Chapter 1 - The Evolutionary Origin of the Plant Spore in Relation to the Antithetic Origin of the Plant Sporophyte

Abstract

The antithetic (interpolational) hypothesis for the origin of the plant sporophyte from algal ancestors calls for the evolution of the embryophytic spore in advance of the vegetative sporophyte. Here we view the fossil record of cryptospores from lower Paleozoic strata in light of this “spores before sporophytes” paradigm. Cryptospores show marked changes between pre-Darriwilian and post-Darriwilian assemblages. These include shifts from smaller to larger spore diameters, topological changes from irregular to isometric spore-body attachment configurations, and the de novo origin of homogeneous sporoderm. These changes in spore morphology and topology appear to reflect the canalization of meiosis in the sporocyte, as karyokinesis and cytokinesis became tightly coupled and meiosis resulted in only four (meio)spores. Indirect evidence of embryophytic sporangia is first found in Darriwilian rocks in the form of homogeneous sporoderm, which is considered to be tapetal in origin. Thus, the fossil record appears to support the progressive acquisition of evolving sporophyte characters with respect to spore formation. If embryophytic sporogenesis first evolved during the Darriwilian, the antithetic hypothesis would predict that the full suite of vegetative characters that defined the first sporophyte must have evolved later. This scenario is consistent with the Homerian origin of land plants as seen in the (mega)fossil record, but it implies that the origin of lands plants, per se, was not a singularity in geologic time but rather required a long period of adaptive selection in subaerial habitats throughout the early Paleozoic.