'',
Galicia, Spain Dictyotales are an order of
brown algae (Phaeophyceae), a lineage of
multicellular photosynthetic protists composed by a
thallus that has a certain level of
tissue differentiation. The reference organism of Dictyotales used to study their general characteristics is
Dictyota dichotoma.
Vegetative growth Dictyotales are composed of flattened,
parenchymatous
thalli. They display apical growth: a single apical cell forms the flattened
thallus, the
vegetative part of the organism, through
mitosis. The mature thallus is composed of three layers: one middle layer of large
cells lacking
chloroplasts, surrounded by two layers of small cells packed densely with chloroplasts lacking
pyrenoids. Some taxa exhibit blue-green
iridescence when submerged. Some species of
Dictyopteris and
Spatoglossum have cellular
vacuoles with very low
pH (between 0.5 and 0.9).
Life cycle and reproduction '' thallus. 1)
Tetrasporangia seen from above. 2) Cross-section of
oogonia. 3) Cross-section of
antheridia. Dictyotales displays, like other
brown algae, an
isomorphic alternation of two generations: the
gametophytes (the
haploid generation), which generate
gametes through
mitosis; and the
sporophytes (the
diploid generation), which generate
spores through
meiosis, both of them similar in shape and structure. The gametophytes form
sex organs (female
oogonia and male
antheridia) in
sori. Both sex organs develop from surface cells. The reproduction is
oogamous, with a small and mobile male gamete (
sperm cell) and a big immobile female gamete (
egg cell). • The female sori, deep-brown in color, usually have 25–50 oogonia, with sterile oogonia localized at the margin. To develop an oogonium, a surface cell divides into a stalk cell and the oogonium proper. Each oogonium produces one egg cell that is released through a gelatinized apex in its wall. • The male sori, resembling white glistening spots, are surrounded by elongated sterile cells regarded as undeveloped antheridia. They can be recognized early in their development due to the disintegration of
chloroplasts in their cells. To develop an antheridium, the surface cells enlarge and divide horizontally into a stalk cell and a primary spermatogenous cell, which in turn divides vertically and horizontally into 650–1500 compartments or locules, each producing one sperm cell. Each sperm cell is pear-shaped, with an anterior
eyespot and only one
flagellum inserted laterally, although two
basal bodies are present, indicating an origin from a biflagellate ancestor. They are released from the antheridium through dissolution of the walls. The egg cell secretes a pheromone,
dictyotene, to attract the sperm cell. After the
fertilization between the two gametes, the resulting
diploid zygote grows into the
sporophyte generation. On their surface, modified
sporangia typically produce through
meiosis four
haploid non-flagellated
aplanospores, but a few genera produce eight spores per sporangium (
Lobophora,
Zonaria) or
flagellated spores (
Exallosorus), two features considered primitive. The spores, while naked are released through the gelatinized apex of the sporangium. Soon after, they grow a
cellulose wall and grow into gametophytes. == Ecology ==