Gender

In some cases the gender of the fish affects the incidence of neoplasia. Often a lack of correlation between tumour development and gender are from studies terminated before or soon after sexual maturity (Hendricks et al., 1995). In Japanese hatcheries, hepatic neoplasms were more common in female salmonids than in males, and neoplasms did not occur until fish were sexually mature (Takashima, 1976). Female brown bullheads from the Black River had higher frequencies of hepatocellular carcinomas and cholangiomas than did males; however, there was no difference in prevalence of cholangiocarcinomas (Baumann et al., 1990). In contrast, male F1 hybrids of southern platyfish (Xiphophorus maculatus) and swordtails (Xiphophorus helleri) had a higher prevalence of hereditary melanomas than did female F1 hybrids (85.2% compared with 55.9%), although almost all fish of both sexes developed melanosis (Siciliano et al., 1971). Only male medaka (Oryzias latipes) developed thyroid neoplasms after exposure to MNNG (Bunton and Wolfe, 1996). There was also a higher incidence of gastric papillomas in male than in female rainbow trout fed 1,2-dibromoethane (DBE) (Hendricks et al., 1995).

Higher rates of certain types of neoplasms in females could be related to oestradiol, which can act as a promoter (Nunez et al., 1989). Predisposition to neoplasia can also result from sex-linked, inherited characteristics; the melanoma locus in Xiphophorus spp. is a well-studied example (F. Anders et al., 1984).

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