Gonadal Tumours in Goldfish x Carp Hybrids

A high prevalence of gonadal neoplasms occurs in hybrids of goldfish x common carp in the Great Lakes (Sonstegard, 1977; Down and Leatherland, 1989). Onset of tumour formation coincides with the age of first sexual maturity, and prevalence increases with age. Overall prevalence was 0.57% in carp, 4.1% in goldfish, and 68% in hybrids; and prevalence was 100% in some samples of hybrids. Sonstegard (1977) hypothesized that this condition was caused by polychlorinated biphenyls or dichloro-diphenyl-trichloroethane (DDT), but Down and Leatherland (1989) found that these neoplasms were as common in areas relatively free of industrial or heavy domestic discharge as they were in polluted locations. Although the cause of these lesions is uncertain, they are undoubtedly related to genetic factors. Ornamental carp (Cyprinus carpio) with complex genetic histories, also develop ovarian neoplasms that may be hereditary (Ishikawa and Takayama, 1977).

Goldfish x common carp hybrids with neoplasms had pronounced hyperplasia of gonadotropic cells of the pituitary, resulting in large amounts of gonadotropin in the pituitary and serum (Down et al., 1990). Serum levels of testosterone and 11-ketotestosterone were also elevated in hybrids with neoplasms consisting of poorly differentiated cells that were probably of Sertoli cell origin. This hormonal imbalance could be related to oncogenesis directly or could result in promotion of preneoplastic changes induced by environmental factors (Down, 1984).

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