Variation in heat shock proteins within tropical and desert species of poeciliid fishes

The 70-kilodalton heat shock protein (hsp70) family of molecularchaperones, which contains both stress-inducible and normally abundantconstitutive members, is highly conserved across distantly related taxa.Analysis of this protein family in individuals from an outbred populationof tropical topminnows, Poeciliopsis gracilis, showed that whileconstitutive hsp70 family members showed no variation in protein isoforms,inducibly synthesized hsp70 was polymorphic. Several species ofPoeciliopsis adapted to desert environments exhibited lower levels ofinducible hsp70 polymorphism than the tropical species, but constitutiveforms were identical to those in P. gracilis, as they were in theconfamilial species Gambusia affinis. These differences suggest thatinducible and constitutive members of this family are under differentevolutionary constraints and may indicate differences in their functionwithin the cell. Also, northern desert species of Poeciliopsis synthesize asubset of the inducible hsp70 isoforms seen in tropical species. Thisdistribution supports the theory that ancestral tropical fish migratednorthward and colonized desert streams; the subsequent decrease invariation of inducible hsp70 may have been due to genetic drift or aconsequence of adaptation to the desert environment. Higher levels ofvariability were found when the 30- kilodalton heat shock protein (hsp30)family was analyzed within different strains of two desert species ofPoeciliopsis and also in wild-caught individuals of Gambusia affinis. Inboth cases the distribution of hsp30 isoform diversity was similar to thatseen previously with allozyme polymorphisms.