Relationships between sex and the size and number of forebrain gonadotropin-releasing hormone-immunoreactive neurones in the ballan wrasse (Labrus berggylta), a protogynous hermaphrodite

This study is the first to examine the brain gonadotropin-releasing hormone (GnRH) cell population phenotype in a protogynous and monandric sequential ly hermaphroditic fish. Male ballan wrasse (Labrus berggylta) had on averag e higher numbers of GnRH-immunoreactive (GnRH-ir) cells within the brain pr eoptic area (POA) than females, a difference not found in GnRH-ir cells in other brain regions. Furthermore, in males, but not females, the number of these POA GnRH-ir cells correlated with body size. Maturational state (pres pawning or postspawning) had marked effects on mean profile sizes (but not numbers) of both GnRH-ir cell bodies and cell nuclei, even when existing di fferences in body size and allometric relationships had been taken into acc ount. Postspawning males tended to have larger GnRH-ir profiles in all brai n regions relative to both prespawning males and females. Moreover, the GnR H-ir cell number in POA, and the cell body profile size in both POA and at the level of the anterior commissure, correlated with gonad size in spermia ted prespawning males, indicating a relationship between both size and numb er of GnRH cells and male gonadal development. These results suggest that t emporary changes in the size of brain GnRH-ir neurones are coupled to the m ale spawning cycle, and that permanent POA GnRH-ir cell number changes are involved in the process of sex change in sequential hermaphrodites. However , smaller males had no more preoptic GnRH-ir cells than equally sized femal es, which may argue against a proximate inducing role of GnRH cell number c hanges in naturally occurring sex reversal. (C) 1999 Wiley-Liss. Inc.