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Giant 100 million year old sperm discovered – the oldest known sperm

Huge sperm illustration

In another fascinating snapshot from deep time, an international team of paleontologists has reported on the discovery of samples of a small crustacean dating back to Chalk (about 100 million years ago), preserved in samples of amber from Myanmar. The most spectacular find is a single woman, which on closer inspection turns out to contain giant sperm in its reproductive tract.

In fact, this is the oldest fossil where sperm have been definitively identified. In addition, the sample represents a previously unknown crustacean species that has been called the Myanmarcypris hui. M. hui was an ostracod, as is clear from the paired calcareous valves that form the carapace, whose shape resembles a seashell.

Ostracods have been around for 500 million years, and thousands of modern species have been described. They are found in the oceans and in freshwater lakes and rivers. Fossilized shells of these crustaceans are by no means rare, but the samples preserved in Burmese amber reveal details of their internal organs, including those involved in reproduction.

“The findings gave us an extremely rare opportunity to learn more about the evolution of these organs,” said Ludwig-Maximilians-Universitaet (LMU) in Munich geobiologist Renate Matzke-Karasz, who played an important role in the morphological analysis of the fossils.

In the Cretaceous, ostriches must have lived in coastal and inland waters of what is now Myanmar, which was surrounded by forests dominated by trees that produced huge amounts of resin. The recently described specimens are among the many organisms trapped in the sticky blobs of the sticky substance.

In recent years, the amber found in the province of Kachin has produced a spectacular trove of fossils, including frogs and snakes, as well as part of a suspected dinosaur (according to new evidence, this specimen may actually represent an unusual lizard). Over the last 5 years, hundreds of previously unknown species have been described based on these inclusions. In fact, many of them have forced evolutionary biologists to reconsider conventional hypotheses about phylogenetic and ecological contexts.

The new ostracod samples were analyzed using computer-assisted 3D X-ray reconstructions. The images revealed astonishing details about the anatomy of these animals, from their small limbs to their reproductive organs. And in a female sample, Matzke-Karasz and her colleagues discovered mature sperm. The cells were detected in the mated sperm containers, in which they were stored after copulation, ready for release when the female’s eggs matured.

“This woman must have mated shortly before it was encapsulated in the resin,” said He Wang of the Chinese Academy of Sciences in Nanjing. The x-rays also revealed the sperm pumps and the pair of penises that male ostracodes insert into the woman’s two gonopores.

The findings in Burmese amber provide unprecedented insight into an unexpectedly ancient and advanced occurrence of evolutionary specialization. “The complexity of the reproductive system in these samples raises the question of whether the investment in giant sperm can represent an evolutionarily stable strategy,” says Matzke-Karasz. The males of most animal species (including humans) produce very large numbers of very small sperm. Relatively few animals, including some fruit flies – and of course ostrich fish – have chosen a different approach. They produce a relatively small number of exaggerated sperm whose moving tails are several times longer than the animal itself.

“To prove that the use of giant sperm is not an extravagant approach on the part of evolution, but a viable strategy that can provide a lasting benefit that enables species to survive for long periods, we need to determine when this state of reproduction first appeared, ”says Matzke-Karasz.

Examples of fossilized sperm are extremely rare. The oldest known ostracod seed (prior to the new discovery) is 17 million years old, and the previous record age, 50 Myr, was held by a worm species. The new evidence extends this age by a factor of at least two.

The fact that animals had already developed giant sperm 100 million years ago suggests that this reproductive strategy may in fact be successful in the (very) long term, Matzke-Karasz points out. “It is a pretty impressive record for a trait that requires a significant investment from both males and females of the species. From an evolutionary point of view, sexual reproduction using giant semen must therefore be a thoroughly profitable strategy. ”

Reference: “Extraordinary Preservation of Reproductive Organs and Giant Sperm in Chalk East Codes” by He Wang, Renate Matzke-Karasz, David J. Horne, Xiangdong Zhao, Meizhen Cao, Haichun Zhang and Bo Wang, 16 September 2020, Procedures of the Royal Society B.
DOI: 10.1098 / rspb.2020.1661

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