VARIATION IN HEAT-SHOCK PROTEINS WITHIN TROPICAL AND DESERT SPECIES OF POECILIID FISHES

The 70-kilodalton heat shock protein (hsp70) family of molecular chape rones, which contains both stress-inducible and normally abundant cons titutive members, is highly conserved across distantly related taxa. A nalysis of this protein family in individuals from an outbred populati on of tropical topminnows, Poeciliopsis gracilis, showed that while co nstitutive hsp70 family members showed no variation in protein isoform s, inducibly synthesized hsp70 was polymorphic. Several species of Poe ciliopsis adapted to desert environments exhibited lower levels of ind ucible hsp70 polymorphism than the tropical species, but constitutive forms were identical to those in P. gracilis, as they were in the conf amilial species Gambusia affinis. These differences suggest that induc ible and constitutive members of this family are under different evolu tionary constraints and may indicate differences in their function wit hin the cell. Also, northern desert species of Poeciliopsis synthesize a subset of the inducible hsp70 isoforms seen in tropical species. Th is distribution supports the theory that ancestral tropical fish migra ted northward and colonized desert streams; the subsequent decrease in variation of inducible hsp70 may have been due to genetic drift or a consequence of adaptation to the desert environment. Higher levels of variability were found when the 30-kilodalton heat shock protein (hsp3 0) family was analyzed within different strains of two desert species of Poeciliopsis and also in wild-caught individuals of Gambusia affini s. In both cases the distribution of hsp30 isoform diversity was simil ar to that seen previously with allozyme polymorphisms.