Polyploidization refers to the multiplication of one or more complete chromosome sets in an organism and represents an important step in evolution and speciation. Polyploidy, i.e. the status when the organism already has a higher number of chromosome sets in every cell, provides beneficial genetic flexibility and broad adaptive responses, i.e., additional gene copies theoretically allow evolution under reduced selective constraint and the acquisition of novel gene functions that contribute to adaptation.

The occurrence of polyploidy in vertebrates is rather sporadic compared to invertebrates and plants, and is mostly restricted to amphibians and ray-finned fishes. The highest chromosome count of any vertebrate to date (2n ~ 446) was documented in Ptychobarbus dipogon, a cyprinid fish from the Tibetan Plateau.

Critically endangered sturgeons, having undergone three whole genome duplication events, represent an exceptional example of ploidy plasticity in vertebrates. Three extant ploidy groups, combined with autopolyploidization, interspecific hybridization, and the fertility of hybrids are important issues in sturgeon conservation and aquaculture.

Solving the CSF grant „Genome duplication in sturgeon evolution and impact on their biology“ (18-09323S), we studied the cases and outcomes of induced polyploidization. Artificial suppression of the first mitotic division alone, or in combination with suppression of the second meiotic division of functionally tetraploid zygotes (4n, C-value = 4.15) of Siberian sturgeon Acipenser baerii and Russian sturgeon A. gueldenstaedtii resulted in fry of various ploidy levels—diploid/hexaploid (2n/6n) mosaics, hexaploid, octoploid juveniles (8n), and dodecaploid (12n) larvae. Counts between 477 to 520 chromosomes in octoploid juveniles of both sturgeons confirmed the modal chromosome numbers of parental species had been doubled. This exceeds the highest previously documented chromosome count among vertebrates in the cyprinid fish Ptychobarbus dipogon and it demonstrates that the sturgeon genome can undergo numerous alterations of ploidy without severe physiological consequences, producing individuals with a range of ploidy levels and extremely high chromosome numbers.

For detailed information see the original article: Lebeda, I., Ráb, P., Majtánová, Z., Flajšhans, M., 2020. Artificial whole genome duplication in paleopolyploid sturgeons yields the highest documented chromosome number in vertebrates. Scientific Reports 10, Article No 19705 (IF = 3.998; Q1; DOI: 10.1038/s41598-020-76680-4).

Flow cytometry analysis of DNA content in a pooled sample of cells from four Siberian sturgeon larvae (2n/6n – mosaic fish; 4n – control; 8n – octaploid; 12n – dodecaploid).

Chromosomes of octoploid Russian sturgeon. (a) Metaphase spread obtained from leucocyte culture counting 520 chromosomes; (b) Corresponding karyotype composed of 224 metacentric/submetacentric chromosomes, 88 acrocentric/telocentric chromosomes, and 208 microchromosomes.