Chromosome analysis in Psygmorchis pusilla (L.) Dodson & Dressier: The smallest chromosome number known in Orchidaceae

Abstract

The neotropical genus Psygmorchis Dodson & Dressier has the smallest chromosome numbers registered for the orchid family. The previous counts in the genus ar.e= 5, 7 for P. pusilla (L) Dodson & Dressier and n=7 for P. glossomystax (Rchb. f.) Dodson & Dressier. In the present work meiosis and mitosis were analysed in two natural populations of P. pusilla and an in vitro cultivated sample. The chromosome analysis also included stain ing with the fluorochromes chromomycin A3 (CMA) and 4’-6-diamidino-2-phenylindole (DAPI). All individuals analysed showed a somatic complement of 12 chromosomes. The meiotic analysis revealed regular behaviour with formation of six bivalents in the natural populations. The Cbanding plus Giemsa staining revealed small centromeric bands, while the staining with fluorochromes did not show any differentially bright region. The chromo some stability in the materials studied suggests that chromosome numbers previously re ported for the species may be incorrect. The reduction of n=7_6, by Robertsonian translo-cation, is the mechanism that would best explain the origin of the karyotype in P. pusilla and suggests that x=7 is the most likely base number in the Oncidiinae. © 1999 Taylor & Francis Group, LLC.