Why do they need polyploidy?
Biologists have found out why the genome of the silver carp consists of six copies
Mikhail Orlov, Naked Science
The silver carp Carassius gibelio is a dangerous invasive species that threatens the native fish of Europe. A successful invader of reservoirs is his ability to reproduce asexually with parasitism on the sperm of other fish and the associated presence of as many as six copies of the genome.
Silver carp (in English it is called Prussian carp, although it originates from Asia – VM) is one of the species of the genus crucian carp from the family of cyprinid fish. Its closest relatives are the common crucian carp C.carassius and the goldfish C.auratus, but zoologists have different views on their taxonomy.
Like goldfish, which were domesticated in Asia many centuries ago and turned into a whole variety of decorative varieties, silver carp are also bred in ponds. And this species is widely dispersed in natural reservoirs, especially in Europe, where it turned out to be the most aggressive representative of invasive fauna.
Populations of silver carp are mostly represented by females, and in some reservoirs consist exclusively of them. How do female silver carp 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 and form a full-fledged embryo without fertilization by a sperm.
In the case of the silver carp, a significant caveat is necessary: for its parthenogenesis, sperm is still needed — only other fish related to this species. Foreign sperm cells trigger the development of eggs, but their DNA is not used in it.
It is worth noting that the ability to reproduce in the usual way C gibelio has not been lost: in the presence of males in individual varieties, the usual fertilization of eggs with sperm occurs.
In addition to "parasitism on someone else's sperm" (really parasitism, because other fish lose their sex cells at the same time), 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 quite a rarity. The silver carp is particularly 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 (Kuhl et al., Balanced evolution of the mixed auto-/allopolyploid haplotype-resolved genome of the invasive hexaploid Prussian carp) obtained the sequence of the C genome. gibelio — each of the 150 chromosomes of six haploid sets. This allowed us to understand how they got into the genome, and establish their connection with the unusual breeding strategy of this animal.
The fact is that parthenogenesis usually leads to a steady accumulation of harmful mutations, but it turned out that the 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 the 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 copies obtained) took place.
It is known that one episode of genome-wide doubling occurred in the ancestor of all ray-finned fish, and the other is associated with the appearance of the carp family. The authors of a new study found that the ancestor Carassius gibelio was just a species with a tetraploid genome — consisting of four haploid sets of chromosomes.
Scientists emphasize the connection between the unusual genome of this fish 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," she explained Dunja Lamatsh from the University of Innsbruck (Austria), head of the study.
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