No abstract

A cytogenetic analysis of chromosomes in mitotic metaphase of Dermatophyllum secundiflorum, a recently synonymized species, collected in the municipality of Cardonal (Hidalgo State, Mexico) was performed. The objective was not only to obtain the karyotype, but also cytogenetic markers such as secondary constrictions and satellites. A conventional method of air drying and Giemsa staining was used in cells from root meristems which confirmed a 2n=18. The formula for the haploid karyotype was 6m+2sm+1st sat . The secondary constrictions and microsatellites were clearly located on the short arm of the st chromosomes, which differs from a previous study that placed them on metacentric chromosomes. Occasionally, metaphase plates exhibited satellite associations involving the secondary constrictions and satellites of the st chromosomes, the role of which is discussed in light of the limited information available.

<p><strong>Background</strong>: Cytogenetic studies in the genus <em>Sesbania</em> 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 in the chromosomes of <em>Sesbania</em> <em>virgat</em>a? Do these regions actively participate in the nucleolar organization?</p><p><strong>Study species</strong>: <em>Sesbania virgate</em> (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.

Antecedentes y Objetivos: Tigridia pavonia fue propuesto como un taxon alotetraploide 2n=4x=28, pero sin evidencia citogenética que sustente su origen híbrido. Es una especie de amplia distribución en México y sus poblaciones carecen de una fórmula cariotípica y un análisis detallado de los cromosomas con satélites como criterio para determinar el número de organizadores nucleolares que confirmen o no dominancia nucleolar. En este trabajo se analiza y describe el número y arquitectura cromosómica de T. pavonia de una población mexicana, en búsqueda de evidencias que soporten o descarten su origen híbrido y se propone una fórmula cariotípica acorde al nivel y origen de ploidía.Métodos: Se utilizó una técnica de extendido en superficie y secado al aire que incluye maceración enzimática y choque hipotónico en meristemos radiculares para obtener los cromosomas en mitosis de seis individuos de T. pavonia, nativa de la Reserva Ecológica del Pedregal de San Ángel, en la Ciudad de México.Resultados clave: Se confirmó un cariotipo bimodal con 28 cromosomas que, de acuerdo a su similitud morfológica, fueron incluidos en siete grupos de cuatro cromosomas homólogos cada uno. Los cromosomas del grupo más pequeño exhibieron constricciones secundarias asociadas a macrosatélites lo que evidenció ausencia de dominancia nucleolar o amfiplastía diferencial. En el grupo de cromosomas grandes se observaron configuraciones que sugieren rearreglos por translocaciones. Se propone la fórmula 6m + 8sm para el cariotipo haploide.Conclusiones: Citogenéticamente, la presencia de cuatro satélites descarta un origen alotetraploide y la evidencia de posibles translocaciones se correlaciona con fragmentos, cromosomas B y centrómeros frágiles observados en otras especies del género. Lo anterior apoya el papel activo de las translocaciones en la conformación del cariotipo bimodal de T. pavonia.