Embryology of Phyllonoma (Phyllonomaceae, Aquifoliales): characteristics and character evolution

Phyllonoma, the sole genus of Phyllonomaceae (Aquifoliales) consisting of four Central American species, has not been well-characterized morphologically. Following a previous study of flower and inflorescence morphology, I here report the embryology of the genus based on P. tenuidens and compare its characteristics with those of other aquifolialean families, namely, Aquifoliaceae, Cardiopteridaceae, Helwingiaceae, and Stemonuraceae. Comparisons indicate that although Phyllonoma resembles all the other families embryologically, it more closely resembles Aquifoliaceae and Helwingiaceae in lacking a vascular bundle in its integument and bearing ab initio Cellular endosperm. The genus especially resembles Helwingiaceae by possessing a tenuinucellate ovule. This result corroborates molecular and floral morphological evidence, supporting the distinctness of Phyllonoma as a family and its sister-group relationship to East‒Asian Helwingiaceae. However, Phyllonoma is clearly distinguished from Helwingiaceae by seed coat structure. In Phyllonoma, the seeds (dispersed in berries) have a thick seed coat composed of irregularly enlarged, thick-walled exotestal cells, whereas the seeds (dispersed in drupes) have a thin membranous seed coat in Helwingiaceae. Taken together with earlier information on pollination (entomophily in Phyllonoma versus ambophily in Helwingiaceae), embryological evidence shows that distinct evolution has occurred in reproductive characters relating to pollination and seed dispersal in Phyllonoma.