Acipenseriformes are composed of 25 sturgeon species and two paddlefish species distributed exclusively in the northern hemisphere. The Danube River and the Black Sea were originally inhabited by six sturgeon species but two are extinct and only four are still reproducing currently in the Lower Danube: Huso huso, Acipenser stellatus, A. gueldenstaedtii and A. ruthenus. Sturgeon species hybridize more easily than other fish and the determination of pure species or hybrid status is important for conservation and for breeding in fish farms. This survey demonstrated that morphological determination of this status is not reliable and a molecular tool, based on eight microsatellites genotypes is proposed. This method, based on three successive statistical analyses including Factorial Correspondence Analysis (FCA), Structure assignation and NewHybrids status determination, showed a high efficiency in discriminating pure species specimens from F1, F2 and two kinds of backcross individuals involving three of the four reproducing Lower Danube sturgeon species.

Several hypotheses explain the prevalence of undifferentiated sex chromosomes in poikilothermic vertebrates. Turnovers change the master sex determination gene, the sex chromosome or the sex determination system (e.g. XY to WZ). Jumping master genes stay main triggers but translocate to other chromosomes. Occasional recombination (e.g. in sex-reversed females) prevents sex chromosome degeneration. Recent research has uncovered conserved heteromorphic or even homomorphic sex chromosomes in several clades of non-avian and non-mammalian vertebrates. Sex determination in sturgeons (Acipenseridae) has been a long-standing basic biological question, linked to economical demands by the caviar-producing aquaculture. Here, we report the discovery of a sex-specific sequence from sterlet ( Acipenser ruthenus ). Using chromosome-scale assemblies and pool-sequencing, we first identified an approximately 16 kb female-specific region. We developed a PCR-genotyping test, yielding female-specific products in six species, spanning the entire phylogeny with the most divergent extant lineages ( A. sturio, A. oxyrinchus versus A. ruthenus, Huso huso ), stemming from an ancient tetraploidization. Similar results were obtained in two octoploid species ( A. gueldenstaedtii, A. baerii ). Conservation of a female-specific sequence for a long period, representing 180 Myr of sturgeon evolution, and across at least one polyploidization event, raises many interesting biological questions. We discuss a conserved undifferentiated sex chromosome system with a ZZ/ZW-mode of sex determination and potential alternatives. This article is part of the theme issue ‘Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)’.

No abstract

No abstract

No abstract