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Gardenfiend
 Supporting Member
My Weather
My Garden
| | Posted on Friday, March 07, 2008 - 03:42 pm EST : |   
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Bees are all over H. ericsmithii! So what exactly is the cause of their sterility?
Gardenfiend
- Germany,
Zone "7a"
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Matthias
Supporting Member
| | Posted on Saturday, March 08, 2008 - 10:07 am EST : |
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It is the same sterility you find in animal- or plant hybrids resulting from natural or, more frequently, artificial crosses between to genetically distant, hence different, species. Their genetic makeup has through different evolutionary pathways become incompatible. In animals, especially birds, genetic, or at least behavioral incompatibility is considered a precondition for speciation (animals actively chose their mating partners  , while plants are passive and are pollinated  ).
Ericsmithii is a hybrid between the very distant groups H. niger and H x sternii which each belong to a different section. It's like horse and donky resulting in a mostly sterile mule. Some hybrids can in some cases be partly fertile.
Mike ... ???
Matthias
- South Germany,
Zone "7"
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Gardenfiend
Supporting Member
My Weather
My Garden
| | Posted on Saturday, March 08, 2008 - 11:02 am EST : |
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Unless it involves genetics on a level a layman (-woman) couldn't follow, is it possible to explain why a viable plant or animal cannot reproduce? In this case the bees prove it must be producing pollen.
Gardenfiend
- Germany,
Zone "7a"
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Addict
| | Posted on Sunday, March 09, 2008 - 11:09 am EST : |
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At a simple level it may be because the number of chromosomes is different for each parent, thus when the offspring undergoes meiosis to produce gametes there will not be evenly matched pairs of chromosomes.. Thus when gametes recombine during fertilisation some chromosomes will be single stranded and therefore many proteins will be uncoded for thus the fertilisation fails.
Production of pollen does not mean that the pollen is fertile (same with sperm actually)
addict Staffordshire zone 8(just) UK |
Kaguyahime
My Favorite Photo
| | Posted on Sunday, March 09, 2008 - 03:04 pm EST : |
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this is a interesting topic, just learned some things i didnt know thanks! Mike and Matthias
Kaguyahime
- Vlaanderen,
Zone "8"
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Matthias
Supporting Member
| | Posted on Monday, March 10, 2008 - 05:06 am EST : |
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Hybrid sterility as a result of crosses between species with different chromosome counts is for example frequent in breeding progammes involving intersectional crosses, e.g. in irises. Mara, my friend Tomas Tamberg in Berlin has produced many beautiful iris hybrids from this kind of intersectional crosses where different chromosome numbers are involved. On the diploid level (the normal double set of chromosomes per cell), the hybrids are sterile as a result of processes Mike has described: The hybrid itself is viable but cannot produce fertile pollen or eggs. Interestingly (and luckily) plant cells can naturally or artificially be converted to the tetraploid level (a doube-double set of chromosomes (4 sets)) which will restore fertility in these tetraploid plants. Tetraploid plants are normally bigger, stronger and showier for those gardeners who like that (I don't need it) but the big advantage for breeders is that breeding lines can be initiated between otherwise incompatible species. An example is Iris sibirica x Iris californica (Calsibe) which have different chromosome counts, but whose offspring is fertile on the tetraploid level.
Now, hellebores all have the same number of chromosomes (2n=32), as far as I am informed (I have not counted them) and a difference in chromosome counts cannot account for hybrid sterility in hellebores. My understanding is that if different groups of organisms have long been separated they have become not fundamentally but significantly genetically different (see flower colours, leaf forms etc) and these genetic differences create problems in the course of gamete (sperm- or egg cell) production. The two different parental sets of genes and gene combinations are incompatible in the process of gamete production. This is a result of evolution and this is, through reproductive isolation, what "makes" species and guaranties their longtime integrity.
I hope this was not too technical ... :-)
Matthias
- South Germany,
Zone "7"
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Gardenfiend
Supporting Member
My Weather
My Garden
| | Posted on Monday, March 10, 2008 - 05:42 am EST : |
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Thank you Mike and Matthias. I do remember some of this from school days (won't say how long ago that was!). It is hard to grasp why, when two distinct and distant species, that you would not expect to be genetically compatible, have been successfully crossed, the resultant organism should be genetically incompatible with itself. But I think I follow Mike's explanation.
Matthias, I have seen many of Tamberg's interesting Iris hybrids. I am with you on preferring diploid flowers (even in the case of Iris barbata). For many years I had a great I. sibirica from Tamberg (named after a common acquaintance) that I loved for being close to the species, only better. Enormously floriferous! RIP: voles devoured it completely this winter 
Sorry to veer off topic...
Gardenfiend
- Germany,
Zone "7a"
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Matthias
Supporting Member
| | Posted on Tuesday, March 11, 2008 - 04:52 am EST : |
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... still OT, Mara, in my former garden voles used to selectively eat ALL my sibiricas in mixed beds, so I simply had to give up on these beautiful plants. If you want to attract voles plant sibiricas :-)
Back to topic:
For sterility in intersectional F1 hellebore hybrids the different-chromosome-number-explanation is not good enough if it is true that all hellebores have 2n=32. In the case of H. niger x (H. argutifolius x H. lividus) one the three partners has been reproductively separated from the other two for several million years which means that they have become genetically so distant that hybrid sterility occurs.
Matthias
- South Germany,
Zone "7"
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Addict
| | Posted on Tuesday, March 11, 2008 - 07:58 am EST : |
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Matthias in the Burrell Knot Tyler book it states
"Peter Brandham of Royal Botanical Gardens Kew told us that all hellebores are tetraploid with 32 chromosomes and 4 copies of every gene."
This suggests that 4n =32.
If chromosomes are of different length and have different an mounts of DNA then this could account for inter species hybrid sterility. It would be possible for the larger chromosome to have the same "genes" as the smaller and thus allow a viable F1 but not F2.Tetraploidy certainly allows for greater possibilities as only 2 of the 4 alleles need to be "compatible"
It seems that niger is the link as other species produce fertile hybrids or not all.
I don't think niger became isolated millions of years ago did it? Thibetanus appears to have about 20 million years ago but the others seem far more recent.
Of course lividus argut. hybrids are fertile.
It is niger that is the quandry.
addict Staffordshire zone 8(just) UK |
Matthias
Supporting Member
| | Posted on Tuesday, March 11, 2008 - 11:07 am EST : |
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Let's assume for a moment that this is true (I have, as I said, not counted chromosomes or sets of genes): then this would indeed mean n=8 and 4n=32.
Yes, I absolutely agree.
Yes, this is a universal observation.
According to a phylogenetic study done at Kew and published in TAXON 50, 2001, the two members of the section Chenopus, i.e. argutifolius and lividus are the oldest living members of the genus Helleborus. The phylogenetic tree shows the following order of development:
(argutifolius - lividus) big gap (vesicarius) gap (foetidus - niger) big gap (thibetanus) gap (orientalis and all other Helleborastrum). I have big doubts about this (foetidus-niger) cluster.
The indicated branch lenghts in the tree show that niger actually is in the middle between the living "fossils" arg-liv and Helleborastrum or in other words the genetic distances from the oldest and youngest living members of the genus are almost equally long. This could account for its capacity to cross with either of the geohistoric (evolutionary) extremes. I don't say that niger became isolated millions of years ago, but I say that niger and arg-liv became isolated from each other millions of years ago. Just look at the distribution patterns.
Matthias
- South Germany,
Zone "7"
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