Do Hydrocarbon Profiles Change

Although it is pretty clear that all insects will have some hydrocarbons on their cuticle, and we can assign a function to some of these hydrocarbons (although this is frequently only via indirect evidence), it is still not known or fully understood if these hydrocarbons change over time in the life of the insect. A phenomenon that will be crucial if these hydrocarbons are to be used in forensic entomology. In a preliminary study, the author with co-worker Dr. Martin studied the hydrocarbon profile of bees of different ages. In this study it became clear that bees just emerging from the cells have a different hydrocarbon profile than worker bees within the colony. This change however is very rapid. Other studies have indicated that the hydrocarbon profile of ants could change with a different diet (Buczkowski et al. 2005). Therefore is seems plausible to assume that the hydrocarbon profile of an insect will fluctuate over time, hence indicating a certain status (age, gender, caste) of the insects. However, this fluctuation should not interfere with biological signalling and should therefore not involve those signalling compounds. However within a hydrocarbon profile only some hydrocarbons are used for communication, while others often have different functions (Martin et al. 2008a,b,c).

Several factors are important for changing the hydrocarbon profile of an insect. In the above section some have already been mentioned. These are genetic factors such as the age of a bee or the gender of a fly. In general the age or gender of an insect can influence the profile of the hydrocarbons present. This genetical component of hydrocarbon production has been well studied in Drosophila spp. (Ferveur 2005). Several genes are known to either induce or reduce hydrocarbon production. Much of this work has been carried out in relation to the sex pheromones in D. domestica that is (Z9)-tricosene. The (Z7) isomer which is produced by the males can either be up-regulated or down-regulated depending on the enzyme. However, the amount of saturated versus unsaturated hydrocarbons can also be altered, as when the PGa14 transposon was inserted within desat1 in D. melano-gaster production of unsaturated hydrocarbons was reduced and saturated hydrocarbon production increased (Marcillac et al. 2005). Savarit and co-workers have found that overexpression of the UAS-tra transgene in D. melanogaster after a heat shock resulted in the complete elimination of their cuticular hydrocarbons (Savarit et al. 1999). Unfortunately this level of detail is not available for all insects, and in many species the production and regulation of hydrocarbons is unknown.

A factor related to the genetic aspect is the developmental stage in which an insect is. In bees it has been shown that the profile can change over time. In ants several studies have confirmed the fact that ants of different casts have different HC profiles, e.g. foragers have more saturated hydrocarbons than ants that work within the nest (Greene and Gordon 2003; Martin and Drijfhout 2009).

Environmental factors such as diet, or temperature could have an effect on the hydrocarbons present. Diet has already been mentioned but this effect is debatable. Geoclimate or temperature can also have an effect of the hydrocarbons present (Rouault et al. 2001; Savarit and Ferveur 2002). In D. pseudoobscura, when they are found in the Mojave Desert, mainly saturated hydrocarbons are found, but when the same ants are reared in the lab mostly unsaturated hydrocarbons are produced (Toolson and Kuper-Simbron 1989). Similarly, the ratio of C35:C37 alkadienes is decreased when D. mojavensis is reared in conditions where the temperature is raised from 17°C to 34°C (Markow and Toolson 1990). All these observations can be explained by the fact that either alkanes (or longer alkanes) are more suitable for waterproofing compared to alkenes (or shorter alkanes).

For living organisms, there is another factor that has been shown to be important, and that is the micro-organisms and fungal pathogens living on, e.g., insects (Crespo et al. 2000). The reader is referred to an excellent review by Pedrini et al. (2007) on entomopathogenic fungi invading their hosts through the cuticle. Napolitano and Juarez (1997) were the first to actually show that entomopathogenic fungi can use hydrocarbons as an energy source. Hydrocarbons extracted from the blood sucking bug, Triatoma infestans appear to be a much better energy source for both Metarrhizium anisopliae and Beauveria bassiana than synthetic hydrocarbons that contained only linear alkanes. Epicuticular hydrocarbon changes were also measured on both Ostrinia nubilalis and Melolontha melolontha due to infections of two ento-mophatogenous strains, Beauveria bassiana and B. brongniartii. In O. nubilalis the total amount of hydrocarbons on the cuticle was reduced from 6.88 ± 1.6 ng to 0.84 ± 0.08 ng in just 96 h after the application of B. bassiana (Lecuona et al. 1991). A similar effect was observed in M. melolontha where the amount of extractable hydrocarbons dropped from 7.71 ± 2.53 ng to 2 ± 0.8 ng in 24 h. After 96 h the amount increased again to 15.2 ± 4.8 ng indicating a restoration of the hydrocarbon profile or deposition of hydrocarbons from the spores on the insect's cuticle (Lecuona et al. 1991). Of the hydrocarbons disappearing, the monomethylalkanes and dimethy-lalkanes were the first molecules to decrease or disappear from the cuticle (Lecuona et al. 1991). This is in accordance with earlier results from Napolitano and Juarez (1997) who showed that linear alkanes was a lesser energy source compared to a hydrocarbon extract that contained some branched hydrocarbons.

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