Short Notes on Parthenogenesis, Advantages of Asexual reproduction, Extra embryonic membranes and Lactation
Parthenogenesis:
Lactation:
Parthenogenesis
is spontaneous activation of mature egg followed by normal egg division and
subsequent embryonic development without fertilization by pricking them with a
needle, by exposing them to high concentration of calcium or by altering their
temperature. Because parthenogenetic eggs are not fertilized they do not
receive male chromosomes. Offspring would thus be expected to have only a
haploid set of chromosomes. In some animals meiotic division is supported, so
the diploid number is conserved. In other animals meiosis occurs, but an
unusual mitosis returns the haploid embryonic cells to diploid condition.
Animals that reproduce parthenogenetically have less genetic variability than
do animals with chromosome sets from two parents. Parthenogenesis plays
important role in social organization in colonies of certain bees, wasps and
ants. In these insects large number of male (drones) are produced
parthenogenetically where as sterile female workers and reproductive female
(queens) are produced sexually.
Advantages of Asexual reproduction:
protists and
some invertebrates can be partially explained by the environment in which they
live. Marine environment is 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. 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.
Large number of
animals, well adapted to a given environment can be produced even if only one
parent is present.
Extra embryonic membranes:
They are amnion,
chorion and allantois.
Amnion and
chorion: Following the neural tube stage, the ectoderm and mesoderm on both
sides of the embryo lift off the embryo and grow dorsally over the embryo. As
these membranes meet dorsally they fuse and form an inner amnion and outer
chorion. The amnion encloses the embryo in a fluid filled sac. This amniotic
cavity protects against shock and drying. The chorion is near the shell,
becomes highly vascular and aids in gas exchange.
Allantois: The
immediate breakdown product of proteins is highly toxic ammonia. This ammonia
is converted to uric acid which is excreted and stored in the allantois, a
ventral outgrowth of the gut tract. Uric acid is semi solid and thus little
water is wasted. The allantois gradually enlarges during development to occupy
the region between the amnion and chorion. In addition, the allantois becomes
highly vascular and functions with chorion in gas exchange.
Lactation:
It includes both
milk production by mammary glands and milk release form breasts. During
pregnancy the breasts enlarge in response to increasing levels of the hormone
prolactin. Before birth, placental secretions of estrogen and progesterone
inhibit milk secretion from the breasts. After the placenta has been expelled
the concentrations of estrogen and progesterone drop, and the breasts begin to
produce copious amounts of milk. Mother’s breasts do not actually release milk
until one to three days after the baby is born. During these first days, the
sucking baby receives colostrums, a high protein fluid present in the breast at
birth. Colostrum contains an abundance of maternal antibodies and thus helps to
strength the baby’s immune system. It also functions as a laxative, removing
fetal wastes called meconium retained in the intestines. After three days
prolactin secreted from the pituitary stimulates milk production. Newborn’s
sucking stimulates the pituitary to release oxytocin as well as prolactin.
Oxytocin triggers milk release from mammary glands.
Comments
Post a Comment