In terms of evolution, sexual reproduction is highly adaptive. By combining a set of genes from two parents in the progeny, populations maintain a high degree of genetic variability. In addition, through genetic recombination, traits of the parents can be expressed in new and diverse combinations which can enhance the adaptability of populations. Given the evolutionary advantages inherent in sexual reproduction, it is perhaps not surprising that both plants and animals have evolved sexual life cycles. Many textbooks, as an aid to student learning, try to emphasize the similarities between the sexual cycles of animals and plants. This is a mistake, for it tends to gloss-over two fundamental points that are essential if you are to grasp the significance of evolutionary changes in plant life cycles:

• Sex in plants evolved completely independently of sex in animals. Despite superficial similarities, due primarily to evolutionary convergence, plant sexual life cycles are fundamentally different (and considerably more complex) than the sexual cycle of animals.

In nature, cells typically have either a single set of chromosomes (N) or a double set (2N). If the cells are 1N they are said to be haploid - which simply means that each cell contains the minimum number of chromosomes (N) characteristic of the species. The haploid condition is typical, for example, in sex cells (gametes), where egg and sperm will each contain a single set of chromosomes. Typically, two gametes are destined to combine (sexual fusion) to produce the next generation of organisms. Since each gamete has a single set of chromosomes, the cell created by sexual fusion is 2N (1N + 1N) or diploid. In essence, all the chromosomes typical of the species are present in pairs. The cell created by fusion of two gametes is termed a zygote. The zygote typically develops into a multicellular adult via a very large number of mitotic cell divisions. Since mitosis simply duplicates the chromosomes of the initial cell, all the cells of the adult organism are diploid (2N) as well. Mitosis is described in detail on another page, which you are not responsible for unless the material has been specifically assigned.

When the time comes for the next generation of, a small number of cells associated with the sex organs undergo a specialized form of cell division known as meiosis. Meiosis is described in detail on another page, which you are not responsible for unless the material has been specifically assigned. Meiosis is also known as reduction division, because it results in a set of four haploid (1N) cells derived from each diploid cell. Thus, meiosis provides a means to transition from the diploid to the haploid phase of a life cycle, while sexual fusion permits the transition from haploid back to the diploid condition.

Although I will make the point that there are very fundamental differences between animal and plant sexual life cycles, it is useful to start with a typical animal sexual cycle since, as animals, we tend to know a little about the basics of animal reproduction.

In all complex animals (including humans) the animal itself is always diploid. In the reproductive organs of adult females (ovaries) and males (testes), meiosis occurs, resulting in the formation of haploid gametes. In animals, meiosis and any later stages resulting in gamete formation are collectively known as gametogenesis. The female gamete is very large and non-motile and is known as an egg. The male gamete is typically very small and motile and is known as a sperm cell. The gametes represent the only components of the haploid phase of an animal sexual life cycle and will typically fuse to form a diploid fertilized egg or zygote. The zygote then undergoes a sequence, first of embryonic and later developmental stages, to become the adult animal. In summary, the diploid phase of the animal life cycle can be said to be the dominant phase, with the gametes being the only haploid elements.

A generalized plant life cycle has both haploid and diploid phases, connected by meiosis and fusion, but in all other respects there are some fundamental differences with respect to animals:

Specific cells in the mature diploid phase of the plant life cycle undergo meiosis, but this does NOT produce gametes. Instead, the haploid cells are known as spores. Since the diploid phase of the life cycle produces spores, it is known as the sporophyte (=spore-plant) phase of the plant life cycle.

• Meiosis in plants does not produce gametes, but rather spores.

• Spores typically develop into multicellular, haploid gametophytes.

• Gametogenesis in plants is mitotic, not meiotic!

1. Both gametes appear identical and both are small and motile (they swim!). This condition occurs in non-vascular aquatic plants known as algae and the condition is known as isogamy (equal gametes). Although the gametes appear identical, there are subtle genetic or physiological differences known as mating types. Typically only cells of different mating types can fuse and, since the gametes look identical, any meaningful distinction of "male" vs. "female" is impossible.
2. Both gametes are motile, but one is distinctly larger than the other - a condition known as anisogamy (unequal gametes). Anisogamy is also typical of some algae. Sometimes the larger gamete is designated "female" and the smaller "male", but this is purely by very crude analogy to animal gametes and such terminology is best avoided.
3. Finally, in more advances algae and all higher land plants, one gamete is large and non-motile (the egg) while the much-smaller gamete is motile (sperm). The "egg" is typically referred to as "female" while the "sperm" is designated "male". The condition is known as oogamy and resembles the situation in animals only as a result of evolutionary convergence - a topic that will be discussed in class.

The following represent examples of progressively more complex plant life cycles that may be assigned during the term.