Odontesthes

Abstract: Specimens presently identified as belonging to Odontesthes perugiae species group from rio Parana, rio Uruguay and rio Negro in Argentina, Brazil and Uruguay are revised. Two species are recognized: Odontesthes perugiae , type species of the genus with Odontesthes orientalis as a junior synonym, and a new species described herein. Odontesthes perugiae is redescribed and its distribution restricted to the lower rio Parana, lower rio Uruguay and rio Negro basins. The new species is endemic from the upper rio Uruguay above Salto do Yucuma (Brazil) or Saltos del Mocona (Argentina) falls. Both species share with the remaining taxa included in the O. perugiae species-group ( O. bicudo , O. ledae , O. mirinensis , and O. piquava ) the possession of teeth of the outer row on dentary and premaxilla shorter than those in the inner row, being distinguished from them by the number of gill rakers, body measurements and some osteological characters.

Abstract: Coastal freshwater fishes provide valuable models for studying the role of the last glaciations in promoting speciation. To date, the great majority of studies are of Northern Hemisphere taxa, and reflect the influence of vicariant events during, or prior to, the Pleistocene. Microsatellite markers and mitochondrial DNA sequences were used to investigate patterns of population divergence and evolutionary relationships in a freshwater group of silverside fishes ( Odontesthes perugiae complex), endemic to the recently formed coastal plain of southern Brazil. Lacustrine morphotypes showed concordant patterns of genetic and morphological divergence consistent with the geographical history of the coastal plain. The results support the proposal of a silverside radiation chronologically shaped by the sea-level changes of the Pleistocene and Holocene. The radiating lineage comprises a minimum of three allopatric and two sympatric lacustrine species. Four species displayed extremely high levels of genetic variation and some of the most rapid speciation rates reported in fishes. These features were related to a marine–estuarine origin of the radiation. To the best of our knowledge, this study represents the first molecular phylogeographic survey of a coastal radiation in South America.

Abstract: Odontesthes (Atheriniformes: Atherinopsidae) is a diverse and widespread genus of silverside fish, containing between 15 and 22 recognized species distributed across marine, estuarine and freshwater environments of temperate South America (Dyer 1993). The marine-estuarine O. argentinensis ranges from the south-east coast of Brazil (25 ° S) to Chubut, in Argentina (43 ° S) (Dyer 1993). It is an economically significant resource for local fisheries in Brazil, Uruguay and Argentina (de Buen 1953) and a suitable organism for bioassay studies (Phonlor & Cousin 1997). The freshwater O. perugiae species group is composed of morphotypes mostly distributed along the complex system of lagoons of the coastal plain of southern Brazil (Dyer 1993). We are using molecular markers to understand historical and recent biogeographic scenarios that have shaped the evolution of Odontesthes. Results based on sequences of three mitochondrial DNA genes revealed that O. argentinensis and O. perugiae are closely related and have recently radiated in the coastal areas of southern Brazil (L. Beheregaray, unpublished). We developed microsatellites for these taxa to identify population structures, clarify taxonomic problems and reconstruct their phylogeographic history. Evidence for the application of these primers in a wide range of Neotropical silverside species is presented. To our knowledge, these are the first microsatellite loci isolated from fish native to South America, a continental area with one of the most diverse ichthyofaunas of the planet. Genomic DNA of O. argentinensis and the O. perugiae species group was extracted by salting-out (Sunnucks & Hales 1996), pooled together, and digested with Hae III, Eco RV and Alu I. Fragments ranging between 430 and 550 bp were cloned into Sma I cut pUC18 and a resulting library of approximately 25 000 colonies was screened with radiolabelled CA/GT and GA/CT co-polymers according to Taylor et al . (1994). Putative positive clones were rescreened, and 31 clones encompassing different hybridization intensities were selected for sequencing. This strategy increased our chances of choosing both long and short repeats as they are expected to inform at different levels of divergence (see below). Thirteen of the clones sequenced contained microsatellites with appropriate flanking regions for primer design. Variation at each of the 13 loci was assessed by the polymerase chain reaction (PCR) using a 10 μ L radiolabelled reaction containing 50–100 ng of template DNA, 12 pmol of each primer, 0.5 units of Taq DNA polymerase (Promega), 200 μ m of dCTP, dGTP, and dTTP, 20 μ m of dATP, 2 m m MgCl 2 , 10 m m Tris-HCl (pH 8.3), 50 m m KCl, 0.1% Triton X-100 and 0.05 μ L [ α 33 P] dATP at 1000 Ci/mmol overlaid with mineral oil. PCR amplifications were performed in an MJ Research PTC100 thermocycler starting with 94 ° C for 3 min, followed by a ‘touchdown’ (32 cycles at 94 ° C/20 s, annealing/45 s and 72 ° C/60 s), and a final step of 72 ° C for 4 min. The annealing temperature of the ‘touchdown’ PCRs decreased two degrees per cycle until reaching the fifth cycle (Table 1). PCR products were separated by 6% polyacrylamide gel electrophoresis and visualized by autoradiography following Taylor et al. (1994). Twelve loci were successfully amplified and showed high levels of polymorphism and heterozygosity (Table 1). Number of alleles per locus ranged from 6 to 33 and expected heterozygosities from 0.42 to 0.94. An ongoing screening shows that loci with short and less variable repeats (e.g. Odont16) better differentiate between marine and freshwater lineages, while loci with long repeats and higher polymorphism (e.g. Odont07) are useful to reveal fine-scale population structures (L. Beheregaray, unpublished). Marine populations have shown a significant larger number of alleles at all loci than freshwater populations (202 and 120 alleles, respectively, P after a Wilcoxon rank test = 0.002). This difference in genetic variability was expected due to the relatively reduced subpopulation sizes of freshwater fish (Ward et al . 1994). All primer sets were also tested without extra optimization in several Odontesthes species (including the commercially important O. bonariensis ) and in the diverged Basilichthys semotilus, a silverside endemic to freshwater environments west of the Andes Mountains. Amplification and polymorphism was extensive (Table 2), indicating that these markers are potentially useful for a wide range of population studies of wild and captive South American silverside species.

Abstract: Allozyme electrophoresis was used to investigate genetic population structure among 245 individuals of the silverside Odontesthes argentinensis sampled from localities in southern Brazil and Uruguay. Striking differences in allozyme frequencies and genetic variability were detected among localities. Analysis of these differences indicated two distinct populations: one distributed in a wide marine coastal zone; and one in the peripheral environment of the Patos Lagoon estuary in southern Brazil. The population in Patos Lagoon appears to have originated from an ancestral marine population and been established in the estuary by a founder effect. The extent and pattern of genetic divergence found in our study, the existence of diverse reproductive strategies in the estuarine silversides, and differences in morphology of eggs and larvae, indicate that speciation is taking place in Patos Lagoon. It is suggested that the two populations should be considered separately when formulating management and con...