Gonadotropin content of the pituitary gland of gonadal tumor-bearing common carp x goldfish hybrids from the Great Lakes, as assessed by bioassay and radioimmunoassay.

The hybrids of carp (Cyprinus carpio) and goldfish (Carassius auratus) collected from the lower Great Lakes between 1978 and 1981 exhibited epizootics of gonadal neoplasm which were rare in the parental species. The pituitary glands of hybrids were, on average, 2.5 times larger than that of carp of similar body size. Histologically, the hypertrophy was due to hyperplasia of proximal pars distalis basophils (the presumptive gonadotropes). Using a carp gonadotropin (GtH) radioimmunoassay it was found that hybrid pituitaries contained more GtH than sympatric carp and that pituitary GtH concentration in hybrids was positively correlated with pituitary weight. Both this and histological evidence suggested that gonadotrope hyperplasia was a progressive event. Two heterologous gonad bioassays indicated that hybrid GtH had biological activity. Whether pituitary hyperplasia in hybrids is a primary problem or secondary to gonadal sterility has not been determined.