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).Â
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)Â 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.Â
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.Â
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).Â
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.Â
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.
Â
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:
