Genetic Predisposition

Although some neoplasms are directly hereditary, genetic predisposition is an important factor affecting the occurrence of all neoplasms. The tendency of certain species to develop particular types of tumours is a well known aspect of oncology (Schlumberger, 1957).

Although frequency of neoplasia varies in different types of fish, there are no taxa known to be insusceptible (Harshbarger et al., 1981). The frequency of reports about neoplasms in various species is undoubtedly affected by several factors other than disease prevalence. For example, although neoplasms occur in sharks (Fig. 3.4) and rays, there are relatively few published reports of neoplasms in these groups. This could be related to the small number of chondrichthyians kept in captivity and the infrequency of experimental oncology with these animals. Sharks with tumours could also be at an extreme disadvantage for capturing prey and for avoiding becoming prey.

Fig. 3.4. Reticulum cell sarcoma in the spleen of a sandbar shark (Carcharhinus milberti). Bar = 25 mm. (RTLA Accession No. 523; submitted by R. O'Gara and V.T. Oliverio.)

The relative importance of genetic predisposition in comparison with species-dependent factors, such as types of food eaten and contact with sediment, is difficult to determine in studies of wild fish. Laboratory experiments have confirmed that there can be differences in sensitivity to carcinogens both between species (Ashley, 1970; Busby and Wogan, 1984; Hawkins et al., 1988a) and within a species (Sinnhuber et al., 1977; Hyodo-Taguchi and Matsudaira, 1984; Schultz and Schultz, 1988; Bailey et al, 1989). Inbreeding (Etoh et al., 1983) and hybridization can also result in predisposition to the occurrence of neoplasms. For example, the various species of Xiphophorus are relatively insensitive to chemical carcinogens and radiation, but certain hybrid Xiphophorus are highly sensitive (Schwab et al., 1979; A. Anders, et al., 1991a,b).

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