Re: Re: Iris attica (was more questions from the newbie)

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Now back to the sets of 8 and 12.  They are related, a gamete with 8 comes in contact with a gamete with 12, many of the chromosomes will try to pair up; however, things aren't quite even and equally matched, so there are usually some things that go wrong, such as extras that don't pair up at all, and some that try to pair with more than one other.  These two groups of species are closely related and they will hybridize; however, the offspring that receive one set of 8 from one parent and one set of 12 from another are generally sterile, because when they undergo meiosis to produce pollen and ovules, the chromosomes don't behave properly and the reproductive cells generally don't mature properly or at all.

 

Now to the miracle of chromosome number doubling.  When you take a tetraploid 32 and cross it with a tetraploid 48, you get two sets of 8 and two sets of 12.  Now they all have somebody to pair up with, and now it works.  You get nice fertile 40 chromosome "amphidiploids".  Generally these hybrids are fertile.  In this way it should be possible to get I. attica genetics (heat and drought tolerance, etc.) into the blood lines of dwarfs and intermediates.  It actually seems likely to me that I. pumila or unreduced I. attica are responsible for the 40 chromomes wild species (I. lutescens and it's close relatives).

 

"Amphidiploid" might be confusing, since on the surface if might seem that these should be called tetraploids, since they have four full sets of chromosomes.  However, now they behave as if they have only two haploid sets of 20.  This twenty is derived from 8 + 12, but now it behaves as if it is one unit or one new basic "building block".  Often, if you cross these with the parents with either 8 or 12, things will get messed up again; there will likely be pairing mistakes, because some of the 8 are very similar to some of the 12, and the chromosomes from the 8 or 12 chromosome parent have trouble pairing up just as mentioned above.  These back crosses are often of reduced fertility or sterile, but sometimes they work.  You can get the first generation with 28 or 32 chromosomes, but a second generation crossed with anything is very difficult to get.  Again notice you have 32 chromosomes, but here it is 12 + 8 + 12 instead of 4 x 8.  Just having the count doesn't always count (pun intended).

 

This can go on and on and on.

 

Again back to the 8 and 12.  It is difficult to know which came first.  Perhaps long ago the basic number was 4 (which would have made diploid plants with 8 chromosomes), and some decendents became diploid with 2n = 8 and others hexaploid with 2n = 24.  After long enough periods of isolation, enough changes could occur to make it so that the basic sets of 4 in each didn't quite match up any more.  However, 4 is a very low number of chromosomes for any plant, and it is very unlikely that this is the case.  It is more likely that either 8 or 12 is the primitive number.  If 8 was the primitive number accidents might have occurred that split one chromosome into two, or extra chromosomes might have accidently gotten passed along in unequal pairing accidents in meiosis.  These extras are called "supernumary" chromosomes.  If these gradually get carried along from one generation to the next until all offspring have extras, it can end up that one extra eventually becomes a pair, two become two pairs, and so on until you have a base number of 12 and a diploid number of 24 (four extra pairs).  It can also happen the other way.  Sometimes through similar accidents, a chromosome is lost.  Usually this would be fatal, but apparently not always.  The number can gradually be reduced.  More likely is that two chromosomes fuse and become one.  Sometimes you see Y-shaped chromosomes instead of the usual linear chromosome, and fusion of two or parts of two chromosomes is often likely the reason for this.  Thus you can get a reduction in number, even if it seems a bit illogical at first look.

 

This means you can have all sorts of odd numbers and combinations.  Generally the rule of thumb is that if a plant has an even number of sets of chromosomes, and if all the chromosomes occur as matched pairs, then the plant will be fertile.  It will produce fertile offspring with plants of the same chromosome makeup.

 

Now is a good time to mention some of the other Bearded Iris (Regalias and Arils).  These have either 10 or 11 as the basic "haploid" number.  They have diploid numbers of 20 or 22.  Apparently the 11 is derived from the 10, with one (duplicated) extra pair.  When you cross a 20 (haploid is 10) with a 22 (haploid is 11) you get a 2n = 21 made from 10 + 11.  These are normally totally fertile, and the extra one chromosome isn't too important.  These have offspring that are either 2n = 20 or 2n = 21.  Eventually through the generations of hybrid offspring, that extra chromosome gets lost.  This is all nice, but things aren't quite as easy with the "true" Bearded Iris discussed above.  You end up with that same old problem of mismatched pairs.  The 10 or 11 aren't quite the same as the 8 or 12, and they don't pair up quite right when hybridized.  So, what's the solution.  Use tetraploids.  Again either natural tetraploids or artificially induced ones can be used.  Then you get those mismatched tetraploids or  "amphidiploids" again, and again they're perfectly fertile.  They come out as:

 

8 + 8 + 10 + 10 = 36

8 + 8 + 10 + 11 = 37

8 + 8 + 11 + 11 = 38

12 + 12 + 10 + 10 = 44 (not the same as the 44 of I. x germanica and I. x albicans)

and so on

 

However, now if you try that fertile amphidiploid 2n = 40 I. lutescens or SDB with one of the tetraploid Aril/Regalia types, you'll have problems.  It all gets out of whack again.  You can't take the basically haploid 20 of one these and match it to the diploid 20 of the Aril/Regalia tetraploids.

 

[8 + 12] + 10 + 10 = 40  Now you have for all practical purposes have three basically different sets trying to match up.  The two sets of 20 behave properly in their respective plants, but they don't match properly and won't pair up properly.

 

And so it goes.  It can all be very interesting, and very confusing.  Sort of like number puzzles.  Which combinations will work, and which won't?  The rules are actually pretty simple, but if you don't have enough information, it is difficult to understand what is happening.  Generally any first generation cross will produce plants, but some of those first generation plants may be sterile because things just aren't sorting out correctly when they try to produce ovules and pollen.

 

By the way, the Psamiris fit in here similarly, but I haven't learned much about them yet myself.

 

Now I'll comment briefly about the Beardless Iris.  The story is pretty much the same; however, it seems that the changes in the actual chromosomes are often not enough to eliminate fertility in mismatched numbers.  It is sort of like the 10 and 11  of the Aril and Regalia types.  The extra one in the 11 is not important.  It is a supernumary chromosome, and it doesn't affect the pairing in negative ways.  It is less like the case of the 8 and 12 where the differences have become so great that they just won't pair up properly.  The extra four aren't just added to the same basic 8, but rather there are great differences in all or most of the chromosomes to the point that things just won't sort out properly.

 

Not all Beardless Iris can be crossed with all others to produce fertile offspring, and some probably cannot be crossed at all.  Generally the more related, the more likely the offspring will be fertile. 

 

I might mention that apparently the Crested Iris (or at least a number of them) are closely related to the Bearded Iris also, at least to the degree that they can be hybridized.  Again, the secret to making these fertile would be to produce amphidiploids or amphipolyploids.  It actually might be quite possible to mix and match traits of Crested and Bearded Iris.  Can you think of a better way to get cold and/or damp tolerance into an Aril Iris?  And, can you imagine the bizzar flowers that might be possible?  Whole new classes could be invented.

 

Well, now I'm dreaming.