Blossfeldialacks cortical bundles and persistent epidermis; is it basal within Cactoideae?

Abstract

The anatomy of Blossfeldia liliputana was compared with that of Notocactus, Parodia, Frailea, and numerous dwarf cacti to investigate the DNA-based hypothesis that Blossfeldia may be basal within Cactoideae (Nyffeler, 2001). Cortical bundles are present in every member of Cactoideae ever investigated except for Blossfeldia. Lack of cortical bundles could represent an ancestral condition because they are also absent from Pereskia, Maihuenia, and all Opuntioideae as well as most vascular plants. If so, Cactoideae would have diverged early into two lineages, one of which gave rise to Blossfeldia, the other of which produced the rest of the Cactoideae after cortical bundles originated. Alternatively, lack of cortical bundles could represent a loss that occurred recently as dwarfism evolved in the Blossfeldia lineage, for example after the Blossfeldia lineage diverged from the Notocactus, Parodia, or Frailea lineage. However, none of the other dwarf cacti investigated here has lost their cortical bundles, so such loss in Blossfeldia would be unique within Cactaceae. The epidermis of Blossfeldia is ephemeral, converting to a cork cambium while only a few weeks old, a feature that also occurs in Pereskia and Maihuenia but almost no other cacti. Stomata and photosynthetic aerenchyma occur only at the base of areole pits, just as occurs in Maihuenia but no other known cactus. All these features are consistent with Blossfeldia being basal in Cactoideae, but do not disprove the alternative theory of its being a highly derived taxon instead.