Insertion polymorphisms may cause stage specific variation in mRNA levels for dopa decarboxylase in Drosophila

Summary

An activity variant in Drosophila, Ddc ⁺⁴, which has been isolated from natural populations is shown to affect the level of messenger RNA for dopa decarboxylase (DDC) in a stage specific manner. Newly hatched first instar larvae and newly eclosed adults have 1.5 times the amount of DDC mRNA as the Canton-S laboratory strain. On the other hand, puparia of the variant have only 0.5 times as much DDC mRNA as Canton-S. Genomic Southern analysis revealed that Ddc ⁺⁴ DNA differed from Canton-S DNA by four small restriction length polymorphisms. To confirm these differences, a library of Ddc ⁺⁴ was constructed in the λ1059 vector. A clone was recovered spanning the DDC region and compared to cloned Canton-S DNA. Acrylamide gel electrophoresis of restriction fragments revealed that one previously identified insertion really consisted of two smaller ones. One of the other differences identified by the Southern analysis was not confirmed in the cloned DNA of Ddc ⁺⁴, indicating some divergence had occurred in the variant strain between the time of its isogenization and cloning. The differences between the cloned Ddc ⁺⁴ DNA and cloned Canton-S DNA consisted of six small restriction length polymorphisms and one restriction site polymorphism. Five of the seven differences lay in the 5′ untranslated leader sequence of the DDC mRNA or in the 4.5 kb of DNA upstream of the transcription start site. The existence of these small (<100 bp) insertion/deletion polymorphisms in a strain exhibiting a complex temporal phenotype for DDC activity, suggests natural populations are an excellent source of variation affecting gene expression. Secondly, subtle restriction length polymorphisms near the 5′ end of genes may well be an important component of the variation upon which selection might be expected to act.