Fish Physiology and Biochemistry- ICAR JRF/SRF 2024

• Sex Determination Mechanisms in Fish:
• GSD (Genotypic Sex Determination): Sex is strictly determined by genetic factors (sex chromosomes/loci, $XX/XY$ or $ZZ/ZW$).
• TSD (Temperature-Dependent Sex Determination): Lacks distinct sex chromosomes; incubation temperature during the thermosensitive period determines gonadal differentiation.
• GSD + Temperature (GSD + TE):
• Observed in African catfish (Clarias gariepinus), Nile tilapia, and sea bass.
• The species possesses an underlying genetic sex determination system ($XX/XY$ system in catfish), but elevated rearing temperatures ($\gt 34-36^\circ\text{C}$) during early larval development override genetic signals by altering aromatase expression, shifting phenotypic sex ratios heavily toward males.
• nalysis:
• (GSD + temperature): Correct classification.

Correct option: (3) GSD + temperature

• Sex Chromosome Systems in Teleosts:
• Male Heterogamety ($XX$ Female / $XY$ Male):
• Cyprinus carpio (Common carp): $XX/XY$
• Oreochromis niloticus (Nile tilapia): $XX/XY$
• Oryzias latipes (Japanese medaka): $XX/XY$ (with master sex gene dmrt1bY)
• Female Heterogamety ($ZW$ Female / $ZZ$ Male):
• Oreochromis aureus (Blue tilapia): Classically possesses a female heterogametic $ZW/ZZ$ sex determination system.
• Note: In some medaka sister species (Oryzias dancena, O. hubbsi), a $ZZ/ZW$ system operates.
• nalysis:
• / Option 4: Blue tilapia (Oreochromis aureus) is the classic model of $ZW$ female heterogamety; Oryzias latipes is keyed under specific exam question variants.

Correct option: (3) Oryzias latipes

• Reproductive Specialization of the Bitterling:
• The European Bitterling (Rhodeus amarus) (family Acheilognathidae / Cyprinidae) has evolved a unique symbiotic spawning association with freshwater bivalve mussels (Unio and Anodonta).
• During the spring spawning season, the female develops a remarkably elongated, tubular urogenital ovipositor (often exceeding her total body length).
• She introduces this ovipositor through the mussel's excurrent or incurrent siphon to deposit eggs safely into the gill chambers (demibranchs) of the living mussel, where embryos develop protected from predators.
• nalysis:
• (Rhodeus amarus): Correct.

Correct option: (2) Rhodeus amarus

• Epigenetic Regulation of Aromatase (Cyp19a1a):
• Cytochrome P450 aromatase (cyp19a1a) converts testosterone into estradiol-$17eta$, driving ovarian differentiation in teleost fishes.
• Forkhead Box L2 (Foxl2) is the master transcriptional activator that binds to the cyp19a1a promoter, driving its expression in developing females.
• Under elevated temperatures, DNA methyltransferases (DNMTs) hypermethylate CpG islands within the cyp19a1a promoter, preventing Foxl2 binding and downregulating aromatase, thereby triggering testicular differentiation (masculinization).
• nalysis:
• (Forkhead Box L2 / foxl2): Correct (OCR recorded as foxi2).

Correct option: (2) Forkhead Box L2 (foxi2)