In eukaryotes, sexual reproduction consists of an alternation of meiosis, which produces haploid cells, and syngamy (or fertilization), which produces diploid cells. This sexual life cycle takes three basic forms, depending on which stage in the life cycle (the haploid stage, the diploid stage, or both) is the dominant stage (the physically largest stage or the stage at which the individual spends most of its life). In some eukaryotes, the haploid stage is dominant, and the diploid stage consists only of the zygote, which immediately undergoes meiosis. In others, the diploid stage is dominant, and the haploid stage consists only of gametes, which immediately undergo syngamy. In still others, both the haploid and diploid stages persist. All three of these types of life cycles are found in the algae. Although there is some specialization towards particular life cycles in some divisions of algae, other divisions exhibit all three types. Therefore, life history patterns are not usually a major characteristic for classification of the various groups. The three life history types are characterized by distinct meiotic patterns:

(1) Zygotic meiosis
Zygotic meiosis occurs in life cycles dominated by the haploid stage: gametes are produced by mitosis and, following fusion of the gametes into a zygote, meiosis immediately occurs, regenerating the haploid condition. Chlamydomonas is a good example of this type of life history, but many other green algae exhibit it. Usually the gametes are motile and may be of three types: isogamous which means the male ( - strain) and female (+ strain) are equal in size and both possess flagella; anisogamous which means that the male gamete is smaller than the female, but both still possess flagella; and oögamous which means that the male gamete is smaller than the female, and the female gamete (sometimes referred to as the egg) lacks flagella.

In some groups of algae that have zygotic meiosis, both the + and - strain gametes lack flagella. Spirogyra is an example of this type and because the gametes and zygote are easier to observe, you will use this alga as an example of zygotic meiosis even though it is not typical of the majority of algae which have flagellated reproductive gametes. Examine prepared slides of conjugation in Spirogyra. Note that the cell contents become condensed and form a ball. When two strains of filaments come close together, the cell walls project towards one another and eventually fuse. Then the contents of the - strain move over to the + strain and fuse, forming a thick-walled zygote, the only diploid stage in this life history. How can you distinguish between zygotes and gametes in this alga?

(2) Sporic meiosis
Sporic meiosis is common type in many groups of the algae. It occurs in life cycles that show alternation of generations. This type of life history is typified by the presence of two vegetative stages: a gametophyte or haploid stage and a sporophyte or diploid stage. The gametophyte produces gametes by mitosis, which eventually fuse (syngamy) and form a zygote that develops directly into the sporophyte. The sporophyte produces haploid spores by meiosis, which are released from the parent plant and settle down to develop into the gametophyte. Both stages may produce asexual cells as well, also called spores, which recycle the parent generation. That is, the gametophyte can either produce gametes which fuse and form the diploid stage or spores which do not fuse and produce another gametophytic stage. Likewise, the sporophyte may produce haploid spores which develop into the gametophyte or diploid spores which act to reproduce the sporophyte generation. Thus, the sporic meiosis type of life history affords many options to the survival of an algal species under different environmental conditions.

(a) Isomorphic vs. Heteromorphic Alternation of Generations
In some algae, the haploid and diploid forms are identical in appearance. It is only through examination of the reproductive structures or observation of the gametes and/or spores produced by a plant, that the sporophyte and gametophyte can be distinguished. Such algae are said to have an isomorphic alternation of generations. See the examples of seaweeds with this type of life history on display. Can you observe any of the reproductive structures on either stage which might be used to separate gametophytes and sporophytes? Another type of sporic meiosis exhibits two vegetative stages which are different in appearance and/or size. This type of life history is called heteromorphic alternation of generations.

In many cases, one life history phase is large while the other is cryptic and/or microscopic. An example of this type of life history is in many kelps. Observe live material of Laminaria, a small kelp of the California coast. The plant attaches itself to the rocks by a root-like structure called a holdfast. Notice a dark brown area at the base of the blade where it meets the stipe. This area is called a sorus and is the site of meiosis and the production of spores. Is this a sporophyte or gametophyte? Examine a prepared slide of a cross-section through a kelp sorus. You will see the swollen sporangia among dense hair-like cells called paraphyses. When spores are released, they settle down on the bottom and develop into small filamentous gametophytes. If available, observe a demonstration slide of a kelp gametophyte. You should be able to see a large cell called the oogonium that contains the egg cell. When fertilized by a male gamete, the egg will develop directly into the giant kelp sporophyte.

(3) Gametic meiosis
Gametic meiosis is less common in algae than the other types of meiosis. In the animal kingdom, this is the most common type of life history. Gametic meiosis is dominated by the diploid stage, with gametes formed directly by meiosis, and their fusion immediately regenerating the diploid condition.

Fucus, the common brown rockweed of the rocky intertidal zone, shows an example of gametic meiosis. Obtain live material of Fucus, and refer to Rust, fig. 24 or V & C fig. 4.50-4.54. The plant thallus or body is the sporophyte stage. Remove the swollen tip or receptacle of Fucus. You will notice many small bumps over the surface. These are called conceptacles and contain the gametes. Using a single-edged razor blade, make thin sections through the conceptacle. Have your instructor demonstrate the "human microtome" technique for making sections. If you have made them thin enough, you should be able to observe with the compound microscope the antheridia and oögonia within the conceptacle. The antheridia containing the male gametes (sperm) are located in branched structures. The oögonia containing eggs appear as globose structures. There are several eggs per oögonium, the number varying with the genus. How many do you count? If your sections are not thin enough, observe the prepared slides for the same features. The eggs usually mature at a different time than the antheridia and are pushed out of the conceptacle where fertilization takes place. The fertilized egg settles down and germinates directly into a diploid plant. Fucus has heavily studied by development biologists because its initial cell divisions are closely controlled by environmental conditions such as light, salinity, and temperature gradients.

Be sure to examine the other seaweeds in the trays and on herbarium sheets. We are lucky to be in northern California where there are over 500 species to be found between Monterey and Humboldt Bay -- and many of them are edible!