Cycling Through Life

Early in the evolution of eukaryotes, a new method of recombination appeared... syngamy. Most textbooks will refer to this process as fertilization. I hope you will leave that out of your vocabulary...pehaps saving it for when you are discussing compost or fertilizers. Males are fertile without a female contribution and vice-versa so we do not make each other fertile. There is no such thing as a "fertilized egg"...once syngamy has occurred it is a zygote... no longer any kind of egg. Hens that have been visited by roosters do not lay eggs...they lay zygotes (or young embryos) in a shell. The only hens that lay eggs are those who live in factory farms that never see a rooster; no amount of incubation will result in their hatching. I think you have the picture!

Syngamy is a two-step process. Initially the two gametes join cytoplasms. This process is called plasmogamy. The cell is called heterokaryotic at that point because it has two different nuclei (one from the egg and the other from the sperm) in the cytoplasm. This condition can also be called dikaryotic for obvious reasons. This phase can be a short-term or a long-term condition. However, syngamy ultimately finishes as the two nuclei join. This second step is called karyogamy.

Of course if syngamy puts two complete genomes together, the chromosome count is twice as much after syngamy. In a few generations the number of chromosomes in the cells would be beyond what is possible and what is observed. The compensatory process happens in the adult. Prior to the formation of gametes, there must be a way to reduce the doubled chromosome count back to the haploid set. This process is known as meiosis.

Review of Generalized Sporic Life History:

This diagram depicts a generalized sporic life history. There is a multicellular diploid adult organism that we call a sporophyte. This plant typically produces some kind of housing for the meiosis process. This container is called a sporangium. The contained diploid cell that will undergo meiosis is called a sporocyte. Meiosis typically produces four haploid meiotic products. These are called spores. The spores are disseminated and under proper conditions germinate and develop into a multicellular haploid gametophyte plant. This plant will also produce housings for gametes; the containers are called gametangia. The gametangia produce gametes by the process of mitosis. Both the gametangia and gametes are haploid. The gametes unite in syngamy and produce a diploid zygote. The zygote matures into a young diploid sporophyte plant to complete the life history.

You might notice that it is possible to have asexual means for "cloning" both gametophytes and sporophytes, depending upon the example under consideration. Yes, plants have evolved ways to clone themselves far before the advent of Dolly the sheep.

You might also notice that two short-cuts are shown in this sporic life history. One short-cut is termed zygotic because the zygote becomes the sporocyte directly; this completely avoids the multicellular sporophyte. The other short-cut is termed gametic because the products of meiosis mature directly into gametes; this completely avoids the multicellular gametophyte.

Many books unfortunately follow an older vocabulary by putting "meiosis" after sporic, zygotic, and gametic. "Sporic meiosis" would appear wrong because the spores do not undergo meiosis--rather they are the product of meiosis. But maybe we can accept sporic meiosis in that meiosis produces spores in this case...of course that is what happens in the zygotic and gametic life histories too...so what does "sporic meiosis" mean? I have no idea.

Well how about "zygotic meiosis"? In the zygotic life history, the zygote becomes the sporocyte...but it is still the sporocyte that undergoes meiosis. Maybe because the zygote so directly enters the meiotic phase, we can accept the term...but it is still the sporocyte that undergoes meiosis!

Perhaps the worst of these three terms is "gametic meiosis." What, the gametes undergo meiosis? Of course that is preposterous. Are gametes the product of meiosis? Not really, the product of meiosis is a spore. That spore might develop into a gamete, but by then meiosis is long over...gametes and meiosis do not go together in any of these life histories!

So for this course I want you to think of these as "sporic life history," "zygotic life history," and "gametic life history." Now looking at the three cycles (the complete cycle and the two short-cuts, we might legitimately ask which might be the plesiomorphic condition. As our fossils and much other information indicates, there were multicellular haploid organisms before the evolution of the diploid condition. This means that probably when syngamy evolved as an effective passage for the life history, the resulting zygote was useless until meiosis could restore the normal condition. This suggests that the zygotic life history is plesiomorphic. This probably transitioned through a sporic history as it ultimately evolved into a gametic life history. The driving force for evolution of diploid dominance in the life history and the reductionism of the haploid was probably the maintenance of a more diverse genetic pool. Diploidy permits recessives that might be disadvantageous in a current environment to remain in the gene pool for generations until an environmental change takes advantage of the possibly useful recessive gene product. If a life history has a long haploid phase, all the genes that are operational in that haploid are subject to severe natural selection. Alleles of current-disadvantage doom the haplophase of the life history to express those alleles into the natural selection forces. The gene pool is diminished. Thus the progression from one-celled diploid to full-multicellularity is an apomorphy from zygotic to sporic histories. The reduction of the haploid in the progression from sporic to gametic histories is the next natural step. We shall see this progression as we examine the natural history of plants. Botany provides a wonderful example of fundamental evolution of the life history.

There is much to learn now that we have moved into sexual reproduction and life history. Please stay tuned!