Evolution of the aquatic habit in Ludwigia (Onagraceae): Morpho-anatomical adaptive strategies in the Neotropics

• We describe selected Ludwigia spp. in terms of their morpho-anatomical characters.
• We evaluate Ludwigia aquatic habit evolution in a molecular phylogenetic context.
• We propose that the strict aquatic habit evolved at least twice in Neotropical Ludwigia.

Ludwigia displays a great variety of life forms adapted to aquatic habitats ranging from humid to water saturated. Morpho-anatomical characters were used to characterize and discriminate among selected Neotropical species of Ludwigia in terms of their root and shoot adaptations. A partial molecular phylogenetic hypothesis was reconstructed using the nrDNA ITS and cpDNA rpl32–trnLUAG regions for Neotropical species. ITS accessions of North temperate species were also included to provide a framework for the placement of our sampled taxa. Mapping of morpho-anatomical characters was performed on the resulting phylogenetic tree. We were able to separate the species in three groups according to their morpho-anatomical adaptations to aquatic habitats. Our results strengthen the support for at least two separate origins for the strictly aquatic habit in Neotropical representatives, in line with previous taxonomic classifications. We also suggest that these species experienced convergence of characters, but also diversified into various life forms adequate to live strictly in aquatic environments. We would like to highlight the great value of Ludwigia as a model system for the study of the evolution of characters that respond to pressures imposed by aquatic ecosystems within the dicotyledons, as well as within aquatic plants in general.

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