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Why are they different genera if they can interbreed?

This is a discussion on Why are they different genera if they can interbreed? within the General Orchid Culture forums, part of the Orchid Culture category; Well genetics is getting more involved daily. The use of DNA is proving some points ...

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  1. #11
    Germinatorman's Avatar
    Germinatorman is offline Senior Member
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    Well genetics is getting more involved daily. The use of DNA is proving some points and making some surprises at the same time. But even DNA is not without its issues...............
    Don

  2. #12
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    I found this article about orchid genetics and DNA sequencing. I can't claim to understand it all. DNA Sequences and Orchid Classification | Selby Gardens As I understand it, some orchids can look very similar and not be closely related, while others can look quite different but be close relatives - so the DNA will help sort that out.

    It looks like there will be more changes to come!

  3. #13
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    Halloamey is offline Senior Member
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    Quote Originally Posted by BearWithMe View Post

    There are no set criteria for defining and distinguishng genera. I thought it was genetic!
    Ok lets address the questions one by one. What you have to consider is that all this classification etc is man-made purely for our convenience and as there are always there are exceptions to every rule. Now all the major classifications done were done in the 1700s when there was no concept of genes, genetics inheritance etc. let alone DNA and genetic code . It went by the simple rule of differentiation and classification based on what could be seen i.e. morphotype or Phenotype. There was a race to find new species and name them, so no one cared for the details, you saw something different, BAM you named it after its morphology or yourself, your loved ones and especially the rich guys who were funding your trips LOL. Plants offcource were classified using flower morphology because vegetative growth is similar with not much to differentiate. There was no way of verifying the identities except for illustrations which could not capture minor details or exaggerated minor differences like color variation etc. So in that age a species was a 1) Morphospecies. Then with development of science and knowledge of genetics came the concepts of Isolation species and reproductive species. 2) Isolation species are basically same species which got isolated from one another due to geographical reasons like islands, seas or continental drifts. Isolation species are almost identical can interbreed but have minor differences in the morphology owing to the differences in their habitat, a good example would be the giant tortoises of the Galapagos islands, many a times isolation species were dubbed as sub-species. Then with crossing experiments and compatibility studies came the theory of 3) Reproductive species, it essentially meant that animals or plants that can reproduce with one another to produce fertile offspring to be considered as a species, this solved some problems but raised many more. For eg. some animals can breed in captivity under artificial conditions to produce fertile offsprings, but these populations never breed together in the wild, like for eg. some insect pollinated orchids. Most species have specific insect pollinators so that these pollinators ensure that the species do not hybridize, but enter man and we can pollinate flowers with artificial techniques which could have been impossible for the insects. Then came many more definitions etc. but now lets jump to the final, most advanced and the ultimate definition which will classify all the species in the future, 4) Genetic species. With the knowledge of gene and DNA, the genetic code of the nucleotides we have a far better understanding of all living things. The sequencing technology has proved to be a boon to the scientific community. The genetic species is defined as two organisms that differ by atleast 2.75% in their genetic composition (Herbert et al. 2004) But these percentages are variable and different for different organism groups. The main problem with this technology at the moment is the huge size of genomes and the high costs involved in sequencing the entire genome. So scientists came up with an idea of just sequencing small portions of the genome (some candidate genes) which are common in all living things and then compare their sequence and its homology. The more similar the sequence the closer are the two organisms related and vice-versa, this was defined as DNA bar coding. MAny candidate genes called as 'markers' have been analysed. For eg. in plants two prominent markers used are 1) Nuclear ribosomal DNA nrDNA and 2) Chloroplast DNA cpDNA. Ribosomes are organelles in all the cells that manufacture all the proteins, enzymes cell components etc so are highly conserved. Similarly all plants have chloroplasts so by comparing genes from the chloroplast people can induce relationships. Many new markers are being found and tested. The only problem with this technology is that with the comparison of the markers we can get a skewed result, i.e for eg. you may find very similar sequences for one marker so you conclude them to be the same species, but for another set of markers the sequences are not so similar so you cannot conclude. This is what happened recently with the phylogeny of water lilies. Using DNA barcoding for a set of chloroplast genes a group of scientists published that the Australian water lilies are a separate group. Then another group challenged them with another set of results for different markers proving contrary, then finally the original group again published a paper which should that their original theory was right and that the markers used by the other group were flawed. But during all this time the Australian waterlilies kept jumping from the subgenus Anecphya to Brachyceras back and forth LOL.
    So the final word would be genetic species determined using many different DNA markers or better whole genomes, and that is definitely going to be possible in the future. At the moment 17 complete plant genomes are published and another 7 are in the process. Genome of Phalaenopsis Equestris will soon be sequenced, it will be the first orchid to be sequenced.
    It will create a lot of problems for us though, there will be many changes LOL !

  4. #14
    PaphMadMan is offline Senior Member
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    Quote Originally Posted by BearWithMe View Post
    Thanks for the comments!

    There are no set criteria for defining and distinguishng genera

    I guess that is why I've been confused. I thought it was genetic!
    For 170 years the primary distinguishing characteristic between Cattleya and Laelia was considered to be the number of pollinia (4 = Cattleya, 8 = Laelia), at the insistence of one influencial botanist. Most of his contemporaries believed that most of those Laelia really belonged with Cattleya. It turns out they were almost certainly correct based on recent genetic studies, and 8 vs. 4 pollinia is a meaningless distinction at the genus level for these orchids, but is useful at the subgenus level. Or you could choose to elevate each subgenus to a new genus. For hundreds of years classification depended on that choice of one or a few physical characteristics to distinguish a genus, and if someone chose wrong... well, it was the best they could do at the time. Genetics will eventually sort out most of the fine distinctions between species and genera, but there is a lot of resistance among both classically trained taxonomists and orchid hobbyists. It will be a very long time before some people accept the recent merger of most Laelia into Cattleya.

  5. #15
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    Thank you Amy for a very careful and detailed description of Orchid taxonomy! It's really great to learn about this issue!
    Also than you KIrk for your addition about the Cattleya / Laelia issue. That was also new to me and now I know!

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