GNRH CELL-SIZE AND NUMBER IN A TELEOST FISH WITH 2 MALE REPRODUCTIVE MORPHS - SEXUAL-MATURATION, FINAL SEXUAL STATUS AND BODY-SIZE ALLOMETRY

Gonadotropin releasing hormone-like immunoreactive (GnRH-ir) cells in both the ganglion of the terminal nerve (TN) and the preoptic area (PO A) have been implicated in the development and maintenance of reproduc tive behavior and physiology in teleost fishes. One marine species, th e plainfin midshipman, Porichthys notatus, exhibits two sexually matur e male morphs (types I and II) which differ with respect to size at se xual maturation, gonad/body weight index, reproductive tactic and voca l motor traits. Type II males become reproductively active at a smalle r body size than either females or type I males. Immunocytochemical te chniques and quantitative analyses were used here to determine the siz e and number of GnRH-ir cells in the TN and POA amongst field collecte d juveniles, sexually mature females, and type I and II males. Mean Gn RH-ir cell size and number in the TN did not vary across the entire ra nge of specimens. However, mean GnRH-ir cell size and number in the PO A were 50-100% greater in sexually mature adults compared to juveniles . Analyses of covariance indicated that increases in cell number, but not cell size, could be explained solely on the basis of changes in bo dy size. However, regression analyses showed that body size had a sign ificant influence on increasing cell number only in the juvenile-type I male transition and the juvenile-female transition, not in the juven ile-type II male transition. The latter suggests that type II males, u nlike the other adult morphs, have 'escaped' from a body size constrai nt imposed on increasing GnRH-ir cell number in the POA. There were al so significant differences among the adult morphs in the size of GnRH- ir POA cells that could not be explained on the basis of differences i n body size but, rather, appear to reflect differences in the temporal onset of sexual maturation. Together, the data suggest that the timin g of changes in POA phenotype may provide a proximate mechanism permit ting the development of alternative male reproductive morphs.