Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 94

Sexual Reproduction
Sexual reproduction in green algae is common and may be isogamous (gametes
both motile and same size), anisogamous (both motile and different sizes, with the
female being bigger), or oogamous (female nonmotile and egglike; male motile).
Many green algae have alternating haploid and diploid phases. The haploid phases
form gametangia (sexual reproductive organs) and the diploid phases form zoo-
spores through reduction division (meiosis). Some do not experience alternation of
generations.
In algae, sexual reproduction involves the conjugation or joining of two haploid
individuals in which one individual provides genetic material to the other.
The gametophyte produces haploid gametes by mitotic division, which unite to
form a diploid zygote that develops into a sporophyte. The sporophyte then under-
goes meiotic division to give rise to haploid spores, which grow into gametophytes.
In this way, the gametophyte and sporophyte generations alter with each other.
Asexual Reproduction in Metazoans
Reproduction by budding or broken-off pieces of sponges occurs in some species of
metazoan but is not widespread. Gemmules, or “internal buds,” are groups of sponge
cells wrapped in spongin capsules. They overwinter in a dormant state and then ger-
minate in warmer weather to develop into new sponges.
Sponges display another variant of budding called internal budding or gemmu-
lation . The buds (gemmulae) form within the body wall and are released when the
sponge dies. Again, this form of reproduction seems to be complementary to sexual
reproduction. Some sponge species form gemmulae that are resistant to cold and
drought. Sponges can also reproduce by regenerating broken-off pieces.
Reproduction by Fragmentation
Fragmentation implies that the branching of sponges may break apart into storms
( Leong and Pawlik, 2010 ). Being sessile organisms, sponges rely heavily on frag-
mentation for dispersion over long distances, while larvae produced by sexual repro-
duction contribute to establishing new populations in the neighborhood ( Maldonado
and Uriz, 1999 ).
Sexual Reproduction in Sponges
Sponges are hermaphrodite metazoans that produce both male and female gametes.
Both gamete types are produced by choanocytes, but in most species, female gam-
etes derive from transformation of archeocytes. Sperm cells carried by water currents