Re: Question About Genomic Duplication, Polyploidy, and Speciation

On 5 Oct 2005 04:06:00 -0700, "Ron O" <rokimoto@xxxxxxx> wrote:

>
>Dale wrote:
>> I found an interesting article while looking for something else. I don't
>> quite understand the lingo, but I think it's saying that a lot of speciation
>> could have happened in a short time as a result of some kind of genomic
>> multiplication event. I don't know exactly what polyploidy is, but I take it
>> that when it occurs it is generally harmless to the offspring that have it.
>> So then the second reference here explains how tetraploids can breed and
>> produce offspring. I'm guessing the offspring cannot backcross for some
>> reason, so a new species is formed of whole cloth in one generation.
>>
>> My question is, how would this new species differ morphologically from the
>> parents? I mean, what exactly does genomic duplication do to the organism?
>>
>>
>> http://www.nature.com/ng/journal/v31/n2/full/ng884.html
>> Our results support the contention that many of the gene families in
>> vertebrates were formed or expanded by large-scale DNA duplications in an
>> early chordate. Considering the incompleteness of the sequence data and the
>> antiquity of the event, the results are compatible with at least one round
>> of polyploidy.
>>
>> http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Polyploidy.html
>> When a newly-arisen tetraploid (4n) plant tries to breed with its ancestral
>> species (a backcross), triploid offspring are formed. These are sterile
>> because they cannot form gametes with a balanced assortment of chromosomes.
>>
>> However, the tetraploid plants can breed with each other. So in one
>> generation, a new species has been formed.
>>
>> Polyploidy even allows the formation of new species derived from different
>> ancestors.
>>
>> --
>> "I'm tryin' ta think, but nuttin' happens!" - Curly Howard
>
>It isn't common among animals, but the common ancestor to all
>vertebrates had a duplicated genome. It was either an amphidiploid or a
>tetraploid species. Amphidiploid species arise from a cross between
>two species and a genome duplication event so that two paired
>chromosomes are created and the total genome is essentially duplicated.
> What you get are four copies of just about everything in the genome,
>but two copies come from one species and the other two come from
>another species. In tetraploids the whole genome duplicates and all
>four copies come from the same species. This type of thing happens in
>plants where both sexes are in a single individual more than in animals
>that have two sexes because matings between diploids and tetraploids
>have trouble producing progeny, and certain systems in animals can get
>screwed up if you put the system out of balance. When I took
>cytogentics 30 years ago the best examples of relatively recent
>tetraploid animals that most people would consider to be "complex" were
>amphibians.
>
>In plants the doubling of genomes is more common. Tobacco is a
>tetraploid. Soybeans and maize (corn) are probably amphidiploids.
>Some strains of wheat are hexaploids. The most noticible thing about
>tetraploids is that they are usually more robust in size than the
>original parent. Certain parts of the plants like the flowers or seed
>heads can be be larger. Humans have selected such traits and we have
>ended up with these examples.
>
>It has had a major impact on biological evolution, but it isn't
>widespread. We wouldn't be here if such an event didn't happen around
>half a billion years ago. The cordate ancestor that gave rise to all
>vertebrates would not have existed. Since then the human lineage
>hasn't gone through another duplication event, but other vertebrate
>lineages have, it just isn't the major speciation mechanism for animals
>in nature.
>

Plants differ from animals in lots of ways, but one is the common
ability to reproduce vegetatively, that is, asexually. One single
plant can subdivide and subdivide and produce a whole population of
clones that can then resume sexual reproduction which involves
meiosis. That, as well as the "selfing" already mentioned (where one
plant has both male and female reproductive organs and hence can
fertilize itself) is why polyploidy is so much more common in plants
than in animals.

Note that many organisms, plants and animals including humans,
normally contain a wide variety of polyploid cells. It is especially
common in malignant cells. However the germ cell line that produces
the sperm and egg (spore mother cell, in the case of plants) is
usually protected from developing polyploidy.



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