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Phylogenetic placement of the Dominican Republic endemic genus Sarcopilea (Urticaceae)

Abstract:

Since the first collection in 1910, botanists have recognized Sarcopilea (Urticaceae) as a dramatic example of convergence with succulent woody members of Crassulaceae. Phylogenetic analyses based on nuclear ribosomal DNA ITS and the trnL-F non-coding plastid DNA regions place Sarcopilea in a Neotropical clade of Pilea. A study of the cystolith, pollen and achene morphology, and of hydathode, hydrenchyma, and stomata position was also performed, confirming the molecular results. Across Rosales, leaf succulence is restricted to the genera Pilea and Sarcopilea. Furthermore, all succulent species of Pilea have an abaxial water-storage hydrenchyma layer in their leaves, a condition unique to the genus. The conditions of adaxial stomata (epistomatic, hyperstomatic), as well as hydathodes restricted to the submarginal abaxial leaf surface, were found in all Neotropical leaf-succulent species. Carbon isotope-ratio mass spectroscopy shows Sarcopilea to have C3 photosynthesis, therefore Crassulacean acid metabolism is still unknown in Rosales. The impact of molecular systematics of Caribbean endemic genera is discussed. In light of the new phylogenetic placement of Sarcopilea, a replacement name is provided (Pilea fairchildiana).