Homologous Versus Antithetic Alternation of Generations and the Origin of Sporophytes

David Haig

doi:10.1007/s12229-008-9012-x

Abstract

The late-nineteenth/early-twentieth century debate over homologous versus antithetic alternation of generations is reviewed. Supporters of both theories, at first, used Coleochaete as a model for the origin of land-plant life cycles. The early debate focused on the morphological interpretation of the sporophyte and on whether vascular cryptogams had bryophyte-like ancestors. The terms of the debate shifted after the discovery that the alternation of morphological generations was accompanied by an alternation of chromosome number. Supporters of homologous alternation now promoted a model in which land plants had been derived from an algal ancestor with an isomorphic alternation of haploid and diploid generations whereas supporters of antithetic alternation favored a model in which land plants were derived from a haploid algal ancestor with zygotic meiosis. Modern evidence that embryophytes are derived from charophycean green algae is more compatible with an updated version of the antithetic theory.

Highlights

Introduction IIIPrelude IV: Homologous and antithetic Alternation expounded V
The ensuing debate focused on the origin of the asexual generation of land plants and is conventionally characterized as a conflict between theories of antithetic and homologous alternation of generations
Lang (1909) presented an ontogenetic theory of alternation that he believed placed the relation of the two generations in a new light. He regarded gametophytes and sporophytes “as homologous, in that they correspond to regularly succeeding individuals, developed from germ-cells which are similar in their morphogenetic powers.”

Summary

Introduction

Introduction IIIPrelude IV: Homologous and antithetic Alternation expounded V. Alternation of generations initially referred to the alternation of sexual and asexual forms in animals, the term is almost exclusively associated with the life cycles of plants, with the alternation of haploid gametophytes and diploid sporophytes.

Results

Conclusion