Sturgeons are famous for the black caviar – the most expensive luxury roe. Nowadays, most of species from Acipenseridae family became critically endangered (17 out of 27 species). Such situation happened because of several factors: overfishing, poaching, pollution of water surface and dam constructions. Nowadays the restoration of the sturgeon populations is dependent on artificial reproduction. However the sturgeon reproduction is not easy. The sturgeons reach maturation quite late. Some species, such as beluga (Huso huso), become matured around 20 years. Therefore it is very important to understand biology of reproduction of these fishes.

Viktoriia Iegorova and colleagues from the Laboratory of Germ Cells, Faculty of Fisheries and Protection of Waters described fertilization aspects, which are unique throughout the whole animal kingdom. Namely two fertilization mechanisms were observed: 1) physiological polyspermy (penetration of numerous spermatozoa into an egg) and 2) karyogamy (fusion of sperm and egg pronuclei) with an additional plasmogamy (fusion of egg cytoplasm with accessory spermatozoa). The study demonstrates that during fertilization, numerous spermatozoa (up to 10) penetrate into the egg cytoplasm due to higher number of micropyles. These spermatozoa have trend to degenerate with time, and usually only one sperm pronucleus fuses with the egg pronucleus. However, some spermatozoa, which did not fuse with the egg pronucleus and escaped from the degradation mechanisms, can also participate in the development and give rise to fishes, called “polyspermic mosaics”. Such mosaics were distinguished by characteristic cleavage pattern, demonstrated a higher number of blastomeres at the 2 to 4 cell stage with a characteristic mosaic haploid/diploid ploidy. In this research it was confirmed that the diploid cells contain maternal and paternal genetic information, while the haploid cells possess only a paternal genome. Surprisingly these mosaics develop normally and survive similar to the control fish.

The obtained discovery gave us a possibility to produce a first viable hybrids originating from three interspecific parents in different combinations between Acipenser gueldenstaedtii, A. baerii and A. ruthenus. In other words, one descendant has one mother and two fathers of different species, where sperm pronucleus of one species fuses with egg pronucleus of second species resulting in diploid line and the sperm pronucleus of third species fuses with egg cytoplasm and give rise to the haploid line.

These findings can open a new approach in biotechnology such as production of clonal gametes.

Detailed information on obtained results can be found in publications:

Iegorova, V., Psenicka, M., Lebeda, I., Rodina, M., Saito, T. Polyspermy produces viable haploid/diploid mosaics in sturgeon. Biology of Reproduction, 2018, 99(4), 695–706.
Iegorova, V., Psenicka, M., Saito, T. First evidence of viable progeny from three interspecific parents in sturgeon. Fish Physiology and Biochemistry, 2018. DOI 10.1007/s10695-018-0553-6.

Sterlet (Acipenser ruthenus) embryo at cleavage stage from 2 to 256 cells. Polyspermic embryos begging the cleavage atypically from 3 (A) or 6 blastomeres (B), while the control embryos show normal beginning of cleavage from 2 blastomeres.

 

 

Larva of sterlet (A. ruthenus) from polyspermic (A) and normal fertilization (B).

 

 

 

 

 

Flow cytometry analysis showed sterlet (A. ruthenus) atypicaly divided (AD) polyspermic embryos to be haploid/diploid mosaics. (A) A control – normally dividing embryo at the hatching stage with a single diploid (2n) cell population. (B) AD embryo with one haploid (1n) and one diploid (2n) cell population. In this embryo, the haploid cell population is more abundant than the diploid one. (C) AD embryo is a 1n/2n mosaic as well but with fewer haploid cells.


 

 

A photography from this work was selected for cover page of the issue in Biology of Reproduction.