Previous investigations of natural populations of the hermaphroditic, self-fertilizing fish species Rivulus marmoratus demonstrated a surprising amount of interclonal differentiation among highly polymorphic "DNA fingerprint" loci. The genetic differentiation observed among clones was thought to be the effect of extreme population mixing because of high rates of migration and population extinction. It was demonstrated that mutation rates at hypervariable loci would have to exceed 10(-4) on average to alone account for the observed interclonal differences. The present study reports that, among laboratory lines of this species, mutation rates at the most unstable set of hypervariable loci are not greater than 3.52 x 10(-4), and are probably lower. Mutation rates at several other sets of loci are even lower. A field transplantation study demonstrated complete clonal stability over several generations. These results suggest that the high interclonal differences observed in natural populations of this species is not caused by a generally higher rate of mutation at these specific loci.

Previously we reported that papillary thyroid carcinomas were predominantly induced at high frequency by a low dose of N‐methyl‐N′‐nitro‐N‐nitrosoguanidine (MNNG) in the hermaphroditic fish Rivulus marmoratus. In the current study, polymerase chain reaction (PCR) amplification and direct sequencing were used to examine the point mutations of Ha‐ and Ki‐ras genes, which may be associated with papillary thyroid tumour development in rivulus. Thirty‐three tumour samples were tested, however, no mutations were detected in rivulus Ha‐ and Ki‐ras genes. In human and rodent models, it has been reported that ras gene mutations in papillary thyroid tumours occurred preferentially in the N‐ras gene in general, while follicular tumours contained activated Ha‐ or Ki‐ras gene mutations. This may explain why papillary thyroid carcinomas in rivulus were not mutated at codon 12, 13 or 61 of exon 1 or exon 2 of the rivulus Ha‐ or Ki‐ras gene. These results imply that another oncogene, such as the N‐ras gene and others, may be preferentially activated in rivulus papillary thyroid carcinomas, and also give valuable information for comparative studies of papillary thyroid carcinogenesis.

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