Low cytomolecular diversification in the genus Stylosanthes Sw. (Papilionoideae, Leguminosae).

Ana Luiza Franco,Lyderson Facio Viccini,Marcelo Ayres Carvalho,Marcelo F Simon,Amanda Figueredo,Lívia De Moraes Pereira,Gustavo Souza,Saulo Marçal De Sousa

doi:10.1590/1678-4685-gmb-2018-0250

Abstract

Stylosanthes (Papilionoideae, Leguminosae) is a predominantly Neotropical genus with ~48 species that include worldwide important forage species. This study presents the chromosome number and morphology of eight species of the genus Stylosanthes (S. acuminata, S. gracilis, S. grandifolia, S. guianensis, S. hippocampoides, S. pilosa, S. macrocephala, and S. ruellioides). In addition, staining with CMA and DAPI, in situ hybridization with 5S and 35S rDNA probes, and estimation of DNA content were performed. The interpretation of Stylosanthes chromosome diversification was anchored by a comparison with the sister genus Arachis and a dated molecular phylogeny based on nuclear and plastid loci. Stylosanthes species showed 2n = 20, with low cytomolecular diversification regarding 5S rDNA, 35S rDNA, and genome size. Arachis has a more ancient diversification (~7 Mya in the Pliocene) than the relatively recent Stylosanthes (~2 Mya in the Pleistocene), and it seems more diverse than its sister lineage. Our data support the idea that the cytomolecular stability of Stylosanthes in relation to Arachis could be a result of its recent origin. The recent diversification of Stylosanthes could also be related to the low morphological differentiation among species, and to the recurrent formation of allopolyploid complexes.

Highlights

Chromosomal evolution, including polyploidization, chromosomal rearrangements, and heterochromatin polymorphisms are known to be important mechanisms promoting speciation in plants (Raskina et al, 2008; Soltis and Soltis, 2009; Reis et al, 2014; Pinto et al, 2016)
The present study was based on material obtained from seeds of 12 accessions of eight Stylosanthes species [S. acuminata, S. gracilis, S. grandifolia, S. guianensis, S. hippocampoides, S. macrocephala, S. pilosa, and S. ruellioides]
Most of the species (S. hippocampoides, S. gracilis, S. macrocephala, S. pilosa, S. ruellioides, and S. guianensis accessions 1480, 1463, 4171, and LC2538), showed two chromomycin A3 (CMA)+/DAPI- signals in the short arms of the smaller chromosomal pair. In addition to those bands, in S. ruellioides CMA-/DAPI+ proximal bands were observed in all chromosomes (Figure 1)

Summary

Introduction

Chromosomal evolution, including polyploidization (auto or allopolyploidy), chromosomal rearrangements, and heterochromatin polymorphisms are known to be important mechanisms promoting speciation in plants (Raskina et al, 2008; Soltis and Soltis, 2009; Reis et al, 2014; Pinto et al, 2016). Pimentel et al (2017) investigated patterns of diversification and chromosomal evolution in Pooideae (Poaceae) in the light of past environmental changes In this group, the haploid basic chromosome number has remained stable, with no direct association of chromosome transitions with diversification shifts. It would be expected that groups of recent diversification would have more stable and uniform karyotypes, while those with older divergences would have time to accumulate more cytomolecular differences, resulting in higher karyotypical diversity. It is not clear if the timing of diversification of a given lineage reflects variation at the cytogenetic level. There are cases of recently evolved lineages with highly variable karyotypes (e.g., genus Nothoscordum [Amaryllidaceae] Souza et al, 2012), and ancient groups showing high karyotype stability (e.g., subfamily Bombacoideae [Malvaceae] Costa et al, 2017)

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Conclusion