RESUMEN
All-female culture of sturgeon is essential for efficient caviar production. However, Russian sturgeon (Acipenser gueldenstaedtii) does not exhibit external sexual dimorphism, and therefore, commercial farms apply gonadal endoscopy or ultrasound at the earliest age of 4-5 years to separate the sexes, with ~90% accuracy. Recently, a dominant genomic marker (AllWSEX2) has been found with association to femaleness in sturgeons. We developed a duplex PCR (dAllWSEX2) with the adjacent bmp7 gene as an internal control, to validate an effective PCR. Robust amplification of control fragments was observed for all samples of our commercial A. gueldenstaedtii stock (n = 337). The dAllWSEX2 assay was significantly associated with sex (n = 43, p < 1.6 × 10-8 ), yet four (18%) of the endoscopy-determined females were genetic males. To examine whether some females display a male genetic profile, we tested 96 egg-producing females, which were all verified as genetic females, indicating that the observed mismatches may be attributed to wrong sexing by endoscopy. Application of dAllWSEX2 on 100 7-month-old fish showed no sex-dependent differences in body weight, indicating that weighing is not an applicable tool for sorting females at a young age. Sanger sequencing of the bmp7 fragment revealed octaploidy and sex-independent variation, suggesting that the critical sex-determining region harboring AllWSEX2 is small. In keeping with a model of a single-ploidy encoding female determination, AllWSEX2 showed no variation despite being a transposase-linked repetitive element. Cross-species conservation of AllWSEX2, and absence of annotated sex-determination genes in this region suggests that, in sturgeons, the sex-determining mechanism is different from mechanisms identified in other fish.