Flatfishes colonised freshwater environments by acquisition of various DHA biosynthetic pathways.

The colonisation of freshwater environments by marine fishes has historically been considered a result of adaptation to low osmolality. However, most marine fishes cannot synthesise the physiologically indispensable fatty acid, docosahexaenoic acid (DHA), due to incomplete DHA biosynthetic pathways, which must be adapted to survive in freshwater environments where DHA is poor relative to marine environments. By analysing DHA biosynthetic pathways of one marine and three freshwater-dependent species from the flatfish family Achiridae, we revealed that functions of fatty acid metabolising enzymes have uniquely and independently evolved by multi-functionalisation or neofunctionalisation in each freshwater species, such that every functional combination of the enzymes has converged to generate complete and functional DHA biosynthetic pathways. Our results demonstrate the elaborate patchwork of fatty acid metabolism and the importance of acquiring DHA biosynthetic function in order for fish to cross the nutritional barrier at the mouth of rivers and colonise freshwater environments.

Unilateral ovariectomy increases egg size and reduces follicular atresia in the semelparous coho salmon, Oncorhynchus kisutch.

Unilateral ovariectomy (ULO, removal of one ovary) is a powerful technique for studying aspects of reproductive physiology, including follicular recruitment and growth. To examine effects of ULO for the first time in a semelparous species, coho salmon (Oncorhynchus kisutch) were unilaterally ovariectomized during mid-vitellogenesis approximately 3 months before spawning. At termination of the study (79 days post-surgery), single ovaries of ULO fish were gravimetrically equivalent to paired ovaries of sham surgery, control fish. There was no evidence of recruitment of new vitellogenic follicles. Instead, the dramatic increase in ovary mass was attributable to hypertrophy of existing vitellogenic follicles (33% increase in volume) and increased fecundity achieved through a greater than two-fold reduction in follicular atresia. The composition of whole ovaries on a dry weight basis from ULO fish was greater in protein, but lower in lipid than that of control fish. Expressing the data on a per follicle basis, however, showed that follicles of ULO fish contained more protein, ash, water, and lipid. The results indicate that ULO of coho salmon induces compensatory hypertrophy of existing vitellogenic follicles, while maximizing fecundity through reduction of atresia. Thus, 3 months before spawning, coho salmon exhibit the ability to adjust final egg size and number when faced with significant depletion of ovarian follicles. This in vivo system provides a platform for further study of physiological mechanisms regulating follicular growth and atresia, and the trade-off between egg size and egg number.

IGF-I and branchial IGF receptor expression and localization during salinity acclimation in striped bass.

The initial response of the IGF-I system and the expression and cellular localization of IGF type-I receptor (IGF-IR) were studied in the gill of a euryhaline teleost during salinity acclimation. Exposure of striped bass (Morone saxatilis) to hyperosmotic and hypoosmotic challenges induced small, transitory (<24 h) deflections in hydromineral balance. Transfer from freshwater (FW) to seawater (SW) induced an initial decrease in plasma IGF-I levels after 24 h in both fed and fasted fish. There was an overall decrease in liver IGF-I mRNA levels after SW transfer, suggesting that decreased plasma levels may be due to a decline in hepatic IGF-I synthesis. No changes were observed in gill IGF-I mRNA, but SW transfer induced an increase in gill IGF-IR mRNA after 24 h. Transfer from SW to FW induced an increase in plasma IGF-I levels in fasted fish. In fed fish, no significant changes were observed in either plasma IGF-I, liver, or gill IGF-I mRNA, or gill IGF-IR mRNA levels. In a separate experiment, FW-acclimated fish were injected with saline or IGF-I prior to a 24-h SW challenge. Rapid regain of osmotic balance following SW transfer was hindered by IGF-I. Immunohistochemistry revealed for the first time in teleosts that IGF-IR and Na(+)-K(+)-ATPase are localized in putative chloride cells at the base of the lamellae, identifying these cells in the gill as a target for IGF-I and IGF-II. Overall the data suggest a hyperosmoregulatory role of IGF-I in this species.

Aromatase cytochrome P450: cloning, intron variation, and ontogeny of gene expression in southern flounder (Paralichthys lethostigma).

Aromatase cytochrome P450 (P450arom) is the enzyme complex responsible for conversion of androgens to estrogens in vertebrates. Consequently, in some fishes its activity appears critical to ovarian differentiation. Southern flounder (Paralichthys lethostigma) is a commercially important flatfish in which females grow larger than males and sex determination is temperature sensitive. Through cloning of the P450arom gene in ovary and quantitative reverse transcription-polymerase chain reaction, we developed a biomarker for early female differentiation in southern flounder. The deduced amino acid sequence for southern flounder P450arom is similar to other teleosts. Comparison of P450arom intron sequences from fish of different populations revealed substantial inter-individual variation. Adult ovary and spleen exhibited high levels of P450arom mRNA, while P450arom mRNA was only weakly detected in testes. Brain, liver, intestine, kidney, gill, muscle, and heart showed little or no P450arom mRNA expression. Gonads of wild and hatchery-produced juvenile flounder of sizes spanning the period of sex differentiation initially exhibited low levels of P450arom mRNA followed by increases in some individuals and bifurcation into two clearly segregated groups (i.e., putative males and females) beginning at approximately 65 mm in total length. Gonadal histology confirmed predictions of sex based on P450arom expression in juvenile flounder, demonstrating that the patterns of P450arom expression observed relate to sex-specific differentiation. This research represents a unique approach to assessing sex differentiation in a natural population, and a powerful technique for better understanding mechanisms of flounder sex determination and rapidly defining conditions for controlling sex for aquaculture.