HYB: Diploid - Tetraploid

Thursday, June 12, 2003 1:08 PM
Jim Gibbons wrote:
"Here's my question: in the Species Checklist, (most current from our SIGNA
folks) i. aphylla is noted as both having both diploid (2n=24) and
tetraploid (2n=48) forms. Does anyone know of the diploid form? I'm assuming
that the aphylla clones I am collecting are tets, as I'm trying to play
around with MTBs, but am also working the diploid lines. Don't want to waste
a bunch of pollen with a mismatch if I have some dips and tets in the
aphylla patch, which is growing..."

Jim,
Something you may not be aware of, is that a large part of the TB genetic
lines originally came from diploid species and hybrids.  These diploid parents
generally entered the tetraploid gene pool when a diploid (most often I.
pallida, but others were used) was crossed with a tetraploid species (or a
previous tetraploid hybrid) to produce more tetraploid offspring.
Normally a triploid would result from a diploid crossed with a tetraploid, but
past history shows that diploids and tetraploids can produce fertile
tetraploid offspring.  [Of course there are other issues such as differing
base chromosome numbers causing incompatibility, but if you are using aphylla
types and MTB diploids, you have base-12 in both cases.]
Tetraploids that should have been triploids can happen when through an
accident in meiosis the diploid gamete is not reduced and is passed on with a
full diploid compliment of chromosomes.  Then a normal haploid gamete is
crossed with a diploid gamete to result in a fertile tetraploid compliment in
the offspring.
Polyploidy can often be induced through chemical treatments (making unreduced
diploid gametes more likely from diploid parents), but I've never fooled
around with this myself.
Normally a triploid with a diploid would result in the following.
2n = 24 diploid parent produces a 1n = 12 gamete.
2n = 48 tetraploid parent produces a 2n = 24 gamete.
If crossed it would be 12 + 24 = 36[3n]
but, if a diploid parent produces an unreduced 2n = 24 gamete, the result of
combining that gamete with a gamete from a tetraploid would be 24 + 24 = 48
[another tetraploid].

Dave

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