Discussion in 'Discus' started by Len, Mar 3, 2007.
Have you seen this?
http://www.practicalfishkeeping.co.uk/p ... ?news=1145
Anyone come across them?
This announcement is nothing to get excited about, as this description is for a fish that is as old as the hills. Red-spotted green discus have now been given this name.
I am work wise heavily involved in the taxonomy field, which is the naming of plants and animals, and the notice that you have come across is the result of a more extensive DNA study in which the authors have decided that these fish are sufficiently distinct to warrant that they call them a different species. However, this does not mean they have discovered a new type of fish, they have just decided to recognize these fish as a separate species.
The name Symphysodon tarzoo was given to a discus in the late 1950's but then the taxonomists decided that the species that it was supposed to describe was not sufficiently different to justify this name and they then went back to an older name to describe it. The name was then "sunk into synonomy" as the taxonomists say. Now it has been resurrected as a valid name.
So, nothing new on the discus scene unfortunately, this the same fish as before.
Genetically, how different do fish have to be to be classified as a separate species? There must be certain markers that have to be different in order for them to be classified differently.
The question that you have asked me, is one that is often hotly debated by biologists. How different must the fishes be to qualify as different species? What is used commonly in discus taxonomy (taxonomy is the science of given something a scientific name) is a count of fin rays and scales. However, for all discus species known, Symphysodon discus (the Heckel discus) and Symphysodon aequifasciatus, with its three subspecies Symphysodon aequifasciatus spp. haraldi (the Blue discus) Symphysodon equifasciatus spp. axelrodi (the brown discus) and Symphysodon aequifasciatus aequifasciatus (the green discus), there is overlap here, so clear boundaries are problematic. In terms of colour very many intermediates have been found and on the basis of colour you can also not lay down clear boundaries. In terms of vertical bars, of which all discus possess nine, Heckel discus have the first, the fifth and the ninth coloured much darker whilst in the other species only the first and the ninth bars are darker and not the middle fifth bar. All of these differences have contributed to the fact that species limits of discus have been heavily debated for over the past almost 40 years. It has also been extensively debated that there must be hybridization between these species and subspecies, which would explain the intermediates, and there is nothing to stop this as the fish can move freely from one area to another. In the aquarium, all of these species and subspecies can be crossed and give fertile offspring (which together with mutations that have occurred has resulted in the often horrific hybrids that flood the market), indicating that they are closely related.
In recent years, an analysis of DNA sequences of a number of different genes has been performed to get an idea of mutation patterns and thereby try to help in setting species boundaries. Here again it is debatable how much difference there must be before you can call them different species, as a matter of fact no rules exist and it is definitely a matter of interpretation. In addition, genes from the mitochondria and from the nucleus are analysed. As nuclear genes are inherited from both parents and mitochondria only from the mother, this can again show up different patterns. This is then exactly what these authors have found in that the newly recognized Symphysodon tarzoo has a difference in the nuclear gene but not in the mitochondrial gene that they analysed. This gene is the same as that of all the other discus. This actually indicates that this new species is of hybrid origin, which again begs the question about how valid it is to name it as a new species.
Eventually, laying down species boundaries becomes a matter of opinion and finer interpretation about which differences between researchers exist. What the general public does also not understand and know is that naming a new species does not make it the new law or rule from that point onwards. It is still a matter of whether the bulk of researchers accept this opinion. It is also the full right of any researcher not to accept a new taxonomy (the new name) and to keep to the old taxonomy. So, with regard to this name change, I am sure that there are going to be differences of opinion and it is not a foregone conclusion that this will be accepted by all.
Although a complex answer, I hope that I have made this issue clearer and not even more fuzzy.
Is it thus posible to have fertile offsprings if the parents are different spesies (in general)?
One aspect of the three currently identifable species of discus (brown&blues / greens / Heckels however you care to name them) is pointed out by Heiko Bleher in his latest book. The three species are very clearly defined territorially and accordingly by general water parameters in those areas. Their territories scarcely overlap, but in the marginal areas where they do there is evidence of hybrids.
So there is clearer distinction between them than simply scale counts, though obviously the species are very closely related to each other.
Thanks Dirk, I think the issue is still really fuzzy but not as fuzzy as before.
Len, I'm no scientist but AFAIK fertile offspring will be reproduced as long as the parents are from the same genus. Hybridization occurs when specimens from different genus' reproduce, which is generally done with the help of man.
But hybridization occurs in nature all the time without the help from man. Take lake Tanganyika Tropheus for instance, are these fish classified as separate species or are they all hybridised fish? Similar to the Discus situation, yes?
Dirk, at what point is it declared descent with mutation (Evolution) or am I getting away from myself here? Is descent with mutation strictly in the genes between known species or can hybridisation affect the 'descent' part? As you state above it seems some Discus folk will reject the newer taxonomic name based on their 'belief system', so where exactly does one draw the line? If it's peer reviewed then surely everyone should accept the 'new' scientific name as the the correct one to use?
I think the Tropheus would be classified as unidentified species.
I don't think you could call it a hybridization because the definition of the word different species while the Tropheus that change are all from the same colony.
But all in all I think I just contradicted my initial point about hybridization occurring between different genus'. So hybridization can occur between different species of the same genus but if the hybrid was from parents of different genus' then they would be sterile.
I think that hybridisaton will affect descent with mutation. You are changing the input genes so the output would be different.
So then, what is the Brichardi complex that occurs in Lake Tanganyika? Many many fish from this lake are classified as separate species and have occured through hybridisation.
Wrong See above. What *defines* a separate species?
Yes it will, but it seems to be a highly sensitive and timely process. I have copied the following quote from another website:
If I understand this article correctly I think it is saying that through LGT the different species that hybridise manage to leave some of their genetic make up mixed in with the other species, *if* the genetic make up is strong enough or compatible enough then the hybrid species is able to produce fertile young and possibly create a separate species. Descent with modification is quite complicated and I've probably got it all wrong, hopefully Dirk can shed some more light on this.
Well is it hybridisation or decent with mutation. Do they all maybe have the same common ancestor? But maybe they are all the same fish but our ways of identifying them are just wrong. hehehe
I guess you could say that hybridisation and decent with mutation are very loosely the same thing. Basically the genetic makeup is changed via a mutation or via different genes being introduced into the pool which would lead to a different genetic make up. hakidy smackidy I'm tired of typing.
Well you can cross Canis lupis with Canis rufus and have fertile offspring for many many generations but you can also cross Equus caballus with Equus asinus(I had to look those two up, Horse and a donkey respectively) and you will have sterile offspring. So to be honest I'm kind of confused.
Now, to go completely OT(please delete this if you feel it is inappropriate) When it comes to humans. Have we gone as far to say that we can be identified as different species or sub species or are we all related closely enough to be the same species. My understanding is that we fall under the latter. Now back to Lake Tang, with regards to Tropheus, could the reason that they are either called T. moorii "colour variant" or T. "colour variant" purely mean that they are the same species but need to be identified or seperated by means of a "race" with the latter not even being able to be identified to the closed speceis ie. moorii or duibosi etc? I think that they are in fact the same species of fish but have evolved in different environments which has caused them to look different.
To be honest I don't know enough about the Brichardi Complex to even comment.
Here are 3 links which may offer some insight.
Nope, no real insight at all.
This topic has jumped from discus to tangs to gene transfer and is becoming very wide and requiring far to many and longwinded answers. I will try to give a few.
I must go back to your first email on this thread Cameron and try to answer those questions in reverse order:
You wrote: "As you state above it seems some Discus folk will reject the newer taxonomic name based on their 'belief system', so where exactly does one draw the line? If it's peer reviewed then surely everyone should accept the 'new' scientific name as the the correct one to use?"
When an article such as the new discus description gets sent to the journal in which they wish to publish it, the editor in most instances sends it to two reviewers. These have to evaluate the content based on whether the methods are correctly applied, whether all the comparisons with all the other specimens have been made properly etc., and then they evaluate the scientific content. If they think it is REASONABLE they will say OK publish the paper. Acceptance of what has been written is however still by the broad scientific community out there, and if they dissapprove, they will then present scientific evidence in another publication in which they try to show that the first authors were incorrect. So, when a paper gets published it does NOT mean that it is immediately accepted or acceptable. (Incidentally I am involved in exactly this sort of issue in a plant group in South Africa at the moment, a new arrangement has been published, I and many of my colleagues are of the opinion it is wrong and will now present our evidence to prove our point.) So, we will have to see what the international feeling is going to be about the new species.....
Then the questions: "What defines a separate species?" and the comment "I think that hybridisaton will affect descent with mutation. You are changing the input genes so the output would be different."
Many, many books have been written about what is a species, Darwin started this idea and the debate has continued. My opinion on the matter is as follows: A "species" is a box that we as humans try to force biological units into. If they are clearly different, we can call then species and everyone is happy. It is when they are not so different that the "species concept" starts running into problems. We cannot then put them into a clear box as a result of which people get unhappy, they want to force them into a box! But biology is not standing still and new species are evolving all the time as a result of which it is often impossible to separate them clearly. To get back to practicalities though, evolutionary biologists must weigh up the total evidence, both based on morphology and on mutations at DNA level to assess whether two entities are sufficiently different to justify that they can be called separate species. New species can be formed by hybridisation where genetic material from two parents is mixed in a new combination to give something which is distinct from both parents. If they are separated geographically this will help to keep them distinct.
One aspect that I do want to clarify is something that I have a number of students working on, i.e. DNA mutations patterns. In plants, the nucleus, the mitochondria and the chloroplasts contain DNA which encodes for different genes. Nuclear DNA and therefore the genes it contains, is inherited from both parents, mitochondrial and chloroplast DNA is inherited via the mother only. If two plant species hybridize, the hybrid will have a mixture of both parents DNA, but only mitochondrial and chloroplast DNA from the mother. What this means is that if we analyse the DNA of a plant species and find chloroplast DNA related to that of one species and nuclear DNA of another we can deduce that it arose from hybridization. This actually happens quite often in plants, but it does occur much more rarely in animals and fishes. Hybrids in the animal kingdom are far less likely to be fertile than in the plant kingdom.
Another aspect is that the mutations in the nuclear DNA and the chloroplast DNA will not be affected by each other. Your comment Cameron: "I think that hybridisaton will affect descent with mutation. You are changing the input genes so the output would be different." In hybridization you are changing the combination of genes as a result of which the product i.e. the hybrid will be different, BUT, hybridization will not affect descent by mutation as you call it. Each of the original groups of genes will carry on mutating as they did before, BUT, there can be exceptions.
Two other things: endosymbiosis only occurred in the two instances that you mention, i.e. assimilation of bacteria into cells to form mitochondria and chloroplasts into cyanobacteria and this can only be viewed to be an extremely rare event. The frequency of lateral gene flow is also very low and is certainly not common. Both of these factors have not contributed to the speciation in Tangs and discus.
I think I must now stop as by now, most folks will have stopped reading this message in any case....
That is a very interesting post. Thanks for taking the time to post it.
Thanks Dirk, that is very insightful.
When you say that hydridisation will not affect decent with mutation, I understand that the specific genes will continue to mutate as they have before but if you take it on an organism level the end result would be different. Ignoring that it has already changed from the hybridisation. Won't the whole mutation path be changed? Or am I just ignorant?
And on the whole defining species story. We try to put them in a box but maybe they aren't supposed to be put into a box, they should be out running and swimming free. Okay, seriously now. Could I be correct in saying that organisms so closely related that they can't separated by species are merely the same species but different "races"?
I can imagine there would be some strong arguments between the 2 camps, so who or what decides on which of the 2 options is correct? Is it a majority rule situation? Over time does one theory get adopted more than the other and becomes accepted as the norm?
Which plant is it, is it indigenous? Is there a paper available on the net? I'd very much like to read it. Spill the beans
Darwin in fact did not start the idea, it was known for decades before by geologists and paleontologists that made a strong case that life had been going on our planet for a very long time, and that it had mutated over time and certain species were no longer with as and considered extinct. If Darwin never brought the theory into fruition then it would just be a matter of time before Wallace presented the exact same theory, I think Darwin beat him to it by a mere few months. Naturalists studying living animals in the early 1800's had discovered much of the evidence that Darwin himself used in his theory. Darwin was however, responsible for carrying out the research and documented that evolution does occur and made the theory acceptable for scientists and the public.
I can see with all the new technology in genetics that it's becoming increasingly difficult to place species 'in a box' as you say, as the lines are so similar it's hard to be distinct. But you have to admit that it's a 'work of art' as far as theories are concerned and it's stood it's ground for over 150 years, it's the best model we have so far. As far as i know it is even being introduced into the local high school curriculum next year, Amen. Even if you look at mathematics and physics we create 'dimensions' in order for us to be able to relate to space and to have some point of origin, but dimensions don't actually exist, they're just created by clever mathematicians in order for us to 'move around'.
If I understand correctly, if the 2 (or more) groups are separated geographically then over time (a very long time) the separated groups would have evolved so much that even if they did get back together the genetic differences would be so vast that they would not be able to create fertile offspring, in essence speciation would have occured?
Don't stop now I find it really interesting, thanks for taking time out to post it, appreciated.
Heavy stuff this, but good!
Jared, to your questions: Once hybridization has occurred the mutation patterns of the two sets of genes, say the one from the nucleus from the one parent and the other from the chloroplast of the other parent can be altered because of the fact that they are now present in this new combination, you are right there. However, because the original two species could not have been so distantly related in the first place to allow them to hybridize, the effect of creating this new combination is not normally so drastic to cause a very large change in the rate of mutation. Interestingly, in the DNA research we do find differences in the rates of mutation even within fairly closely related groups. Clearly, these rates of mutation are influenced by many factors which we as scientists do not understand yet.
You are also quite correct that once we cannot separate a species into two species, we are dealing with races or as we also refer to them as populations. Scientifically, persons studying populations within a species would be called population geneticists, persons studying the relationships between species are called systematists. What now happens is that when dealing with closely related species, you need the tools of both of these fields of research to get to the bottom of the matter. A group of colleagues and I have in recent years been analysing the relationships between two species of plants (the genus is called Streptocarpus, South African relatives of African violets) that occur in Kwa-Zulu Natal and coastal Transkei in the one species, and the other species occurs from East London further south to George and then inland from East London again to Matatiele in the Transkei interior. We had to use systematic and population genetic techniques to unravel the whole lot, and to top it all we could also detect hybridization. I have a pdf of this paper if someone may be interested, but I think it is too far off topic and it would be contravening copyright for me to attach it.
Will answer Camerons questions after this to shorten this post.
Now to your questions:
Quote: I can imagine there would be some strong arguments between the 2 camps, so who or what decides on which of the 2 options is correct? Is it a majority rule situation? Over time does one theory get adopted more than the other and becomes accepted as the norm?
It often happens in science when the editor of a journal sends out a manuscript to two reviewers that the one reviewer might say that he accepts the manuscript, whilst the second reviewer says he thinks that major portions of the manuscript are unfounded and he does not find them suitable for publication. A good editor would then send the manuscript to a third reviewer to make sure about the matter, whichever way, OR he can use his own discretion and either thereafter reject the manuscript or asks the authors to make the changes the reviewer suggest. I as an author, then still have the right to write back to the editor saying that I think the reviewer suggesting the changes is wrong, and he may even accept this is given properly supported arguments. After publication of a paper, however, it still remains up to the broad scientific community out there if they accept what is written or not. What is important to realize is that science will always listen to an argument based on sound evidence, if thereafter new evidence is found showing that the first ideas may be wrong that is fine, but up until that happens the first idea may be the one that is viewed as correct.
With regard to the plant group that we are working on, I will remain tight-lipped, because if that author were to google for just this topic, he might find this thread and find out which gene I might be sequencing to get more information to prove my point....!
You are quite right about who came up with the species concept, remember Linneaus developed the binomial system of species name before Wallace so these ideas have been thought about by many scientists.
The new analysis techniques for unravelling species and population relationships are very powerful. The Malawi cichlids, have been shown to have evolved from one common ancestor, have been intensively studied in recent years and some beautiful results, illustrating speciation concepts have come out of this work. What is amazing in Lake Malawi cichlids is that they are all actually evolved very recently, yet they have developed into such distinctly different forms that we would have no problem in recognising different species based on what they look like. This is also the reason why Malawis can hybridize to easily.
We have a technical term for species that evolve in close proximity versus ones that are separated from each other. This is called sympatric versus allopatric speciation. We tend to think of large distances when it comes to separation, but it can mean separation by a very short physical distance, but in terms of habitat it could be large, let us say from the shaded base of a tree to getting up to the branches of the tree.
Another interesting thing is something called convergent evolution and I will name an example. Two disa species of which we have analysed the relationships look very similar to each other, both look like red hot pokers (Kniphofia). Both occur in the grassveld. One occurs in East and Central Africa, called Disa ochrostachya and the other occurs in southern Africa, called Disa chrysostachya. According to morphology, all botanists would say that they must be closely related. Our DNA analyses revealed this not to be so as a matter of fact they are only distantly related. What happened was that their ancestors reached these areas, they mutated to show a red flower colour and then sunbirds started pollinating them. Because sunbirds here and in central Africa go for the same thing, these two species were selected by the birds to look almost identical. So the evolution was convergent and they look very similar, but genetically they are very different. There are numerous instances where this has happened in the animal and fish world as well. Don't you think this is fascinating!
After all this, I think I must do some work....
Separate names with a comma.