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An adult coelacanth or Latimeria chalumnae. This specimen is on loan to the National Museum of Natural History, from the South African Institute of Aquatic Biodiversity (SAIAB). Photograph by Don Hurlburt.

The coelacanth leads a monogamous life

The coelacanth is a 'living fossil' fish species having lived on earth 300 million years! Yet, unlike most fish, it appears females spawn with only one father for all of their offspring.

Scientists have successfully analysed the genetic make-up of the offspring of pregnant coelacanth females for the first time—and found that the likelyhood of offspring being fathered by one single individual is very high – unlike with many other fish species.

Coelacanths were thought to be extinct
Until December 23, 1938, scientists had been convinced that coelacanths were extinct. Only a few fossil prints gave evidence that the animals had existed more than 300 million years ago.

Then, fishermen by the South-African coast discovered a greyish blue fish with a length of some 1.50 m and a weight of some 52 kg in their trawl net—the first specimen of a living coelacanth.

Today, the existence of some 300 specimen has been proved. It is very difficult to get hold of tissue samples of a complete litter of a pregnant coelacanth. This latest identification was made possible by the co-authors of “GEOMAR” in Kiel and Tutzing who have been researching the coelacanths' habits and occurrence for many years.

Dr Kathrin Lampert from the Ruhr-Universität Bochum and Prof Dr Manfred Schartl from the University of Würzburg, Germany, together with their colleagues reported their findings in the journal Nature Communications. The research team gave analyses of the genetic make-up of two pregnant females.

Analysis of the microsatellite DNA
The pregnant coelacanth females studied by the researchers were about to give birth to their offspring.

One female had accidentally ended up in a trawl net on the Mozambique coast and was carrying 26 embryos. Another one caught, unintentionally, by fishermen in Zanzibar waters, was carrying 23. When comparing 14 characteristic spots in genetic make-up of the females and their offspring to each other, the researchers found numerous overlaps. They deployed a method also used for conducting paternity tests in humans, namely microsatellite analysis.

Microsatellites are short DNA sequences, consisting of only a few units that, typically, may recur up to 50 times. They do not usually carry any genetic information, but are passed down from both parents.

“As we know the mother's genotype, we were able to demonstrate by means of the microsatellite analysis that the coelacanth offspring had one single father.”

Manfred Schartl.

Consequently, coelacanth females must be monogamous – at least for a certain period of time. The team also reconstructed the “hypothetical genotype,” i.e. the hypothetical genetic make-up of both fathers.

Coelacanths do not take advantage of multiple mating
It is not clear why the females mate with one single male each.

Mating with several males increases the chance of successful fertilisation, results in a higher genetic variability in the offspring and ensures that the best genes are passed on.

It is possible that the advantages of multiple mating do not outweigh the costs for the female: increased energy input when searching for new males, danger of falling prey to predators, and an increased risk of infection.

No mating with relatives
The researchers discovered another interesting detail in the coelacanth's genetic make-up: father and mother of the offspring were not more closely related than the majority of random couples in a coelacanth population. This could mean that the females avoid mating with close relatives. Or that other features are more relevant in the choice for a suitable mate, for example size and frame or resistance against parasites.

Three years of pregnancy
In many fish species, fertilisation takes place outside the body. The females lay their eggs in a quiet spot in their aquatic environment; subsequently, the males – it can be several – add their semen. The offspring grow up in the water without their parents' protection – whereas the coelacanth gives birth to fully developed young. Scientists estimate that the “pregnancy” takes about three years.

Abstract
Latimeria chalumnae, a ‘living fossil,’ is of great scientific interest, as it is closely related to the aquatic ancestors of land-living tetrapods. Latimeria show internal fertilization and bear live young, but their reproductive behaviour is poorly known. Here we present for the first time a paternity analysis of the only available material from gravid females and their offspring. We genotype two L. chalumnae females and their unborn brood for 14 microsatellite loci. We find that the embryos are closely related to each other and never show more than three different alleles per locus, providing evidence for a single father siring all of the offspring. We reconstruct the father’s genotype but cannot identify it in the population. These data suggest that coelacanths have a monogamous mating system and that individual relatedness is not important for mate choice.

Title catalogue
K.P. Lampert, K. Blassmann, K. Hissmann, J. Schauer, P. Shunula, Z. el Kharousy, B.P. Ngatunga, H. Fricke, M. Schartl (2013): Single male paternity in coelacanths, Nature Communications, doi: 10.1038/ncomms3488

Editorial journalist
Dr. Julia Weiler
Press Office Ruhr University Bochum

Original press releas:http://aktuell.ruhr-uni-bochum.de/pm2013/pm00245.html.en