About # of K, species and subspecies

[Croquin]

Here is some of the "saffron crocuses" with corresponding amount of chromosomes such as listed in the literature.

o   Crocus pallasii subsp. pallasii, 2n=14
o   Crocus pallasii subsp. dispataceus, 2n=14
o   Crocus pallasii subsp. haussknechtii, 2n=16
o   Crocus pallasii subsp. turcicus, 2n=12
o   Crocus sativus, 2n=24

Questions:

1) How can all subspecies of C. pallasii not have the same amount of K (2n=12, 14, 16) and still be classified as the same species?  
2) C. sativus is triploid, therefore built on 3 sets of 8K instead of 2 sets of 12K: why writing 2n=24 and not 3n=24?

Thanks for any inputs  

[Janis Ruksans]

I only can recommend you to read some basic genetic and botanic books...
Janis

[Croquin]

This is why I come here seeking your light, by asking - obviously - the wrong questions.
Please Sir, give me very simple answers that I my brain can grasp
These are after all 2 very simple questions that anyone having read basic genetic and botanic books can answer.



Edit by moderator to remove personal remarks.

[Pascal B]

I don't think Janis his answer was meant in any patronizing way, it is just that your questions, although short and simple, don't have simple answers. To illiustrate this by answering your first question a bit, even at species level chromosome counts can differ so at subspecies level they will sometimes differ too. You also have to remember that species and subspecies are usually based on morphology (the "way they look") and distribution ranges and whether that morphological assesment is correct has to be investigated on a molecular level. No doubt a proper phylogenetic reconstruction will question the status of some of the species or subspecies and I am sure some incorrect lumping and splitting has happened in Crocus (or Colchicum for that matter) but it is known that the phylogenetic signal of chromosome count is poor. Multiplication and gene loss are driving forces behind evolution and speciation and an incredibly complex process.

Your second question is really basic genetics and genetic notation and like Janis says, a book on genetics will teach you a lot more than any of us on the forum can do explaining about base count, origin of triploids etc.. In short, a triploid is the result of a cross beween a diploid (2 n, having 2 identical chromosomes) and a tetraploid (4 n, having 4 identical chromosomes). During the formation of the gametes the number of chromosomes is halved in each parent and during fertilization the 2 gametes are combined and yes, the proper notation for a triploid is 3n=...
However it is not a question of 3 x 8 or 2 x 12 making up the total number, it depends on the chromosome base count of the species so it most likely is 3 x 8. But to show you why your question has no simple answers. In evolution there are several processes that will reduce or increase the number of chromosomes that not necessary affect the way plants look therefore explaining the fact that some species occur as diploid and tetraploid of which the tetraploid no longer can be divided with the same amount to come to base number.

The thing you have to understand is that chromosome counts don't say much about relationships between species or between subspecies. There were so many factors involved in time and evolution for a species or subspecies to arrive at a specific chromosome count that you really need to read into it to at least understand these factors. And what you are trying to do is correlate the chromosome counts to the various subspecies. Which is a nogo because as I said, the phylogenetic signal for chromosome count is weak, or in layman terms, it doesn't say much about the relationships at a subspecies, species or even sectional level, too many variables that made up the chromosome count end result.

And to give you a question in return, try to figure out which tetraploid was one of the parents of the triploid Crocus sativus ..... It might not even exist anymore or is a tetraploid of a usually diploid species that still exists. In saying that, to my knowledge a species can not have 2 different values for the base count so the subspecies of pallasii look a bit odd to me. But I don't know enough about that to be sure, my genetic knowledge comes from studying biochemistry and is basic.

[Gerry Webster]

While I think that Pascal’s account is basically correct there is a relevant question which he does not address, namely, ‘what is a species?' There is no general agrrrement on the answer to this.

‘Traditional’ taxonomies, in principle, are constructed primarily on the basis of morphology using  shared characters which are deemed to be important or essential. It employs, basically, a traditional logic of classes (compare the class of all red  objects) - so, a species is a class, a genus is a more inclusive class & so on  In the case of crocus, chromosome number seems not to be regarded as an essential character - at least by Mathew. There are many problems with this view. Especially, given the variability of organisms, which characters are to be regarded as essential. This is one reason for the proliferation of sub-specific classes.

For those who believe that evolution theory is the central theory in biology (not all biologists),  a taxonomy is a picture of evolutionary (phylogenetic) relations; as the late Helen Spurway put it - “to claim that two species belong to the same genus is to claim that they have a common ancestor”.  A species is no longer a class but a population of individuals which, in principle, need not share any morphological (or other) characteristics. All that is required is that they possess a common ancestor, which is difficult or imposible to determine directly. From this perspective, as Pascal suggests, it is thought by some that traditional taxonomies might be ‘corrected’ by molecular (DNA) investigations. One problem with this is that, as classical genetics demonstrated, there is no simple relation between genetic composition (genotype) & morphology (phenotype)

I hope this  brief & necessarily inadequate account will indicate that these questions are far too complex to be properly addressed on an internet forum - especially one whose primary interest is horticultural.

Those who want to investigate some aspects of these problems examined from as non-Darwinian perspective might look at :
Brian Goodwin, How the Leopard Changed its Spots, Weidenfeld & Nicolson, London
(1994).
Although this is a ‘popular’ book, it does presuppose some knowledge of biology.

[Pascal B]

Gerry, I decided not to point that out because the question "what is a species" is an even simpler question with an even more complex answer and goes back to the basics of classification. But for those interested in that question, there is an article by Mayden called "The Hierarchy of Species Concepts" in which an overview is given of all the different concepts of what "a species" might constitute, something many forum members take for granted or never question themselves. If interested, send me a pm and I can send it as .doc. In it he lists 22 different concepts..... That plethora of concepts is also the reason why various taxonomists look at plants differently and part of the reason why there are "splitters" and  "lumpers". Not to mention the botanists that don't work with any concept and nickpick at specific characters in one plant and completely ignore it in a related one without having any general approach to a genus or taxonomy for that matter.

Just to make it clear, I do not believe molecular analysis as it stands is the definitive answer for classification for several reasons I will not mention now and I regard it as a powerful additional diagnostic tool for refinement of classifications. If lucky and the markers are correctly chosen it IS sometimes possible to correlate morphological characters to clades (= related plantgroups sampled) that have come out of a phylogenetic reconstruction (see the reinstatement of the genus Sauromatum by Renner et al.) but it also depends on the characters used. I highly doubt "stamens slightly excerted" "at level with" etc. like I often read with Crocus would still stand once a thorough molecular analysis is done.

[Maggi Young]

An interesting post on the same themes from a Cactus thread inthe NARGS Forum:

http://nargs.org/smf/index.php?topic=636.msg12735#msg12735

[David Nicholson]

In a way I'm glad I don't have the intellect to understand most of this (whilst I have every admiration for those who do understand it) I just grow 'em because they look nice and make me feel good.

[Janis Ruksans]

Pascal & Gerry,
Many thanks. My English isn't so good for explaining all this so clear as you did. In Latvian - no problems, of course, although I learned genetics 40 years ago...
Janis

[shelagh]

David I'm with you on this one I'm too old now to take in (and retain) botancial info.

[Croquin]

Thanks to all for trying to bring answers to these 2 questions.
The questions are very simple, but I have not expressed what's behind clearly enough and I will explain again with more details.

1)
many concepts of species do exist because the set of living beings will always come with an exception invalidating a given concept.
the first question, before relating to this level of debate, has to do with the mechanics of chromosome pairing during reproduction.

1a)
if one subspecies has reproductive cells with 8K (like Crocus pallasii subsp. haussknechtii) and another subspecies has reproductive cells with 7K (like Crocus pallasii subsp. pallasii or Crocus pallasii subsp. dispataceus), it seems that there will be a mismatch preventing fecundation or ending in a non viable organism.
therefore, the crossing of 2 subspecies with such K differences should be sterile, i.e. no possible interbreeding between some of the C. pallasii subspecies.
Is that the case ?

1b)
if it is the case, then we can move forward discussing the next level in the question, which is the notion of species: one generally good definition of what a species is (because it is basic and simple and works on a large scale), derives from the possibility of individuals to mate and reproduce (biological species concept).
if this is acceptable and assumptions in 1a) are correct, then

1c)
how to explain straightforwardly and simply that all those C. pallasii are variants of the same species according to taxonomists ?
Please note that the question is for crocus experts involved in taxonomy of this particular case of crocuses, and answers will hardly be found in basic textbooks of genetics (or give references).
The discussion can be fuelled with the article "A phylogeny of the genus Crocus (Iridaceae) based on sequence data from five plastid regions", TAXON 57 (2) • May 2008: 487–499
The phylum of the crocuses genus based on these phylogenetic analyses shows in Fig1 that C. pallasii subspecies are not all placed under a sublevel of what seems to be the level for species (except turcicus et haussknechtii), but maybe I'm reading the graph incorrectly (C. cartwrightianus and C. sativus would be subspecies ? all subspecies of C. pallasii but turcicus and haussknechtii would be species ?)

2)
2n=24 is a question regarding nomenclature, and more specifically related to the meaning of n in the equation.
It has nothing to do with the meiotic/mitotic mechanisms or the basics about what a di/tri/tetraploid organism is, which I'm already aware of.
I had initially supposed that n was the amount of homolog K in an equation like 2n=24 before realizing with the case of C. sativus that it could not be that.
for a diploid being, it works well, we get n=24/2=12 K as the amount of homolog chromosomes.
for a triploid being, however, writing 2n=24 means that the amount of K can't be n because we would still get 12, but it should be 8.
if n is not the amount of K what amount is n representing ? where is the amount of homolog K represented in this equation ?
n can't even represent the amount of K in haploid sexual cells produced by a triploid organism: with 2n=24, this number will vary randomly between 8 and 15.

Thanks again.

[Gerry Webster]

1b)
if it is the case, then we can move forward discussing the next level in the question, which is the notion of species: one generally good definition of what a species is (because it is basic and simple and works on a large scale), derives from the possibility of individuals to mate and reproduce (biological species concept).
if this is acceptable and assumptions in 1a) are correct, then

Why is this a "good definition"? In relation to taxonomy, why should we accept the biological species concept rather than the morphological species concept since both are fraught with major problems. Traditional taxonomy assigns individual to taxa on the basis of comparison with type specimens which are frequently dead, hence incapable of mating or reproduction.  

I have great difficulty in understanding your problems. With regard to your question 1c: I think you may be misinterpreting the cladogram which simply serves to demonstrate the purported phylogenetic relationships  between known taxa. May I suggest you read the discussion on p491.

[Pascal B]

The biological species concept fails the moment intergeneric hybrids are considered like the cross between a member of the orchid genus Ryncholaelia and a member of the orchid genus Cattleya (= Rhyncholealiocattleya)....

Every species concept has it flaws which underlines the fact that a "species" is a human invention which nature cares little about. Nature messes up every definition for a species we can think of.......

No matter which concept is used, 1b) always is an assumption rather than an general agreement and very much dependent on the view an individual has with respect to a species. I maintain that your questions are not simple, they only are if you assume certain things and try to reason to a conclusion.

The comments in 2) are confusing to me because you are saying "2n=24 is a question regarding nomenclature". Nomenclature deals with the principles and procedures related to names, it has very little to do with the 2n, 3n or 4n notation, taxonomy often uses chromosome counts but the notation does not originate from taxonomy or nomenclature, it originates from genetics. Most likely you are referring to this article:

http://www.globalsciencebooks.info/JournalsSup/images/Sample/FPSB_4%28SI2%291-14o.pdf

In table 2 you will see several times the notation "2n=3X=24; X=8" occurs. The 2n notation in Crocus sativus probably comes from the view it might be an autotriploid instead of an allotriploid?. With an autotriploid all 3 chromosome sets come from the same parent species and I think another notation is "(2n + n)=..." for an autotriploid but I am not sure.... If the origin is a cross between a diploid and a tetraploid species following the rules the correct notation should always be 3n=24.

In this case the confusion arises from the choice the authors made in terms of notation based on the fact they most likely think it is an autotriploid rather than an allotriploid. Irrespective of the kind of triploid, the chromosome base number is the total chromosome count divided by 3 for ANY triploid. Which would result in a base number of 8 for sativus equal to cartwrightianus and thomasii.

Regarding 1c), a cladogram shows how close taxa genetically seem based on the base pairs they have examined, it should not be interpreted as a identification key. Like Gerry says, the discussion is where all the data comes together and if possible, conclusions are drawn.

[Croquin]

Thanks for all clarifications.

Concerning q1, please let me know about c. pallasii subspecies' hybridisation patterns -measured for instance by produced seed rates, I think that it will help answering cascaded questions. I'm not willing to engage confrontational religious debates concerning your belief systems on crocus taxonomy although I profusely thank you for any methodlogical information on how classification is achieved. I have found something hard to understand because of inconsistencies and I'm trying to make my way to knowledge. In this perspective I appreciate your kind efforts to unravel reality.

Concerning q2, nomenclature may not be the proper lexical choice, this is where I reach my limits with your language I'm sorry. However, you got the picture this time and kindly provided an answer. 2n+n or 3n makes perfect sense. Considering that we don't know about the auto/allo nature of C. sativus triploidy (although auto from C. cartwrightianus is more likely based on the natural geographical distribution of potential ancestors: they need to grow in the same area) and that mathematically 2n+n = (2+1)n = 3n, I will not feel wrong writting 3n=24.

Do you think that C. sativus could have been engineered by man through artificial breeding (which would make allotriploidy possible) ?

Someone mentioned earlier in this thread that crossing diploid with tetraploid crocuses creates a triploid one.
But a tetraploid crocus would have had bigger stigmas than a triploid, why would it have been more interesting to create a triploid ?
Wouldn't the saffron producer have rather selected the tetraploid plant ?

I read in one of Grilli Caiola's papers that C. sativus was probably resulting from the fusion of a non reduced (16 K) and a reduced (8K) gametes, which seems to be a possible natural genetic accident ?

[Martin Baxendale]

Triploids tend to be very vigorous, growing more strongly and increasing faster. In the bulb world, they are often selected out for those properties. It's more likely that a triploid crocus with strong commercial value would have been spotted and selected as a chance seedling rather than having been deliberately bred, especially if we're talking way back in history when nothing much was known about genetics, including the ploidy level of plants. A Tetraploid C. sativus would not necessarily have been bigger or have had bigger stigmas. The triploid offspring would quite possibly have been bigger and stronger than the tetraploid. I can't explain exactly why this is often the case, but it seems it is.