Comparative spore wall architecture and development across hornwort diversity and across time

Abstract Hornworts are a small group of plants exclusively characterized by asynchronous sporogenesis and a sporangium that grows from its base. Among the most widely used and reliable features that delineate hornwort taxa are those related to spores, especially shape, color and spore wall ornamentation. The present study provides comparative data on the development and microanatomy of spores from all genera, representing the range of hornwort diversity. A key finding is that aspects of hornwort spores and sporogenesis are conserved across taxa (except the derived Dendroceros ) and differentiate hornworts from other bryophytes. With isolated exceptions, spores are tetrahedral with a flattened proximal face and trilete mark or ridge that overlies an aperture with reduced sporopollenin. Monoplastidic meiosis is exhibited by all taxa and spore maturation occurs within the spore mother cell wall. The spore wall is typically three-layered (two exine layers with sporopollenin and one intine layer). A band of outer exine is laid down first, usually on tripartite lamellae, and this layer delineates the sculptoderm, regardless of the external topography. An inner exine forms next and contains granular to globular sporopollenin that is progressively deposited as the spore matures. The intine varies in thickness and forms late in development, followed by deposition of a thin fourth layer of remnant spore mother cell wall on the spore surface. Ultrastructural and developmental deviations associated with modified hornwort spores are identified in taxa such as Anthoceros fuciformis (round spores) and Dendroceros (multicellular spores). Although sparse, the fossil record provides insight on the evolution of hornwort spore characteristics and the occurrence of hornwort spores in the Cretaceous and beyond.