No abstract

Chromosomes in prometaphases and typical metaphases obtained from root meristems of samples of Crotalaria incana, collected in the Mexican municipalities of Tamasopo (San Luis Potosí) and Martínez de la Torre (Veracruz) were analyzed, paying special attention to the possible cytotypic variation due to differences in number and nature of the chromosomes with secondary constrictions and satellites. The chromosomal number 2nϭ14 (in both populations), the great uniformity in the total haploid chromosomal length, the karyotypic formula (8 mϩ6 sm), the chromosomal length interval, and a slight difference in the asymmetry index were corroborated with the use of the "air drying method" (splash). It was observed that the 14 chromosomes, particularly in prometaphase, exhibit uncoiled terminal regions which could be confused with secondary constrictions. These findings are compared to previous records on the fine chromosome morphology of C. incana and other species of the genus. The existence of proposed cytotypes for the species is discarded, and their evolution as a function of gene changes or genic complexes is discussed.

A combination of modified squash and splash techniques was used for a cytogenetic study of mitotic and meiotic cells of Prosopis laevigata. The mitotic karyotype was obtained for the species, and meiotic behavior was monitored from prophase I through the synaptonemal complex (SC). Analysis using light microscopy showed 2n=28, with chromosomes that, depending on the technique employed, varied in length from 0.84 to 1.84 pm in squashed specimens and from 1.0 to 1.82 pm in splashed specimens. These values are larger than those previously recorded for other South American species of the genus. The karyotype was studied by the splash method. The frequency of tetraploid cells was about 11% of 2052 metaphase cells examined. Meiosis was normal, because 14 bivalents were seen at metaphase I. The SC in pachytene showed regular features and an asynchronous pairing pattern with some short segments of lateral elements apically or interstitially unpaired, and occasional partner exchanges between three lateral elements. A constant association of the telomeric end of a bivalent with the nucleolus in mid-and late-prophase suggests the location of an active nucleolus organizer region (NOR) and its relation with a unique pair of mitotic chromosomes with satellites. The techniques employed here are recommended for species not amenable to cytogenetic studies with conventional techniques.

The mitotic chromosomal complement of Prosopis juliflora from a population of Mexico was analyzed using the method of air drying and Giemsa staining. In addition to an unequivocal and constant chromosome number 2n=4x=56, detailed characteristics of its general chromosomal morphology and karyotype are described for the first time, where metacentric (m) and submetacentric (sm) chromosomes and few subtelocentric (st) predominate. Only two SAT chromosomes were recorded with microsatellites on short arms in apparently st chromosomes and their nucleolar organizer region (NOR) condition was verified by locating them close to or embedded in the nucleolus of prometaphase cells. A maximum number of two nucleoli in interphase nucleus and rarely traces of two additional micronucleoli were also corroborated. The average chromosome size exhibits a notable reduction compared to that recorded in diploid species of the genus. Taken together, these results show P. juliflora as an allotetraploid taxon whose complements show amphiplasty or nucleolar dominance. Finally, it is proposed that the current distribution of this species, adjusted mainly to coastal environments, responds to an adaptive and functional novelty as a result of a hybrid condition.

Se utilizó la técnica de splash para analizar las características citogenéticas de Tamarindus indica en meristemos radiculares de semillas provenientes de individuos silvestres y de variedades cultivadas de México. El número cromosómico observado en metafases mitóticas 2n = 2x = 24 concuerda con recuentos anteriores, aunque difiere de otros obtenidos en algunas zonas paleotropicales, por lo que se discute el papel de la disploidía en la evolución del género. La fórmula cariotípica establecida aquí como 16m + 6sm + 2stsat y los tamaños cromosómicos obtenidos confirman que en Caesalpinioideae predominan los cariotipos simétricos y cromosomas de tamaño pequeño (< 3 µm). La constante asociación de los satélites con un nucleolo único confirma que los cromosomas subtelocéntricos (st) son portadores de la región del organizador nucleolar (NOR). Diferencias en las longitudes cromosómicas totales sugieren que las semillas utilizadas para su cultivo en México proceden de regiones geográficas diferentes o bien reflejan un proceso de intensa selección artificial aplicada en las variedades cultivadas. Se requiere evaluar la extensión de la variación en el número y morfología cromosómica en otras poblaciones del Nuevo Mundo.