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Melanism in Female Gambusia Holbrooki


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#1 appozulp

appozulp
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  • Stuart, FL

Posted 13 March 2018 - 07:18 PM

I've been observing the melanistic-spotted mutation in a wild population of mosquitofish in the Loxahatchee River District of Southeast Florida. From what I've read the mutation is documented as naturally-occurring  in the phenotype of male G. holbrooki wild populations.

 

Wild females with the speckled phenotype are very rare? According to a decades-old article (I attached below) they believed they had found the first recorded incidence. This article claims that pollution was not a likely factor contributing to this female specimen's phenotype but a previous study suspected hypermasculinization (and therefore melanism) in wild mosquitofish populations could be caused by pollution. 

 

I've been finding many melanistic females in this population. I'll share some photos (caught and released, there are so many with this trait and easy to find in this area) below and please let me know if I'm missing something simple here, or could this be a new observation?
I was wondering if I stumbled across a population of introduced Phalloceros caudimaculatus but as you can see from the photo of one of the spotted males I found the gonopodium is not hooked at the end like P. caudimaculatus. 
The literature on melanism in the wild males suggests it is a sex-linked trait... so am I correct in hypothesizing that either these females are hypermasculinized,  or their melanism is possibly due to a completely different genetic/environmental mechanism?
 

Attached Images

  • KIMG1666.JPG
  • KIMG1650_02.JPG
  • KIMG1675.JPG
  • KIMG1669.JPG
  • KIMG1662.JPG

Attached Files



#2 gerald

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  • Wake Forest, North Carolina

Posted 13 March 2018 - 07:48 PM

Are these really females, or young males whose anal fin has not yet metamorphosed into a mature gonopodium?

When you find a black-spotted female that is clearly gravid (or delivers young!), THAT will be news-worthy!


Gerald Pottern
-----------------------
Hangin' on the Neuse
"Taxonomy is the diaper used to organize the mess of evolution into discrete packages" - M.Sandel


#3 appozulp

appozulp
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  • Stuart, FL

Posted 13 March 2018 - 08:16 PM

Are these really females, or young males whose anal fin has not yet metamorphosed into a mature gonopodium?

When you find a black-spotted female that is clearly gravid (or delivers young!), THAT will be news-worthy!

 

That's what I was thinking, the ones I photographed that were lacking gonopodium are still smaller than adult females but bigger than other males with gonopodium. Is there a way to distinguish immature males and females?

 

Next time I am in that area I'll focus on looking for gravid females. 



#4 gerald

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  • Wake Forest, North Carolina

Posted 13 March 2018 - 08:42 PM

Keep them and grow them up is the only way I know.  Some males just mature later than others, and are often bigger when they go through "puberty".


Gerald Pottern
-----------------------
Hangin' on the Neuse
"Taxonomy is the diaper used to organize the mess of evolution into discrete packages" - M.Sandel


#5 appozulp

appozulp
  • NANFA Guest
  • Stuart, FL

Posted 16 March 2018 - 10:13 AM

I got in touch with a local native fish guy who has caught wild melanistic females in Florida over the years and incorporated them into a captive strain, so it does appear to be a heritable mutation in females as well since he was able to produce melanistic offspring.

 

I'm going to seek out some more primary sources and keep tossing around the question of whether this could be a case of a wild population expressing a new/different genetic hypermelanism than the sex-linked one?

 

I've heard that melanism in G. affinis is expressed in females, so these could be hybrids, or at least this type of bisexual melanism be the same that occurs in affinis? In previous threads someone mentioned wanting to cross affinis and holbrooki specifically to produce a line with melanism in both sexes. In Florida there are accounts of G. affinis occurring nonindigenously.

 

Another thing that I'm unsure if it's possible, due to the morphological similarities I was wondering if these could be a case of introduced Phalloceros caudimaculatus, the speckled mosquitofish of South America, which do express melanism in both sexes. But the shape of the gonopodium is different, it has a distinct hook, and only 6 anal fin rays....so that's probably not it?

Attached Images

  • melanistic female.jpg

Edited by appozulp, 16 March 2018 - 10:14 AM.


#6 gerald

gerald
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  • Wake Forest, North Carolina

Posted 16 March 2018 - 11:40 AM

Yes that seems possible there might be a sex-linked mutation and a less common non-sex-linked mutation that can both cause melanistic spotting, or maybe some combination of mutations that affect gene expression.   Besides the downward hooked gonopodium in Phalloceros, are there other features (fin ray counts) that separate Gambusia from Phalloceros?  I guess the "teardrop" and rows of dots on the dorsal and caudal fins seen in normal Gambusia don't really work on melanistic fish.  USGS doesn't have any Phalloceros records yet in its invasive species database:  https://nas.er.usgs....up/default.aspx


Gerald Pottern
-----------------------
Hangin' on the Neuse
"Taxonomy is the diaper used to organize the mess of evolution into discrete packages" - M.Sandel


#7 fundulus

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  • Global Moderator

Posted 16 March 2018 - 04:26 PM

Are we really talking about mutations here, or relatively rare alleles with different frequencies in different populations? Something similar exists in Trinidad and the Caribbean coast of Venezuela and Surinam with Poecilia picta (Swamp Guppy), a guppy relative. There is a low-frequency red morph in many populations only in males that isn't sex-linked, strangely enough. Anna Lindholm and Felex Breden have done some published work with this, and it remains a mystery to my knowledge.


Bruce Stallsmith, Huntsville, Alabama, US of A

#8 Auban

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Posted 18 May 2018 - 07:35 PM

hmm...  well, if you are ever in clay county, specifically in and around lake brooklyn in keystone heights, look for the spotted females.  i used to catch them there from time to time.  

 

for some reason, they seemed to come in waves.  one year, there would be a whole lot, the next year, hardly any.  


"The ecologist is continually having to look at the aspects of nature with which he is unfamiliar and perforce must be an amateur for much of his working time.... professionals may carp at omissions, misconstructions, or even downright errors in these pages. perhaps ultimately they may forgive them for the sake of the overall vision that only the amateur, or the ecologist, blithely sets out to experience."G. Evelyn Hutchinson



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