A floral ontogenetic study on the sister group relationship between the genus Samolus (Primulaceae) and the Theophrastaceae

The former Primulales used to be subdivided into the woody Theophrastaceae and Myrsinaceae, from the tropics and subtropics, and the herbaceous Primulaceae, which are mainly found in the temperate regions of the northern hemisphere. Recent analyses based on morphological as well as molecular data revealed a close relationship between the genus Samolus L. of Primulaceae and the monophyletic family Theophrastaceae. We studied the floral development of six species from four different genera of Theophrastaceae and compared it to floral ontogenetical data of Samolus valerandi L. to find support for a close relationship. Samolus and the members of Theophrastaceae share the presence of staminodes and a similar development of the placenta and the ovules. Apart from the different habit and distribution, however, we also observed some major differences between both lineages, such as the absence of common primordia in Theophrastaceae, the development of a gynoecial cap in Samolus, and the difference in development, shape, and structure of the staminodes. Therefore, we propose to keep Samolus separated from the genera of the Theophrastaceae, and we suggest that it be raised to family level.