Throughout North America, up to 27 % of adult walleye (Pisces: Stizostedion vitreum) are seasonally affected by a dermal tumor termed walleye dermal sarcoma (WDS) which is associated with C-type retrovirus-like particles. A homogenate from 20 pooled tumors was fractionated by sucrose density gradient centrifugation and retrovirus particles were identified by electron microscopy in fractions with high reverse transcriptase (RT) activity. RNA was Isolated from a sucrose density gradient fraction which contained a major portion of the RT activity and which had a density of 1.18 g ml-' Denaturing gel electrophoresis showed the presence of a minor 12 kb RNA species, presumed to represent the undegraded genomic RNA of walleye dermal sarcoma virus (WDSV). Digoxigeninlabeled cDNA synthesized from this viral RNA preparation specifically hybridized with a 13 kb linear unintegrated viral DNA species present only in DNA from walleye tumors. These findings strongly suggest that WDS is the result of an infection caused by a unique exogenous retrovirus of which the unusually large genome is predominantly unintegrated in tumor cells.

The walleye dermal sarcoma is a mesenchymal tumor which seasonally affects up to 27 % of adult walleye fish (Stizostedion vitreum). It arises multicentrically in the dermis, in which its development remains restricted. We report the molecular cloning of a type C retrovirus from this tumor. The genome of this virus (13.2 kb) is larger than that of all retroviruses and in that respect is approximated only by the recently characterized spumaviruses. In tumors, the predominantly unintegrated linear viral DNA has a single-stranded gap region which is similar to the structure found in some lentiviruses and all spumaviruses. The presence of at least four viral transcripts suggests that this virus has the capacity to encode accessory functions and is reminiscent of the transcriptional complexity of lentiviruses and spumaviruses.