Re: NANFA-- Cold Tolerance of North American Livebears

Bruce Stallsmith (
Sun, 09 Jan 2000 23:52:48 EST

Rob, I think you're probably right. The best studied example of this
phenomenon of different populations in a species with different temperature
tolerances is the mummichog, _Fundulus heteroclitus_. They are found on the
east coast from northern Florida up to Newfoundland, obviously a wide range
of temperature conditions. Two subspecies of _F. heteroclitus_ are
recognized, a northern and a southern, with the dividing line approximately
Delaware Bay/south Jersey. The original division into subspecies was based
on various morphological differences, but recent work has shown that there
are also measurable differences in gene frequencies in genes that affect an
organism's ability to tolerate extremes of cold or hot. These gene frequency
differences are very closely correlated to the different subspecies. One of
these genes is for lactase dehydrogenase, a key enzyme in metabolizing
sugars. Different structural variants of this enzyme have a greater or
lesser ability to function in cold temperatures, especially. A surprisingly
large literature on this has developed, some of which I was reading last
spring and have in my office.

Mummichogs are one of the most studied fish, largely because they're used in
toxicology studies. I suspect that you would find a similar difference in
gene frequencies between northern and southern Gambusia, and probably Het.
formosa; for that matter, many of the fish that we're concerned with.

This is actually an interesting application of molecular biology! I would
urge anyone on this list who's a biology major to take biochemistry and
molecular biology, even if you'd rather be collecting in the field. You can
learn some powerful tools for answering interesting questions. Enough

--Bruce Stallsmith
Huntsville, AL
"I survived biochemistry"

>After some reflection,(and consultation with "Ecology and Evolution of
>Livebearing Fishes") I am of the opinion that there are "northern" and
>"southern" population morphs of both Gambusia and Heterandria strains
>with VERY DIFFERENT temperature tolerances.
>I would bet good money, that if you got some H. formosa from say Miami,
>FL...and put them in Moon`s Het. pond in Wilmington, N.C., that they
>would go "belly-up" first good freeze.
>Same with Gambusia strain (holbrooki and affins)...Miami and say New
>Simply put, these fish have adapted to local climates through the ages.
>And I think there is an incredible lack of knowledge and documentation
>in this area, and with native fishes in general.

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