RE: Chromosome Count

 

Here is a very marginal answer.  Somebody else here can probably get more specific.

Even if the chromosomes are different in number, if the species are closely related enough, they may hybridize with little difficulty.  Usually the species with the larger number has the same basic chromosomes, little different from the one with fewer, it just has more of them.  Sometimes the ones with more are simple polyploids, having multiples of the exact same chromosomes.  Sometimes there are supernumerary chromosomes, which are usually extra copies of one or more of individual chromosomes, or which may represent a chromosome that split into two.  Sometimes most or all of two chromosomes might fuse together into one, to make a lesser total number.  Sometimes the numbers are shifted due to crossing of species that have different base numbers that become combined into a new larger full set (these may include duplicates, but they behave as if different).  Regardless of why the numbers differ, if the chromosomes match up well enough, a plant can result from crossing two related species, regardless of chromosome numbers.  Sometimes though the chromosomes are different enough, or enough errors occur in pairings, that some or all attempts at a given cross will fail.  Sometimes supernumeraries will join up with like ones to make three-somes, sometimes the extras just hang around and don't pair up.  I don't know all the details of what happens with Iris, but they should follow the same rules as most any other plants.

The next part of the question is "are the hybrids fertile".  If there are enough discrepancies in the chromosome pairings, or if there are other incompatibilities, they may be unable to produce offspring, and end up being basically genetic dead ends; or, at least the successes will be few and far between.  Sometimes if you can double the hybrid's number (if you can get unreduced gametes), you can restore fertility with others that have like sets of chromosomes (or when the hybrids are self-pollinated).

Sometimes in higher ploidies, there are so many copies of the same chromosome, that the numbers start to become almost irrelevant, as long as some full sets are represented in gametes, and can pair up when fertilized.  (In some groups of plants having just full sets seems to be more important to fertility than it is in other groups of plants).

It can get pretty complicated when all the ramifications and combinations are considered, but maybe this is enough to help a bit.

I need to learn more about chromosomes specifically as they occur in Beardless Iris before I could elaborate more.  I'm not even sure what the base haploid number is for "water Irises", nor whether some of them have fused chromosomes or supernumeraries in their compliments.  It appears that the ones you listed have very unrelated numbers, but when broken down more, they probably would make more sense.

Dave Ferguson
in New Mexico

To: iris-species@yahoogroups.com
From: eleanore@mymts.net
Date: Sat, 1 Jan 2011 14:17:35 -0600
Subject: [iris-species] Chromosome Count

 

Can someone please give me a simple explanation on hybridizing, using the chromosome count.
 
For example, I was just checking I. versicolor on SIGNA, where it mentions the chromosome count is 2n=108.  It easily crosses with the laevigatae series, which has a chromosome count of 2n=32, while I. virginica has 2n=70 or 2n=72, as does I. virginica var shrevei.
 
Thank you!
 
El

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