A 180 Myr-old female-specific genome region in sturgeon reveals the oldest known vertebrate sex determining system with undifferentiated sex chromosomes.

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)'.

Chronic exposure to the ß-blocker metoprolol reduces growth and alters gene expression of gonadotropins and vitellogenin in Nile tilapia (Oreochromis niloticus).

Knowledge of the occurrence and impacts of human pharmaceuticals in the aquatic environment is increasing since many years. Ecotoxicological studies mainly focus on acute effects though; chronic exposure studies are still rare. ß-adrenergic receptor antagonists (ß-blockers) are widely detected in the aquatic environment and likely alter the physiology of aquatic vertebrates due to a well-conserved adrenergic system. In this study, Nile tilapia (Oreochromis niloticus) were exposed to four different concentrations (4×10-10M, 4×10-9M, 4×10-8M and 4×10-7M) of metoprolol (ß1-blocker) from fertilized egg until 80 days post-hatch. Hatching and survival were not affected but growth was reduced almost dose-dependently after 30 and 80 days post-hatch. Histopathological evaluation of the gills revealed the tendency of mild alterations with proliferation of mucous/chloride cells and infiltration by leucocytes as the main findings. The transcriptional responses of both pituitary gonadotropins (luteinizing hormone and follicle stimulating hormone) as well as the estrogenic biomarker vitellogenin indicated moderately altered endocrine processes due to metoprolol exposure at the concentrations chosen. In contrast, hepatic detoxification mechanisms displayed only little to no effects. Based on this study, the overall toxicity of metoprolol in fish at environmentally relevant concentrations seems to be rather low.

Chronic diclofenac exposure affects gill integrity and pituitary gene expression and displays estrogenic activity in nile tilapia (Oreochromis niloticus).

Oreochromis niloticus has been exposed to diclofenac (DCF), a nonsteroidal anti-inflammatory drug prevalent in the aquatic environment, for 80 days post-hatch (dph). Concentrations ranged from environmentally relevant (0.1 µg L-1 and 1 µg L-1 DCF) up to 100-fold thereof. Population relevant endpoints (hatching, survival, growth) as well as gill histopathology were analyzed. On this level of examination only gills exhibited mild to moderate alterations. On the contrary, biomarkers associated with reproduction were affected due to DCF exposure, indicating the potential to affect sexual differentiation and gametogenesis by acting as an estrogenic endocrine disrupting compound (EDC) in tilapia. Vitellogenin (VTG) gene expression was significantly induced at 1 µg L-1 DCF. In order to find an explanation, gene expression patterns of key enzymes of the biotransformation phases I, II, and III have been analyzed. It seems very likely that the detoxification metabolism is induced in a dose dependent manner at higher concentrations of DCF leading to the expression pattern of VTG mRNA. Our results suggest that DCF at environmentally relevant concentrations adversely affects O. niloticus gill histopathology and pituitary gene expression, and has the potential to act as an estrogenic EDC. The sensitivity of various endpoints, however, differs and therefore these endpoints should be linked.