Topic 21.1 Evolution has Favored Diploidy in Plant Life Cycles

Topic 21.1 Evolution has Favored Diploidy in Plant Life Cycles

Land plants probably evolved from a freshwater algal ancestor, with a life cycle similar to that of extant charophycean algae (e.g., Chara fragilis), the algal taxon most closely related to land plants. These algae have multicellular haploid bodies that produce the gametes. After fertilization, the diploid zygote, the only sporophytic stage, undergoes meiosis directly to produce haploid spores. In primitive nonvascular land plants, such as liverworts, mosses, and hornworts, the haploid gametophyte generation is still the only free-living generation, but the diploid sporophytic stage has become larger and multicellular. Beginning with the vascular plants, the diploid sporophyte becomes free-living and is the dominant generation. Seed plants, which possess the most elaborate sporophytes and the most reduced, dependent gametophytes, represent the final stage in the transition from a predominantly haploid to a predominantly diploid life cycle.

The selective advantages driving the shift from haploidy to diploidy in the plant life cycle are still being debated. One potential advantage is the ability of heterozygous diploids to “mask” the effects of deleterious recessive mutations through the presence of dominant wild-type alleles. A second advantage is that the meiosis/syngamy reproductive system generates greater genetic diversity when the sporophytic stage is multicellular. Meiosis can only occur in diploid cells, and when the diploid organism becomes multicellular, the number of meiotically dividing cells increases, and the potential for genetic diversity is greatly increased. For this reason the evolution of sex has been considered to be one of the major trends in evolution.