Research Description
HEAT-SHOCK PROTEINS AND GENES
Summary: My laboratory investigates the heat-shock protein Hsp70, its encoding genes, and its regulation in Drosophila as a model system for understanding evolutionary adaptation. Hsp70 is a molecular chaperone that deters stress-induced protein aggregation, but has numerous other functions. Hsp70 is necessary for full-strength tolerance (in terms of survival, normal development, normal function) of high temperature. Such tolerance is critical in nature, where non-adult Drosophila undergo harmful to lethal high temperatures. In nature, Drosophila populations vary in stress tolerance and Hsp70 levels. Our current major focus is on understanding the genomic basis for this variation. The number of hsp70 gene copies and evolution of the hsp70 coding sequence are partial or inadequate explanations. Evidently cis-regulatory regions such as proximal promoters underlie intraspecific variation in Hsp70 levels. Repeated insertion of mobile genetic elements into these promoters is a recurrent mechanism of evolution.
Why are heat-shock proteins important?
Are heat-shock proteins important in nature?
How is natural variation in Hsp70 expression encoded at the genomic level?
Are heat-shock proteins important in nature?
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Outside the laboratory, Drosophila
melanogaster oviposits and
develops on rotting fruit. |
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When rotting fruit is sunlit,
temperatures can increase high enough to imperil Drosophila developing in the fruit and can result in Hsp70
expression. Below to the left are
temperatures of rotting fruit in an orchard in Indiana, USA, in July 1997; orange
line indicates air temperature.
Below to the right are body temperatures of Drosophila recorded on a similar day in 1994. Red crosses indicate heat-killed
larvae.
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During such exposures, Hsp70
levels in larvae increase dramatically (below left). Exposing larvae with or without extra hsp70 gene copies to natural heat shock demonstrates the
importance of increasing Hsp70 levels for survival.
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Additional support for the importance
of Hsp70 in nature comes from work in my lab, Michael
Evgen'ev 's, and Volker Loeschcke's showing
that Hsp70 levels in natural Drosophila populations are correlated with environmental gradients in stress. For example, in Evolution
Canyon in Israel, Drosophila
inhabit opposite slopes differing dramatically in microclimate. Flies from the hotter,
south-facing slope express significantly more Hsp70 and are more thermotolerant
than flies from the cooler north-facing slope.
In another group of Drosophila, Drosophila lummei replaces Drosophila virilis along a latitutindal gradient. Strains of the low-latitude Drosophila
virilis both express more Hsp70 and
have greater thermotolerance than strains of the high-latitude Drosophila
lummei. The same is true in a second species pair (D.
novamexicana and D. texana).
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Laboratory selection for
inducible thermotolerance yields similar results. By contrast, laboratory culture at warm temperatures without
heat shock or in similar natural settings yields decreased Hsp70
expression. We interpret
this as the outcome of an evolutionary trade-off of the beneficial impact of
Hsp70 during extreme thermal stress and the deleterious consequences of high Hsp70
in the absence of extreme thermal stress.
For a complete list of
laboratory publications on the above topics, go to