Spores of relictual bryophytes: Diverse adaptations to life on land

• Thick-walled meiospores and cryptospores were key to the evolution of life on land.
• We provide comparative data on spores of eight early-divergent or relictual bryophytes.
• Tripartite lamellae and multilamellar layers are plesiomorphic in sporoderm development of bryophytes.
• Distinct proximal apertures evolved in mosses and hornworts but not liverworts.
• Ancestral spores were tetrahedral and trifacial with less ornate proximal than distal surfaces.

Sporogenesis and the evolution of thick-walled meiospores were of paramount importance in the establishment of the first life on land nearly half a billion years ago. Indeed, the first evidence of plant life on land is the sudden appearance of spores (palynomorphs or cryptospores) about 470 million years ago, predating the first megafossils by some forty million years. Affinities of these fossil spores with living embryophytes are often difficult to assess due to significant gaps in knowledge of spore diversity in critical extant taxa. The bryophytes (liverworts, mosses and hornworts) are the most ancient living plant lineages. To fill in data and provide a framework in which to evaluate spore evolution, we conducted ultrastructural and developmental studies of eight early-divergent or relictual bryophytes, as identified by cladistic/molecular and morphological studies. Spore wall data are presented for the liverworts Apotreubia, Haplomitrium, and Blasia; the mosses Takakia, Sphagnum, Andreaea, and Oedipodium; and the hornwort Leiosporoceros. Wall ontogeny is based on tripartite lamellae (TPL) in all taxa except Andreaea, which has a unique spongy exine. While a single foundation TPL is typical of bryopsid mosses, Takakia has a multilamellate layer (MLL) composed of numerous appressed segments of TPL. Spores of Sphagnum, liverworts, and the hornwort Leiosporoceros have a MLL layer composed of continuous, not segmented, TPL. We conclude that TPL and multilamellate layers are plesiomorphic in sporoderm development in land plants. Proximal wall modifications evolved into distinct apertures in mosses and hornworts but not in liverworts. Analysis of relictual bryophyte species indicates that the ancestral spore types would be tetrahedral with trifacial proximal surfaces having less ornamentation than the distal surfaces. Identification of features specific to relictual liverwort, moss and hornwort spores provides a foundation for determining fossil spore affinities and interpreting spore evolution and diversification in the earliest land plants.

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