Two dominant genes conditioning resistance to the root-knot nematode Meloidogyne arenaria were identified in a segregating F2 population derived from the cross of 4x (Arachis hypogaea x Arachis cardenasii)-GA 6 and PI 261942. Mae is proposed as the designation for the dominant gene restricting egg number and Mag is proposed as the designation for the dominant gene restricting galling. The high levels of resistance in GA 6 were introgressed from A. cardenasii and, therefore, a search to identify A. cardenasii specific RAPD markers that are tightly linked to these resistance genes was conducted utilizing bulked segregant analysis. One RAPD marker (Z3/265) was linked at 10 +/- 2.5 (SE) and 14 +/- 2.9 cM from Mag and Mae, respectively. The marker was mapped to linkage group 1 at 5 cM from Xuga.cr239 in the backcross map in an area where introgression from A. cardenasii had previously been reported. This fragment was cloned and used to generate a pair of primers that specifically amplified this locus (sequence characterized amplified region, SCAR) and as a RFLP probe. Their close linkage with the resistance genes will be useful in marker-based selection while transferring nematode resistance from introgression lines into elite breeding lines and cultivars. The Z3/265 marker associated with the genes Mae or Mag was not found in other highly resistant Arachis species (Arachis batizocoi or Arachis stenosperma), in progenies of interspecific crosses with A. cardenasii that were moderately resistant, or in the resistant A. hypogaea lines PI 259634 and PI 259572. These represent the first molecular markers linked with a resistant gene in peanut and the first report of two physiological responses to nematode attack associated with two genetic factors.

Forty-six introgression lines (F10C9) from a cross between Arachis hypogaea L. (2n = 4x = 40) and A. cardenasii Krapov. & W. C. Gregory (2n = 2x = 20) were analyzed for the introgression of A. cardenasii chromosome segments. Seventy-three RFLP probes and 70 RAPD primers, expressing from one to four A. cardenasii-specific bands, were used to evaluate the set of introgression lines. Thirty-four RFLP probes and 45 RAPD primers identified putative A. cardenasii introgressed chromosome segments in one or more lines. Introgressed segments were detected by RFLP analysis in 10 of the 11 linkage groups; the smallest introgressed fragments were detected by single RFLP markers and the largest were detected by three or four adjacent markers and represented introgressed segments of 30-40 cM. Similar results were obtained with RAPD markers, although markers detecting introgressed fragments could not be placed on the peanut linkage map. Introgression into both A. hypogaea genomes was detected and its implication in breeding for disease resistance is discussed.

The annual killifishes of the genus Cynolebias Steindachner, 1876, include one of the well-supported clades composed of the Cynolebias adloffi species complex of the Ban˜ados del Este Biosphere Reserve Site, from the Atlantic coastal wetlands in Uruguay. One member of this clade, Cynolebias charrua, has been considered as an intergradation complex of populations between C. adloffi and C. viarius by showing high levels of morphological variability. Systematic affinities between both taxa have been historically discussed. In the present study, a phylogeographic approach based on mitochondrial cytochrome b haplotype analysis and chromosome data are presented. According to the present data a scenario of a multiple simultaneous speciation process and perhaps reticulation events is possible in the C. adloffi species complex, followed by divergence associated with Quaternary marine transgressions in this region. This hypothetic scenario could explain the remarkable genetic diversity and the lack of phylogenetic resolution among clades in C. charrua, as well as the disconcordance in the phylogenetic relationships among Cynolebias from different data sets. Several parameters of DNA polymorphism and chromosome data have also suggested that populations of C. charrua from southern Laguna Merin, in eastern Uruguay and southern Brazil, could represent a relictual group from a widespread ancestral population. Chromosome rearrangements like pericentric inversions in isolated populations could have triggered cladogenetic events from this highly polymorphic population. Present analysis suggests that the metapopulation could represent the management units in these endangered killifishes and that the populations of C. charrua from the Laguna Merin basin could be included in a high-priority conservation programme.

The annual killifish genus Austrolebias includes approximately 38 species distributed throughout the Paraná-Plata basin and Patos-Merín system. Within the Austrolebias adloffi species complex, the Uruguayan populations of Austrolebias charrua were considered as an intergradation between A. adloffi and Austrolebias viarius populations. Austrolebias charrua presents an intermediate phenotype between both taxa and high levels of morphological and chromatic variability. In the present study, we incorporate different methodological approaches (molecular, morphology, and gamete ultrastructure) to elucidate the pattern of differentiation among the parapatric taxa (A. charrua, Austrolebias reicherti, A. viarius) distributed in a Biosphere Reserve Site. Analyses of cytochrome b sequences show high values of DNA polymorphism, in particular for A. charrua. This is in accordance with both morphological and gametic variation. Using a statistical parsimony network based on these sequences and analysis of morphological data, past fragmentation and range expansion involving perhaps secondary contact between A. charrua and A. reicherti could be proposed. Coloration pattern and morphometric analyses showed an unexpected higher similarity between the most distantly-related taxa, A. viarius and A. charrua. This could be the result of retention of ancestral polymorphisms, especially in A. charrua populations from ponds of higher elevation, or to directional selection acting in similar ecological environments. Because these annual killifish species are considered endangered, our work reinforces the high priority need to include them in a conservation programme.

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The extent to which genome sizes and other nucleotypic factors influence the phyletic diversification of lineages has long been discussed but remains largely unresolved. In the present work, we present evidence that the genomes of at least 16 species of the neotropical rivulid killifish genus Austrolebias are unusually large, with an average DNA content of about 5.95 ± 0.45 picograms per diploid cell (mean C-value of about 2.98 pg). They are thus larger than the genomes of very nearly all other diploid, i.e. non-(paleo) polyploid species of actinopterygian fishes so far reported. Austrolebias species appear to be conventional diploids in all other respects and there is no reason to believe that they arise from polyploid ancestors. The genome sizes reported for other rivulid killifishes, including a putative sister group, are considerably smaller and fall within the range typical of most other cyprinodontoid species. Therefore, it appears that the ancestor(s) of contemporary Austrolebias have undergone one or more episodes of genome expansion encompassing sudden speciation process during the Pleistocene. In addition, these findings are consistent with the hypothesis of a positive correlation between species richness and genome size.

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