Variations in Insect Pheromone Structure and Evolution

Chemical substances make insect pheromones what they are: compounds made from precursor molecules through chemical reactions. In most pheromones, there is more than one active component – that is, two or more constituents act by making a mixture. The nature and amount of insect pheromone elements are highly speciose, and even within close groups pheromone elements can be very different.

The chemical compositions of the identified insect pheromones were compound and derivative elements (alkanes, alcohols, aldehydes, ketones, esters, anhydrides, terpenes, amines, epoxides, nitriles, etc), which operate individually or in a limited ratio. Pheromones are similar to each other mostly in the following ways:

And first is the length and alteration of the carbon skeleton. Pheromones of insects are carbon-skeleton-based. There are 5 to 30 carbon atoms in typical pheromone compounds. It's larger, for instance, the carbon skeleton of the male adult pheromone 2-methyl-triacontane of the Mexican fruit fly Anastrepha ludens has 30 carbon atoms. Volatility of chemicals is another critical parameter in determining pheromone release rates. Molecular weights that are too big (more than 30 carbon atoms) or too small (less than 5 carbon atoms) don't work well, so most airborne pheromones have molecular weights from 80 to 300 D. Even similar-functioning pheromones differ in length of carbon skeleton across species. Sex pheromone of Drosophila melanogaster, for instance, consists of 7-twenty-triene, sex pheromone of D simulans consists of 7-twenty-pentene, and 7-twenty-triene and 7-twenty-pentene differ by 2 carbon atoms. Alkanes' skeleton could be broken down and translated into further pheromone molecules. Oxidation to alcohols, aldehydes, ketones and acids, dehydrogenation and cyclization to produce terpenes, addition of halogens or amino acids to produce halogen substitutions and amines, dehydration condensation of carboxylic acids and alcohols to produce esters or epoxides. Such changes have amplified the varieties of pheromones.

Second is unsaturation and double bond position. Desaturase also forms unsaturated alkanes and its derivatives. It is the unsaturation number between 1 and 4 for insect pheromones, and the orientation of double bonds varies. Green-headed flat moth Planotortrix excessana uses (Z)-5-tetradecene-1-ol acetate and (Z)-7-tetradecene-1-ol acetate for pheromone components, and sister moth Planotortrix octouses uses (Z)-8-tetradecene-1-ol acetate and (Z)-10-tetradecene-1-ol acetate. They are both dienes but they are placed in different places around the double bond. While (Z,Z)-7,11-heptadiene is the principal component of Drosophila melanogaster pheromone in most of the world, the principal component of Drosophila melanogaster pheromone in Africa and the Caribbean is (Z,Z)-5,9-heptadiene, and the double bond position differs too. The double bond difference in pheromones has to do with the dehydrogenase or deunsaturase in the body, and the double bond difference eventually has an effect on how pheromones work.

Third is chiral carbon and component proportion. Chiral carbonation in insects also influences pheromone activity. Pollinating insects, for instance, track plants using the pheromone carvone oxide, and the chiral carbon chemistry of trans-carvone oxide and cis-carvone oxide is not the same. The antennae of two pollinating insects, Euglossina E. at-leticana and E. niveofasciate, respond better to trans-carvone oxide than to cis-carvone oxide, and both species may be pollinating in a different plant with different results. Some insects even react to stimulation of pheromone isomers without pheromone. For instance, pheromone of the stingless bee Melipona solani has only cis-2-heptanol and no trans-2-heptanol, but insect antennal potential indicates that its group is sensitive to both cis-2-heptanol and trans-2-heptanol stimulation. Moreover, the proportion of different pheromone elements will influence pheromone function. A bark beetle called Gnathotrichus sulcatus in Douglas fir in the United States. It wasn't responsive to either cis-6-methyl-5-heptene-2-ol or trans-6-methyl-5-heptene-2-ol, but to a combination of the two, suggesting that the concentration of each pheromone is capable of different purposes.

The changes in insect pheromones are synchronized with the evolution of insect species. Different insects evolved from a common ancestor, so their pheromones must have certain similarities or connections with each other. Through the analysis of pheromones of various insects, it was found that most insect pheromones are transformed from precursor compounds, but only changed in composition and ratio. At present, it is believed that there are two ways of pheromone evolution, as follows:

The first is a slow change, with the composition of the pheromone changing somewhat – say, the amount of one component of the pheromone increases or decreases, or the ratio of those components changes. The pheromone molecules of several species of insects in the same genus, for instance, are nearly identical, or even exactly the same, just in different proportions. For instance, the four harvester ant species in the Pogonomyrmex genus all have three pyrazines used as a sign for where to find food, but proportions of the three components are different in the four ants; the Chinese honey bee Apis cerana cerana cerana and the Italian honey bee Ap.mellifera ligustica of the same genus, although they differ in behaviour and physiology, they both employ four chemicals, trans-9-o-2-decenoic acid (9-ODA), trans-9-hydroxy-2-decenoic acid, cis-9-hydroxy-2-decenoictic acid and methyl paraben, as queen mandibular gland pheromones that prevent the bee colony from driftingpheromone, though the proportions of all four substances are different in the two bee species. Evolution of species is slow. If we can establish the law of pheromone shifts across species, then we can measure kinship and evolutionary dynamics between species and start to have a way to think about biological system evolution.

The second way is that pheromones change so dramatically that the pheromones of the two species are either hugely different or completely different. The So Tomé fruit fly D santomea and the Yakuba fruit fly D yakuba are sisters on the island of So Tomé. Both species were analysed using gas chromatography-mass spectrometry, their surface hydrocarbons were identified as consisting mostly of (Z)-7-tricosene, and the rest were very obviously unrelated so that the male adults of each species were unable to detect the female adults of the other, and they became reproductively isolated. The apple moth Y padellus and the plum moth Y evenymellus of the genus Y ponomeuta are similar, but their pheromones are entirely different, and so diverged. It could be that it's different because the outside world or your living environment has altered. According to the original pheromone pathway, insects can also make a new element using a few simple chemical reactions, so there are no big leaps you can take in pheromone composition.

Please kindly note that our products and services are for research use only.