On 14th December 2001 a baby shark was born in the zoo at Nebraska, USA. Now baby sharks are regularly born in zoos all over the world. But this baby aroused the curiosity of scientists worldwide. Why? Because the mother shark had never come in contact with a male in her entire life! Scientists who investigated the baby shark found that the female pup’s DNA did not have any male DNA. The pup was born by virgin birth!
In today’s VERITAS we will study Parthenogenesis which is composed of two Greek words: parthenos(virgin) and genesis(birth).
There are several creatures capable of producing offspring by Parthenogenesis or virgin birth. Most of them are invertebrates like some bees, some species of scorpions etc. Some vertebrates are capable of virgin birth. These are Komodo dragons, some snakes, lizards. Very rarely sharks and birds also produce offspring by parthenogenesis.
We have seen an example of parthenogenesis in an earlier VERITAS: Fibonacci series in Nature, March 15, 2010. In that article we saw that a bees can have either one or two parents! Fertilized eggs of a queen bee result in a worker bee(female)/or queen. If the egg is left unfertilized then it hatches to become a drone( male bee). Parthenogenesis is thus very common in bees and other lower insects.
In higher creatures like reptiles, birds(eg turkeys) and sharks it is rare. One of the reasons why it may happen is when there is a scarcity of males to fertilize the eggs. Another probable reason could be an error in the genetic code of the egg caused by infection or mutation of the normal sequence.
An offspring born through parthenogenesis is always a clone or half clone of its mother. As you know, all creatures carry two sets of each chromosome. This is called diploid chromosome number. For example we carry 23 pairs of chromosomes- of each pair one is from the mother and the other is from the father. In parthenogenesis two possibilities exist: either the egg will have only half of its mother’s chromosome set( meiosis has occurred) and the chromosome set doubles during development thus resulting in a diploid chromosome number. This would cause the offspring to be a half clone of its mother. Another possibility is that meiosis does not occur i.e the mother’s chromosome set does not become half in the egg – it copies itself whole. The offspring is, in this case, a full clone of its mother.
Parthenogenesis has a great advantage over sexual reproduction: these is absolutely no need for the male and this can result in rapid reproduction and therefore super-fast population increase. But there is a huge disadvantage: the offspring of parthenogenesis is always a clone( or half clone ) of its mother and therefore more susceptible to diseases. Parthenogenesis does not result is variation of the gene pool and over a few generations the offspring is unable to cope with changes in microbes around itself and therefore loses the battle of survival. Thus sexual reproduction is a must for genetic variation. Another way to look at it ( though some of us may not like the blunt fact): males and sex exists only because there are germs and diseases in this world!
There is an interesting fact related to parthenogenesis: All offspring born this way in some creatures are always of the same sex. In others it may be of the either sex. For reptiles, for example ,the offspring from parthenogenesis can be male or female. But for sharks the offspring from virgin birth is always female. Lets now understand the reason for this.
There are two ways in which sex is determined in an individual: XY and ZW. In humans we have the XY system- the 23rd chromosome pair can be XX or XY. If it is XX then it is a female, if XY then it is a male. So in XY system the identical sex chromosome causes the offspring to be a female, else it is a male. Sharks follow the XY system. In reptiles and birds there is a different system: Males have the identical sex chromosome and the females have the different pair. To differentiate from XY system this is called the ZW system. So in reptiles and birds a female is ZW and the male is ZZ in terms of the pair of sex chromosome.
So now lets consider parthenogenesis from a female reptile which carries the ZW pair. The offspring may be ZZ( male – half clone of mother) or ZW( full clone of the mother). If the offspring is WW then it is a female but rarely survives.
But in a creature like shark which follows the XY system the offspring from parthenogenesis is always XX ie always female. The XX female cannot ever produce a male by virgin birth!
Some people use the phenomenon of parthenogenesis to explain that Jesus Christ’s virgin birth is a scientific possibility. This is not possible because of two reasons: first parthenogenesis has never been observed in humans. Second, even if parthenogenesis was possible in humans, the offspring born of such a birth would always be female since human sex is determined by the XY system. A XX mother can never give birth to a XY male from parthenogenesis!
Go wondrous creature, mount where science guides
go measure earth, weigh air, state the tides,
instruct the planets in what orbs to run
correct old time, regulate the sun