X-rays caused neuroblastoma in hybrid Xiphophorus after multiple, whole-body exposures (Schwab et al., 1979). The only fish to develop neuroblastomas were those carrying the 'lineatus' locus; however, the parent species carrying this trait (X. variatus) and the hybrid used to produce the susceptible backcross did not develop this type of neoplasm. The increased susceptibility of backcross fish is presumably related to the absence of repressor genes as discussed for genetically related melanomas. Schwab et al. (1979) found that neuroblastomas also occurred in these backcross hybrids exposed to N-methyl-N-nitrosourea (MNU).
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