The Roles of Kisspeptin System in the Reproductive Physiology of Fish With Special Reference to Chub Mackerel Studies as Main Axis

Abstract

Kisspeptin, a novel neuropeptide product of the Kiss1 gene, activates the G protein-coupled membrane receptor G protein-coupled receptor 54 (now termed Kiss1r). Over the last 15 years, the importance of the kisspeptin system has been the subject of much debate in the mammalian research field. At the heart of the debate is whether kisspeptin is an absolute upstream regulator of gonadotropin-releasing hormone secretion, as it has been proposed to be the master molecule in reproductive events and plays a special role not only during puberty but also in adulthood. The teleostean kisspeptin system was first documented in 2004. Although there have been a number of kisspeptin studies in various fish species, the role of kisspeptin in reproduction remains a subject of controversy and has not been widely recognized. There is an extensive literature on the physiological and endocrinological bases of gametogenesis in fish, largely derived from studying small, model fish species, and reports on non-model species are limited. The reason for this discrepancy is the technical difficulty inherent in developing rigorous experimental systems in many farmed fish species. We have already established methods for the full life-cycle breeding of a commercially important marine fish, the chub mackerel (cm), and are interested in understanding the reproductive function of kisspeptins from various perspectives. Based on a series of experiments clarifying the role of the brain–pituitary–gonad axis in modulating reproduction in cm, we theorize that the kisspeptin system plays an important role in the reproduction of this scombroid species. In this review article, we provide an overview of kisspeptin studies in cm, which substantially aids in elucidating the role of kisspeptins in fish reproduction.

Expression patterns of kiss2 and gpr54‐2 in Monopterus albus suggest these genes may play a role in sex reversal in fish

“…Both genes are generally highly expressed in fish brain , where they probably have a broad range of functions . The expression of gpr54 ‐ 2 in the brain of M. albus is in full agreement with this, with similar (high) expression levels among all three sexes, which suggests that functions in brain do not vary between sexes.…”

“…This indicates that kiss2 appears to undergo a ‘reset’ in the expression as M. albus undergoes the sex change. This might be in correlation with the proposed important role of the kisspeptin system in modulating gonadal sex differentiation, pubertal timing, and reproduction in teleost fishes . In the light of these findings, it would be of interest to assess the constitutive expression of kiss2 in juvenile (prepubescent) M. albus in future studies.…”

“…A number of gene (and genome) duplications (and subsequent gene losses) in the evolutionary history of fish and the existence of compensatory mechanisms make the studies of kiss/gpr54 system in fish very complex. Both genes ( kiss2 and gpr54‐2 ) identified in the genome of M. albus are comparable to those characterized in other teleost fishes in terms of size . There is evidence that kiss1 and kiss2 have different functions in other fish species ; for example, in medaka ( Oryzias latipes ), only the kiss1 system, but not the kiss2 system, shows expression dynamics strongly indicative of its direct involvement in the HPG axis regulation .…”

“…This is in agreement with high kiss2 expression in the gonads of male M. albus . Overall, studies suggest that kisspeptin peptide modulates gonadotropin secretion and influences gonadal development in fish, but the effects on ovarian development appear to be slower than the effects on testicular development . This may bear importance for the interpretation of our results, as the expression in female gonads was much lower than the expression in male gonads.…”

Phoenixin-20 Stimulates mRNAs Encoding Hypothalamo-Pituitary-Gonadal Hormones, is Pro-Vitellogenic, and Promotes Oocyte Maturation in Zebrafish

“…In agreement with the observations in the hypothalamus, an upregulation of kiss1, kiss1rb and kiss 2 (only in the male treatment group) mRNAs was identified in the gonads of zebrafish treated with PNX-20. Studies conducted in different fish species suggest that kisspeptin and gonadal development are positively correlated 20 . The upregulation of the kisspeptin system in the gonads of zebrafish is another possible mechanism by which PNX-20 influences zebrafish reproduction.…”