Comparative cytogenetics in Cyclopogon (Orchidaceae)

Abstract

A cytotaxonomical description of Cyclopogon (Spiranthinae, Orchidaceae) is carried out through a deep karyotype analysis of four species from NE Argentina. Distinctive karyotype parameters concerning the chromosomes number, morphology, size and symmetry and the genome size associate to each taxon. Cyclopogon calophyllus (2n = 2x =28; 18m + 10sm), C. congestus (2n = 2x = 32; 26m + 6sm), C. elatus (2n = 2x = 28; 18m + 10sm) and C. oliganthus (2n = 4x = 64; 40m + 24sm) possess symmetrical karyotypes (i-mean = 40. 01-42. 84; A1 = 0. 24-032; r>2 = 0. 06-0. 29) and excluding C. congestus (A2 = 0. 26; R = 2. 62) unimodality is the rule (A2 = 0. 12-0. 20; R = 1. 73-1. 92). Diploid taxa show a terminal macrosatellite in the m pair no. 2 (large arm) and share a comparable mean chromosome length (ca. 2. 75 μm) and genome size (ca. 40 μm), superior to the tetraploid C. oliganthus (ca. 2 and 32 μm, respectively). The novel data added to preceding cytological, morphological and molecular approaches involving Cyclopogon and those related taxa of Spiranthinae largely based on x = 23 support the hypothesis that the unusual 2n and the karyotype morphology of Cyclopogon is an evolutionary advance within Spiranthinae with a basic reduction to x = 14 or 16 by chromosome fusions. A polyploid-dysploid series added to dibasic hybridization explain the extant 2n diversity though a paleopolyploid series on x = 7-8 is also possible.