Alternation of Generations

  All plants and most algae have a sexual life cycle that consists of an alternation between a haploid (n= one copy of each chromosome) gametophyte generation, and a diploid (2n= two copies of each chromosome) sporophyte generation. The haploid gametophyte generation produces haploid gametes (eggs and sperm), which unite to form a diploid zygote. The gametophyte is usually multicellular, while the gametes and the zygote are single cells. The zygote grows by mitosis into the diploid multicellular sporophyte. The diploid sporophyte contains specialized regions where meiosis occurs, creating haploid spores. The spores, once dispersed, germinate into haploid gametophytes, starting the cycle anew. Some of the differences that will be found between plant phyla include relative dominance, size, and independence of each of the two generations. Make sure you understand the basic concept of alternation of generations before proceeding, as you will be learning concepts that build on your understanding of this basic sexual reproduction cycle.

STUDY HINT:

·         the gametophyte generation always produces gametes

·         the sporophyte generation always produces spores

Forms of Reproductive Life Cycles

The two extremes:

·         In the haplontic (or haploid dominant) life cycle, after the gametes (n) fuse to form the zygote (2n), the zygote immediately undergoes meiosis to produce spores (n), which regenerate the gametophyte generation (n). Thus, only one single cell, the zygote, is ever diploid. This type of life cycle is seen in some green algae.

·         In the diplontic (or diploid dominant) life cycle, on the other hand, ALL cells are diploid, with the exception of the gametes. This life cycle will be familiar to you since it is what is seen in humans and most other animals: only the egg and sperm are haploid, all other cells are diploid. The diplontic life cycle is not seen in algae or plants.

The various life cycles seen in most of the algae and in plants are somewhere in between these two extremes. In these cases both the sporophyte (2n) and gametophyte(n) generations are multicellular. Sometimes they are indistinguishable from each other in any way except for their chromosome number (eg. some species of Phaeophytes), while in other cases either the sporophyte or the gametophyte generation will be larger and more familiar than the other.

Alternation of Generation in Nontracheophytes
(Liverworts, Hornworts, and Mosses)

In nontracheophytes, the gametophyte generation is the dominant generation, and is the more prominent green “leafy” structure. This gametophyte (n) produces gametes (n) in multicellular gametangia. If the gametangia is female, and produces eggs (n), it is known as the archegonium, while if it it male and produces sperm (n), it is known as the antheridium. Sperm are released from the antheridium and swim to the archegonium, where they fertilize the egg to form the zygote (2n). The zygote develops into the sporophyte embryo (2n), which as it grows eventually becomes nutritionally independent from the gametophyte. Mature sporophytes (2n) produce spores (n) by meiosis in sporangia; the spores are eventually dispersed by the wind. When a spore germinates, it grows into another gametophyte (n), starting the cycle all over again.

Alternation of Generation in Non-seedbearing Tracheophytes
(Club Mosses, Horsetails, Ferns)

All nonseed tracheophytes reproduce by means of spores. The sexual reproduction lifecycle shows an alternation of generations between gametophyte and sporophyte generations just as in the nontracheophytes, however, in the tracheophytes (nonseed and seed), the diploid sporophyte generation is dominant, not the gametophyte generation. The leafy green plant you think of when you think of ferns, for example, is the sporophyte generation. Gametophytes of nonseed tracheophytes are very tiny, but are free living (live separately from the parent sporophyte). Water is still required for the sperm to reach the egg in nonseed tracheophytes.

Reproduction in Seed-bearing Tracheophytes
(Gymnosperms and Angiosperms)

Today, seed bearing plants vastly outnumber the nonseed plants. These species are very successful for several reasons.

·         First, they have bypassed the need to have water in their environment for fertilization to occur, by the formation of pollen; very small male gametophytes that are transported to the female gametophytes by wind or insects. Thus the sperm, instead of having to swim to the egg, is brought to the egg by the male gametophyte. Upon arrival the male gametophyte produces sperm, which are carried to the egg inside its protective structure (the archegonium), by a pollen tube.

·         Second, the spore (which was the means of dispersal in nonseed tracheophytes) is replaced by the seed as the means of dispersal. Seeds consist of a seed coat, the embryo, and stored food. Seeds protect the young sporophytes and provide them with food reserves.

·         Finally, the evolutionary trend has been towards reduction of the gametophyte to being of extremely small size, and dependent on the sporophyte for nutrition.

Reproduction in Gymnosperms

Gymnosperms include cycads, gingkos, conifers, and gnetophytes. All of these species produce seeds that are covered by a seed coat but not a fruit. These species DO NOT produce flowers. All produce pollen, which can be transported either by the wind, or by wind and insects in some species. Pollen makes it unnecessary for the plant to live in a moist environment; the sperm no longer needs water to swim to the egg, rather the whole male gametophyte (pollen grain) is transported to the female. Gymnosperm gametophytes are smaller than those in the nonseed tracheophytes, and are not free-living. This reproductive cycle still shows alternation of generations, however the haploid gametophyte is very reduced in size and no longer lives independently of the sporophyte.

Reproduction in Angiosperms

In flowering plants the gametophyte generation is more reduced than in any other plant phylum. The male and female gametophytes are composed of even fewer cells than in gymnosperms, and are completely dependent on the sporophtye generation. The male gametophyte is the germinated pollen grain. Unlike in other plant phyla (except Gnetophyta), TWO male gametes, both contained within a single pollen grain, participate in the fertilization process. One male gamete (sperm) combines with the egg to form the zygote, while the other combines with two haploid nuclei of the female gametophyte, to form a triploid nucleus. This triploid nucleus divides to form endosperm. Endosperm is a nutritive tissue formed only in seeds of angiosperms. It stores starch, lipid reserves, and other compounds for use by the developing sporophyte. Fertilization that involves two male gametes is known as double fertilization.

After fertilization in angiosperms, the ovary, together with its seeds, develops into a fruit. Fruits are formed only in angiosperms. A fruit can consist only of the mature ovary and its seeds, or it can include other flower parts. Many fruits play a role in seed dispersal. Animals eat many fruits, the seeds of which travel unharmed through their digestive systems, and usually deposited away from the parent plant, along with a good supply of fertilizer! Other fruits (eg. dandelions, milkweed) are carried on the wind to a new location, aided by specialized parts of the fruit that enable it to be carried by the wind. Other fruits have specialized hooks and barbs which allow them to stick to animal fur, to be carried with the animal until they finally fall off or are taken off by the animal (eg. burdock, tick-seed clover).