Cytomolecular diversity of the subtribe Laeliinae (Epidendroidae, Orchidaceae) suggests no relationship between genome size and heterochromatin abundance

Abstract

The subtribe Laeliinae (Orchidaceae) comprises approximately 2000 species and 38 genera, with numerically stable karyotypes of 2n = 40 (x = 20) in most species. Cytomolecular and chromosome banding analyses have not yet been combined with phylogenetic studies, making the understanding of chromosomal evolution in the group more difficult. The present work sought to perform comparative cytogenetic analyses of representatives of the subtribe Laeliinae, followed by the interpretation of that data within an evolutive context. We analyzed 23 species from seven genera for their chromosome numbers and morphology, the distribution of CMA and DAPI bands, fluorescent in situ hybridization (FISH) with 5S and 35S rDNA and DNA quantification by flow cytometry. All of the species showed 2n = 40, suggesting an apparent macrostructural karyotypic stability, except the polyploids Encyclia alboxanthina, E. jenischiana, E. seidelii, Laelia gouldiana and Prosthechea faresiana (2n = 80). At least two CMA+/DAPI− terminal bands were observed in all analyzed species. Additionally, CMA+/DAPI− blocks were observed in the pericentromeric region of species of the genera Brassavola, Cattleya (except C. trianae) and Laelia. Terminal CMA−/DAPI+ bands were observed in representatives of Brassavola, Laelia and Guarianthe. The clade Brassavola + Cattleya + Laelia, that presented high number of heterochromatic bands, also showed expansions of the numbers of 5S rDNA sites. DNA contents varied from 2C = 2.85 pg (Prosthechea fragrans) to 2C = 5.76 pg (Laelia gouldiana) without relation with number of CMA/DAPI bands or 5S rDNA sites. The relationship between the amplification of repetitive DNAs (viewed here as rDNA sites or heterochromatic bands) and genome size variation is discussed and compared with other plant groups.