Chromosome counts in irises vary a lot. Ignoring a few triploids and
odd-numbered species, the genus includes at least 2n= 16, 18, 20, 22,
24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 48, 50, 54, 70, 72, 84,
86, and 108.
Just among the series Laevigatae, you've got ensata = 24, laevigata =
32, pseudacorus = 34, virginica = 70 or 72 and versicolor = 108.
Versicolor is explained as a natural tetraploid hybrid between
virginica v. shrevei and hookeri (2n= 38, series Tripetalae), since 38
+ 70 = 108. I see in the SIGNA database that the backcross of
versicolor to virginica (Ãrobusta) is healthier and only fertile when
virginica is 2n=72, despite the ancestry mentioned above.
Apparently 2n=90 is somehow healthier than 2n=89, with one chromosome
of virginica pairing with one of versicolor's hookeri chromosomes?
Just from the numbers I would suppose that virginica is itself a
tetraploid derived from a common ancestor with the Eurasian species.
Given all the unusual crosses out there it seems like anything is
worth attempting, especially if you don't care about sterility.
Sean Z
Quoting David Ferguson <m*@msn.com>:
>
> 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
> Fergusonin New Mexico
> To: i*@yahoogroups.com
> From: e*@mymts.net
> Date: Sat, 1 Jan 2011 14:17:35 -0600
> Subject: [iris-species] Chromosome Count
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> Can someone please give me a simple explanation on
> hybridizing, using the chromosome count.
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> 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.
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> Thank you!
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> El
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