Prochilodontidae fish are widely distributed throughout South America and have been reported in almost all South American hydrographic basins (Mago-Leccia, 1972).
Thirteen species of the Prochilodus genus are recognized (Castro, 1990) and three species, P. marggravii, P. affinis, and P. vimboides, live in the São Francisco basin (Britski, 1988), which comprises an area equivalent to 7.4% of Brazil's territory (Paiva, 1982; 1983). The two former species are migratory and are considered very important species for inland fishing in this river basin (Paiva, 1983). However, a dam built in 1960 on the main channel of the São Francisco river in the municipality of Três Marias (MG) prevents migratory fish from lower areas reaching most of the river's upper tributaries. There is no information on the effect of the construction of a new dam on the populational genetic structure of Neotropical migratory fish fauna. It is known that migratory fish require favorable conditions, determined by endogenous and exogenous factors, which guide migration and spawning. Free fish migration across a dam is often imparied and genetic variation and allelic frequency of migrating populations can be changed. Moreover, the environmental conditions prevailing in this region's hydrographic system have undergone profound changes both upstream and downstream from the dam (Sato and Osório, 1986). Drastic environmental conditions, caused mainly by reduced water flow, lower water temperature and oxygen rates just downstream from the dam, may be influencing some of the biological features of the living organisms in this area (Sato et al., 1995). Histological studies of gonads in P. marggravii have revealed reproductive disturbances in the specimens caught in the downstream area nearest the dam when compared to individuals from more distant regions (Sato et al., 1995).
Little is known about the population genetics of Prochilodus. Previous enzyme electrophoresis, used to quantify the genetic variations of P. lineatus collected from three sites in the Paraná river basin, detected no genetic divergences among the three samplings, suggesting a unique genetic pool of P. lineatus spread along the studied area (Revaldaves et al., 1997).
Another useful methodology to assess genetic variations in fish populations is the RAPD (Random Amplified Polymorphic DNA) technique, which is used in fishery management and conservation genetics of wild populations. Based on the amplification of genomic DNA by PCR (Polymerase Chain Reaction) with arbitrary nucleotide sequence primers, RAPD can detect high levels of DNA polymorphisms and can produce fine genetic markers (Williams et al., 1990; Welsh and McClelland, 1990). Nevertheless, RAPD analysis has some limitations that must be considered. It shows dominant inheritance and marker/marker homozygotes cannot be distinguished from marker/null heterozygotes. In addition, it is unable to assign bands to specific loci unless a previous pedigree analysis is performed. In applying this method, it is assumed that populations are under the Hardy-Weinberg equilibrium, that polymorphic bands segregate in the Mendelian way, and that marker alleles from different loci do not co-migrate to the same position in the gel (D'Amato and Corach, 1996).
The potential use of RAPD in genetic mapping and population genetics has been widely documented for a large variety of organisms, including fish (Postlethwait et al., 1994; Foo et al., 1995; Bielawski and Pumo, 1997; Caccone et al., 1997; Dergam et al., 1998; Nadig et al., 1998; Liu et al., 1999).
In order to assess the genetic variation of P. marggravii in the area of influence of the Três Marias dam on the São Francisco river, RAPD reactions were performed on samples from three collection sites. The results promise to be useful for the fishery management, aquaculture and stock conservation of this species.
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