Differences in response to a chemical carcinogen within species and clones of the livebearing fish, Poeciliopsis.

Freshwater viviparous fishes of the genus Poeciliopsis are evaluated as a model for testing water-borne carcinogens. The unique property of this model lies in the clonal form of reproduction of some of these fishes. Interspecific hybrids between P.monacha and P.lucida occur in nature as all female 'species' which produce only female offspring and transmit the P.monacha genome intact as a single linkage group. Without sacrificing the integrity of the wild P.monacha genome, a naturally occurring 'hemi-clone' can be brought into the laboratory and rendered isogenic in the next generation by providing an inbred strain of P.lucida as the male parent. The genetic mechanism of P.monacha-lucida hybrids provides a tool for duplicating wild genomes from heterogeneous populations of these fishes for broad and continuous investigation. In the present study susceptibility to induction of liver tumors is compared in inbred strains of two species Poeciliopsis lucida and P.monacha, and among nine P.monacha-lucida hybrid clones exposed to diethylnitrosamine (DEN). Incidence of fish with liver tumors was significantly higher in P.lucida M61-31 (89.0%) than in P.monacha strains S68-4 (18.9%) and S68-5 (36.8%) and the nine hybrid clones of P.monacha-lucida wherein tumors occurred in a graded series (15.6 to 66.7%). Tumors were not found in any untreated fish reared as controls. The incidence of treated fish with large tumors (occupying more than one eighth of the liver) was significantly higher in P.lucida (72.7%) than in the P.monacha strains S68-4 (9.4%) and S68-5 (10.5%). The difference in frequencies of large tumors among the nine clones ranges from a low of 3.8% in M65-24 PCx to a high of 52.9% in M61-35 PCz. Hemangiopericytomas were induced in P.lucida and in five of the nine P.monacha-lucida clones; they were not found in either strain of P.monacha. Lymphosarcoma was induced only in P.monacha S68-5. An unclassified sarcoma was induced in both P.monacha strains and in two of the nine P.monacha-lucida clones, but not in P.lucida. The intrahepatic cholangiocarcinoma was the most frequent tumor type induced with DEN in this study. It occurred in all twelve genotypes, and with one exception, in the highest incidence. In contrast to a previous study wherein responses of different strains of P.lucida were within a narrow range, substantial differences exist among P.monacha genotypes.