Biologists have discovered why the silver carp genome consists of six copies

(ORDO NEWS) — The silver carp Carassius gibelio is a dangerous invasive species that threatens the native fish of Europe.

What makes it a successful invader of water bodies is the ability to reproduce with parasitism on the sperm of other fish and the presence of as many as six copies of the genome associated with them.

In a new study, biologists have obtained the DNA sequence of silver carp and found out the evolutionary roots of this unusual fish.

Silver, or Prussian crucian carp Carassius gibelio is one of the species of the genus crucian carp from the family of carp fish. Its closest relatives are common crucian carp C. carassius and goldfish C. auratushowever , zoologists have different views on their taxonomy.

Like goldfish , which were domesticated in Asia many centuries ago and turned into a whole host of decorative varieties, goldfish are also bred in ponds. And this species has also spread widely in natural water bodies, especially in Europe, where it turned out to be the most aggressive representative of the invasive fauna.

Prussian carp populations are mostly represented by females, and in some reservoirs they consist exclusively of them. How do female goldfish manage to reproduce without the help of males?

Like some other organisms (mostly plants and invertebrates), they are capable of parthenogenesis, that is, same-sex reproduction. In this case, the egg (egg) can begin to split up and form a full-fledged embryo without fertilization by a spermatozoon.

In the case of silver carp, a significant reservation is necessary: ​​for its parthenogenesis, sperm is still needed – only other fish related to this species. Alien spermatozoa trigger the development of eggs, but their DNA is not used in it.

It should be noted that C. gibelio has not lost the ability to reproduce in the usual way: in the presence of males, in some species, the usual fertilization of eggs by sperm occurs.

In addition to “parasitism on someone else’s sperm” (really parasitism, because other fish lose their germ cells), the silver carp has another strange trait associated with the first. We are talking about its polyploidy, that is, the presence of more than two haploid sets of chromosomes in the genome.

Many polyploids are known among plants , but for animals (especially vertebrates) this is a decent rarity. Silver carp is especially surprising in this regard, because its genome consists of as many as six copies, which are slightly different from each other.

To deal with the complex evolutionary past, the authors of a new article in Nature Communications obtained the sequence of the C. gibelio genome – each of the 150 chromosomes of six haploid sets.

This made it possible to understand how they got into the genome, and to establish their connection with the unusual reproduction strategy of this animal.

The fact is that parthenogenesis usually leads to a steady accumulation of harmful mutations, but it turned out that silver carp successfully fights this due to the special organization of the genome and the presence of “spare copies” that make it more stable.

It turns out that silver carp received six copies of its genome in two different ways. In the evolutionary past of this fish, both allopolyploidization (the introduction of a set of chromosomes of another species) and autopolyploidization (doubling of its own genome followed by a gradual change in the resulting copies) took place.

It is known that one episode of genome-wide doubling occurred in the ancestor of all ray- finned fish, and another is associated with the appearance of the Cyprinidae family. The authors of the new study found that the ancestor of Carassius gibelio was just a species with a tetraploid genome – consisting of four haploid sets of chromosomes.

Scientists highlight the connection between this fish’s unusual genome and its even more unusual reproduction.

“Probably, at some point during all these crosses, there were problems with the formation of germ cells. This could be the beginning of same-sex reproduction, ” explained Dunja Lamatsh from the University of Innsbruck (Austria), head of the study.

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