John Hewitson john_hewitson at breathe.co.uk
Sat Sep 4 03:32:04 EST 1999

>I am going over some evolutionary theory in my plant bio class, and it
>occurred to me that, although I know that polyploidy occurs much more
>often in plant speciation events than in animals, I do not know *why* this
>is true.  Is it a quality of the plant genome itself, or a result of
>products similar to colchicine (that can prevent reduction-division during
>meiosis) being more common in plant cells, or because of some other

My reading of the situation is that speciation by polyploidy arises as a 
result of TWO rare event.

1) an interspecific hybrid is formed as a result of the fusion of two 
gametes from different species.  This is a rare event.  eg Spartina 
stricta (2n=56) crossed with Spartina alterniflora (2n=70).  Gametes 
contain 28 and 35 chromosomes respectively.  Hybrid carries 63 
chromosomes which are all different and cannot therefore pair up to 
undergo meiosis, but gives no problems for mitosis.  Thus a sterile 
hybrid is produced.  This does happen in animals eg horse + donkey -> 
sterile mule.

2) the plant's trick is that this hybrid then thrives by asexual 
reproduction so that there is plenty of it about and it becomes more 
likely that the second rare event of the chromosomes doubling but nucleus 
not doubling can occur.  This gives rise to a nucleus in which there are 
PAIRS of chromosomes and CAN undergo meiosis.  (This is what colchicine 
can do.)  Thus a fertile hybrid is formed which has twice as many 
chromosomes as the original species = Spartina townsendii (2n=126).  I 
suppose this would need to happen in two animals in the same place before 
they both died whereas self fertilisation is a possibility in plants in 
an emergency. 

Now this may only be part of the picture....

Dr. John Hewitson (Oundle School)
Bramston Garden Cottage, Oundle, Peterborough PE8 4BG UK
Phone and Fax +44 (0) 1832 272209
email john_hewitson at breathe.co.uk


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