I've been trying for some time to get captive bred melanistic Gambusia. I've had half a dozen beautiful white, black and yellow males for some time now (six months). They're sharing a 30 gallon tank with some swamp darters. To this tank I've added at different times a number of young females. I've always tried to add them while they are small enough to preclude them already being pregnant with a wild-type father. Once they start to show signs of pregnancy the females are removed to separate buckets maintained outside. The tubs are large enough to provide a constant supply of live food (I think it's about the time of year I expect the bloodworms to drop off even in SC, so I do supplement their diets). Once they drop fry, I move the adult back to the breeding tank to try again.
So far I have no spotted fry, but some of the young have now grown large enough to add them to the tank and attempt to cross them back with the parents. My problem is that occasionally I've had a "female" fry develop a gonopodium while in the tank. I'm wondering
1) Have I not kept them segregated long enough for development to be visible, or is it possible for them to change gender?
2) If I remove them as soon as a gonopodium is obvious, is there a chance of them impregnating one of the females, or does it take more time for gonadal development?
3) Anybody know the approximate expected rarity of spotted offspring from a spotted father?
4) Right now I'm using female fry only, because I assume by the fact that I see only males with the spotted phenotype that the trait must be inherited in a way that would ensure its expression if present in male offspring. Is there a chance of wild-type males being carriers of the gene, or should I just feed them to the pickerel?
Sexual maturity of Gambusia holbrooki
Started by
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, Nov 14 2009 05:18 PM
22 replies to this topic
#2
Guest_fundulus_*
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Posted 14 November 2009 - 06:11 PM
There is documentation of the masculinization of female Gambusia exposed to paper mill effluents in Florida. They still had functional ovaries, apparently, but developed very convincing gonopodia. There may be some sort of chemical exposure that could trigger some degree of intersex. Or, more simply, individuals may just differentiate later.
As to the melanism... isn't that a recessive trait, maybe carried on the X chromosome? So I guess you would have to do some back/in-crossing to get a higher degree of expression.
As to the melanism... isn't that a recessive trait, maybe carried on the X chromosome? So I guess you would have to do some back/in-crossing to get a higher degree of expression.
#3
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Posted 14 November 2009 - 07:35 PM
That was my assumption about the heredity of melanism, but I didn't actually know, I just assumed based on the observed distribution of the trait. If true then there's no reason to save males not expressing it, but if it's a simple single-gene recessive carried on X then wouldn't I expect some of the F1s to display it?
I'm curious also if there's any downside to putting a huge number of females in the tank. It's about a 2-1 mix right now, but not all are quite breeding age yet.
Thanks for the info.
I'm curious also if there's any downside to putting a huge number of females in the tank. It's about a 2-1 mix right now, but not all are quite breeding age yet.
Thanks for the info.
#4
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Posted 14 November 2009 - 08:42 PM
If it's an X-linked recessive, no males in the F1 would get it because they would only receive a Y from their fathers; it would be passed on to F2 males from their mothers, the same mechanism responsible for the transmission of color-blindness or hemophilia in humans. And females would need to be homozygous for that X-linked allele to show the trait.That was my assumption about the heredity of melanism, but I didn't actually know, I just assumed based on the observed distribution of the trait. If true then there's no reason to save males not expressing it, but if it's a simple single-gene recessive carried on X then wouldn't I expect some of the F1s to display it?
As to more females... sure, why not?
#5
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Posted 14 November 2009 - 10:17 PM
If it's an X-linked recessive, no males in the F1 would get it because they would only receive a Y from their fathers; it would be passed on to F2 males from their mothers, the same mechanism responsible for the transmission of color-blindness or hemophilia in humans. And females would need to be homozygous for that X-linked allele to show the trait.
As to more females... sure, why not?
Does this species operate with an XX / XY system as in mammals? My understanding of other more studied and related livebearers is that the system is not the same as in mammals in that crossing over of alleles (possible target of tudy) between sex chromomes readily occurs and that sex determination can have an environmentally controlled mechanism (i.e. some females can become males when abundance of latter is low).
#6
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Posted 14 November 2009 - 10:32 PM
Yes, poeciliids have an XX/XY genetic sex determination system. But it's more of a homogametic rather than heterogametic system, with about 70% of the two chromosomes still potentially recombinant. The literature on this sex-linkage goes back to the 1920s, mostly with guppies. A majority of genes that affect the male aspects of sexual selection are Y-linked but a good number are X-linked recessives. The (relatively) high level of recombination is why you can create guppy strains like the snakeskins, where even females have a surprising amount of color and pattern.
The truth is that I'm not absolutely certain that the melanistic locus is X-linked, that's my recollection.
EDIT: I really meant to say heteromorphic and homomorphic for the poecilliid X & Y chromosomes, but homogametic also describes them.
The truth is that I'm not absolutely certain that the melanistic locus is X-linked, that's my recollection.
EDIT: I really meant to say heteromorphic and homomorphic for the poecilliid X & Y chromosomes, but homogametic also describes them.
#9
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Posted 15 November 2009 - 01:21 PM
Yes, they do. I wasn't familiar with that word, so it's a good thing you did spell it right 
I'm not sure if it's technically superfetation or if they have the ability to store sperm for later use. Other livebearers have been noted to become pregnant as much as six months after last contact with a male, which is far beyond the expected 40 days for a pregnancy.
Placing them back in the males' tank has as much to do with preventing cannibalism of the fry as anything, but also I'm not completely sure in all cases that they weren't impregnated by other males either before capture or due to inadvertent reintroduction of male offspring. Is there any harm in trying to maximize their exposure to the males I want? They seem enthusiastic about mating pretty much every day.
I'm not sure if it's technically superfetation or if they have the ability to store sperm for later use. Other livebearers have been noted to become pregnant as much as six months after last contact with a male, which is far beyond the expected 40 days for a pregnancy.Placing them back in the males' tank has as much to do with preventing cannibalism of the fry as anything, but also I'm not completely sure in all cases that they weren't impregnated by other males either before capture or due to inadvertent reintroduction of male offspring. Is there any harm in trying to maximize their exposure to the males I want? They seem enthusiastic about mating pretty much every day.
#10
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Posted 15 November 2009 - 01:31 PM
Allowing more frequent / recent mating with male you want to be sire should increase odds you get what you need. Yes, you are correct in the term superfetation is not the most accurate description for what your fish do. What is the best term for multiple / discrete pregnancies enabled by storage of sperm from a single mating?
Are you linebreeding or simply breeding F1 females back to any spotted P1 male?
Are you linebreeding or simply breeding F1 females back to any spotted P1 male?
#11
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Posted 15 November 2009 - 02:43 PM
All the males are in the same tank. I prefer that they breed with a different spotted male than the parent, but I don't have enough space for extra tanks to ensure that happens all the time, so the fish pick for themselves. I don't think it matters much except for purposes of trying to reduce negative effects of inbreeding. The problem is melanistic males are so rare in the wild where I am that I don't have much new blood to work with.
The ones I've caught as adults have a quite striking pattern of black, white and yellow, but there is one that I caught with a scoop of very small fry way back when I was trying to feed a 2" pickerel. Growing up in my tank may be the variable, but he has less extensive black spotting and no yellow. He's mostly a pasty corpse white. Time will tell if that's genetic.
The ones I've caught as adults have a quite striking pattern of black, white and yellow, but there is one that I caught with a scoop of very small fry way back when I was trying to feed a 2" pickerel. Growing up in my tank may be the variable, but he has less extensive black spotting and no yellow. He's mostly a pasty corpse white. Time will tell if that's genetic.
#12
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Posted 15 November 2009 - 07:42 PM
If your females are mated back to sire, which is line breeding, then the inbreeding effects will not be as intense as breeding her to a full sibling as employed with most inbreeding efforts.
What is your generation time? If short enough and longevity of original sire long enough, then you should be able to to breed sires to grand daughters and great granddaughters.
What is your generation time? If short enough and longevity of original sire long enough, then you should be able to to breed sires to grand daughters and great granddaughters.
#13
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Posted 15 November 2009 - 09:12 PM
Seems like it's about 3 months between broods, but I haven't made a very concerted effort to track which individuals are spawning.
I don't have any visibly pregnant F1s yet, but they are about the right size and I expect to see signs really any day. As to longevity, it's really irrelevant because I have no idea of the age of any of the wild-caught adults, though they appeared full-grown. The one which was young when I caught him is now approximately a year old.
There is no risk of breeding to a sibling as the project is currently configured, since I am removing all F1 males. Once I get spotted offspring there will be a risk of that. Hopefully by then I can get more wild adults, and maybe I'll be sufficiently enthusiastic to dedicate another tank. The pickerel isn't proving as demanding as I once expected, so I may be able to ease up on the number of Convict grow-out tanks
I don't have any visibly pregnant F1s yet, but they are about the right size and I expect to see signs really any day. As to longevity, it's really irrelevant because I have no idea of the age of any of the wild-caught adults, though they appeared full-grown. The one which was young when I caught him is now approximately a year old.
There is no risk of breeding to a sibling as the project is currently configured, since I am removing all F1 males. Once I get spotted offspring there will be a risk of that. Hopefully by then I can get more wild adults, and maybe I'll be sufficiently enthusiastic to dedicate another tank. The pickerel isn't proving as demanding as I once expected, so I may be able to ease up on the number of Convict grow-out tanks
#15
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Posted 16 November 2009 - 08:23 AM
The answer to a minimal sustainable population in the modeling and practice of conservation genetics is n=500, where that n is the number of adult females. Your population may be somewhat better adapted for a smaller founding population because if they were all from the same area they may have already sorted out many nasty recessive alleles, but in truth they never disappear entirely. If the trait is X-linked you'll have to work much harder to get true fixation in the population, since all females will have to be homozygous for the trait. If you're lucky it's on the Y chromosome, which would almost guarantee father to son transmission.
#16
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Posted 16 November 2009 - 11:49 AM
Dang, that's a big number.
I did find an interesting paper on the heredity pattern of this trait if anyone's interested: http://jeb.biologist...int/209/24/4938
They theorize that there are two genes involved, one linked to Y and another dominant, but rare, that is not sex-linked. They also note that expression may be controlled by temperature in some, but not all, populations.
I did find an interesting paper on the heredity pattern of this trait if anyone's interested: http://jeb.biologist...int/209/24/4938
They theorize that there are two genes involved, one linked to Y and another dominant, but rare, that is not sex-linked. They also note that expression may be controlled by temperature in some, but not all, populations.
#17
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Posted 16 November 2009 - 04:33 PM
The paper reads to me like you could, with relative ease create a population of melanistic males that will breed true. The key for rapid fixation will require progeny testing of females. Some females will throw a much larger percentage of melanistic male offspring than others. Females of those lines should ultimately enable founding of lines that produce only melanistic males. Where the sex ratio is skewed, does that indicate to you that a the Y-version can be lethal or otherwise decreases viability in womb or during development prior to maturation?
Would be interesting to see how Y-linked and autosomal alleles coding for the melanistic phenotype would interact in the same individual.
Would be interesting to see how Y-linked and autosomal alleles coding for the melanistic phenotype would interact in the same individual.
#19
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Posted 16 November 2009 - 09:55 PM
The sex ration was skewed in all cases in favor of males, so that doesn't seem to suggest a negative survivability effect from Y.
There has to be more going on than what those authors described though, since I so far have zero melanistic offspring. It's possible that expression is completely prevented by the warm summer temperatures here in SC. We'll see what happens over the winter. The tanks are in an attached garage with no heating or air, so it will lag outside temperatures somewhat but will ultimately probably be a close analog of winter temperatures in the wild.
There has to be more going on than what those authors described though, since I so far have zero melanistic offspring. It's possible that expression is completely prevented by the warm summer temperatures here in SC. We'll see what happens over the winter. The tanks are in an attached garage with no heating or air, so it will lag outside temperatures somewhat but will ultimately probably be a close analog of winter temperatures in the wild.
#20
Michael Wolfe
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Posted 16 November 2009 - 10:48 PM
...since I so far have zero melanistic offspring.
This is an interesting statement... in breeding melanistic fundulus chysotus, I have noticed that they seem to get more dark spots as they get older... so seeing no melanistic fry might not be a problem... it might just be becasue they are fry... and as they mature they will get darker... maybe? I know they are quite different kind of fish, but might be a similar process?
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