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.
.jpg)