Tarsi of Male Heliothine Moths Contain Aldehydes and Butyrate Esters as Potential Pheromone Components

The Evolutionary Importance of Intraspecific Variation in Sexual Communication Across Sensory Modalities

The evolution of sexual communication is critically important in the diversity of arthropods, which are declining at a fast pace worldwide. Their environments are rapidly changing, with increasing chemical, acoustic, and light pollution. To predict how arthropod species will respond to changing climates, habitats, and communities, we need to understand how sexual communication systems can evolve. In the past decades, intraspecific variation in sexual signals and responses across different modalities has been identified, but never in a comparative way. In this review, we identify and compare the level and extent of intraspecific variation in sexual signals and responses across three different modalities, chemical, acoustic, and visual, focusing mostly on insects. By comparing causes and possible consequences of intraspecific variation in sexual communication among these modalities, we identify shared and unique patterns, as well as knowledge needed to predict the evolution of sexual communication systems in arthropods in a changing world.

Functional identification of fatty acyl reductases in female pheromone gland and tarsi of the corn earworm, Helicoverpa zea

Most moths utilize sex pheromones released by the female to attract a mate. Females produce the sex pheromone in the pheromone gland in a biosynthetic pathway which consists of several key enzymes. Fatty acyl-CoA reductase is one of the key enzymes, which catalyzes the conversion of fatty acyl-CoA to the corresponding alcohol, playing an important role in producing the final proportion of each pheromone component. In Helicoverpa zea, (Z)-11-hexadecenal is the major sex pheromone component in female pheromone glands and previously a large amount of hexadecanal was also found in female and male tarsi. In our previous study, we compared the transcriptome between pheromone glands and tarsi and found 20 fatty acyl-CoA reductases in both tissues. In this study, we functionally characterized four FARs which were expressed at high levels according to the transcriptome of pheromone glands and tarsi. Fatty acyl-CoA reductase 1 was homologous to other moth pheromone gland specific fatty acyl-CoA reductases, and it was also present in male tarsi. Functional expression in yeast cells indicates that only fatty acyl-CoA reductase 1 was able to produce fatty alcohols. In addition, a decreased mRNA level of fatty acyl-CoA reductase 1 in female pheromone glands and male tarsi by RNAi knockdown caused a significant decrease in the production of (Z)-11-hexadecenal in pheromone glands and hexadecanal in male tarsi. This study is the first to demonstrate the direct function of a fatty acyl-CoA reductase in male tarsi and also confirms its role in sex pheromone biosynthesis in H. zea.

Transcriptional comparison between pheromone gland-ovipositor and tarsi in the corn earworm moth Helicoverpa zea

The corn earworm, Helicoverpa zea, utilizes (Z)-11-hexadecenal as the major sex pheromone component. The saturated fatty acid derivative hexadecanal is also found in the pheromone gland and recently a large amount (0.5-1.5 μg) was found in male tarsi with lower amounts (0.05-0.5 μg) in female tarsi. In this study, we compared the transcriptome between female pheromone glands (including the ovipositor) and female and male tarsi to identify differences between these tissues, particularly the genes involved in sex pheromone biosynthesis and chemosensation. We found transcripts encoding 9 fatty acyl-CoA desaturases, 20 fatty acyl-CoA reductases, 8 alcohol oxidases, some G protein-coupled receptors and many transcripts involved in signal transduction and pheromone transportation. Also we found gustatory and olfactory receptors associated with the tarsi and ovipositor. Differential expression analysis showed that there were many genes differentially expressed between tissues, including the candidate desaturases, fatty acyl-CoA reductases, and alcohol oxidases. We discuss how some of these genes produce proteins that could be involved in the biosynthesis of hexadecanal in tarsi and (Z)-11-hexadecenal in the pheromone gland and the possible role of proteins in chemosensation of the tarsi and ovipositor.

Molecular elements of pheromone detection in the female moth, Heliothis virescens

Pheromones play pivotal roles in the reproductive behavior of moths, most prominently for the mate finding of male moths. Accordingly, the molecular basis for the detection of female-released pheromones by male moths has been studied in great detail. In contrast, little is known about how females can detect pheromone components released by themselves or by conspecifics. In this study, we assessed the antenna of female Heliothis virescens for elements of pheromone detection. In accordance with previous findings that female antennae respond to the sex pheromone component (Z)-9-tetradecenal, we identified olfactory sensory neurons that express its cognate receptor, the receptor type HR6. All HR6 cells coexpressed the "sensory neuron membrane protein 1" (SNMP1) and were associated with supporting cells expressing the pheromone-binding proteins PBP1 and PBP2. These features are reminiscent to male antennae and point to congruent mechanisms for pheromone detection in the two sexes. Further analysis of the SNMP1-expressing cells revealed a higher number in females compared to males. Moreover, in females, the SNMP1 neurons were arranged in clusters, which project their dendrites into a common sensillum, whereas in males there were only solitary SNMP1-neurons and only 1 per sensillum. Not all SNMP1 positive cells in female antennae expressed HR6 but instead the putative pheromone receptors HR11 and HR18, respectively. Neurons expressing 1 of the 3 receptor types were assigned to different sensilla. Together the data indicate that on the antenna of females, sensory neurons in a subset of sensilla trichodea are equipped with molecular elements, which render them responsive to pheromones.

Experimental evidence for chemical mate guarding in a moth

In polyandrous species, males seek to maximize their reproductive output by monopolizing their mate. Often the male transfers substances to the female that suppress her sexual receptivity or antagonize the behavior of competing males; both are usually transferred in seminal fluids and represent forms of chemical mate guarding. In moths, more long-range female sex pheromones have been identified than in any other animal group, and males often display with close-range sex pheromones, yet odor-based post-copulatory mate guarding has not been described in moths so far. We tested the hypothesis that the male sex pheromone in the noctuid moth Heliothis virescens perfumes the female and functions as an anti-aphrodisiac. Indeed, virgin females perfumed with male pheromone extract, or with its main component, mated significantly less than control virgin females, and this effect persisted for two successive nights. This chemical mate guarding strategy was disadvantageous for H. virescens females, because the reproductive output of twice-mated females was significantly higher than that of once-mated females. Since the female and male sex pheromones are biosynthetically related in this and other moth species, chemical mate guarding may also impose selection pressure on the long-range female sex pheromone channel and consequently affect the evolution of sexual communication.