Drosophila melanogaster cuticular pheromones consist of unsaturated hydrocarbons 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 populations like the Tai strain, females possess low levels of 7,11 HD and high levels of its positional isomer 5,9 HD. We have previously isolated a desaturase 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 involved in the difference between 7,11 HD and 5,9 HD morphs. Therefore, we have searched for different desaturase isoforms in both strains. We first cloned desat1 in the Tai strain and report here functional expression of desat1 in CS and Tai. In both strains, the Desat1 enzymes have the same ⌬9 specificity and preferentially use palmitate as a substrate, leading to the synthesis of 7 fatty acids. Also found was a desaturase sequence, named 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 expressed only in females and acts preferentially on myristate, leading to the synthesis of 5 fatty acids. We suggest, therefore, that desat2 might play a control role in the biosynthesis of 5,9 HD hydrocarbons in Tai females and could explain the dienic hydrocarbon polymorphism in D. melanogaster.acyl-Coa desaturase ͉ pheromone biosynthesis U nsaturated fatty acids are structural components of membrane lipids. In animals, fatty acids are desaturated by a membrane-bound enzyme complex involving cytochrome b5, cytochrome b5 reductase, and a desaturase (1-3). Desaturases catalyze the introduction of a double bond into the hydrocarbon chain of a fatty acyl-CoA, at a position determined by the enzyme specificity. A low number of animal desaturases have been both molecularly and functionally characterized by heterologous expression in yeast or in Arabidopsis: ⌬9 and ⌬6 desaturases from rat (4, 5), ⌬6 desaturase from mouse (6), 3, ⌬6, and ⌬5 desaturases from Caenorhabditis elegans (7-9), and ⌬11 and ⌬9 desaturases from the moth Trichoplusia ni (10, 11).In the Drosophila melanogaster subgroup, flies use double bonds in cuticular hydrocarbons among other structural parameters to signal sex or species (12, 13). Males have high levels of monoenes, whereas females are rich in dienes, for example 7-tricosene (7T) and 7,11 heptacosadiene (7,11 HD) in the Canton-S strain (CS) (14).Studies in D. melanogaster strongly suggest that the biosynthesis of male monoenes and female dienes follows the same elongation-decarboxylation mechanism characterized in Musca domestica (15, 16) and shares early steps up to vaccenic acid, a common precursor (refs. 17 and 18; Fig. 1). The involvement of a desaturase with a ⌬9 specificity to introduce the common double bond has also been hypothesized (18). We have previously isolated a CS D. melanogaster desaturase gene, desat1, expre...

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