Senna Mill. is one of the most representative genera of Caesalpinioideae, with wide geographical distribution. Some species have been cytogenetically characterized, but there is no information on population level. Therefore, the present work had the objective of characterizing and comparing chromosomal number and meiotic behavior of 39 accessions belonging to 11 species of Senna Mill. from southern Minas Gerais state, Brazil. Meiotic preparations were obtained by cell suspension technique and stained with 10% Giemsa solution. All meiotic phases were analyzed and the meiotic index was obtained. The species showed different chromosome numbers (n = 12, 13, 14, and 28). Meiotic behavior of four species is described for the first time. The chromosomal numbers reported for most species coincide with those described for populations from either Southern Brazil or from state of Pará. Intraspecific variation for chromosome number was observed in S. rugosa (G. Don) H. S. Irwin & Barneby (n = 14 and n = 28). This variation along with multivalent occurrence suggests polyploidization as an important event in the evolution of this species, as described for many others, mainly in Caesalpinioideae. Most of the analyzed species showed regular meiotic behavior, despite the occurrence of some irregularities.

Senna rugosa (G.Don) H.S.Irwin & Barneby (Leguminosae) is a species from the Brazilian Cerrado commonly used for medicinal purposes. Chromosome variability (n = 14 and 28) described for this species indicates the occurrence of chromosome races and recent polyploidization in its evolution history. The aim of this study was to verify the relationship between ploidy level and occurrence of polyembryony in 5 accessions of S. rugosa. Cytogenetic analysis revealed the type of interphasic nuclei, chromosome number and morphology, and nuclear DNA content. Frequency of polyembryony was estimated by germination test. Pollen grain viability was estimated by different techniques. There was variability among the accessions of S. rugosa for interphasic nucleus type (semireticulate and nonreticulate), genome size (2.57 to 2.80 pg/2C), and polyembryony frequency (from 2 to 4 embryos). Pollen grain viability was 2.5% to 20%, depending on the technique used. Numeric chromosome variability was found among meristematic cells from each individual (2n = 28 to 2n = 112), with prevalence of 2n = 56. In mitotic metaphases with 2n = 56, all chromosomes were metacentric and of similar size. The data described, combined with previous descriptions of irregularities in chromosome pairing, suggest that S. rugosa is an autopolyploid facultative apomictic species, with interpopulation variability for level of apomixis expression.

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ABSTRACT

The aim of this study was to perform chromosome counts and nuclear