Caracterização cromossômica comparada no gênero Stylosanthes (Fabaceae)

Abstract

Stylosanthes (Leguminosae) comprises about 50 species being some of them with great economic importance. Some species are widely used as forages and, to recover degraded areas and as a green fertilizer due to its adaptability to low fertility soils and nitrogen fixing capacity . The genus is distributed in tropical and subtropicals areas. Brazil is considered the larger diversity center, with great inter and intraspecific variation. Breeding studies have been increased although the taxonomic situation of the group remains controversial. The occurrence of polyploidy, hybrids and many additional synonyms reinforce the importance to add new information about the genus. Cytogenetics data about the genus are scarce, even considering as an important tool to give relevant information about the genome. Here we describe, for the first time, karyotipic aspects and the DNA content of diploid species (S. grandifolia, S. gracilis, S. hipocampoides, S. acuminata and four varieties of S. guianensis) by using molecular cytogenetic techniques (CMA/DAPI banding and in situ hybridization) and flow cytometry. Seeds germination was induced by temperature variation (18° to 40°C) and growth hormones (CEPA 0.1mM) in a germination chamber. Meristematic roots were used to slides preparation. Chromosomes were stained with CMA/DAPI for banding patterns analysis and ribosomal 5S, 45S rDNA sites were mapped by in situ hybridization. To estimate the DNA content, Pisum sativum were used as a reference standard in flow cytometry analysis. The chromosomal morphometry showed a symmetrical karyotype with few variations between species. The localization of the CG rich regions was similar (two bands at terminal position) in S. gracilis, S. hipocampoides, and in three of the S. guianensis varieties (1480,1463 and 4171). However, one variety of S. guianensis (C.V mineirão), S. grandifolia and S. acuminata exhibited four CMA+ marks. Regarding 5S and 45S rDNA, two sites were observed in most species, excepting for S. acuminata, with fours 45S marks. Some variations were also observed among S. guianensis accessions. In 4171, metaphases with three or four 45S rDNA sites were observed. DNA content estimation was about 2.50pg in all species. Nevertheless, for S. grandifolia, cells with two DNA C-values were observed suggesting a variation in the DNA content. Taking together, the data constitutes an important contribution in the study of evolutionary history of the group, revealing the genome stability when chromosome and DNA content information were considered. The study also contribute for breeding programs of species with economic importance.