Abstract
BackgroundEvolution of sturgeons and paddlefishes (order Acipenseriformes) is inherently connected with polyploidization events which resulted in differentiation of ploidy levels and chromosome numbers of present acipenseriform species. Moreover, allopolyploidization as well as autopolyploidization seems to be an ongoing process in these fishes and individuals with abnormal ploidy levels were occasionally observed within sturgeon populations. Here, we reported occurrence of Siberian sturgeon (Acipenser baerii) male with abnormal ploidy level for this species, accessed its ploidy level and chromosome number and investigate its potential sterility or fertility in comparison with normal individuals of sterlet (A. ruthenus), Russian sturgeon (A. gueldenstaedtii) and Siberian sturgeon (A. baerii).ResultsAcipenser ruthenus possessed 120 chromosomes, exhibiting recent diploidy (2n), A. gueldenstaedtii and A. baerii had ~245 chromosomes representing recent tetraploidy (4n), and A. baerii male with abnormal ploidy level had ~ 368 chromosomes, indicating recent hexaploidy (6n). Genealogy assessed from the mtDNA control region did not reveal genome markers of other sturgeon species and this individual was supposed to originate from spontaneous 1.5 fold increment in number of chromosome sets with respect to the number most frequently found in nature for this species. Following hormone stimulation, the spontaneous hexaploid male produced normal sperm with ability for fertilization. Fertilization of A. baerii and A. gueldenstaedtii ova from normal 4n level females with sperm of the hexaploid male produced viable, non-malformed pentaploid (5n) progeny with a ploidy level intermediate to those of the parents.ConclusionThis study firstly described occurrence of hexaploid individual of A. baerii and confirmed its autopolyploid origin. In addition to that, the first detailed evidence about fertility of spontaneous hexaploid sturgeon was provided. If 1.5 fold increment in number of chromosome sets occurring in diploids, resulted triploids possess odd number of chromosome sets causing their sterility or subfertility due to interference of gametogenesis. In contrast, 1.5 fold increment in number of chromosome sets in naturally tetraploid A. baerii resulted in even number of chromosome sets and therefore in fertility of the hexaploid specimen under study.
Highlights
Evolution of sturgeons and paddlefishes is inherently connected with polyploidization events which resulted in differentiation of ploidy levels and chromosome numbers of present acipenseriform species
We reported the occurrence of a spontaneous hexaploid male among hatchery stock of Siberian sturgeon, Acipenser baerii, a species of recent tetraploid level 4n with ~245 chromosomes; its experimental hybridization with normal females of A. baerii and A. gueldenstaedtii; and analysis of resulting viable progeny
The relative DNA content in juveniles from purebreeding of A. baerii and A gueldenstaedtii (Figure 3a), from the normal x spontaneous hexaploid A. baerii and from the normal A. gueldenstaedtii x spontaneous hexaploid A. baerii hybridizations (Figure 3b) revealed 100% normal ploidy in the purebred juveniles compared to 100% intermediate ploidy level of the F1 hybrid juveniles
Summary
Evolution of sturgeons and paddlefishes (order Acipenseriformes) is inherently connected with polyploidization events which resulted in differentiation of ploidy levels and chromosome numbers of present acipenseriform species. Paddlefish, the fishes of the genera Psephurus, Polyodon (Acipenseriformes: Polyodontidae), Acipenser, Huso, Scaphirhynchus, and Pseudoscaphirhynchus (Acipenseriformes: Acipenseridae) provide the most remarkable examples of evolution of ploidy levels among vertebrates [7]. Several well divided groups of acipenseriform species can be recognized depending on DNA content and the number of chromosomes in their cell nuclei. They include species with ~120, ~240 and ~360 chromosomes, corresponding to elevated DNA content [10]. Recent investigations suggest two scales of ploidy levels in Acipenseriformes: the ‘evolutionary scale,’ which presumes tetraploid - octaploid - dodecaploid relationships among species, and the ‘recent scale’, which presumes diploid–tetraploid–hexaploid relationships [11]
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