Origin of Alternation of Generations

Alternation of generations is a common component of all land plants, vascular and nonvascular; it does not express itself in the same manner in any other organisms. Given the other overwhelming similarities in land plant biochemistry, photosynthetic pigments and tissue organization, it seems that the easiest assumption is that they are derivatives of a common ancestor. The origin of alternation of generations is, however, unclear. Considerable evidence suggests that the land plants (both vascular and non-vascular) originated from a green algal ancestor that was haploid for most of its lifecycle, and only after gametic fusion and fertilization was it briefly diploid. The resulting zygote was the only diploid part of the life cycle, and it was immediately followed by meiosis. Thus, the zygote directly gave rise to spores and the haploid generation was re-established. The gap from four derivatives to more than four has already been bridged in some algal species by an intercalated mitotic division. But the spore producing structure remains that of the former egg in a unicellular organ. It could hardly be termed a sporangium, although it still had the same function.