Re: Re: Iris abicans - chromosomes

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I do not have the various papers on chromosomes in front of me; must have left them at work.  I just have Randolph's comments here.  However, my recollection is that Simonet? and definitely Mitra came to the conclusion that the parentage of the various 44 chromosome "species" was derived from 40 chromosome species crossed with tall bearded species.  This was based on karyotype analyses in the case of Mitra, who attempted to identify the individual chromosomes, and compared them between various species. 

 

The number of bivalents and univalents does not provide reliable identification of the chromosomes, it can help as a tool, but it is useful primarily for identifying only analogous chromosomes through pairing as bivalents.  Bivalents should be made up of analogous chromosomes, but they cannot identify with certainty which parent each chromosome came from.  Potentially analogous univalents cannot be identified with certainty, only guessed at through comparisons.  Which chromosomes came from which parent are best identified, with a degree of uncertainty, by comparison with chromosomes from suspected parents.  However, when the chromosomes match closely one for one with those of suspected parents in the proper proportions, it can be reasoned with some degree of certainty which species the parents are.

 

Now I'll go with the idea that 44 chromosome plants such as I. germanica and albicans derived from a 40 chromosome species crossed with a TB (probably tetraploid).

 

If you have a 40 chromosome [probably dwarf] species contributing 8 + 12 [20] combined with the contribution from a 48 chromosome [probably TB] tetraploid (or unreduced gamete from a diploid) species of 12 + 12 [24], it may seem logical that all three sets of 12 would contain analogous chromosomes, but it isn't necessarily so.  It would seem that you would get 12 pairs and 20 univalents, but this is not necessarily so either.  Some of the odd sets will be analogous and will pair, some most likely will not.  Often some will even form multivalents.

 

Beyond this, the 12 in the 40 chromosome species may have a different origin from those in the 48 (or unreduced 24) species.  Even if the origin is the same, they may well have undergone changes in both species since the time their origins diverged.  Therefore some of the 12 from the 40 chromosome species may no longer be able to pair with those from the TB, but some will.  It is also possible that all are analogous.  So, some of the odd set of 12 (potentially all) will likely pair with some of the two sets of 12 from the other parent, leaving some of those as univalents or to pair with the some of the set of 8. 

 

Also, if the TB is a tetraploid, and has been such for long enough, or is of hybrid origin also, the two sets of 12 from it may not even all be analogous to each other!  It may actually be functionally diploid.

 

This confusing mess is part of why hybrids with mismatched chromosomes are often of reduced fertility.  Things tend to get all messed up when they are trying to produce gametes.

 

There are other things that may occur as well, lots of them.

 

So, bottom line is that the number of bivalents vs. univalents most likely does not indicate how many sets of how many chromosomes are really represented.

 

Though it is not relevant to this example, I should add that even in pure species individuals, chromosome pairing sometimes goes off kilter, and you can get fragments, univalents, and multivalents; so, you can get some drift in chromosome numbers from the norm in some individuals, and even in whole populations.  This is part of why numbers can be different between related species

 

Now if we went to the supposition that in 'Albicans' we have 8 + 8 + 8 + 8 + 12, that would seem less likely to result from any single hybridization event.  However, an unreduced gamete from I. pumila with a gamete from a tetraploid TB or an unreduced gamete from a diploid TB would give the correct number.  However, the karyotype analyses did not come up with this result, and I would personally think that the resultant plant would look a lot more like I. pumila and a lot less like a TB than does 'Albicans'.

 

Also, the older [mostly] IB's with 44 chromosomes which are derived from 40 chromosome parents crossed with TB's look very much like I. germanica, and often even more like 'Albicans', and this seems to add morphological evidence by comparison for the 20 (from 40) plus 24 (from 24 or 48) theory.

 

I can't help wondering if anybody knows of a documented 44 chromosome plant that is derived from I. pumila crossed with a 48 chromosome tetraploid?  I know a lot were recorded as such, but they were later pretty much all verified to have the "pumila" parent (really I. lutescens) misidentified.

 

There are lots of 40 chromosome I. pumila X I. tetraploid TB, the more expected result (mostly SDB in size).

 

It's interesting to do all the number crunching and see what combinations can give what results.

 

Dave

 

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