Integrative Taxonomy of the Model Liverwort Riella Helicophylla (riellaceae, Sphaerocarpales) Reveals Its Extreme Rarity and an Overlooked Widespread New Species, R. Macrocarpa

Riella (Riellaceae) is a genus of rare, ephemeral, aquatic liverworts that includes some taxa specialized in growing submerged in brackish, seasonal water ponds and lagoons. One of these species, Riella helicophylla is the type of the genus. Apart from being of conservation concern, it has been used as a model organism for cytological and physiological processes. The species was described from Algeria in the mid-nineteenth century and was not found elsewhere for almost a century, when it was discovered in central Spain. Nonetheless, those new populations were considered morphologically different and served to describe a new variety (var. macrocarpa), and a form (f. latealata) of the latter. The number of population records increased considerably in the following decades, especially in the late twentieth century; at the same time, it was discovered in several countries throughout the Mediterranean basin. Those reports, however, did not account for its infraspecific distinction. As part of a broader study of the genus, in this work, we have revisited the taxonomy of R. helicophylla based on an integrative approach considering morphological and molecular DNA sequence data. Our broadly sampled study revealed that R. helicophylla includes two morphotypes corresponding with the previously recognized varieties of this species. These varieties differ consistently in a number of morphological gametophytic and spore traits. Riella helicophylla was resolved as paraphyletic in phylogenies based on nuclear and plastid DNA sequences. The accessions clustered in two monophyletic clades in concert with morphology, supporting their recognition at the species level. Accordingly, a new combination is proposed, R. macrocarpa, which includes the neglected R. helicophylla var. macrocarpa and R. helicophylla var. macrocarpa f. latealata. Lectotypes are designated for the two latter taxa and for R. helicophylla. The two species are fully described and illustrated, and an updated identification key of the Mediterranean species is included. Updated information on the conservation status of both species is provided.