Reproduction, Alternation of Generations and
Reproduction, Alternation of Generations and
During sexual reproduction two gametes, each of which is haploid , unite to form a single-celled zygote , which is diploid . As a consequence of the chromosome doubling that occurs during fertilization, at some point in the organism's reproductive cycle meiosis, or reductive cell division, must also occur to restore the haploid condition. In many organisms, including most animals, the zygote develops into a multicellular individual, and meiosis occurs during gamete production. In such organisms, gametes are the only haploid cells in the life cycle. In many algae and fungi, in contrast, the diploid zygote undergoes meiosis immediately to form haploid cells, called spores. Spores subsequently grow into multicellular haploid individuals. In both of these life cycles there is only one multicellular phase. In some algae and in all plants, however, there are actually two multicellular phases, one haploid and one diploid, which alternate with each other in the life cycle. This type of reproductive cycle is referred to as alternation of generations.
In organisms with alternation of generations, the diploid generation, or sporophyte, is formed by mitotic divisions of the diploid zygote, just as in animals. When mature, the sporophyte produces asexual reproductive organs called sporangia. Meiosis within the sporangia produces the one-celled, haploid spores that are released when the sporangia open. Each spore then gives rise to a multicellular haploid individual, or gametophyte. The game-tophyte produces the sexual reproductive organs, or gametangia, in which haploid gametes are formed by mitosis . Gametes then fuse to form the zygote, completing the cycle.
Occasionally, sporophyte and gametophyte generations look identical, as in many red and some green and brown algae, in which case alternation of generations is described as isomorphic. In other algae and all plants, the two generations are structurally different, and alternation of generations is said to be heteromorphic.
It is notable that isomorphic alternation of generations occurs only in certain algae and aquatic molds, while heteromorphic alternation of generations is the rule in land plants. In bryophytes the gametophyte is the ecologically persistent, independent generation, and the sporophyte is ephemeral and dependent upon the gametophyte for its nutrition. In all other plants the sporophyte dominates the life cycle. The fern gametophyte, for example, is a small thalloid plant, which is soon destroyed by the growth of the large, leafy sporophyte. In gymnosperms and angiosperms , the gametophyte is reduced to but a few cells of the pollen grain (the male gametophyte) and the embryo sac (the female gametophyte).
Two theories have been proposed to explain how alternation of generations evolved. Both theories hypothesize that the haploid generation is ancestral and that the diploid generation developed as a consequence of mitosis replacing immediate meiosis in the unicellular zygote. One theory proposes that originally the developmental potential of the diploid zygote was identical to that of the haploid spores, resulting in isomorphic sporophytes and gametophytes. Sporophytes became structurally different from gametophytes as a result of spores and zygotes being exposed to different environmental pressures. In land plants, for example, spores are released as unicells into the environment, while zygotes begin their development within the confines of the female gametangium. As a consequence, gametophytes, which develop from spores, and sporophytes, which develop from zygotes, are structurally very different.
The second theory proposes that the sporophyte generation evolved gradually by stepwise delays in zygotic meiosis, accompanied by the elaboration of vegetative diploid cells. The first sporophytes were little more than single sporangia, probably embedded in the much larger gametophytes. As evolution progressed, sporophytes became larger and larger, and gametophytes became more and more reduced. Even today, there is no consensus as to which theory best explains the diversity seen in modern organisms.
see also Algae; Angiosperms; Bryophytes; Ferns; Gametophyte; Gymnosperms; Reproduction, Fertilization and; Reproduction, Sexual; Sporophyte.
Graham, Linda. "The Origin of the Life Cycle of Land Plants." American Scientist 73 (1985): 178-86.