Many organisms have life cycles in which diploid stages (sporophytes)
alternate with haploid stages (gametophytes)— an
alternation of generations.
Meiosis marks the beginning of the
gametophyte stage and fertilization marks the beginning of the
sporophyte stage. The sporophyte generation has assumed greater and greater
importance throughout plant evolution.
Gametophyte: Any multicellular haploid body.
Gametophyte phase or generation: Haploid portion of life cycle, from
meiosis to fertilization.
Sporophyte: Any multicellular diploid body.
Sporophyte phase or generation: Diploid portion of life cycle, from
fertilization to the next meiosis.
- Either the sporophyte or the gametophyte phase (or both) can be
conspicuous. Most microorganisms have dominant haploid phases. Higher plants
and most animals have dominant diploid phases.
Evolution of life cycles:
- The unicellular green alga Chlamydomonas has a dominant haploid
phase. The diploid zygote exists only briefly before it undergoes meiosis
and releases new haploids. Gametes of opposite (+ and -) mating types look
the same, a condition called isogamy.
- Life cycles vary greatly among other algae. Gametes are often dissimilar
(anisogamy). Often, the male gametes are small and motile, while
eggs are much larger and nonmotile (oögamy).
- Moss plants are gametophytes, which contain sex organs and produce
gametes. Sporophytes grow as virtual parasites on the gametophytes.
Meiosis produces haploid spores, which grow into new gametophytes.
- The small fern gametophyte resembles a heart-shaped leaf; sex organs
are produced on its surface. Fertilization produces a sporophyte, which
is the conspicuous fern. Meiosis produces haploid spores, which grow
into new gametophytes.
- In seed plants, the conspicuous plant is always a sporophyte.
Angiosperms produce microscopic female gametophytes of only a few cells
within the ovaries of the flower. Male gametophytes are even smaller
and are contained in the pollen grains. The seed that results after
fertilization contains a young sporophyte.
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