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:
