| Several studies conducted in higher organisms such as birds and mammals have recently suggested that sexual dimorphism may exist in learning and memory, particularly just prior to or during mating season. The teleost fish, Lythrurus fasciolaris (scarlet shiner), is known to be both a seasonal breeder and sexually dimorphic. In this study, we have collected scarlet shiners from their natural environment at specific intervals throughout the breeding season to better reflect changes that occur in nature. Using these fish we explored the morphological and molecular changes that may underlie sexual dimorphism in three key regions of the brain: the cerebellum, optic tectum and the telencephalon, which has a structure homologous to the hippocampus in humans. Allometric analysis of whole brain mass relative to body mass showed that male teleost fish are significantly more variable (R 2 = 0.29, p ≤ 0.01) during the breeding season than females (R 2 = 0.84, p ≤ 0.01). Males were also found to have larger average volumes than females in all three brain regions relative to total brain mass. Because N-methyl-D-aspartate receptors (NMDARs) are involved in neuronal maturation, synaptic formation and learning and memory, we asked if there was a change in the quantity and location of NMDARs in relation to sex and reproductive viability. Four groups were classified morphologically: dominant males, recessive males, reproductive females and non-reproductive females. At the beginning of the breeding season, dominant male shiners have at least a 3.5-fold higher expression of NMDARs than recessive males in all three brain regions (ANOVA, p ≤ 0.01). In female fish, reproductive females exhibit 4-fold higher NMDAR in telencephalon, but only 2-fold higher expression in the optic tectum. Non-reproductive females were seen to have 4-fold higher NMDAR in the cerebellum then reproductive females. At the peak of the breeding season, the telencephalon region of non-reproductive female fish exhibits 3.5-fold higher expression of NMDAR than recessive males. Dominant males and reproductive females both exhibit about a 2-fold higher expression than recessive males. In the cerebellum, both female groups exhibited a 2-2.5-fold higher expression level of NMDAR than either male group. In the optic tectum, the non-reproductive females showed the highest difference with nearly a 25-fold increase of NMDAR than recessive males. Reproductive females exhibited a 10-fold higher increase and dominant males an 8-fold increase. This study suggests that NMDARs undergo both temporal and sex-specific regulation. These findings illustrate that there are quantifiable morphological and molecular changes in sexually dimorphic teleost fish may correlate to changes in breeding and dominance requirements, which has implications for all higher animals, including man. |
