Thismia americana, the 101st Anniversary of a Botanical Mystery

Abstract

Premise of research. One hundred and one years ago, Thismia americana was discovered in a prairie near Chicago in the United States. This tiny mycoheterotrophic plant was observed at the type locality for a few years but is now considered to be extinct. The occurrence of the mainly tropical genus Thismia in temperate North America remains a mystery, particularly because T. americana’s putative most closely related species occurs in Australia and New Zealand. Here we evaluate the affinities of T. americana using morphological data and the relationships of the two other species in Thismia subgen. Thismia sect. Rodwaya with molecular data.Methodology. A 21-character morphological data set of 50 Thismia species was analyzed with parsimony and Bayesian phylogenetic analyses. A data set of eight species of Thismia and allied taxa containing sequence data of the nuclear 18S and mitochondrial atp1 regions was analyzed with maximum likelihood and Bayesian phylogeny inference methods, including a Bayesian relaxed molecular clock analysis.Pivotal results. Morphological data indicate that T. americana is most closely related to the Old World species of Thismia. Nuclear and mitochondrial data suggest that Old World species of Thismia are part of a single clade but also show that the section Rodwaya is not a monophyletic group.Conclusions. The results suggest that T. americana diverged from an Old World ancestor, but the morphological similarity of T. americana to other species in the section Rodwaya may result from the presence of plesiomorphic characters or convergent evolution rather than close phylogenetic affinity. Northern Hemisphere species of Thismia may be more plausible close relatives of T. americana. Under the latter scenario, T. americana would be part of an Amphi-Pacific Northern Hemisphere disjunction, a more commonly observed distribution pattern in plants.