Alternation of generations
The sea lettuce Ulva is a green algae that lives in
saltwater. The life cycle of Ulva is described as alternation of
generations because one generation of Ulva is produced sexually by
gametes while the next generation is produced asexually by
zoospores. The gametophyte generation is a haploid thallus from
which small, flagellated gametes are released into the water. They
pair off and fuse. Each fused pair of gametes forms a zygote that,
after a short time, becomes a diploid thallus of a new generation of
Ulva. The diploid thallus is the sporophyte generation. It produces
haploid zoospores that develop and grow into a haploid thallus,
which is now the gametophyte. Many other simple organisms also have
an alternation of gametophyte and
sporophyte generations.
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