Low specific activity of rare allozymes of α-glycerophosphate dehydrogenase in Drosophila

Abstract

ELECTROPHORETIC surveys of Drosophila species¹ show that many gene–enzyme systems are essentially monomorphic. Very often, however, one or more populations may contain rare alleles, that is, at frequencies less than 5%. Are these alleles selectively neutral and rare because of historical accidents² or are they deleterious and maintained in populations only through mutation? The dimeric enzyme α-glycerophosphate dehydrogenase (αGPDH) is a good example of an essentially monomorphic Drosophila gene–enzyme system. Out of the 101 species of Drosophila surveyed thus far for electrophoretic variation of αGPDH (refs 3–5), only D. melanogaster is polymorphic. This polymorphism seems to be maintained by selection with temperature being, or strongly correlated with, the selective agent^6–8. Microcomplement fixation experiments indicate that this enzyme is evolving slowly among Drosophila species and has accepted relatively few changes in its structure⁹. We propose that the rare alleles of the αGPDH locus found in some species (see ref. 5) may be at low frequencies because the products of these alleles (allozymes) are catalytically less efficient than their ‘wild-type’ counterparts.