Convict Cichlid Marble Genes

Last updated: 2012-09-17 — Created: 2010-09-08

I’ve been playing with various colour combinations of convict cichlids to apply the theories of genetic inheritance. This post concentrates on the marble gene for the convict cichlid. Specifically, whether or not the mutation that causes convicts to come out marbled (i.e. pink with splatters of black on them rather than completely colourless (pink) or black (striped/wild-type)) is on the same gene locus as pink/black or in a different spot altogether.

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PART 1 – Marble-Pink Interaction

Previous research has shown us that the black gene is dominant over the pink one, that is, if B represents black and b represents pink, crosses occur as follows:

true black (BB) x pink (bb)
 x  | B  | B  |
----+----+----+
 b  | Bb | Bb |
----+----+----+
 b  | Bb | Bb |
----+----+----+
= 100% black (but carrying a pink gene)

black, carrying a pink gene (Bb) x pink (bb)
 x  |  B |  b |
----+----+----+
 b  | Bb | bb |
----+----+----+
 b  | Bb | bb |
----+----+----+
= 50% black (but carrying a pink gene)
= 50% pink

two blacks, both carrying a pink gene (Bb)
 x  |  B |  b |
----+----+----+
 B  | BB | Bb |
----+----+----+
 b  | Bb | bb |
----+----+----+
= 75% black (25% true black, 50% carrying a pink gene)
= 25% pink

Two true blacks or two pinks will breed true, of course.
BB x BB = 100% true black
bb x bb = 100% pink

With this quick intro behind us, let’s move forward with my results…
My initial pairing resulted in fry that were 50% marble and 50% pink – no surprises there. Two of these fry are shown below as adults…

When these two marbled siblings were paired, something funky happened…

If you look closely at the fry in the above photo, you can see some fish that are lightly-marbled, some that are darkly-marbled and some that are plain pink. These fry are all from the same spawn, suggesting that the marble gene is co-dominant, that is, it has an  additive effect and works in “doses” – zero doses is pink, one dose is light marble, two doses is dark marble. A similar phenomenon can be seen in angelfish genetics for the dark gene – zero doses is silver (+/+), one dose is black lace (D/+), two doses is true black (D/D).

Comparing the (grand)parents to the fry would suggest that each marbled (grand)parent carries only one copy of the marble gene. From these results, it would suggest the interaction between marble and pink genes is as follows, where b’ represents the marble gene and b represents pink:

two light marbles (b'b)
 x  | b'  |  b  |
----+-----+-----+
 b' | b'b'| b'b |
----+-----+-----+
 b  | b'b | bb  |
----+-----+-----+
= 25% dark marble (b'b')
= 50% light marble (b'b)
= 25% pink

This will need to be verified through counting fry in future spawns. Those results may confirm this light-dark model, or it could very well be that the marble gene is subject to variable penetrance or expressivity, which would require a different way of looking at things altogether.

Further questions I would like to answer through future crossings…

1. Do two dark marble convicts produce 100% dark marble offspring?
2. Does a dark marble crossed with a pink convict produce 100% light marble offspring?
3. Does line breeding these dark marble convicts have any effect on their marbling amount or colour intensity?

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PART 2 – Marble-Black Interaction

Shown here is a true black (wild-type) convict. He has fathered spawns with multiple females and all of them came out 100% black, so he carried two black genes (BB).

Pairing him with a female (light) marble mate was the next logical step in this exercise. Their fry were all black like dad, showing that marble genes are masked by black genes in the same way that pink genes are. Black genes are dominant to both marble and pink.

The next thing to try was to mate one of these black x marble offspring to a pink convict in order to paint a clearer picture of how the three colours interact.

For this part of the experiment, I was able to mate an unrelated pink jellybean convict (which I believe to NOT be a hybrid, but a pure convict with a balloon-like body mutation, similar to a balloon ram, balloon platy, etc, and also carried by the blood parrot — this will be explained in other posts) to a black x marble specimen. The offspring were raised for a number of weeks, until their body patters could be clearly distinguished.

The fry were separated and counted by visual appearance, and the counts were as follows: 25 total, including 12 black, 13 marble and 0 pink. All of the offspring were normal-bodied. Within expected error, this indicates that the result of this pairing is 50% black convict and 50% (light?) marble convict.

Further observation of this spawn to follow.

September 27th, 2010 – Inspection of the spawn today showed some of the “marble” specimens as almost completely striped, with a few breaks on a pink background, while some had barely any marbling on a pink background. Since it is improbable for any of these fish to have inherited a second marble gene from the pink jellybean convict father, the marble gene would seem to appear to have variable expressivity/penetrance. Further observations to follow. An analysis of parent gender and colour should be made as well to determine if the expressivity of the marble pattern is sex-linked.

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PART 3 – Confirmation of Findings

Two siblings from the black (BB) x marble (b’b) offspring above were mated together. These two fish could have either genotype Bb’ or Bb, but there is no way to tell for certain just by looking at them since they both appear to be normal black convicts (with some ridiculously-blue hues).

The first spawn did not yield very many survivors, so a definitive statement about the black-marble gene interaction cannot be made at this time. However, the offspring that were produced were predominantly-black, though there were at least some that were light marble. No pink offspring were noticed. Interestingly enough, there was one short-bodied mutant in the mix, which adds a little bit of weight to the theory that there is no cross-breeding involved in creating jellybean convicts. Unfortunately, this small slow-growing individual was joined in his protective habitat by a larger specimen and devoured.

Two additional spawns from these parents are currently growing out, so a statement is forthcoming. It is expected that the results will be as follows:

 x  |   B  |  b   |
----+------+------+
 B  |  BB  | Bb   |
----+------+------+
 b  |  Bb  | bb   |
----+------+------+
= 75% black, 25% pink

OR    

 x  |   B  |  b'  |
----+------+------+
 B  |  BB  | Bb'  |
----+------+------+
 b  |  Bb  | b'b  |
----+------+------+
= 75% black, 25% light marble

OR 

 x  |   B  |  b'  |
----+------+------+
 B  |  BB  | Bb'  |
----+------+------+
 b' |  Bb' | b'b' |
----+------+------+
= 75% black, 25% dark marble

Since the marbled offspring that were initially produced appeared to be of the light marble variety, it would seem that the one parent was Bb’ and the other Bb, and it will be the second case above will likely be the outcome when the younger offspring are large enough to count accurately.

October 21st, 2010 – The two spawns (combined) were counted and observed today. Had I cooked the numbers, the counts couldn’t have come out any better! Out of 101 fry, 76 were black and 25 were marble, a 3:1 ratio pretty much exactly. This shows that the gene for marble must be on the same locus as the gene for black. You can see in the above photo the marble babies in the floating box and the black ones in the tank underneath.

Further experiments are underway, but my current position is that…

– The pink gene is recessive to black. (established)
– The marble gene is on the same locus as the genes for black and pink. (confirmed)
– The marble gene is dominant over pink, but recessive to black. (confirmed)
– The inheritance of these three gene options is not sex-linked and completely Mendelian.
– The marble gene behaves co-dominantly with itself (i.e. one copy of the gene gives lighter marbling patterns, where two copies present darker marbling).
– So,
—> BB, Bb’ and Bb represent striped/black convicts
—> b’b represents a lightly-marbled convict
—> b’b’ represents a darkly-marbled convict
—> bb represents a pink convict.

December 13th, 2011 – A spawn from (I think) my marble-gene black convicts was counted today and of the seventeen fry, thirteen were black (76.47%) and four were marbled (23.53%), again showing a 3:1 ratio pretty much spot on.

December 13th, 2011 – A spawn from my marble-gene black convict male and marble female was counted and of the twenty-two fry, nine were black (40.91%) and thirteen were marbled (59.09%). The expected ratio was 1:1, which these numbers approximate.

The marbled fry from the above two spawns were combined to grow out, while the black fry were used as food.

March 30th, 2012 – Above are the grow-outs from four separate spawns, combined – two each from a marble-gene black x marble pair and a marble-gene black x marble-gene black pair. Both pairs spawned at either end of the same tank almost simultaneously, however, the first pair’s fry were removed first in each case. Assuming all four spawns had equal survival rates (not a chance), the numbers should work out as follows:

MGB x M = (50% black, 50% marble) x 50% of total +
MGB x MGB = (75% black, 25% marble) x 50% of total
= 62.50% black, 37.50% marble

I’m pretty sure some of the older fry jumped in from or out to the surrounding tank and the older fry chewed on some of the younger. Since all things are far from equal, I don’t want to put too much stock in these data, but they are included for completeness anyway. Not-so-meticulous counts were 15 marble (34.09%) and 29 black (65.91%) of the 44 fry counted. Not too far off.

June 25th, 2012 – This spawn seemed to throw a wrench in the gears, but I’m hoping the results are more of an anomaly than anything. From a marble male, paired with a pink female, of 16 fry, ten were marble (62.5%) and six were pink (37.5%). It would seem I have more work to do on this part of the experiment!

September 17th, 2012– Here’s a grow-out from a pink male mated to a marble-gene black female. The count was perfect, thirty-two fry total, sixteen marble, sixteen black! (There was one little guy that could have been a black instead of a marble, so maybe it was 15 and 17, but still, it very close to 50% of each!). Mom passed a black gene on to 50% of her fry and a marble gene on to the remaining 50%. Dad’s recessive pink genes were masked by the marble and black genes, as expected.

 x  |   B  |  b'  |
----+------+------+
 b  |  Bb  | b'b  |
----+------+------+
 b  |  Bb  | b'b  |
----+------+------+
= 50% black, 50% light marble

[— More spawns to come! —]

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PART 4 – Marble Gene Expressivity

February 28th, 2012 – No two marble convicts are alike, each having its own unique splattering of black patches. Some have very intense marbling, while others have barely any. I’ve theorised that the marble gene works in doses: zero doses (bb) giving a pink fish, one dose (b’b) giving a lightly-marbled fish, and two doses (b’b’) giving an intensely-marbled fish. However, I’m not entirely convinced it works like this, wondering if the gene instead has variable expressivity based on some other factor.

Shown here is a new store-bought pink female with a home-bred marble male that has barely any marbling on him, just a few spots on his right flank. They have recently spawned. I’m very curious to know if the degree that the marble gene is expressed a parent affects the expressivity of the gene in that parent’s fry, so this spawn will be grown out to observe and answer this question. The male was produced from a brother and sister marble pair, that were produced from store-bought marble male and a store-bought pink female, with both of these pairs shown in photos at the top of this post.

Something interesting happened as the male above aged, the few black specks disappeared, leaving him totally pink. He and the pink female managed to spawn one more time before she killed him off, so their final batch of fry is growing out now. I suspect they will all come out nice and plain pink.

September 2nd, 2012 – I’ve found one really nice male that has some really intense marbling on him. I’m growing him out and hoping to find a similar-looking female to pair with him in order to see if their fry come out just as intense.

[— Results to follow! —]

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References:

• Mendelian Inheritance
• Dominance Relationship
  
• The Angelfish Society