The Odd One Out: A Comparison of Flower and Fruit Development in Holoparasitic Pholisma arenarium (Lennoaceae, Boraginales) to That in Closely Related Ehretiaceae

Abstract

Premise of research. Lennoaceae is a small root-parasitic plant family comprising four species in two genera. They are included in Boraginales, but their position within the order has long been questionable. Only recent phylogenomic studies have placed Lennoaceae within Ehretiaceae with highest support. Because of their holoparasitic lifestyle, Lennoaceae are morphologically aberrant, and their morphological evolution is not fully understood. The position of Lennoaceae within Ehretiaceae invites a study of their development and morphology in comparison to closely related species of that family. Methodology. We investigate the flower and fruit development of Pholisma arenarium from the Lennoaceae and two species from Ehretiaceae (Bourreria succulenta and Ehretia dicksonii) using scanning electron microscopy and micro–computed tomography. Pivotal results. The three species share a number of common morphological and architectural features. Most noteworthy are the absence of a stylar compitum and the formation of false septa, which are involved in pyrene formation. Pholisma arenarium and B. succulenta share more similarities with each other than with E. dicksonii. Conclusions. Our findings are in line with recent phylogenomic analysis and underscore that Lennoaceae should not be treated as a separate family. Aberrant fruit morphology of P. arenarium appears to be driven by its holoparasitic lifestyle, but fruit anatomy clearly reflects a position in Ehretiaceae. Increased seed production is ensured by carpel multiplication as an obvious escape route to the evolutionary dead end of seed and pericarp integration. Changes in merosity of the outer floral organs are a result of an increased floral apex size.