Re: Is parthenogenesis a result of a meiosis 'spin'?

Hello Francisco,

Parthenogenesis is defined as reproduction in which offspring are produced without fertilization, 
and this can occur in one of several different ways.

	One way is known as automixis.  In this case, the first step is normal meiosis, giving rise to 
a haploid cell.  This haploid cell can then do one of two things:
A) fuse with another haploid cell within the mother, resulting in a diploid cell that can develop 
into an embryo.

B) divide, giving rise to two identical haploid cells which then fuse, resulting in a diploid cell that 
is homozygous at every locus.  In 2006, a female Komodo Dragon lizard in London reproduced in 
this manner, prompting some media coverage.
 
In neither (A) or (B) is the offspring a clone of the mother.

	A different way is known as apomixis, in which the eggs are produced through mitotic 
oogenesis.  In other words, there is no meiosis, the egg is essentially made by normal cell 
division.  In this case, the offspring will be genetically identical to the mother (a clone), with the 
exception of any mutations that may have occurred in the process.  Animal and plant breeders 
would love to be able to induce this kind of parthenogenesis in selected livestock and crop plants 
(to produce clones of exceptional individuals), but no luck yet!

There are also cases in which meiosis proceeds normally, and a haploid egg develops into a 
haploid offspring.  This is seen in honeybees.

In the laboratory, parthenogenesis can be artificially induced.  For example, in zebrafish (Danio 
rerio), unfertilized eggs can be mixed with sperm whose DNA has been destroyed.  The 
interaction between egg and sperm "activates" the egg to begin the second meiotic division.  At 
this point, the egg is subjected to very high pressure (8000 psi), which causes the meiotic 
spindle to break down, so the chromosomes can not segregate.  The result is a diploid cell that 
has no DNA from the sperm, but has been triggered to develop.  This gives rise to a viable fish 
that is not a clone of the mother.  This is the closest thing to an "error" in metaphase that I could 
find.  I was not able to find documentation of this kind of parthenogenesis (failure of the spindle 
during meiosis II) occurring naturally.

This web site gives some more background on different types of parthenogenesis: http://www.faculty.ucr.edu/~legneref/biotact/bc-15.htm

Alex Brands
Lehigh University