Isozyme Number in Subtribe Oncidiinae (Orchidaceae): An Evaluation of Polyploidy

The Oncidiinae has attracted attention because of the variation it exhibits in chromosome number, n = 5–30, which is greater than the range in the rest of the Orchidaceae. The genus Psygmorchis, with n = 5 and 7, has been a particular focus of controversy, and many authors have suggested that 5 and 7 are the base numbers for the subtribe. The other taxa in the subtribe presumably evolved through hybridization and polyploidy. Other workers have found that the lowest counts correlate with derived morphological conditions and have hypothesized that these low numbers result from aneuploid reductions, while higher numbers are associated with ancestral morphologies and are not the result of polyploidy. These two hypotheses were evaluated by determining isozyme numbers for 13 enzymes in species that span the chromosomal range known for the Oncidiinae (n = 5–30). Isozyme number has been shown to be a reliable indicator of polyploidy in angiosperms because polyploids display isozyme multiplicity relative to diploids. This analysis revealed no differences among species in isozyme number for the enzymes examined. Therefore, our data reject the hypothesis that species with higher chromosome numbers are polyploid.