Re: I. sibirica X I. pseudacorus

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I can try to create a visual example using text...

Say you've got diploid species 1, with paired chromosomes (AA BB), and
diploid species 2 with a higher chromosome count (aa bb cc). The egg
and pollen are haploid, with only one chromosome from each pair: (A B)
and (a b c). The species are related, so A and a are similar enough to
pair, and a cross between the two ends up as (Aa Bb c), with one
unpaired chromosome, probably rendering it infertile.

diploid (AA BB) x (aa bb cc) = unbalanced diploid (Aa Bb c)

If species 1 and 2 are converted to tetraploids, they each have two
pairs of each chromosome, (AA AA BB BB) and (aa aa bb bb cc cc). The
gametes are produced as normal, so each pair contributes only one
chromosome (A A B B) and (a a b b c c). Once these single chromosomes
are recombined in the offspring, though, they can pair with their
duplicate from their own species, forming (AA BB aa bb cc). The
gametes of this amphidiploid will all be (A B a b c), just as if it
was a normal diploid with 5 pairs of chromosomes, and you have
essentially created a new, fertile species.

tetraploid (AA AA BB BB) x (aa aa bb bb cc cc) = amphidiploid (AA BB aa bb cc)

A tetraploid plant has two pairs of each chromosome instead of one. An
amphidiploid plant only has one pair of each chromosome, but has a
full set of chromosomes from each of two species. Crossing two
tetraploids of the same species produces tetraploid offspring, while
crossing two tetraploids of different species produces amphidiploid
offspring. HOWEVER, depending on how similar the parents are, I assume
an amphidiploid often functions as a tetraploid.

Hopefully that explanation makes some sense...

Sean Z