Evolutionary study of one-leaf plants (Gesneriaceae)

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Monophylly is exhibited in Monophyyllaea and in Streptocarpus (Gesneriaceae). Their plant body consists of a single large cotyledon. They never form stems or foliage leaves throughout their life history, and they produce inflorescences at the base of the cotyledon lamina. This research aims to clarify the evolution of the monophylly based on the molecular phylogenetic analysis and comparative morphogenesis. Research materials include not only anisocotylous Gesnerioideae but also isocotylous Cyrtandroideae. They were collected in Borneo, Malay Peninsula including Thai, Madagascar, South Africa, and Brazil on three botanical expeditions. The embryo of anisocotylous Gesnerioideae remains at the torpedo stage, and during or after seed germination the root apex and then shoot apex are formed from the hypocotyl tip and between the two cotyledons respectively. The shoot apex grows too slow to form the vegetative shoot, and in monophyllous plants a rudimentary meristem called groove meristem b … More ecomes directly the reproductive shoot apex. The groove meristem is considered to be homologous to the shoot apex, not a de novo meristem. The accrescent cotyledon is produced by the prolonged activity of the basal meristem located on the cotyledon base. The evolutionary course of the basal meristem and the underlying mechanism still remains to be solved. However, morphological analysis of caulescent species of Streptocarpus and Chirita shows that there is a similarity in the developmental mode between their large cotyledon and the foliage leaves ; the both leaves grow mainly due to the meristematic cells in the basal portion. Their foliage leaves do not have an apparent plate meristem typical of other dicotyledon-leaves. It seems likely that the suppression of the plate meristem occurred more strongly in their cotyledon than the foliage leaves, resulting in the establishment of the basal meristem in anisocotylous Gesnerioideae. In conclusion, one-leaf plants may have evolved through retardation of the embryogenesis and consequent suppression of the vegetative shoot growth. Less