Melanoma in Xiphophorus Hybrids

Melanomas can result from matings between southern platyfish from different populations (Gordon, 1948; Kallman, 1975) or between Xiphophorus of different species (Figs 3.5 and 3.6). The most frequently studied Xiphophorus hybrids are inbred strains of southern platyfish X swordtail (F. Anders et al., 1984; Schwab, 1986; Vielkind et al, 1989; A. Anders et al, 1991b; Morizot et al., 1991; Malitschek et al., 1995), but other Xiphophorus species have also been used. Similar melanomas sometimes occur in certain strains of purebred Xiphophorus spp. (Kazianis and Borowsky, 1995; Schartl et al., 1995). Melanomas in hybrids of Xiphophorus were reported in 1912-1913, and early studies on genetics of these hybrids were published in 1927-1928 (Schwab, 1986; F. Anders, 1991).

A key feature of the Xiphophorus melanoma model is the macro-melanophore, a distinctive type of pigment cell. Macromelanophores are up to 500 mm in diameter compared with normal melanophores which are about 100 mm in diameter (Gordon, 1959). Macromelanophores form conspicuous clusters or spots because they are closely spaced; these cells do not seem to be subject to distance-dependent regulation affecting spacing between normal melanophores (F. Anders et al., 1984). The presence of macromelanophores is sex-linked and causes various pigmentation patterns that are determined by Mendelian dominant genes (Gordon, 1931; Kallman, 1975). The presence of macromelanophores identifies broodfish carrying the gene for melanoma.

Fig. 3.5. A melanoma from a Xiphophorus hybrid. The densely pigmented melanoma has invaded the dermis and underlying musculature. Bar = 100 mm (RTLA Accession No. 230, specimen contributed by I.L. Gorman.)
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