1. Do all the plants within a given species have the same basic "X" number of
chromosomes?
=Yes. The basic chromosome number ( X or N ) is assigned to a given species or
genus.
Does this mean a species within the genus Landoltia, for example, might have a
normal 2n chromosome count of 40 with haploid (2n = 20), diploid (2n = 80),
etc., variations, but not 2n = 46?
= Normally, individuals in a species share the same chromosome number. However,
some deviation in the number can naturally occur. Aneuploidy is the one with a
change in chromosome number (for example 2n = 2N+1, = 2N+1+1, = 2N-, 2N - 1-1)
while euploidy is the one with the change in number of chromosome set (for
example 2n = N, = 3N, = 4N). By the way, 2n is the chromosome number normally
counted from root tip while n (haploid number) is the chromosome number
normally counted from anther. In your case, 2n = 80 should be tetraploid, not
diploid.
So if you have two plants, one with 2n = 40 and another with 2n = 46, do you
know you have two different species?
= From the chromosome number you gave, it is most likely to be 2 different
species. The difference (6) is too high to be an aneuploidy, but you can not
tell. In Aglaonema costatum, 2n = 40 and 48.
2. Can plants with different chromosome counts be cross fertile?
= Yes, it can, especially with the help of embryo rescue technique. Many
slipper orchid species with different chromosome number yielded a number of
interspecific hybrids, both primary and complex ones.
Can a 2n = 40 plant produce viable seed with its own diploid?
= Yes, it can, especially 2n = 2N = 40 since it is diploid. However, the
fertility declines if it is tetraploid (2n = 4N).
How about with a plant in the same genus with a chromosome count of 2n = 46?
= It should be fertile.
3. What happens when a plant messes up and undergoes aneuploidy.
Aneuploidy is defined in the text as when some type of evolution takes place
where the offspring end up with a few extra chromosomes here and there. Doesn't
this mess up the plant? If not, why not?
= Normally, an aneuploidy has low fertility. However, a few offsprings of its
can regain the fertlity through chromosome doubling. In evolution process, the
fittest will survive and establish an population. If the aneuploidy can perform
better than the original in survival and reproduction in a new environment, it
may evolve to be a new species in a due course.
4. Similarly, there is a term called dysploidy where a few chromosomes don't
make it into the new plants, or where old chromosomes get pieces chewed off,
somehow. Don't plants need at least a portion of the information contained in
the chromosome arms? Doesn't this mess up the plant? If not, why not?
= Most of higher plants today are polyploid. It is hypothesized that plants
with N > 10 are eupolyploid. Missing a chromosome from eupolyploids may not be
lethal.
5. Can a plant that has experienced aneuploidy or dysploidy produce viable
seed with a normal-count plant? Or does the plant have to reproduce vegetatively
for a while until there is another receptive brother or
sister with whom to mate?
= As stated above, it is possible that aneuploid can produce some seeds.
6. How much messing around with chromosome numbers does it take before the
morphological differences are large enough to produce a new genus?
= It is not just the chromosome number. Reproduction organ and crossability are
the key different that separate two population to a genus level.
You may try look at chromosome number of the genus Aglaonema which varies within
the species and genus. The genus is vegetatively propagated in natural
condition.
I hope my answer can help you clear up your doubt.
Regards,
Surawit
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