This info is not the latest, but there was a stufy in Copeia on swordtails a
number of years ago considering the role of different sized swordtails.
There were those who started showing their swords when they were small.
Their body growth slowed down precipitously. There is a correlation between
sexing out and body growth. But they were available to inseminate females
X. helleri females preferred the company of large males (those big studs
which sexed out last) because the big male would keep the small sneaker
males away (most of the time) from her, allowing her to forage for more food
and spend less energy avoiding the sneakers - which could then be used for
fry and growth.
Sometimes sex changes have been claimed for fish which just kept growing and
growing and sexed out as males. Of course if the big boys got picked off by
predators, there were always the sneakers willing to help with procreation.
John Dawes, in his (1991) book on livebearers notes that there are a couple
of genetic combinations whcih determine sex. There are XY males, also W,X,
and Y chromosomes. He further mentions a Z chromosome (I know this sounds a
tad like Dave Barry.) Locations of specific genes on a chromosome may also
be a variable. I haven't described this well. (Come to tomorrow's CKA
meeting, I will lend you the book and bibliography.)
Toki-O Yamamoto has also discussed males and females which have secondary
characterists of the other gender, The person which mentioned this to me
went on to suggest that the researcher felt that the livebearers were still
functionally on gender or another, just confusing to the observer.
However I also recieved a note from a correspondant suggesting that in
"Ichthyology" by Lagler, et. al. "Sex-reversed males presumably bearing XX
chromosomes have been noted in the guppy; their genetic sex was female,
their real sex, male.... when one of these exceptional XX males was mated to
an XX female, all the resultant offspring were females."
I've not looked that up yet.
Aging livebearers may show the influence of a shifting hormonal balance.
There certainly are Poecilid photos of previously functional females
showing male traits such as larger fins, some more color, a thickening of
the first anal fin rays)
However, female guppies and mollies, platys, swordtails, variatus and
Poecilia actually lose a number of bones reaching from their ribs to the
anal fin when they develop female features. (There is a drawing of this in
the latest ALA Livebearers #179) That allows the females to have room in an
expanding gravid area for fry.
Males need to keep that pelvic girdle of bones in order to move the
gonopodium around. That is why some livebearer experts question whether a
female could change to a functional male because they still would not have
the bone structure necessary to effectively use a modified anal fin.
There are several articles in Ecology and Evolution of Livebearering Fishes,
a 1989 set of studies on Poecelids edited by Gary K Meffe and Franklin F.
Snelson Jr. which also address some of these issues. I don't have it handy.
(Interlibrary loan probably could get you a copy in a couple of weeks.)
The Xiphphorus home page also has an on-line bibliography. Haven't used it
Much of this is not real recent, but maybe a start.
All the best!
> What is the latest scientific conclusion on the sex change in freshwater
livebearers like Xiphophorus sp.?
> Is it true sex change? Is it considered an apparent sex change with
late-developing males? Is it a change in hormone levels in females causing
them to develop male-like characteristics?
> References would be great, if you have them.
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