Abstract: A plant seed begins with a single cell, called a zygote. You, a human, started as a zygote too. And your first cell, just like the one that is a plant zygote, was formed when a sperm nucleus joined an egg. The saga of how that single cell (the zygote) becomes a seed, and how that seed grows up and makes its own sperm and eggs is the life story of each generation - what we call the life cycle.
But the life cycle of plants is more embellished than ours. Plants have a secret life that plays out before our very eyes, hidden in plain view. To appreciate this extra bit of complexity, you must get past some bothersome but simple math. Plants and humans function with two sets of chromosomes. To reproduce sexually, those chromosomes must be reduced to a single set, not halved in a willynilly way, but reduced to a single matching set (we call this a single complement, designated by the letter "n" because different organisms have a different number in the complement.) This single complement is all of the chromosomes an egg or a sperm will need. It is the recombining of sets, an egg from your ma and a sperm from your pa, that brings two sets back to the party, two sets for the new generation - which is you. If that makes sense, then you are ready to work through the following tale.
The Story: It is a great word - zygote. It is the essence of beginning, the name we use for the first cell of a new generation. Formed by the union of sperm with egg, the zygote magically re-sorts and restores the full genetic complement necessary to operate most of the plants around us.
I was thinking about the idea of a zygote because that is where I began a botany class the other day for Yvonne Savio's Los Angeles County Master Gardener trainees. It just seemed like the right place to jump into the story of plant life cycles because the green flowering plants that make up the flora of our vegetable gardens all began their current generations as zygotes.
Beginning discussion with this diploid (having two sets of chromosomes) cell belies its recent history. Each zygote is a new being, a new combination of life with two sets of chromosomes that resulted from sexual union. Sex, by this narrow half-definition, happens when the two famous gametes, a sperm and an egg, join. In reality, the joining itself is fairly ignominious. The male pollen tube basically dumps a nucleus into the egg cell. But that comes after some serious travail, is the product of some serious limitations, and (of course) is just the headliner; many other things happen that are as crucial.
Going backward we get to these curious gametes, the sperm nucleus and egg. Like Voldemort, the egg and the sperm are no longer whole. They are simply the final products of a frail and arcane generation that lived a kept-life with only a single set of chromosomes - a haploid life. Yes, between one pumpkin and the next generation of pumpkin live these arcane, horcrux-like haploid beings which botanists call "Gametophytes" - so named because these are the plants that create the gametes (sex cells) we call sperm and egg. But they are so unseen as to be nearly unheralded.
The Male Gametophyte is the easier to envision of the two kinds. It is the pollen grain, which eventually becomes a free-living, free-ranging, speck-like non-green plant with only one set of chromosomes (the condition we call haploid.) When grown up, it has two or three nuclei. The egg belongs to a similarly haploid plant that you will never see, all tucked inside an ovule in a flower pistil (eventually a pea in a pod.) Though hidden, the Female Gametophyte has a much more exhuberant life than the pollen grain. It grows up to become what we call an embryo sac, a microscopic, haploid "plant" with several nuclei (often 8), one of which is "the egg."
So where did these Male and Female Gametophytes come from? As Harold Bold (who studied plant morphology at The University of Texas and at Vanderbilt University) would have taught you, they came from spores - little haploid spores. In their flowers or cones, the "normal", Diploid plants make special haploid spores through a curious kind of cell division called meiosis. Meiosis is every bit as dramatic as the process by which Voldemort split his soul, except it is real. And it happens millions of times a day all around you, in special cells in anthers and pistils of flowers and in the scales of cones. To make pollen grains, special cells each divide and turn out four little, equal-sized microspores - each of which now has one set of chromosomes. In the pistil, those special diploid cells divide unevenly; producing four haploid cells, but three will be failed, empty cells and one will be a larger megaspore that (along with its single set of chromosomes) gets all of the cell content. A microspore grows up to be a pollen grain. A megaspore grows up to be an embryo sac. Since it is the Diploid plants that make these microspores and megaspores, Botanists call them Sporophytes. As a sidebar, this is also how we decide what to call male and what to call female. When there are two different products of meiosis in a life cycle, the spores portioned out as four equally sized and active cells (the microspores) are considered "male" - while the single megaspore that was invested with all of the original cell contents is "female."
The implication of these even and uneven divisions is profound. Having preferentially received all of the cell contents, the embryo sac (now the Female Gametophyte because it will produce the female gamete) has more than a nucleus, it has all of the other stuff a normal plant cell needs in its cytoplasm (mitochondria, plastids, membranes, etc.) The egg (which is the female gamete, the "sex cell") therefore brings everything to the party - a nucleus with a single set of chromosomes as well as the other structures and genetics a plant cell needs to function. The male spore began as a smaller cell in the first place, which became a pollen grain. The pollen grain lands on a stigma and grows its pollen tube to the embryo sac- but in the end, it delivers only its nucleus to the egg, only its set of chromosomes. The result is what we call "maternal inheritance." Characteristics derived from structure and genetic information in various components of the cytoplasm came with the egg, the female parent. That can be curiously important; both chloroplasts and mitochondria are in this camp, reproducing themselves from plastids that are part of the cytoplasm - not from genetic information carried in the nuclear chromosomes.
So the real beginning of sex is meiosis. In plants, this first half of sex yields spores that have their own, frail but modestly exhuberant lives before producing the sex cells, the gametes. The end of sex becomes the union of sperm nucleus and egg nucleus (which we call syngamy.) Thus we sometimes talk about plants as showing "alternation of generations" - from diploid normal beings to half-hidden haploid plants (from Sporophyte to Gametophyte,) before being "restored" as a new generation of diploid plants. For humans the alternation does not apply. In human males, meiosis yields four cells which are themeselves the gametes, the sperm cells. In human females, meiosis yields a single well-outfitted cell called the egg (while discarding the other three haploid sets of chromosomes.) Humans go straight from diploid cells to egg and sperm, the gametes that unite right again to make new zygotes.
Starting with the zygote can be a good beginning for a lecture, but it is just one point in the great progression of life - bearing heavily on the question of when life begins. Life does not start with a zygote, it does not begin with a single cell, because life never ended. The sperm and egg are as alive as the zygote - they are frail and totally dependent - you may call them helpless, or you may think of them as parasites. But they are a crucial link in the chain of life, carrying all of the genetics and structure and life processes that characterize living things. What we might call new "lives" are simply the most recent diploid beings that were generated by this remarkable sexual system. But "life" itself doesn't really "start" because it never ends. Breaking the chain of life is not merely death, it is extinction.
