Starfish: External Characteristics

Parthenogenesis

When development of organism takes place without fertilization, the process is called as parthenogenesis. Parthenogenesis is of two types viz.

1.      Natural

2.      Artificial

Natural parthenogenesis:

·         Eggs that develop without fertilization naturally, the process is called as natural parthenogenesis.

·         It is commonly seen in Aphids, Phyllopods and Rotifers at sometimes of the years not always

·         Whereas in case of bees and wasps males are always produced from unfertilized egg. Thus, males are always haploid and females are diploid because females are produced from fertilized egg.

·         In some species such as Tenthredinidae, reproduction is alays parthenogenetically and males of this species are not known.

Artificial parthenogenesis:

·         In normal conditions some eggs do not develop unless they are fertilized.

·         In such conditions, when ripe egg is induced artificially the development starts. This is called as artificial parthenogenesis of induced parthenogenesis.

·         Eggs of many animals such as Sea urchin, Starfish, Moths, Fish, Frog etc have been activated by various ways. Following are the ways:

-          Chemical treatment with hypo or hypertonic solutions, weak organic acids, alkalis, chloride salt of Ca, K, Na, and Mg and other chemicals like chloroform, ether, alcohol, urea, sucrose etc.

-          Physical or mechanical agents like heat, cold, electric shock, pricking and shaking can also induce parthenogenesis.

-          Fertilization by genetically inactivated (UV treated) sperm.

·         The exact mechanism of activation of egg by these methods is difficult to explain. But, it has been suggested that, the treatment cause damage to the egg cortex by proteolysis and hence egg is activated.

·         Generally, such eggs do not usually develop beyond blastula or gastrula but sometimes they do form normal looking like adults.