AR: Chromosome Segregation

From: Sharon McAllister <73372.1745@compuserve.com>

Bill Shear wrote:

>  Versicolor has so many chromosomes, I suspect that the ensata genome
gets
>  "crowded out" by meiotic drive in just a few generations, and you get
what
>  is essentially pure versicolor.  So it would be neccessary to select
>  rigorously for those that show ensata characteristics.

>  Perhaps Sharon can speak to this but I believe it can also happen in
>  arilbreds, where the onco or regelia chromosomes segregate out in
>  successive generations.

O.K.  But I suspect I'm about to displease everyone on the list.  Those
who've worked with wide crosses will know I'm over-simplifying this, while
those who haven't will probably think this discussion is far too
complicated....

The world of advanced-generation inter-species crosses can be a very
interesting one -- especially when there's a significant disparity between
the ancestral types.   And, yes, it's very easy to breed back to the type
that has more chromosomes.  My experience is with arils & arilbreds, so
that's what I'll use for my example.

For the first generation, imagine crossing a 20-22 chromosome diploid aril
with a 48-chromosome TB.   The offspring are most likely to be relatively
infertile ABB-type triploids, with one set of aril chromosomes and two sets
of bearded ones.   

This is because the B sets tend to pair on meiosis, while the A chromosomes
tend to be distributed at random.  As a result, most gametes are unviable
-- but the viable ones tend to have one set of each, or just one set of B
chromosomes.   In reality,  there's also the possiblity of an unreduced
ABB-type gamete, and there is evidence that viable BB-type and A-type
gametes may sometimes be formed -- but to keep this as simple as possible
I'm going to ignore those more remote possibilities.  

So let's just look at the three types of crosses that can be made using
these first-generation, ABB-type  offspring:

1.      Backcrossing to diploid arils, which produce only A-type gametes. 
The result will usually be chaff.  But any seedlings that do result will
most likely be AAB-type triploids [relatively infertile] or  AB-type
diploids [also infertile].  No pure arils.

2.      Intercrossing the F1 hybrids.  When both parents are relatively
infertile, hybridizing quickly becomes a frustrating exercise.  The odds
are against getting any offspring at all, but the few seedlings that do
result are apt to be ABB-type triploids [like the parents], or AABB-type
tetraploids.  The latter are fertile -- what the early arilbred hybridizers
were working for.  [THEORETICALLY, if a colony of these F1 hybrids were
left alone long enough natural selection would favor the AABB-type
offspring and enough  would eventually appear that it would become a
self-sustaining, fertile family.  I don't know of anyone who has actually
tried this, and wouldn't even hazard a guess as to how many decades -- or
even centuries -- it would take.] 

3.      Backcrossing to tetraploid TBs, which produce BB-type gametes. 
This time, seedlings will tend to be ABBB-type tetraploids or BBB-type
triploids, both relatively infertile -- but the latter are pure TB and
either type can produce fully fertile BBBB-type tetraploids if crossed back
to TB lines.  


Now, if you're still with me -- assume that instead of using them in a
hybridizing program you have a number of these F1 hybrids planted for open
pollination.  The bees can bring TB or aril pollen to their flowers as
readily as they can other F1 pollen.   What will happen?

In the first year, you might get AB-type diploids; AAB-, ABB-, and BBB-type
triploids; and ABBB- and AABB-type tetraploid seeds.   Everything from
predominantly aril to pure TB.  Assuming you plant all of them -- what
should you expect in the next generation, if it is also subjected to open
pollination?  

This time, the possible combinations range from pure aril to pure TB --
I'll let you figure out the details.  But in the real world, the most
PROBABLE combinations involve the tetraploids.  And the distribution
depends on the number of available pollen parents of the three tetraploid
types.  In this model, AABB will be rare while ABBB will be a bit more
common and BBBB will dominate.  So unchecked evolution will be toward the
BBBB-type tetraploid.    Just as Bill described.

The solution is also as he described:  

1.      Mark the desirable plants and collect their seeds.

2.      Rogue the undesirable plants and don't let them go to seed.

If you do this for several generations, there's a good chance you'll
establish a self-sustaining, fertile family.  

OTOH, if you let nature take its course there's a good chance that the
seedlings will "revert" to the ancestral species that contributes the most
chromosomes.

Sharon McAllister
733712.1745@compuserve.com


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