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Early gonadal development in Russian (Acipenser gueldenstaedtii Brandt, 1869) and Siberian sturgeon (Acipenser baerii Brandt & Ratzeburg, 1833) reared in controlled conditions from dph 1-439 (days post hatching) was studied using histological methods. Migrating primordial germ cells (PGCs) were observed in both species on dph 1. On dph 25 in both species gonad primordia developed in gonadal ridges. In dph 115 juveniles two types of gonad were distinguished, putative ovary and putative testicle. Meiotic activity was observed in ovaries on dph 197, while in testicles the first meiotic divisions were observed much later, on dph 439 and only in several Siberian specimens. The presence of Vasa protein was confirmed exclusively in germ cells during gonadogenesis, indicating that Vasa is a good marker of germinal cells also in Acipenseridae. Vasa protein displayed a nuage association or nuage-like localization. Some differences in the level of Vasa protein expression during male and female gonad development were also observed. In the gonads of both investigated sturgeon species the PCNA (proliferating cell nuclear antigen)-positive reaction was observed first in oogonia, while in spermatogonia at approximately 115 dph which was correlated with active mitotic divisions of germ cells. Surprisingly, meiotic oocytes arrested in diplotene also demonstrated strong PCNA-positive germinal vesicle.

Among sturgeons, the occurrence of individuals with gonads containing both testis and ovary components is considered pathological, and such fish are described as intersex individuals or intersexes. Intersexes are observed in both wild and cultured populations of sturgeon, usually at low frequencies. In the present study, intersex Russian (Acipenser gueldenstaedtii) and Siberian (Acipenser baerii) sturgeons constituted 30% of the studied populations. Macroscopically, intersex gonads were recognizable from 500 days posthatching (dph). Initially, gonads with predominantly male characteristics (testis-ova) were observed, but in older fish gonads with predominantly female traits (ova-testis) were more frequent. Using microscopic analysis, intersex gonads were discernible by 130-200 dph. Observations of intersex germinal epithelium development and analysis of sex distribution in the study populations indicated that feminization was occurring. Histological analysis revealed that differentiation of the germinal epithelium in such gonads was accompanied by various morphological alterations (transformations) that were described using quantitative and localization criteria. The most common type of transformations, massive subepithelial transformations, was manifested by the presence of abundant female germinal tissue located under the gonad surface epithelium in the developing testis. These transformations were identified in the early development stage (100-200 dph). In this type of transformation, differentiation of female germinal tissue at the gonad surface and male tissue at the mesorchium/mesovarium resulted in complete formation of both male and female germinal epithelia within the same gonad.

Expression of the dmrt1 and vtg genes was described using the real-time PCR (rt-PCR) method from 25 to 1600 days post-hatch (dph) in cultured Russian sturgeon Acipenser gueldenstaedtii. The level of dmrt1 transcription in gonads in subsequent studied periods increased exponentially while vtg expression increased in gonads and livers of A. gueldenstaedtii examined, but in later stages of development. Both dmrt1 and vtg genes showed elevated expression in intersex individuals probably caused by dietary exposure to phyto-oestrogens.

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