<p><strong>Background</strong>: Cytogenetic studies in the genus Sesbania show lack of agreement among the researchers about the precise number and position of secondary constrictions and satellites as well as their relation to the organization of the nucleolus. The lack of this information makes it difficult to carry out reliable comparative cytogenetic studies and chromosome evolution in this genus.</p><p><strong>Questions</strong>: Where are the secondary constrictions and satellites located

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... in the chromosomes of Sesbania virgata? Do these regions actively participate in the nucleolar organization?</p><p><strong>Study species</strong>: Sesbania virgate (Cav.) Pers.</p><p><strong>Study site</strong>: Municipality of Tlacotalpan, Mexico and Province of Salta, Argentina.</p><p><strong>Methods</strong>: Surface spreading and air-drying technique was applied to obtain chromosomes in prometaphase and typical metaphase from radicular meristems.</p><p><strong>Results</strong>: Each population exhibited a different karyotype and only two secondary constrictions associated with microsatellites in the short arms of the smallest chromosome pair and not in long arms as was suggested by other authors. The inclusion of secondary constrictions and satellites in the nucleolus of cells in prometaphase allowed to corroborate their active participation in the formation of this one. This information was used to reevaluate the position of the nucleolar organizer regions "NOR´s".</p><strong>Conclusions</strong>: Our results agree with the predominant point of view on the location of the "NOR´s" in the short arms of plant species, particularly in legumes. In addition, given that the populations under study are geographically isolated, we suggest that an active process of speciation manifested in the two found cytotypes whose differences are attributed to changes in the proportion of arms of the satellite chromosomes is favored.