A Swordtail Mystery Solved
The Puzzle Piece
This blog has been incorporated in the "Guppy Color Manual."
It is well known in guppy circles that you cannot have a true half-black swordtail. Those who report that they have seen a half-black swordtail invariably mistake a dark blue peduncle composed of iridophores for a black peduncle composed of melanophores (black color cells).
I have always thought that the reason you cannot have a hb swordtail is because the half-black pattern actually extends its influence into the caudal fin, a fact I discovered is even true on strains (like the half-black yellow) that appear to show no black in the caudal fin. I discovered in a cross I did between a wild guppy and a half-black yellow that in fact even though you cannot see black in a half-black yellow caudal fin, the genetics are there and the area of influence of the gene is there.
This old illustration from the Russian geneticist Kirpichnikov shows the pattern as it is expressed when it is not inhibited from expressing itself in the caudal fin.
And here is the male that resulted from the cross between a wild male and a half-black yellow female.
F1 Wild Male X Half-Black Yellow Female
Notice the almost identical pattern in the caudal fin of this F1 male and Kirpichnikov's illustration.
The theory I proposed at the time was that the half-black pattern was inhibiting the lengthening of the fin rays in the swordtail. If you look at the edge of the fins in the half-blacks shown above you will see that the likely culprit is associated with the black color cells.
The theory has some corroboration in the work of Dr. Nayudu who published a paper in 1979 on genes affecting the development of the caudal fin (Genetic Studies of Melanic Color Patterns and Atypical Sex Determination in the Guppy, Poecilia reticulata (Copeia 1979, 2 pp. 225-231). Specifically she noted that the Ni gene (the scientific notation for the half-black gene) was regulated by the Flavus (Fla) gene.
A student’s test showed that the area of the tail fin of male sibs carrying the genotype Fla, Cp /Fla + was significantly smaller than those of the genotype +, Cp/Fla, + (Genetic Studies, p. 228)
This little gem of information has been around since 1979, over three decades. But few have read the paper. In fact I find myself returning to the paper again and again.
But it was not until today that Nayudu's insight came home to me. It was not until today that a new puzzle piece added to previous work finally made the emerging picture clear.
The guppy you see at the top of this blog, the "Puzzle Piece" was discovered when I was culling a tank of F1 hybrids of a recent cross. The cross was between a half-black red and a snakeskin. (I am trying to reverse engineer a Micariff.) I had noticed that the males from this cross had ragged fins before but did not pay too much attention to them. Then I saw this male. It was not until I plucked him (and a few of his brothers) out of the cull tank and photographed them that I realized another genetic puzzle piece had fallen into place.
If you look carefully at the trailing edge of his caudal fin you will see that the fin rays have extended in some cases and in some cases have not. What do you notice about the pattern? Wherever there is black on the trailing edge (black melanophores) the fin rays have failed to lengthen beyond the wild type. Where the fin is composed of iridophores and red color cells they extend beyond wild type size. The black color cells in the caudal fin are involved in suppressing fin extensions!
I'll bet that the reason why you cannot have a half-black sword is because the half-black pattern is expressed along the trailing edge of the caudal fin, and prevents the sword extensions from developing. Nayudu in her paper speculates that this is because of intercellular signalling...the messaging system between cells. I think she was right, although I might recast her speculation in terms of gene regulation. I would say that the particular black melanophores created by the half-black mutation (i.e. not all melanophores) prevent the regulation of the fin rays into longer lengths. In other words it is the mutant color cells of the half-black Ni gene that inhibit the extension of fin rays. That is something worth thinking about, because it may in fact give us a crucial insight into the half-black mutation itself. But that is a whole other subject, too dense to discuss here!
So what do we make of the fact that magenta guppies also have shorter fins, along with Stoerzbach and other metal mutants? That will have to wait for a deeper exploration in the Color Manual. But my hunch is that it has to do with black color genetics. Black magic.
I have photos of a whole range of males from the same drop, all who show the peculiar ragged fin shape. And in every case it is the presence of black on the trailing edge that causes the fin rays to fail to develop. Good thing I did not throw out the culls before taking one last good look at them. My goal to decipher Micariff genetics may end in failure. But all is not lost. I just found a key piece of fin color genetics. Now I am going away to see what I can do with it.
Addendum
As I have said in the past, the advantage of organizing your thoughts about something you observe in your fish room, and composing them into a short formal essay is that you often discover connections between observations that would be obscure if you never went through the act of writing. I am not talking about posting observations as comments on facebook or messages on a website forum. I think the act of essay composition activates memories and create new connections between thoughts that just do not occur when you are free associating. It is as if the structure of a short essay or extended book supports these kinds of insights. Facebook comments and website forum messages have much smaller and free flowing structures, like "thinking out loud." They often do result in new insights when they induce extended exchanges, but the quality of the insights is just not there.
Also, crucially it is the act of revision where thought develops coherence and depth. Facebook comments and website forum messages usually are "off the top of the head."
A case in point is this blog and its addendum. After posting the blog, and reading it, I realized that the pattern you see in snakeskin crosses, which is usually is an irregular pattern of dots and mosaic shapes, is itself evidence for a theory I have been developing in the Color Manual. The theory is that the snakeskin is fundamentally a disorganized reticulation pattern. That is, the reticulation pattern superimposed on the scale pattern of wild type guppies loses its relationship to the scale edges and forms an independent chain link pattern in the snakeskin. When you look at the snakeskin pattern under the microscope you see that is islands of melanophores in a sea of platinum. So it forces the melanophores to form clumps instead of sticking together along the edges of the guppies scales. Or it has lost the landmark (scale edge) that normally it pins itself to.
Snakeskin is a mutation affecting the migration of the melanophores from the neural crest during embryonic development to areas of the body. What I realized after writing the blog is that the half-black trait has a definite distribution pattern in the tail, a crescent shape at the base and a crescent shape on the trailing edge of the fin. What the snakeskin gene(s) does is interfere with this pattern producing the spotted pattern you see in the Puzzle Piece male at the top of the blog. The gaps in the fin pattern are a result of the snakeskin pattern's attempt to force the melanophores into islands.
You see why it is important to analyze your own observations? You need to integrate your insight into the discussion of color genetics you find in the scientific journals. That is the only really reliable way of putting your observations and insights on firm and steady ground.
The other thing I realized upon reflecting what I originally wrote in this blog is the reason why half-black yellow guppies have no black in their caudals. It is because the black color has been converted to white. See my blog "No Such Thing as a Bad Cross." In that blog I speculate that there is a suppression gene for black in the caudal. But I realized in thinking about it that black is in fact present, but it has changed color. It is white! This is another idea I explore in the Color Manual. Amazing what happens when you formalize your observations by developing connections with other observations and put them out for public scrutiny using the conventions of academic and scientific publishing.
And Additional Thoughts
The elongation of the caudal fin is an excellent example of gene regulation. In gene regulation you can have what is called a "cascading network of genes." What this means is the over time master genes activate secondary genes which in turn turn on tertiary genes and so on. Genes at the local level may encounter conditions requiring local adaptation so the system of genes being turned on or off according to local conditions is very useful.
The limbs of animals are an example of this. The arms and legs of humans have the same general plan, but feet and hands are adapted to very different uses. The growth of the arms and legs are activated by the same genes, but by the time they mature, the anatomy of hands and feet are due to different genetic factors turned on at the local level by gene regulation. In the animal world there are examples of what happens when gene regulation goes awry. There are fruit flies whose antennae are legs. Even though they share the same basic plan, legs and antennae are under different gene regulation. So it is with fins of guppies and other fish. The variety of guppy fin shapes (half moon, triangle, sword tail, pin tail, merah, and so on) are evidence that the individual fin rays are under separate genetic regulation. (Interesting enough human appendages and fish fins actually share the same basic genetic toolkit,) So technically what is going on with the shape of the Puzzle Piece male is that the normal regulation of the fin shape is disrupted by erroneous or missing signals coming from the malformed half-black melanophores. The snakeskin and other genes may also contribute to the "signal noise" happening at the local level of the trailing edge of the caudal fin.
I thought of an analogy for gene regulation, but I am not sure of its validity. The analogy is to a military operation during a war. There is a general, who is analogous to a master gene that initiates development of a certain plan, a military plan or in the case of guppies the development of the caudal fin from an embryonic bud. During the development of a military operation, there is a chain of command right down to the local level, which is the platoon in the case of a military hierarchy and the lengthening of a fin ray in the case of the guppy. In military terms the general's original command, Attack! is a generalized plan of action. But an effective military operation, at least in modern times, is one where the original order is interpret locally. The local commander interprets the general plan as an adaptation to local conditions. So it is with the lengthening of the fin ray.
I suspect there is a biological basis for fin extensions in guppies. For one, they are found on males and not females, which suggests they have a role to play in sexual display. This means that colorful fin extensions would be more impressive than clear fin extensions, which would be non-adaptive since the male's ability to swim would be compromised. In other words, colorful fin extensions may give males an advantage in sexual display while giving them a disadvantage by making them more conspicuous to predators and less able to dart away. Clearly (pun intended) the fin ray is most effective in a display if it is colored. A clear fin extension would be largely invisible and have none of the advantage or a colored extension in sexual display. So there may be a genetic factor that extends the fin ray when color cells migrate from the body into the fins. Because the black color cells of the half-black mutation are mutant, they may not behave according to plan. Instead of passing on a signal for the fin ray to extend, they garble the message and the fin rays never get the signal to continue growing.
What do you think?
And More!!
I came across this image when researching my new book. It is a picture of a wild-type strain I once had. I shot some of the males with odd shaped fins.
Wild-type male with a mini sword tail!
As you can see there is a little "mini" sword tail at the bottom of the caudal fin. Notice that at the point where the "swords" divide there is a black spot. There is also some grey color around the area of the swords.
The black spot must be inhibiting the extension of the rays, creating the false sword shape. But what is interesting is that the overall color of the area may have promoted the extended rays in the first place.
Comments
Your constant,playfu l invitations have finally got through.I finally begin to realise the importance of writing and interpretation, or diagnosis from the evidence, is what empowers the tailor to work the fabric.
I'm going to try now to document my games and share some definite results.
A look at my profile will show the guppies I have available.
Any suggestions both welcome and timely.;)
I looked at your profile. You have some interesting genes in your fish room. Exploring the Endler hybrids would be fascinating. The patterns are quite different from guppies. I don't know much about them, so there are some fundamental issues to explore. Are there X-linked Endler specific genes, such that crossing Endler females to guppies produces unique patterns? Also I think the Mg gene in your Full Platinums is a huge area to explore. I think the gene might have a role in many well known guppy strains.
I don't have any pure Endlers now...only the fish that have Magenta,Metalli c Yellow,Pingu and Snakeskin unfortunately.
My Albino Full Platinum Swallow Tails were imported from Thailand.Their fry have now matured but none are Albino.In fact,they all look very similar with black bodies,yellow caudals (with black spots)and white dorsals,with platinum white shoulders.I will post some pics of both parents and fry and see if we can work out some ways to explore the fish.
In the same vein of trying to test this idea, one might expect that mutations that impact melanophore number would showed similarly reduced potential for caudal extension (in the same genetic background). Do they?
According to zebrafish research, fin extension occurs at the base of the caudal, where segments are added. The black at the trailing edge actually began its migration to the trailing edge at the base of the caudal. So that is where the melanophores are having their effect. Same result, but different location for the fin length suppression.
When I get more ragged fin males, I think I will do as you suggest and ablate the fin trailing edge. None of the males I photographed show black at the base of the caudal. So theoretically the fins should grow out even if the black areas are ablated along the trailing edge. The other possibility is the snakeskin pattern will produce another round of black dots emerging from the base. Theoretically I should get ragged fins again.
Nayudu's article brings in another dimension. She notes that certain genes suppress the spread of black color of other genes during development (from birth to sexual maturity). So the black pattern formed during fin regeneration may not indeed behave like migrating black color cells during sexual development. I think fin regeneration studies in zebrafish shows that local "stem color cells" are recruited. So that would be another good reason for the ablation. Will I get a different pattern?
I will watch for these ragged fin males in the F2 which is weeks old. And grab a few for the test. Thanks for the great idea!
The difficulty with your second idea is that most mutations affect melanin and melanosomes (the black pigment bodies) and not melanophore number. I once found what I thought were very pale guppies in a petstore but they died almost right away and I have not seen them since.
Guppies with pure yellow fins exist, which represents a complete absence of melanophores. There are no half-black yellow swordtails. Since there are yellow platinum swordtails, it must be the case that the fin length regulation is occuring at the base of the caudal where the half-black color cells are present.
So maybe the real key here is the snakeskin gene and its organization of black color cells into islands. At the base of the caudal these islands pass through creating the ragged fin effect by turning fin lengthening on and off.
I was planning to incorporate the golden gene into a snakeskin line. It exaggerates the reticulation pattern on the guppy and re-organizes other black color cells. I wonder what effect it would have on the ragged fin guppy phenotype?
Do any of the albino mutants in guppies reduce or eliminate the number or melanocytes, or are amelanotic melanocytes produced in all cases?
Philip
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