ASEXUAL REPRODUCTION
• Single parent is involved.
• The daughter individuals are genetically identical (clones) to the parents.
• Gamete formation is generally absent.
There are three modes of such reproduction.
(1) By fission
(2) By budding
(3) By fragmentation
(1) By fission - It is of two types, i.e. binary fission and multiple fission
(A) Binary Fission – In such fi ssion two daughter individuals are produced. The binary fission can be.
(a) Simple binary fission -
• This fission is random or asymmetrical.
• Nucleus shows mitotic phases but nuclear membrane does not disappear at metaphase.
eg. Amoeba and Entamoeba
(b) Transverse binary fission -
• In such fission the two daughter individuals are formed by transverse division.
eg. Paramecium (ciliate protozoan ), In Ceratium (the binary fission is oblique).
• Transverse fission occurs in Planaria (Platyhelminthes).
(c) Longitudinal binary fission -
• In Euglena (A fl agellate protozoan) the two daughter individuals are produced by longitudinal division of the parental organism.
(B) Multiple Fission –
• This fission may occur without encystment, or after encystment, if the environmental conditions are not favourable.
• In Plasmodium, the schizogony is a type of multiple fi ssion which occurs without encystment.
• In Amoeba, the multiple fi ssion occurs by sporulation.
(2) By budding – It can be of two types, i.e. exogenous and endogenous budding.
(I) Exogenous Budding –
• Such budding occurs in most of the Porifers and few Coelenterates (Cnidarians).
• Such buds are formed outside the body.
• In most of the sponges and few Coelenterates (e.g. Physalia), which are colonial, the colony is formed by budding.
• In chordates the budding occurs in the members of Tunicata/ Urochordata.
(II) Endogenous Budding –
• Such buds are internal and are also called gemmules. They are produced in fresh water sponges like Spongilla and Ephydatia.
(3) By fragmentation –
• In such asexual reproduction the parental individual gets fragmented into a number of pieces. Each piece has the capacity of forming a complete daughter individual.
• Such fragmentation occurs both in Hydra and Planaria.
(Amongst lower non-chordates, the asexual reproduction is absent in Ctenophores and Nematodes).
SEXUAL REPRODUCTION
• Two parents are involved.
• Gametes formation occurs.
• The daughter individuals are genetically different from both the parents.
• The parental individuals may be unisexual (dioecious) or bisexual/ hermaphrodite (monoecious)
e.g. Paramecium, Plasmodium, Hydra, tape worm and earthworm.
In case of Monocystis the gametes are identical and their union is called isogamy. In other animals the two gamates are dissimilar and their union is called anisogamy.
In Paramecium sexual reproduction occurs by conjugation process, in which gametic nuclei are exchanged between conjugating parents. Each Paramecium after conjugation produces 4-daughter individuals.
