Sporophyte Structure in the Neotropical Hornwort Phaeomegaceros fimbriatus: Implications for Phylogeny, Taxonomy, and Character Evolution

The first detailed structural description of the sporophyte of a Neotropical hornwort is presented herein. The morphology and ultrastructure of the placenta, basal meristem, and aerial sporophyte are described in populations of Phaeomegaceros fimbriatus, a poorly known but widespread species, from Panama, Venezuela, Costa Rica, and Ecuador. Gametophyte cells with wall ingrowths and haustorial sporophyte cells lacking ingrowths are closely intermingled in the placenta. Through anticlinal divisions, the basal meristem generates a protoderm, several rows of assimilative cells, and a single ring of archesporial cells surrounding a columella of 16 cells. Further periclinal division in the archesporium produces two or three rows of spore mother cells intermixed with elater initials, both of which are separated and suspended in mucilage as the sporophyte elongates. Spores are finely vermiculate with seven to 10 dimples on the distal surface. Spore walls comprise a perine-like outer layer derived from the spore mother cell wall, a two-layered exine, and a translucent intine. An aperture of thickened exine and sparse sporopollenin deposition defines the conspicuous trilete mark. The mature sporophyte consists of elongated, thick-walled epidermal cells with stomata, sporogenous tissue, a central columella of up to 30 cells, and seven to 13 layers of assimilative cells with chloroplasts lacking pyrenoids. This study reveals a sharp morphological separation of plants from Venezuelan paramos from the rest of the populations, indicating recent evolutionary events. Superficial structural features such as foot shape, spore color, and stomata in the sporophyte align P. fimbriatus with Phaeoceros, while more elaborate development of histological regions, such as massive sporogenous tissue and the ultrastructure of plastids, suggests affinities with Megaceros. The unique mixture of Phaeoceros and Megaceros traits defines the genus Phaeomegaceros, including P. fimbriatus, and provides further evaluation of the taxonomy and evolutionary diversification of hornworts.