Nonapeptides oxytocin (oxt) and arginine vasotocin (avt) are evolutionarily conserved in vertebrates and have arisen through an ancestral vasotocin gene duplication. Taking advantage of recently generated homozygous zebrafish knock-out lines (KO) for oxytocin (oxt ^-/-) and arginine vasotocin (avt ^-/-), the author tested preliminary findings that avt ^-/-, but not oxt ^-/-, KO affects reproductive success in a female-specific manner. There is a significant reduction in the average number of eggs spawned in the avt ^-/- line 2, but not oxt ^-/- line 2, compared to the wildtype (wt). Furthermore, mating trials with WT backcrosses make it evident that this is a female-specific effect, because male avt ^-/- crossed with female WT exhibited normal spawning success, while female avt ^-/-crossed with male WT produced a reduced number of eggs like the double KO mating results. This affect does not appear to be mediated by AVT-dependent changes in reproductive behaviour, as indices of courtship behaviour in line 2 avt ^-/- females are not different from wt. Histological data shows that the effect is furthermore linked to female egg maturation than release, as avt ^-/- lines have a premature arrest in the egg maturation stages 2-3. This is further corroborated at the gene expression level as nanos2, a primordial germ cell marker, does not exhibit different ovarian transcript expression between WT and avt ^-/-, suggesting a similar reservoir of germ cells. Conversely the expression of the luteinizing hormone receptor lhr revealed a significantly reduced expression in ovaries of avt ^-/- females, suggesting that decreased peripheral responsiveness to LH may contribute to the female reproductive phenotype in this KO line. Together, these data support a role for vasotocin in the female male zebrafish reproductive physiology and success, which has implications for aquaculture and ecotoxicological research.

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Keywords: Nonapeptides, Zebrafish, Double KO, Vasotocin, Oxytocin

Author: Kusum Sharma

Department of Biology, University of Ottawa, Ottawa, Canada