Polyploidy and apomixis in accessions of Senna rugosa (G.Don) H.S.Irwin & Barneby

Abstract

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.