theory wanted

Brian Fristensky frist at ccu.umanitoba.ca
Tue Nov 15 11:47:00 EST 1994


In article m7n at wumpus.cc.uow.edu.au, mai at wumpus.cc.uow.edu.au (Van Dao Mai) writes:
> > I recently watch a TV show that raised the new idea about the way new
> species of plants coming into existence. It is against the old idea of
> genetic mutation. It suggests that different species co-habit and
> eventually come together to form new plants.

It's hard to answer your question with such little information, but I'd
guess that the show was talking about the process of polyploidization,
that is, the addition of one or more complete chromosome complements
to a population. There are two kinds of polyploidization:

autopolyploidy - a duplication of chromosome number within a species.
  Can be due to 1)multiple fertilizations by one egg 2) failure of mitotic
  reduction in diploid germline cells 3) failure in meiotic reduction,
  resulting in diploid gametes

allopolyploidy - addition of one or more chromosome complements from a
  different species. This would be the result of a rare intercross 
  between two closely-related species. The important point here is
  that the two 'species' must be interfertile. What this really means
  is that these species are recently diverged in the first place,
  and there hasn't been enough time interfertility to be lost. The 
  diagram below illustrates the process:

                             ancestral species (2n)
                                     |
                                     v
         -------------------------------------------------------
         |        divergence of subpopulations through          |
         v                random mutations                      v
         |                                                      |
         v                                                      v
 species A (2n)                                         species B (2n)
         |                                                      |
         v                   hybridization                      v
         --------------------------------------------------------
                                      |
                                      v
                             new species (4n)

   where n is defined as the haploid number of chromosomes
   (eg. in peas, n=7, so the somatic cells are diploid with 2n=14,
    and gametes are haploid, with 1n=7 chromosomes.)

The point is that, by the strictest definition, species A & B
are really still one species, because they are still interfertile.
If fact, when we identify separate plant species, what we are
really doing is identifying separate populations that have been
reproductively isolated long enough to have diverged with
respect to morphological characteristics. It can often (but not
always) take a long time before interfertility is lost. In fact,
polyploidization is itself a good mechanism for eliminating
interfertility, since a zygote resulting from the mating of
a diploid and a tetraploid will usually be triploid, which
is often nonviable, and almost always infertile. 

> The problem I have in mind is that a new species must be able to create a
> genetic blue print including the genes of all the participants as well as
> information for them to work together. How does it fit together with the
> general frame work of the evolution theory? 
Polyploidization is nothing new. It is a well-know mechanism for generation
of genetic diversity as well as speeding the process of speciation.

> > It is easier to imagine that these species are present in the environment,
> and they simply possess the ability to merge and grow together as one
> reality. It is difficult to make assumptions that modern plants evolved 
> out of this process.
> 
> Is there any book written on this topic? What is the formal technical terms
> used for this theory?

Any introductory textbook on Cytogenetics, and probably also Plant Breeding,
will discuss polyploidization in depth. It occurs often in plants, one
of the best natural examples being the oilseed Brassica species:

                               B.nigra
            --------------------  n=8-------------------
           |                                            |
           v                                            v
     B.carinata                                    B. juncea
        n=17                                            n=18
           ^                                            ^
           |                                            |
         B.oleracea                                     |
               n=9                                      |
           |                                            |
           +----------------> B.napus  <--------------B.rapa
                                n=19                   n=10


   For example, B. carinata represents a hybridization between B.nigra
and B. oleracea, and B. napus resulted from hybridization between
B. rapa and B.oleracea.

There have been many man-made allopolyploids as well. Modern wheat has a 
hexaploid genome resulting from the intercrossing of three grass species
several thousand years ago. Triticale is a synthetic cereal derived from
a cross between wheat and rye. Various synthetic tobacco species have
also been generated by hybridization.

Just as an aside, plants seem to tolerate polyploidy, as well as other
aneuploidies (changes in chromosome number) much better than animals. 
Not surprisingly, polyploidy is seen much less often in animals. 
We might infer that plants are better at compensating for having too
many or too few copies of chromosomes. 

===============================================================================
Brian Fristensky                | 
Department of Plant Science     |  Life doesn't imitate art,
University of Manitoba          |  it imitates bad television. 
Winnipeg, MB R3T 2N2  CANADA    |  
frist at cc.umanitoba.ca           |  
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