Re: Re: inducing tetraploidy

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Dennis,

I have never really understood the spec-x category, other than as a catch-all for the award programs.  As far as I can read, it does not present a real category, rather an opportunity for hybrids to compete on a level other than physical size and current modes of beauty.  In this sense, a nice thing, but in the biological sense, nonsense.

As far as fertility in wide crosses, or any cross from plants with differing chromosome counts, we have a variety of reasons why two plants may be unlikely parents together.  First, the differing number of chromosome is not actually the big hurdle, rather the differences in general genetic makeup.  If the chemistry is compatable enough, then odd chromosome counts will produce off-spring.  They are generally infertile, as they are incapable of a proper meiosis, which is required to produce gametes for the next generation.

What is more likely to happen, and this has only been recently understood, is a chemical incompatability between the two species/parents which tends to prevent them producing any viable seed.  This is not directly associated to the chromosome count, rather the chromosome/cell make-up/chemistry.  In short, the ability of the two groups of genetic material to communicate is hindered or prevented totally (laymans terms).  I can't explain all the intimate factors involved, as I am not learned enough in the field, but we are dealing which incompatability beyond the chromosome count.

If two plants have similar enough chemistry, despite un-equal chromosome counts, they may well produce off-spring. (look at Thomas Tamberg's primary diploid hybrids)  Due to the odd combination of chromosomes in the off-spring, these plants will not produce balanced gametes.  What I mean, here, is the ability of the available chromosomes to pair-up during meiosis and produce sex cells.  If the chromosomes are not sufficiently similar, they will not find a partner and thus not be directed through the seperation process to produce the gametes. 

As you know, gametes/sex cells contain only half the compliment of chromosome found in the parent.  The idea is they will then pair with another gamete with the same or similar compliment and fuse into a viable embryo.  It is at the point of this primary production where hybrids prove to be infertile.  The produced gametes, if any, do not have regular amounts of genetic material and can thus rarely find a companion.  The result is strong infertility.  Complete sterility is rare, but the odds are so slim, we tend to think of these plants as sterile, while they are actually only limited to the extreme.

By doubling the number of chromosomes, as in tetraploid conversion, your are providing partners for the chromosomes during meiosis.  In essence, as the conversion creates duplicates of the available chromosome indirectly by preventing the meiotic cell from actually dividing into two after the chromosome replication, one has doubled/reproduced the genetic materials within the cell. They will now be able to divide into a gamete with half a compliment from one parent and half from the other parent.  The resulting offspring will thus remain tetraploid and fertile (in most cases).  This type of tetraploid is refered to as an amphidiploid, as it is actually a tetraploid composed by the fusion of two diploid sets of chromosome representing two different chromosome sets.  A classic tetraploid has 4 sets of essentially the same haploid chromosome set.  If we do a tetraploid conversion on a species, say I. siberica, we will have a plant with 4x the same base chromosome set.  If we create a tetraploid of a CalSib, we will have an apmphidiploid with 2x the california haploid set and 2x the siberian haploid set.  Genetically very different plants and they will behave differently in hybridizing.

Sharon McAllister wrote some very informative material on the fertility amongst aril irises.  Here is a link to one of her pages.  The site is full of interesting material concerning iris genetics.  Although she is discussing arils, the principles are the same for all plants.

  http://www.smcallister.com/hybridizing/fertile-families.htm

What you wrote in #5 is essentially correct.  I would change the wording from 'chromosome counts' to 'chromosome incompatability', as this is a bit more general, and, as I mentioned, the actual count is not necessarily key, rather the general cell chemistry involved.

If I have been unclear explaining, just let me know.  The first time I researched this I was a bit confused and then it all clicked.  The a-ha effect.

cheers,

Jamie

Am 09.01.2014 00:22, schrieb Dennis Kramb: