Paleontology: The oldest known sperm cells

Date:
September 16, 2020
Source:
Ludwig-Maximilians-Universita"t Mu"nchen
Summary:
An international team of paleontologists has discovered giant
sperm cells in a 100-million year-old female ostracod preserved
in a sample of amber.

Clearly, the tiny crustacean had mated shortly before being entombed
in a drop of tree resin.



FULL STORY ==========================================================================
An international team of paleontologists has discovered giant sperm
cells in a 100-million year-old female ostracod preserved in a sample
of amber. Clearly, the tiny crustacean had mated shortly before being
entombed in a drop of tree resin.


==========================================================================
In another fascinating snapshot from deep time, an international team of paleontologists has reported the discovery of specimens of a minuscule crustacean that dates back to the Cretaceous (about 100 million years
ago), conserved in samples of amber from Myanmar. The most spectacular
find is a single female, which turns out on closer examination to contain
giant sperm cells in its reproductive tract. In fact, this is the oldest
fossil in which sperm cells have been conclusively identified. Moreover,
the specimen represents a previously unknown species of crustacean, which
has been named Myanmarcypris hui. M. hui was an ostracod, as clearly
indicated by the paired calcareous valves that form the carapace, whose
form recalls that of a mussel shell. 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 specimens
preserved in Burmese amber reveal details of their internal organs,
including those involved in reproduction. "The finds gave us an extremely
rare opportunity to learn more about the evolution of these organs,"
says Ludwig-Maximilians-Universitaet (LMU) in Munich geobiologist Renate Matzke-Karasz, who played a major role in the morphological analysis of
the fossils.

During the Cretaceous period, ostracods must have lived in the coastal
and inland waters of what is now Myanmar, which were fringed by forests dominated by trees that produced huge quantities of resin. The newly
described specimens are among the many organisms that were trapped in the oozing blobs of the gooey substance. In recent years, the amber found
in the province of Kachin has yielded a spectacular trove of fossils,
including frogs and snakes, as well as part of a putative dinosaur
(according to new evidence, that specimen may actually represent an
unusual lizard). Over the past 5 years, hundreds of previously unknown
species have been described based on these inclusions.

Indeed, many of them have forced evolutionary biologists to reconsider conventional hypotheses concerning phylogenetic and ecological
relationships.

The new ostracod specimens were analyzed with the aid of computer-assisted
3D X-ray reconstructions. The images revealed astonishing details of
the anatomy of these animals, ranging from their tiny limbs to their reproductive organs. - - And in one female specimen, Matzke-Karasz and
her colleagues discovered ripe sperm. The cells were discovered in the
paired sperm receptacles in which they were stored after copulation,
ready for release when the female's eggs matured.

"This female must have mated shortly before being encased in the resin,"
says He Wang of the Chinese Academy of Sciences in Nanjing. The X-ray
images also revealed the sperm pumps and the pair of penises that male ostracods insert into the twin gonopores of the females.

The finds in Burmese amber provide unprecedented insights into
an unexpectedly ancient and advanced instance of evolutionary
specialization. "The complexity of the reproductive system in these
specimens raises the question of whether the investment in giant
sperm cells might represent an evolutionarily stable strategy, says Matzke-Karasz. The males of most animal species (including humans)
produce very large numbers of very small sperm. Comparatively few animals, including some fruit flies -- and of course, ostracods -- have opted for
a different approach. They make a relatively small numbers of oversized
sperm, whose motile tails are several times longer than the animal itself.

"In order to prove that the use of giant sperm is not an extravagant
whim on the part of evolution, but a viable strategy that can confer an enduring advantage that enables species to survive for long periods of
time, we must establish when this mode of reproduction first appeared,"
says Matzke-Karasz.

Examples of fossilized sperm cells are extremely rare. The oldest known ostracod sperm (prior to the new discovery) are 17 million years old,
and the previous record age, 50 Myr, was held by a species of worm. The
new evidence extends that age by a factor of at least two. The fact that animals had already developed giant sperm 100 million years ago implies
that this reproductive strategy can indeed be successful in the (very)
long term, Matzke-Karasz points out. "That's a pretty impressive record
for a trait that requires a considerable investment from both the males
and females of the species. From an evolutionary point of view, sexual reproduction with the aid of giant sperm must therefore be a thoroughly profitable strategy."

========================================================================== Story Source: Materials provided by
Ludwig-Maximilians-Universita"t_Mu"nchen. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. He Wang, Renate Matzke-Karasz, David J. Horne, Xiangdong Zhao,
Meizhen
Cao, Haichun Zhang, Bo Wang. Exceptional preservation
of reproductive organs and giant sperm in Cretaceous
ostracods. Proceedings of the Royal Society B: Biological Sciences,
2020; 287 (1935): 20201661 DOI: 10.1098/ rspb.2020.1661 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200916090532.htm

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