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These findings suggest the occurrence of a structured population and have important …

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- RAPD markers indicate the occurrence of structured populations in a migratory freshwater fish species

Ten primers were tested prior to the analysis, with the amplification products ranging from 300 to 3000 bp. A low variable pattern was observed among P. marggravii individuals when the OPP-4 and OPP-18 primers were used, while the remaining primers produced polymorphic patterns (Table 1). No characteristic and/or diagnostic bands were found for any sampling site. All the primers examined produced reproducible RAPD fragment patterns. Figure 2 shows an amplification pattern obtained with the OPP-12 primer. Four of the 10 tested primers were chosen (OPP-7, OPP-10, OPP-12, OPP-15), based on profile sharpness and on adequate variation levels for statistical tests. A total of 31 loci were analyzed and the results of Fisher's Exact Test revealed five loci with significantly different frequencies among the sampling sites (Table 2).

The similarity coefficients of Jaccard (1901) within and between sampling sites are given in Table 3, which shows that the lowest value of the similarity coefficients within samples was obtained for individuals from region C (0.442), whereas regions A and B presented Jaccard's coefficient values of 0.636 and 0.604, respectively. The Kruskal-Wallis test revealed that the mean similarity coefficients of Jaccard for each primer among the sampling sites were statistically significant (KW = 7.538; p = 0.011). Pairwise comparisons (Dunn's Multiple Comparisons test) demonstrated that the differences observed between regions A and B and between B and C were not significant (p > 0.05), but a statistically significant difference was found between regions A and C (p Table 3).

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