Two theories of origin of the land-plant sporophyte: Which is left standing?

Abstract

Questions concerning the two competing theories of the development of alternating generations in land plants, the homologous theory and the antithetic theory, have never been fully resolved. In the majority of recent accounts there appears to have been increasing de facto support (if one considers the ontogenetic processes and phylogenetic consequences discussed) for the antithetic theory. However, this preference is usually not plainly stated (as such) in these discussions, and some support has also continued for the homologous theory. The crux of both theories (homologous and antithetic) centers upon how the sporophyte may have originated in the life cycle. One problem with the homologous theory is that it is not made explicit how the development of a dependent sporophyte could have occurred in the life cycle (when the precedent organisms are considered to have had free-living, putatively similar, gametophytes and sporophytes). The antithetic theory, by contrast, offers a definite ontogenetic mechanism or process (retention of the zygote on the gametophyte, delay of zygotic meiosis, with zygotic mitoses occurring first) by which a dependent sporophyte might have originated and persisted, in the context of a life cycle formerly lacking a sporophyte generation. Also, a review of a variety of evidence (morphological, cytological, biochemical, etc.) would appear to lend more support to the antithetic theory than to the homologous theory. In discussing types of algae now known to be most clearly related to land plants (i.e., charophytes, particularly advanced forms), the type of life cycle exhibited by these particular algae (haplontic, with zygotic meiosis; no sporophyte present) suggests that only an antithetic origin of the sporophyte in land plants is actually feasible.