Claude Wicker-Thomas

Relations between cuticular hydrocarbon (HC) polymorphism, resistance against desiccation and breeding temperature; a model for HC evolution in D. melanogaster and D. simulans

D. simulans and D. melanogaster present two types of polymorphism in their cuticular hydrocarbon (HC) composition. Especially both sexes of D. simulans, and D. melanogaster males display 7-tricosene (7T) as the major compound type [7T]s and [7T]m, or 7-pentacosene (7P) [7P]s and [7P]m. D. melanogaster females display 7,11-heptacosadiene (7,11HD) as the major compound: [7,11HD]m, or 5,9-heptacosadiene (5,9HD): [5,9HD]m. The [7P]s, [7P]m and [5,9HD]m are mainly present in central Africa. A significant correlation was found between latitude and the proportion of compounds with 23 and 25 carbon atoms, especially 7T and 7P in both sexes of D. melanogaster. [7P]m type of D. melanogaster, characterized with an excess of C25 compounds, presents a higher resistance against desiccation than [7T]m type, where C23 compounds are more abundant. These differences can be correlated with calculated HC fusion temperatures. Moreover, increasing the breeding temperature from 18 to 29°C induces in D. melanogaster males an increase in 25C compounds and a decrease in 23C compounds, but the opposite effect in D. simulans. A mathematical model of biosynthesis, based on kinetics of elongation and decarboxylation enzymes, suggests that a simple variation of the efficiency of an elongation enzyme may account for the differences observed between the [7T]m and [7P]m types of D. melanogaster and [7T]s and [7P]s types D. simulans. Finally on the basis of the geographical distribution of the HC types of both Drosophila species, an evolutionary dispersal pathway is proposed and discussed in relation to the environment and reproductive behavior.

A gene responsible for a cuticular hydrocarbon polymorphism in Drosophila melanogaster

Drosophila melanogaster is polymorphic for the major cuticular hydrocarbon of females. In most populations this hydrocarbon is 7,11-heptacosadiene, but females from Africa and the Caribbean usually possess low levels of 7,11-heptacosadiene and high quantities of its position isomer 5,9-heptacosadiene. Genetic analysis shows that the difference between these two morphs is due to variation at a single segregating factor located on the right arm of chromosome 3 near map position 51.5 and cytological position 87C-D. This is precisely the position of a desaturase gene previously sequenced using primers derived from yeast and mouse, and localized by in situ hybridization to the polytene chromosomes of D. melanogaster. Alleles of this desaturase gene may therefore be responsible for producing the two hydrocarbon morphs. Mating tests following the transfer of these isomers between females of the two morphs show that, in contrast to previous studies, the hydrocarbon profiles have no detectable effect on mating behaviour or sexual isolation.

A female-biased expressed elongase involved in long-chain hydrocarbon biosynthesis and courtship behavior in Drosophila melanogaster

Drosophila melanogaster produces sexually dimorphic cuticular pheromones that are a key component of the courtship behavior leading to copulation. These molecules are hydrocarbons, with lengths of 23 and 25 carbons in males (mainly with one double bond) and 27 and 29 carbons in females (mainly with two double bonds). Here, we describe an elongase gene, eloF, with female-biased expression. The 771-bp ORF encodes a 257-aa protein that shows the highest sequence identity with mouse SSC1 elongase (33%). The activity of the cDNA expressed in yeast was elongation of saturated and unsaturated fatty acids up to C30. RNAi knockdown in Drosophila led to a dramatic modification of female hydrocarbons, with decreased C29 dienes and increased C25 dienes accompanied by a modification of several courtship parameters: an increase in copulation latency and a decrease in both copulation attempts and copulation. Feminization of the hydrocarbon profile in males by using targeted expression of the transformer gene resulted in high expression levels of eloF, suggesting that the gene is under the control of the sex-determination hierarchy. There is no expression of eloF in Drosophila simulans, which synthesize only C23 and C25 hydrocarbons. These results strongly support the hypothesis that eloF is a crucial enzyme for female pheromone biosynthesis and courtship behavior in D. melanogaster.

A female-specific desaturase gene responsible for diene hydrocarbon biosynthesis and courtship behaviour in Drosophila melanogaster

Drosophila melanogaster shows sexually dimorphic cuticular hydrocarbons, with monoenes produced in males and dienes produced in females. Here we describe a female‐specific desaturase gene, desatF. RNAi knock‐down led to a dramatic decrease in female dienes and increase in monoenes paralleled with an increase in copulation latency and a decrease in courtship index and copulation attempts by the males. The desatF gene was also expressed in females from D. sechellia, rich in dienes, but not D. simulans, which produce only monoenes. When hydrocarbons were feminized in D. melanogaster males by targeted expression of the transformer gene, the expression of desatF occurred. These results strongly suggest that desatF is a crucial enzyme for female pheromone biosynthesis and courtship behaviour in D. melanogaster.

Drosophila melanogaster Acetyl-CoA-Carboxylase Sustains a Fatty Acid–Dependent Remote Signal to Waterproof the Respiratory System

Fatty acid (FA) metabolism plays a central role in body homeostasis and related diseases. Thus, FA metabolic enzymes are attractive targets for drug therapy. Mouse studies on Acetyl-coenzymeA-carboxylase (ACC), the rate-limiting enzyme for FA synthesis, have highlighted its homeostatic role in liver and adipose tissue. We took advantage of the powerful genetics of Drosophila melanogaster to investigate the role of the unique Drosophila ACC homologue in the fat body and the oenocytes. The fat body accomplishes hepatic and storage functions, whereas the oenocytes are proposed to produce the cuticular lipids and to contribute to the hepatic function. RNA–interfering disruption of ACC in the fat body does not affect viability but does result in a dramatic reduction in triglyceride storage and a concurrent increase in glycogen accumulation. These metabolic perturbations further highlight the role of triglyceride and glycogen storage in controlling circulatory sugar levels, thereby validating Drosophila as a relevant model to explore the tissue-specific function of FA metabolic enzymes. In contrast, ACC disruption in the oenocytes through RNA–interference or tissue-targeted mutation induces lethality, as does oenocyte ablation. Surprisingly, this lethality is associated with a failure in the watertightness of the spiracles—the organs controlling the entry of air into the trachea. At the cellular level, we have observed that, in defective spiracles, lipids fail to transfer from the spiracular gland to the point of air entry. This phenotype is caused by disrupted synthesis of a putative very-long-chain-FA (VLCFA) within the oenocytes, which ultimately results in a lethal anoxic issue. Preventing liquid entry into respiratory systems is a universal issue for air-breathing animals. Here, we have shown that, in Drosophila, this process is controlled by a putative VLCFA produced within the oenocytes.

Involvement of desat1 gene in the control of Drosophila melanogaster pheromone biosynthesis.

Cuticular pheromones in Drosophila melanogaster are unsaturated hydrocarbons with at least one double bond in position 7: 7-tricosene and 7-pentacosene in males and 7,11-heptacosadiene and 7,11-nonacosadiene in females. We have previously shown that a desaturase gene, desat1, located in chromosome region 87 C could be involved in this process: the Desat1 enzyme preferentially leads to the synthesis of palmitoleic acid, a precursor of ω7 fatty acids and 7-unsaturated hydrocarbons. Therefore, we have searched for P–elements in the 87 region and mapped them. One was found inserted into the first intron of the desat1 gene. Flies heterozygous for this insertion showed a large decrease in the level of 7-unsaturated hydrocarbons, comparable to that observed in flies heterozygous for a deficiency overlapping desat1. Less than 1% of flies homozygous for this insertion were viable. They were characterized by dramatic pheromone decreases. After excision of the transposon, the pheromone phenotype was reversed in 69% of the lines and the other excision lines had more or less decreased amounts of 7-unsaturated hydrocarbons. All these results implicate desat1 in the synthesis of Drosophila pheromones.

Genetic studies on pheromone production in Drosophila

Publisher Summary This chapter focuses on the genetic studies on pheromone production in Drosophila. The hydrocarbon systems of mature D. simulans of both sexes are very similar and consist of 7-monoenes. A geographical polymorphism concerning linear hydrocarbons with 23 and/or 25 carbons (especially 7-T and 7-P) occurs in both sexes. When 29 populations of D. simulans were compared for their cuticular hydrocarbons, only flies around the Benin Gulf in Africa showed higher levels of 7-P compared to those of 7-T. The cumulative amounts of both hydrocarbons (7-T + 7-P) were constant and the Seychelles strain was found to be of the general type. This system favored the study of genetic control of pheromone production, as there is no sexual isolation between the two D. simulans strains. Also, the absence of qualitative sexual dimorphism allowed both sexes to be used for analyses. The chapter also provides mutational analysis of the structural genes in Drosophila melanogaster, which is a viable approach to the understanding of the involved biosynthetic pathway.

Sexual Communication in the Drosophila Genus

In insects, sexual behavior depends on chemical and non-chemical cues that might play an important role in sexual isolation. In this review, we present current knowledge about sexual behavior in the Drosophila genus. We describe courtship and signals involved in sexual communication, with a special focus on sex pheromones. We examine the role of cuticular hydrocarbons as sex pheromones, their implication in sexual isolation, and their evolution. Finally, we discuss the roles of male cuticular non-hydrocarbon pheromones that act after mating: cis-vaccenyl acetate, developing on its controversial role in courtship behavior and long-chain acetyldienylacetates and triacylglycerides, which act as anti-aphrodisiacs in mated females.

Flexible Origin of Hydrocarbon/Pheromone Precursors in Drosophila melanogaster

In terrestrial insects, cuticular hydrocarbons (CHCs) provide protection from desiccation. Specific CHCs can also act as pheromones, which are important for successful mating. Oenocytes are abdominal cells thought to act as specialized units for CHC biogenesis that consists of long-chain fatty acid (LCFA) synthesis, optional desaturation(s), elongation to very long-chain fatty acids (VLCFAs), and removal of the carboxyl group. By investigating CHC biogenesis in Drosophila melanogaster, we showed that VLCFA synthesis takes place only within the oenocytes. Conversely, several pathways, which may compensate for one another, can feed the oenocyte pool of LCFAs, suggesting that this step is a critical node for regulating CHC synthesis. Importantly, flies deficient in LCFA synthesis sacrificed their triacylglycerol stores while maintaining some CHC production. Moreover, pheromone production was lower in adult flies that emerged from larvae that were fed excess dietary lipids, and their mating success was lower. Further, we showed that pheromone production in the oenocytes depends on lipid metabolism in the fat tissue and that fatty acid transport protein, a bipartite acyl-CoA synthase (ACS)/FA transporter, likely acts through its ACS domain in the oenocyte pathway of CHC biogenesis. Our study highlights the importance of environmental and physiological inputs in regulating LCFA synthesis to eventually control sexual communication in a polyphagous animal.

Control of female pheromones in Drosophila melanogaster by homeotic genes

We have investigated the role of the Antennapedia and Bithorax complexes (ANT-C and BX-C) on the production of cuticular hydrocarbons in Drosophila melanogaster. In males, there is little, if any, influence of these complexes on the hydrocarbon pattern. In females, there are large and opposite effects of these complexes on diene production: two ANT-C mutations cause an increase in diene production and a reduction of monoenes, whereas most BX-C mutations result in a decrease in dienes and an increase in monoenes, although their sum remains constant. The effect is the highest in Mcp and iab6 females. It is suggested that a factor originating from the prothorax might activate the conversion of monoenes to dienes in females. The abdomen seems to have a crucial role in the production or control of pheromones: abdominal segments four to seven have the main effects, with a most dramatic effect for segments four and five.