Origin of sex, Most
primitive algae the cyanophyceae (Myxophyceae) reproduce only the
method of asexual and vegetative reproduction. Sexual reproduction is
entirely absent in their case. In forms like Nostoc and oscillatoria, it
takes place by means of hormogones in which the plant body breaks up
into group of few cells. In class xanthophyceae, chrysophyceae,
cryptophyceae and dinopphyceae, sexuality is rare and has not much
evolved beyond the stage of isogamy.
 |
| Spirogyra (Watersilk) |
 |
| Chlamydomonas |
In
higher classes of algae reproduction takes place by vegetative, asexual
and sexual methods. The commonest method of asexual reproduction is by
means of Zoospores. The possibility of gametes from zoospores can
evolved from the following description.
In
genera like ulothrix, ordogonium, cladophora, zoospores were probably
produced before the origin of sexual reproduction. In ulothrix three
types of swarm spores are formed.
(i)
Quadriflagellate macro-zoospores: They are produced either singly or in
small numbers from a single cell. These swarmers are capable of
developing into normal plants.
(ii)
Macro-zoospores: They are either quadriflagellate or biflagellate and
are produced in greater number from each cell than macro-zoospores. They
give rise to vary weak plants which are smaller in size.
(iii) Gametes: These are always biflagellate, smaller than the
micro-zoospores due to more division in the protoplasts of the cell and
hence are largest in number. They are unable to develop into any plant
individually but if two of these come together they behave as gametes
and fuse leading to the formation of a zygote which germinates into a
normal plant. |
| Oedogonium |
Thus
a gradual loss in vitality of swarmers can be traced, the end product
(i.e. gametes) being largest in number but smallest in size and
physically in capable of developing into usual plants. It is believed
that there was a by chance fusion between these swarmers previously
accustomed to reduce vegetatively and then the plants regularly took to
the method of fusion or sexual reproduction. It may be concluded that
the gametes are ordinary zoospores but reduced in size and unable to
germinate without the stimules of sexual fusion. These biciliate gametes
unite in pair (isogamy) forming a zygospore which forms a new plant.
This proves the derivation of gametes from asexual swarmers.
 |
| Chlamydomonas reinhardtii |
Oogamy
is most advanced type of sexuality where the two gametes are so
dissimilar in shape, size, structure and function that one of them is
known as oogonuim and other as antheridium e.g.: volvox, chlamydomonas
and ordogobium. In chlorophyceae evolution of sex may be seen from
isogamy to oogamy when we study individual genera like chlamydomonas (C.
media iroamous, C. brounii anisogamous, C. Coccifera oogamous) or order
volvocales in which pandorina and endorina are anisogamous and volox is
oogamous. In order ulotrichales family ulvaceae genus enteromarphs
sexually ranges from isogamy to oogamy.
In
Rhodophyceae isogamy and anisogamy are found. In Polysiphomia advanced
oogamy is present in phacophycal ectocarpus shows isogamy and anisogamy.
Anisogamy is also seen in cutariales. Fucus shows primitive type of
oogornium. Sargassum shows advanced oogany because only one ovum is
produced in oogonium after fertilization takes place.
It
may concluded that in the evolution of sex in algae, the
differentiation of gametes is associated with the differentiation of sex
organ. It also shows that evolution of sex in algae has taken place
from simplest type of the highest evolved type.
*pictures shared from various internet sources, thankfully.
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