The Haploid Life

It was probably when I was taking a course on vascular plants in my senior year of college that I learned that the gametophyte generation of ferns was a free-living multi-cellular organism. This revelation about the alternation of generations helped me to understand a crucial difference between plants and animals, clarified the relatedness among major phyla, and made meiosis and mitosis a lot less abstract.

Alternation of chromosome numberThe gametophyte (literally “gamete plant”) has only half the number of chromosomes (n; haploid) as the sporophyte generation of a given taxon. In ferns and what used to be called “lower plants” the sporophyte produces spores by meiosis, a process of cell division that creates cells with only one copy of each chromosome in them. In the ferns these spores undergo mitosis, a mode of cell division that preserves the number of chromosomes as each new cell is formed. The number of chromosomes is doubled just before the cell divides.

Gametophyte of Onclea sensiblis, a fernThe gametophyte fern is considerably smaller and less differentiated than the more familiar sporophyte plant. The gamete divides to create a prothallus, basically a single photosynthetic leaf. This multi-celled organism produces both male and female one-celled gametes (sperm and eggs) via mitosis. Like animal sperm this fern sperm is flagellated and motile; it fertiizes the eggs on the surface of the prothallus to produce a 2n (diploid) cell, which then begins to divide by mitosis to create the sporophyte plant.

The process of sexual reproduction is a bit different in animals. The gametes remain one-celled, never undergoing mitosis to produce a free-living haploid organism; there is no alternation of generations because there is no haploid generation. Also, the male and female gametes are derived from different organisms in animals rather than one.

In the mosses the gametophyte and sporophyte plants are of roughly the same size and complexity, while the liverworts actually have a dominant gametophyte stage and the sporophyte generation lives as part of the gametophyte rather than fully independent. That is, the complete inverse of the animal situation. All these variations on the relationship between the haploid and diploid stages of the life cycle illustrate the evolutionary pathways that were taken by different phyla.

In the plants you can see a continuum from the mosses to the ferns to the “seed plants” of gradual dominance of the sporophyte generation as vascular structures appear in the transition from mosses to ferns and then grow more complex in the seed plants.

The liverworts would seem to be on an entirely different pathway, which is also suggested by their lack of certain introns in their mitochondrial DNA that are shared by other plants, even the mosses, which were formerly thought to be more closely related to liverworts.

Contemplation of the alternation of generations can lead to interesting exercises in developing “what if” scenarios for evolution. For example, consider how strange it would be to have free-living, multi-celled haploid generation animals running around. The actual path of evolution has left us with female diploid animals carrying around a finite number of haploid cells in their ovaries and male diploid animals producing millions of haploid cells in their testicles. These haploid cells have no independent existence. Consider the alternate scenario in which animals released haploid spores into the environment, which then proceeded to undergo mitosis and cell differentiation, and developed into separate haploid creatures.

Families would consist of two sets of organisms that would (if ferns are any indication) look quite different from one another. The diploid generation would never actually have to engage in sexual congress because it would be up to the haploid generation to produce haploid gametes that would fuse into a diploid animal.