Asexual reproduction invertebrates, its advantages and disadvantages
Asexual
Reproduction:
It is a
reproduction without the union of gametes. It the only means by which primitive
organisms could increase their number. But these species that reproducing
asexually tends to evolve very slowly, due to less genetic diversity for
evolutionary selection. It is common among protozoa as well among lower
invertebrates such as sponges, jelly fishes, flatworms and many segmented
worms. It is rare among the higher invertebrates. The ability to reproduce
asexually often correlates with marked capacity for regeneration.
In the lower
invertebrates its most common forms are fission, budding (internal and
external) and fragmentation. Parthenogenesis which is comparatively uncommon
also occurs in few invertebrates.
Fission:
Protists and
some multi-cellular animals (cnidarians, annelids) may reproduce by fission.
Fission is the division of one cell, body or body part into two. In this
process, the cell pinches in two by an inward furrowing of the plasma membrane.
Binary fission occurs when the division is equal; each offspring contains
approximately equal amount of protoplasm and associated structures. Binary
fission is common in protozoa for some; it is their only means of reproduction.
In fission the
plane of division may be asymmetrical, transverse or longitudinal, depending on
the species. For example Planarian reproduces by longitudinal fission. Some
flatworms and annelids reproduce by forming numerous constrictions along the
length of the body; a chain of daughter individuals results. This type of
asexual reproduction is called multiple fission.
Budding:
In cnidarian Hydra
and many species of sponges, certain cells divide rapidly and develop on the
body surface to form an external bud. The bud cells proliferate to form a
cylindrical structure which develops into new animals, usually breaking away
from the parent. If the buds remain attached to the parent, they form a colony.
Internal budding (as in freshwater sponges) produces gemmules, which are
collections of many cells surrounded by body wall, when the body of parent dies
and degenerates each gammule gives rise to new individual.
Fragmentation
(regeneration):
It is a type of
asexual reproduction where by a body part is lost and then regenerates into new
organisms. Fragmentation occurs in some cnidarians, Platyhelminthes, Rhychocoels
and Echinoderms. For example in sea anemones, as the organisms moves, small
pieces breakdown from the adult and develop into new individuals.
Parthenogenesis:
It is
spontaneous activation of mature eggs followed by normal egg divisions and
subsequent embryonic development without fertilization by pricking them with a
needle by exposing them to high concentration of calcium or by altering their
temperatures. E.g. Flatworms, Rotifers, Round worms, Insects, Lobsters, some
lizards and some fishes. Because parthenogenetic eggs are not fertilized, they do
not receive male chromosomes. The offspring would thus be expected to have only
a haploid set of chromosomes. Parthenogenesis plays an important role in social
organisation in colonies of certain bees, wasps and ants. In these insects
large number of male (drone) are produced parthenogenetically where as sterile
female workers and reproductive females (queens) are produced asexually.
Advantages of
Asexual Reproduction:
(1) The
predominance of asexual reproduction is protists and some invertebrates can be
partially explained by the environment in which they live.
(2) Marine
environment is usually very stable. Stable environments may favour this form of
reproduction because a combination of genes that matches the unchanging
environment is an advantage over greater number of gene combinations, many of
which do not match the environment.
(3) Asexual
reproduction is seasonal. The season during which asexual reproduction occurs
coincides with the period when the environment is predicately hospitable. Under
such conditions it is advantageous for the animal to produce asexually large
number of progeny with identical characters.
(4) Large number
of animals, well adapted to a given environment can be produced even if only
one parent is present.
Disadvantages:
(1) Due to this
reproduction a population of genetically identical animals can be devastated by
single disease or environment insult such as long drought.
(2) Asexually
reproducing animals can cope with changing environment only through rare
spontaneous mutations that prove to be beneficial. But most mutations are
lethal and here in lies one of the greatest disadvantages of asexual
reproduction.
(3) All such
mutations are passed on to every offspring with the normal, unmutated genes.
Consequently, typical asexual animal may have only one ‘good’ copy of each hereditary
unit (gene); the one on the homologous chromosomes may be a mutated form that
is lethal.
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