Molecular Studies on Evolution of Salmonid Populations

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Previously, our group proposed that SINEs are derived from a "strong stop DNA" with a primer tRNA that is an intermediate in the reverse transcription of certain retroviruses (1). This model suggests that a certain group of SINEs may have been generated by horizontal transmission, although it is not clear whether information was transmitted via a similar retrovirus or via an RNA or DNA of a SINE (1).Recently, our group obtained two experimental data which may suggest horizontal transmission of SINEs. The first finding is as follows. In the subfamily Coregoninae (whitefishes), a new family of SINEs was discovered. Members of this family (designated SRW family) are almost identical to those of the Sma I family, that is restricted to chum salmon and pink salmon in the subfamily Salmoninae. The similarity between these two families is 97%. In contrast, the genetic distance deduced from synonymous changes in a growth hormone gene of chum salmon (Salmoninae) and C.lavaretus (Coregoninae) is … More about 10%. The result suggests that the Sma I family may have been generated by horizontal transmission of the SRW family, if SINEs have no functions and behave like pseudogenes.Second, fourteen members of Hpa I subfamilies that had been specifically amplified in particular species were isolated from genomic libraries established for chum salmon, kokanee, coho salmon, masu salmon and steelhead. Alignment of the sequences of these 14 members, three of which were previously demonstrated to have been amplified specifically in certain lineages (2), revealed the presence of five subfamilies with particular diagnostic nucleotides. The amplification of members of the same subfamily in different salmonid lineages and the amplification of members of the different subfamilies in the same salmonid lineage suggest that multiple dispersed loci were responsible for amplification. However, these results are also interpreted on the asumption that SINEs can be transmitted horizontally between species. The horizontal transmission model and the multiple dispersed loci model are not mutually exclusive for an explanation of the phylogenetic distribution of the 5 subfamilies of the Hpa I SINEs in the salmonids.(1) Ohshima et al., (1993) Proc. Natl. Acad. Sci. USA 90, 6260-6264.(2) Murata et al., (1993) Proc. Natl. Acad. Sci. USA 90, 6995-6999. Less