant sperm. Our results reveal that the reproductive repertoire for reproduction, which is associated with significant morphological adaptations, has remained unchanged for at least 100 million years – an overarching example of evolutionary stasis. These results also double the age of the oldest unambiguous sperm from fossil animals. This discovery highlights the amber’s ability to document invertebrate soft parts that are rarely detected by other depositional environments.