A Delta 9 desaturase gene with a different substrate specificity is responsible for the cuticular diene hydrocarbon polymorphism in Drosophila melanogaster

Drosophila melanogaster cuticular pheromones consist of unsaturated hydroca rbons with at least one double bond in position 7: 7 tricosene (T) in males and 7,11 heptacosadiene (HD) in females. However, in many African populati ons like the Tai strain, females possess low levels of 7,11 HD and high lev els of its positional isomer 5,9 Ho. We have previously isolated a desatura se gene, desat1, from the Canton-S strain (CS), a 7.11 HD-2-rich morph of D . melanogaster. This desaturase is located in 87C, a locus that has been in volved in the difference between 7,11 Ho and 5,9 Ho morphs. Therefore, we h ave searched for different desaturase isoforms in both strains. We first cl oned desat1 in the Tai strain and report here functional expression of desa t1 in CS and Tai. In both strains, the Desat1 enzymes have the same Delta 9 specificity and preferentially use palmitate as a substrate, leading to th e synthesis of omega 7 fatty acids. Also found was a desaturase sequence, n amed desat2, with a homologous catalytic domain and a markedly different N- terminal domain compared with desat1. In CS genome, it lies 3.8 kb upstream of desat1 and is not transcribed in either sex. In the Tai strain, it is e xpressed only in females and acts preferentially on myristate, leading to t he synthesis of omega 5 fatty acids. We suggest, therefore, that desat2 mig ht play a control role in the biosynthesis of 5,9 Ho hydrocarbons in Tai fe males and could explain the dienic hydrocarbon polymorphism in D. melanogas ter.