Abstract
Egg quality in fishes has been a topic of research in aquaculture and fisheries for decades as it represents an important life history trait and is critical for captive propagation and successful recruitment. A major factor influencing egg quality is proper yolk formation, as most fishes are oviparous and the developing offspring are entirely dependent on stored egg yolk for nutritional sustenance. These maternally derived nutrients consist of proteins, carbohydrates, lipids, vitamins, minerals, and ions that are transported from the liver to the ovary by lipoprotein particles including vitellogenins. The yolk composition may be influenced by broodstock diet, husbandry, and other intrinsic and extrinsic conditions. In addition, a number of other maternal factors that may influence egg quality also are stored in eggs, such as gene transcripts, that direct early embryonic development. Dysfunctional regulation of gene or protein expression may lead to poor quality eggs and failure to thrive within hours of fertilization. These gene transcripts may provide important markers as their expression levels may be used to screen broodstock for potential spawning success. In addition to such intrinsic factors, stress may lead to ovarian atresia or reproductive failure and can impact fish behavior, fecundity, and ovulation rate. Finally, postovulatory aging may occur when eggs become overripe and the fish fails to spawn in a timely fashion, leading to low fertility, often encountered during manual strip spawning of fish.
Highlights
Reproductive fitness is a critical component to the success of wild and domestic animals and egg quality of fishes has been a topic of research for decades in regards to aquaculture and fisheries
The intent of this review is to provide an overview of oogenesis, yolk formation, ovarian maturation, and ovulation with special attention paid to our understanding of how these processes may become dysfunctional leading to poor egg quality
In zebrafish (Danio rerio), a teleost exhibiting no lipid droplets in the oocytes [135], apparent expression of lpl transcript was hardly observed in the ovarian follicle cells by in situ hybridization, lpl transcripts were present in the vitellogenic and non-vitellogenic ovaries at detectable levels by PCR amplification [136]
Summary
Reproductive fitness is a critical component to the success of wild and domestic animals and egg quality of fishes has been a topic of research for decades in regards to aquaculture and fisheries. In zebrafish (Danio rerio), a teleost exhibiting no lipid droplets in the oocytes [135], apparent expression of lpl transcript was hardly observed in the ovarian follicle cells by in situ hybridization, lpl transcripts were present in the vitellogenic and non-vitellogenic ovaries at detectable levels by PCR amplification [136] Taken together, these results strongly support the hypothesis that VLDL is metabolized by LPL action at the exterior of the oocyte, especially within the granulosa cells of the follicle. Histological tracing studies using dual fluorescent-labeled VLDL revealed that only the lipid components of VLDL were incorporated into oocytes, while protein components of VLDL (apolipoproteins) remained outside of the oocyte proper showing that VLDL is not endocytosed into the cell [7] This lipase-dependent, non-endocytotic pathway was evident in cutthroat trout, medaka, black skipjack tuna, and pipefish, indicating that the pathway may be common in teleosts that accumulate lipid droplets within the oocytes. The mechanisms regulating oocyte lipidation may be different from that regulating vitellogenesis in fishes
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