Artificial whole genome duplication in paleopolyploid sturgeons yields highest documented chromosome number in vertebrates

Abstract

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. Here we demonstrate that the sturgeon genome can undergo numerous alterations of ploidy without severe physiological consequences, producing progeny with a range of ploidy levels and extremely high chromosome numbers. 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 progeny 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 2n ~ 446 in the cyprinid fish Ptychobarbus dipogon.

Figure 1

Time-lapse of the first two cleavage divisions of Russian sturgeon zygotes treated with mitotic heat shock showing delayed development of functionally octoploid zygotes. The first photo was taken at the first cleavage division 3 h after fertilization with subsequent time-lapse capturing in 15 min. Incubation temperature 16 °C. A – unfertilized egg; B – tetraploid zygotes; C – putative octoploid zygotes.

Figure 2

Example of flow cytometry analysis Siberian sturgeon larvae subjected to mitotic and/or meiotic shock and control. (a) Control sample, Siberian sturgeon, functionally tetraploid (4n) is at channel 54.95 (CV 3.19); (b) Functionally octoploid (8n) is at channel 110.96 (CV 2.02) and 4n control; (c) Functionally dodecaploid (12n) is at channel 166.05 (CV 1.93) and 4n control; (d) Diploid/hexaploid mosaic (2n/6n is at channels 26.67 and 81.54 (CV 3.15 and 1.88 respectively) and 4n control.

Figure 3

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

Figure 4

Chromosomes of octoploid Siberian sturgeon. (a) Metaphase spread obtained from leucocyte culture of functionally octoploid Siberian sturgeon counting 501 chromosomes; (b) Corresponding karyotype composed of 200 metacentric/submetacentric chromosomes, 96 acrocentric/telocentric chromosomes, and 205 microchromosomes.