Rory Atteridge 0603612E Sex Pheromones in the Insects Introduction Sex pheromones are chemicals or odours given off by an individual in order to invoke a sexual response or behaviour change on individuals of the same species (Shorey 1973). These chemicals can be released by males and/or females, depending on the species (Gieselhardt et al. 2008, Ayasse et al. 2001). Pheromones generally consist of a carbon backbone of between 10 and 20 carbon atoms (in Lepidoptera), these are however arranged to form multitudes of different compounds, ranging from methyl groups and alcohols to fatty acids and acetates.
These include isomers and stereoisomers of the same compounds allowing for a large diversity and chance for high species specificity (Raina 1993, Ayasse et al. 2001, Roelofs and Brown 1982, Lanier 1970). Sex pheromones have been studied in greater detail over recent years, but most historical work was done on Hymenoptera and Lepidopteran species. However, work has been done on Coleoptera, Dictyoptera and Mantodea(Perez 2005, Ayasse et al. 2001). Pheromones are released from many anatomical sites of the insect body.
Some of the more obvious and common places include in pheromone glands between the 8th and 9th segments of the lepidopteran abdomen and the aedaegal glands of most insects (Gieselhardt et al. 2008, Roelofs and Brown 1982, Ayasse et al 2001). There are many other, more unexpected, locations at which pheromones are released or stored. These include the antennae of many parasitic wasps (Isidoro et al. 1996), the cuticle of insects in the form of lipids (Jurenka et al. 2007), venom glands of ants and mandibular glands of some flightless bees (Ayasse et al. 2001). Detection and tracking of pheromones
Other location mechanisms include the orientation of an insect towards the source point of a pheromone. In the Desert Tenebrionid Beetle, once the male detects the sex pheromone of the female, he stops and walks in a circle in what seems to be an orientation to the source of the scent (Geiselhardt et al. 2008). This behaviour seems to be a relatively common method for determining pheromones in walking insects. Flying insects have another way of locating the source of a sex pheromone. This method entails an insect flying in a zigzag pattern towards the source of the pheromone, narrowing he track of the zigzag as it approaches the source. This has been documented in many moth species and most recently in the Cowpea weevil by Kuenen and Rowe (2006). In order to track a source point of a pheromone, recent research by Baker et al. (2008) postulates that pulsing release of pheromones, a common observation in many species, is necessary in order for receptor mechanisms not to be over run by large amounts of a particular stimulus. This could be corroborated by the research done on half life of certain sex pheromones (mainly bombykol) by Vogt et al. 1985), showing that there is a decrease in half life of bombykol once it has come into contact with degrading enzymes on the antennae of moths. Should there, however, be a constant flow of pheromone, especially from distance, there could be a loss of orientation of an individual in a “cloud” of pheromone. Effects of sex pheromones Sex pheromones can have many different effects on conspecifics. These however can be separated into two major groups; those that aggregate the opposite sex around the pheromone releasing individual, and those that start sexual courting behaviour.
Attractant pheromones are released, generally in higher quantities than the courting ones, to cause either an aggregation of mates around the source point, to go hand in hand with calling behaviours or attracting mates in lower population density areas (Baker et al. 2008). With courting pheromones, the smaller amounts of pheromones are released and change the behaviour of the receiving insect (at a shorter distance) to a more sexual behaviour, initiating courting behaviour and eventually copulation (Baker et al. 2008, Geiselhardt et al. 2008).
The advantage of having a long and a short distance sex pheromone can be solidified with what I mentioned earlier, in that a large amount of pheromone could disorientate or lead to the sensory overload of an individual tracking towards a point source. Thus if two or more compounds constitute a pheromone in a high to low ratio, there could be tracking from distance, followed by a much more accurate tracking in close quarters. This can possibly reduce the potential sensory overload Pheromone specificity Sexual pheromone specificity was described back in 1970 by Lanier.
More importantly, in 1973, Klun et al. describe sex pheromones as not a single compound, but as ratio of at least two compounds, in their case the two compounds were geometrical isomers. By not only showing that sexual interest and aggregation was decreased when exposed to pure samples of either compound, than when both compounds were present, they showed that this ratio had to be nearly exact. This opened a new door to specificity research in sexual pheromones. Nevertheless, it is not just the ratios of pheromones that make up the big picture.
It may be important for specificity within a family or genus, but when looking more broadly, different compounds do play a large role in specificity of sex pheromone communication (Ayasse et al. 2001). Good examples of this specificity are the bees. Certain species will have near to identical pheromone constituents, with merely one component being an isomer or a slightly different compound (Ayasseet al. 2001) Advantages to pheromone specificity Genetically there is a large advantage to being very species specific with the respective sex pheromones. Below are a couple examples showing this.
In Canada, three closely related species of Bark Beetles (_Ips_) reside within the same vegetation types and within the same locations. These beetles have a very specific sex pheromone that allows them to keep the three species separate instead of becoming one hybrid species. Forced cross species breeding in the laboratory led to new, hybrid species which were fertile and produced fertile young (Lanier, 1970). A field study also undertaken showed that hybrid species were extremely rare. This shows that the specificity of pheromones can keep similar species separate, thus not lead to a hybrid species.
In a somewhat converse way, female flies (_Drosophila_) tend to choose to mate with males who are unrelated as opposed to those that are more closely related genetically (Smith 1983). This is also seen with halictine bees, where the male will mate with a female once and will then not mate with her again or even with other females who are closely related to the originally mated female. This is likely to be due to a specific sex attractant that changes slightly in its genetic make up with every generation and the bees can detect these differences (Smith 1983).
This allows for the expansion of a male’s genes throughout the population in question or to lead to a greater overall population genetic diversity. In a dissimilar strategy to the previous example, tsetse flies, which have a very long life cycle and low numbers of offspring, have a pheromone that remains the same genetically between populations (Jurenka et al. 2007). This strategy may vary from those insect with a high population turnover as described by Smith (1983), but it has fundamentally the same purpose.
By keeping the same pheromone across populations, the individuals of either population can find each other and breed, allowing for a constant gene flow between population (Jurenka et al. 2007), and again a high genetic diversity between and within populations. The specificity of sex pheromones has another advantage apart from just a genetic component, the pheromone concentrations and compound could be varied depending on the individual’s strength and thus the strength of sexual selection (Rantala et al. 2002).
Female mealworms seem to discriminate on males based on their pheromones, as these pheromones relay important information relating to the males. Rantala et al. (2002) tested, and showed that immunocompetence information is communicated through pheromones. Other information is also communicated with pheromones, including parasite burden (Penn and Pots 1998), male aggressiveness (Moore and Moore 1999) and female fitness (Moore et al. 2001). The communication of these signals through sex pheromones could be seen as a mechanism by which a population increases its strength and possibly increase general immunity to certain threats.
Thus, pheromones can play a large role in mate selection within species and can relate directly to sexual selective forces. Interesting uses of sex pheromones There are many species of insect that use sex pheromones in different ways when compared to the norm. Butterflies have pheromone signalling, but some species use coloration for long distance communication and to cause aggregation of the opposite sex, only then do these species use sex pheromones (in close quarters) in order to cause the onset of sexual or courting behaviours (Raina 1993).
The bagworm moth does not release her sex pheromone into the air as would be considered normal. Instead she keeps the compound in hairs on the underside of her thorax. Being a flightless moth, these hairs rub off on a surface, leaving a trail to where she is (Leonhardt et al. 1983). Furthermore, during pupation, it seems that sex pheromone is stored inside the pupae capsule and, at ecdysis, this pheromone is released into the environment before the moth has emerged. This causes an aggregation of males and mating happens almost instantaneously (Leonhardt et al. 983). This could play a large advantage in decreasing risky movement for copulation and thus decrease the chances of predation. Social insects (mainly ants and bees) have very complicated systems of sex pheromones (Ayasse et al. 2001). The aggregation of numerous male flying ants at a specific time is thought to be mediated by pheromones. These aggregations occur not only within nests but between nests concurrently. This could allow for an increase in the dispersion, and interaction of genetic properties between populations.
The queen of a nest will undergo calling posture and release sex pheromones from within or around the nest, and mating of the queen will occur (Ayasse et. Al 2001). The sex pheromone that these queens release is undercane, which interestingly is also implicated in alarm pheromones of other ant species (Ayasse et al. 2001). This sexual aggregation pheromone can also be found in the venom gland of some ant species, showing a possible evolution of the pheromone from being an alarm pheromone to a sexual one.
In weevil species, this is also seen, where the three components making up their sex pheromones are well documented defensive chemicals in many arthropod species (Geiselhardt 2008) Bee queens also have sex pheromone stores in their mandibular glands. This compound has an extremely strong effect on drones of the hive. In stingless eusocial bees it has been documented to be able to attract drones from 60m (Ayasse et. al. 2001). The strength of this sex pheromone shows that there is a big effect of sex pheromones on the social behaviour of bee species. Conclusion
In conclusion, sex pheromones are very varied substances, comprising of many different compounds and causing many different behaviours of the receiving individual. These pheromones are not necessarily bound to being a sex pheromone just based on composition, as many sex pheromones of one species are the defensive pheromone of another. There are many advantages to using pheromones, especially when there is high species specificity. From keeping genetics separate (as seen in the Ips, Drosophila and halictine bee) to allowing for the out breeding between populations (Tsetse flies) and maintaining genetic flow.
Pheromones do have a normal method of dispersion and perception, however there are many examples in which these vary and allow us to infer lots about a species or a general behaviour. References Isidoro N, Bin F, Colazza S, Vinson SB (1996). Antennal Gustatory Sensilla and Glands in some Parasitic Hymenoptera: a Critical Morpho-Functional Approach. Journal of Hymenopteran Research. 5:206-239 Klun JA, Chapman OL, Mattes KC, Wojtkowski PW, Beroza M and Sonnet PE (1973).
Insect Sex Pheromones: Minor Amount of Opposite Geometrical Isomer Critical to Attraction. Scence. 181:661-663 Lanier GN (1970). Sex Pheromones: Abolition of Specificity in Hybrid Bark Beetles. Science. 169:71-72 Penn DJ and Potts WK (1998). Chemical Signals and Parasite-Mediated Sexual Selection. Trends in Ecological Evolution. 13:391-396 Moore AJ and Moore PJ (1999). Balancing Sexual Selection Through Opposing Mate Choice and Male Competition. Proceedings of the Royal Society of London. 266:711-716
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