EVIDENCE FOR POST‐TRANSLATIONAL MODIFICATION OF TRIOSE PHOSPHATE ISOMERASE (TPI) IN ISOËTES (ISOËTACEAE)

As part of an electrophoretic study on Isoëtes , a number of Neotropical and North American species were examined for allozyme variation in TPI. Three of these species— I. storkii, I. flaccida , and I. mexicana —exhibit three distinct zones of TPI activity. The two most anodally migrating zones are comparable to the two zones found in most angiosperms and in several other species of Isoëtes. The single or three‐banded phenotypes produced at these loci correspond, respectively, to the homozygous and heterozygous patterns typical of a dimeric enzyme. The most cathodal zone (zone III) differs in producing either single or two‐banded phenotypes. Analyses of these three zones indicate a nearly perfect correlation between zones II and III in putative allelic constitution and relative allelic mobility. Explanations involving TPI gene duplications and/or null alleles fail to account for the peculiar banding characteristics and origin of activity zone III. An alternative hypothesis involving a protease duplication and differential post‐translational modification is postulated. This hypothesis adequately explains the zone III phenotypes and fixation of the third activity zone in the species examined. Amino acid sequencing is suggested as the most direct test of this hypothesis. The taxonomic distribution of TPI III generally supports a previous, morphologically‐based, hypothesis on species relationships in Isoëtes. The presence of this zone is regarded as an independent synapomorphy for a major clade of Neotropical Isoëtes.