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In Apiaceae, embryos of most species have two cotyledons, but some species are consistently monocotylar. Traditionally, the monocotyly has been considered as taxonomically important at the generic level, despite its presumably multiple origins in the family. In this study, a survey of the published literature and our new findings on cotyledon number, embryo and seedling morphology and nrDNA ITS sequence data are presented to provide modern insights into the taxonomic distribution and phylogenetic relationships of monocotylar taxa. A molecular phylogenetic tree representing much of the diversity of monocotylar Apiaceae was produced to re-evaluate the potential implication of monocotyly for systematics and to elucidate its evolutionary significance in the family. Our data document the presence of monocotylar seedlings in 59 species representing 15 genera, in three species and one genus (Postiella) of which monocotylar seedlings are reported for the first time. Analysis of ITS sequence data indicates that monocotyly in Apiaceae has arisen independently in at least seven different lineages encompassing five of 41 major clades of subfamily Apioideae, but not in early-diverging lineages. Parallel evolution has resulted in a remarkable morphological similarity in monocotylar embryo and seedling organization, especially in the proportion of the cotyledon length to the axis of the embryo and the multifunctional cotyledonary tube in the seedling. These features could be considered as adaptations to a geophilic life form, as all monocotylar species are perennial herbs with tuberous underground organs distributed mainly in the Ancient Mediterranean region. The single cotyledon in Apiaceae, as in most other monocotyledonous eudicots, could be interpreted as two united cotyledons (syncotyly), but further developmental studies are needed to test this hypothesis.

A taxonomic survey of monocotylar Apiaceae and the implications of their morphological diversity for their systematics and evolution