Life History Biology

There remain a number of significant gaps in our understanding of the transitional steps required to move from a charophycean algal ancestor to a land plant. One of these involves significant differences in the life histories of these two groups of organisms (Nishiyama, 2007). In the haplobi-ontic life cycle of a charophycean alga, zygotic meiosis leads to the formation of haploid zoospores, each of which develops into a mature haploid organism. In this life history the only cell that is diploid is the zygote. This is in marked contrast to the life history displayed by vascular plants, in which the multicellular sporophyte, that is the diploid organism, is the dominant phase (Graham et al., 2000), or the life cycle of bryophytes, in which a multicellular sporophyte is produced which is dependent on the gametophyte (Chapter 5). Historically there have been two different theories on the evolution of the alternation of generations in land plants (reviewed in Blackwell, 2003; Haig, 2008).

Homologous Theory

According to the homologous or transformation theory, both the gametophyte and sporophyte phases of land plant ancestors were morphologically identical (FIG. 6.34), that is they possessed an isomorphic alternation of generations. These two phases differed only in that the gametophyte was hap-loid and produced gametes, whereas the sporophyte was diploid and produced spores. The presence of extant green algae with isomorphic alternation of generations, and some bryophytes with photosynthetic sporophytes, has been used to support the homologous theory. The proponents of this idea suggest that, during the course of land plant evolution, in particular the evolution of vascular plants, the sporophyte phase eventually evolved as the dominant, nutritionally independent phase.

Antithetic Theory

The second idea, termed the antithetic or interpolation theory, begins with the premise that the gametophyte phase was primitive and that the sporophyte phase was later added to the life cycle as a result of a delay in zygotic meiosis (FIG. 6.35). This theory, which was championed by the eminent

Alternation Generation Gametophyte

figure 6.34 Hypothesized stages in the origin of the alternation of generations according to the homologous theory. Beginning with a green alga with an isomorphic alternation of generations, the sporophyte becomes structurally and physiologically more complex, whereas the gamete producing phase becomes reduced. (From Taylor and Taylor, 1993.)

figure 6.34 Hypothesized stages in the origin of the alternation of generations according to the homologous theory. Beginning with a green alga with an isomorphic alternation of generations, the sporophyte becomes structurally and physiologically more complex, whereas the gamete producing phase becomes reduced. (From Taylor and Taylor, 1993.)

Homologous Theory

figure 6.35 Hypothesized stages in the origin of the alternation of generations according to the antithetic theory. Beginning with a haplontic green alga with zygotic meiosis, a delay in meiosis results in the interpolation of a new multicellular sporophyte that becomes heterotrichous. Meiosis occurs and the spores give rise to a multicellular gametophyte. (From Taylor and Taylor, 1993.)

figure 6.35 Hypothesized stages in the origin of the alternation of generations according to the antithetic theory. Beginning with a haplontic green alga with zygotic meiosis, a delay in meiosis results in the interpolation of a new multicellular sporophyte that becomes heterotrichous. Meiosis occurs and the spores give rise to a multicellular gametophyte. (From Taylor and Taylor, 1993.)

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  • Gloria Denton
    What is homologous theory states about evolution of bryophytes?
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