Short communication Species polyphyly and mtDNA introgression among three Serrasalmus sister-species

Understanding the processes that generated pattern of DNA variation in natural populations may be a difficult task. Since migration and gene flow may superimpose to genetic drift and divergence, evolutionary forces responsible of shared polymorphism may be difficult to identify (Pamilo and Nei, 1988; Nielsen and Wakeley, 2001). In this context, the raise of the coalescent theory constituted a significant improvement in the comprehension of the theoretical framework behind gene genealogies (Kingman, 1982; Tajima, 1983) and its application to the analysis of DNA sequences has proven to constitute an informative approach to the problem of shared polymorphism (Chiang, 2000; Takahashi et al., 2001; Machado and Hey, 2002; Rokas et al., 2003; Bowie et al., 2005). The coalescent theory predicts that haplotype sharing will persist at the incipient stage of species divergence between species that founded from the same gene pool (Rosenberg, 2003). This stage of shared polymorphism without gene flow has been previously formalised as the lineage sorting period (Hoelzer et al., 1998). This step is characterised by the occurrence of coalescent events between alleles from isolated groups leading to erratic genealogies (Pamilo and Nei, 1988; Funk and Omland, 2003). However, recently diverging groups may still exchange genes and distinguishing between gene flow and ancestral polymorphism may be a difficult task (e.g. Nielsen and Wakeley, 2001). The piranha belongs to the characidae subfamily of Serrasalminae (Buckup, 1998). Currently including 28 species ranging from 130 to 420 mm standard length, the piranha genera Serrasalmus and Pygocentrus constitute the most speciose group of large carnivorous Characiformes (Jégu, 2003). DNA sequences from mitochondrial DNA (mtDNA) recently evidenced that these genera constitute a monophyletic group originating 9 million years ago (Ma) and that Serrasalmus splits into three distinct clades, all distributed throughout the Amazon, Orinoco and Paraná watersheds (Hubert et al., 2007). The biogeography of the Amazon freshwater fish fauna has been largely influenced by the Miocene marine incursion that happened at 5 Ma (Hubert and Renno, 2006; Nores, 1999). The analysis of mtDNA sequences within the Piranha evidenced that the colonisation of the Upper Amazon by the genera Serrasalmus and Pygocentrus occurred after the marine retreat, during the last 4 million years, from the Miocene freshwater refuges of the Brazilian and Guyana shields (Hubert and Renno, 2006; Hubert et al., 2007). The Madeira is one of the major Andean tributary of the Amazon and previous phylogeographic studies evidenced that the piranha genera Serrasalmus and Pygocentrus colonised the Andean tributaries of the Amazon during only the last 2 Ma (Hubert et al., 2007). Although the colonisation of the Upper Madeira is recent, molecular phylogenetic results suggested that speciation occurred in Serrasalmus within the Upper Madeira watershed (Hubert et al., 2006). This may be related to the existence of varied water types in the area as a function of the relative