Alternation of Generations
Alternation of generations (AoG) refers to alternate asexual and sexual reproductive modes during a multicellular organism’s life cycle. For algae, plants, fungi, and slime molds, AoG also involves different genetic forms at different stages of life: alternately haploid (1 set of chromosomes) and diploid (2 sets of chromosomes).
One generation is predominant during life. The other form is typically part of early development.
Many invertebrates alternate between sexual and asexual generations; notably protozoa, jellyfish, and flatworms.
Alternation of generations for animals refers solely to reproductive mode: sexual (heterogamic) or asexual (parthenogenic) reproduction. Parthenogenesis may happen in nematodes, parasitic wasps, some bees, some scorpions, and a few vertebrates, including fish, amphibians, and reptiles.
In some species, switching between heterogamy and parthenogenesis may be triggered by the season (aphids, some gall wasps), by a lack of males, or by conditions that favor rapid population growth (e.g., rotifers and water fleas).
Asexual reproduction occurs either in summer (aphids), or as long as conditions are favorable. Asexual reproduction allows a successful genotype to spread quickly without the fuss of sex or wasting resources on males who can’t reproduce.
Animals are always diploid. As such, animal AoG has a limited context when contrasted against the genomic gyrations of plants.
All land plants have a genetically complex life cycle that alternates between sporophytes (spore-producing organisms) and gametophytes (gamete-producing organisms). Whereas sporophytes are diploid, gametophytes are haploid.
The earliest plants – liverworts, mosses, and hornworts – have the gametophyte generation predominant in their life cycle. At the onset of life, a minute and nutritionally dependent sporophyte grows upon the body of a gametophyte.
In ferns, a (diploid) sporophyte produces (haploid) spores via meiosis (cell division which cuts the number of chromosomes in half). A spore grows into a gametophyte by mitosis (still haploid, but bigger).
The fern gametophyte is a prothallus: a small, flat, free-living organism bearing reproductive organs and feeding itself via photosynthesis. The gametophyte produces gametes via mitosis, often fabricating both sperm and eggs on the same prothallus.
A motile flagellate sperm fertilizes an egg that remains attached to the prothallus. The fertilized egg is a diploid zygote that grows (via mitosis) into a fern plant. Thus, the sporophytic phase predominates the fern’s life cycle.
Gymnosperms (e.g., conifers) and angiosperms (flowering plants) generally follow the fern plan. The major difference for seed plants (from ferns) is that the gametophyte is even more reduced: sometimes only 3 cells, and the gameotpyhte is entirely dependent upon the sporophyte for all its nutrition. There are a diversity of variations to the overall scheme, including the sizes of spores and gametes and the means by which sperm and egg meet in fertilization.
Fungi, Slime Molds, Algae
Land plants descended from algae, many of which exhibit alternation of generations (AoG). Fungi and slime molds also practice AoG.
Fungal mycelia are typically haploid. Each mycelium has a sexual orientation. When mycelia that may mate meet, they produce a pair of multinucleate cells which fuse to form diploid nuclei, in a process termed karyogamy.
Karyogamy produces a diploid zygote. This short-lived sporophyte soon undergoes meiosis, forming haploid spores. The spores germinate, developing into new mycelia.
Slime mold AoG is similar to fungal mode. Haploid spores germinate into myxamoebae: swarm cells that find each other and create a colony.
Swarm cells fuse to form a diploid zygote, which develops into a plasmodium. A plasmodium matures, whereupon producing 1 or more fruiting bodies (depending upon species) which shed haploid spores.
Genetic alternation of generations is a hedge against adverse environmental conditions, as well as a nod to the benefit of genetic diversity via sexual reproduction. Spores can outlast inhospitable habitats, waiting until their time has come. AoG practitioners can gauge growth prospects and time their ontogeny (course of development) accordingly.
Especially for plants, growth and tissue differentiation in each generation is governed by different genetic programs, which are initiated either by fertilization (haploid to diploid) or meiosis (diploid to haploid).
Having 2 distinct ontologies demands a rich genome: far beyond the needs of animals, with a single ontogeny that is, however mind-boggling, relatively simple. Yet development is triggered by a few key genes capable of jiggering regulatory networks.
To take full advantage of AoG, its practitioners rely upon environmental cues, from which genetic connection is made via existential epigenetics. One might otherwise call it “life experience,” but spores are not exactly living it up.