Introduction
Pheromones contain blends of chemicals specific within a species, and they primarily function to attract mates or cause aggregation among the species. Although chemical communication serves an incredibly important purpose in the survival and success of all kinds of organisms, very little is known about its diversity and evolution. Part of this is due to the fact that understanding the true origins of chemical communication is very difficult. However, recent studies on the chemical communication of organisms have provided data for analyzing the origins of the chemicals as well as their relation to the evolution of the organisms themselves. Studies regarding pheromone evolution often use bark beetles to investigate the method of evolution of these chemical blends.
Bark beetles survive by attacking and overwhelming a tree’s defenses, and they rely heavily on their ability to signal other members of their species through the release and reception of aggregation pheromones [5]. The large reliance of bark beetles on trees as a habitat and food source often leads to the death of the tree as it is taken over by beetles [5]. For this reason, bark beetles are of particular interest by agricultural and foresting industries, and studies are therefore often funded by forestry programs and other organizations with agricultural interest in these insects. There are also many species in existence, allowing for evolutionary comparison between the different species.
It is believed that speciation greatly affects the diversification of these chemicals used for chemical communication. The high species-specificity in pheromone signaling prevents hybridization and mismating in organisms, and it also prevents one species from attracting a different species [4]. Despite this, it is unlikely that speciation alone causes such diversity of pheromone production [1]. Many species release substances for non-communicative functions, and this offers a starting point for investigating the methods of evolution of pheromones [1].
Cues and Signals
It is important to first differentiate between two forms of communication. Cues are typically defined as chemicals that inadvertently provide information, despite their release serving a different purpose [1]. Signals are used in intentional communication and likely arise from the evolution of cues [1]. An example of such a cue is a chemical that is released to announce the presence or location of an organism, but inadvertently tells of the organism’s sex, health, reproductive state, or dominance [1]. The receiver of the chemical can use the additional information to their benefit, but it may also have no effect. In the case that the cue elicits a neutral selectivity, no evolution toward the chemical’s effectiveness would be expected [1]. On the other hand, if the receiver responded favorably to the cue, it would likely lead to evolution of the cue as a more effective way to communicate.
The case of cue to signal evolution can take many paths, as non-communicative chemicals can lead to an array of signals through various modes. Exaptations involve many functional shifts, like metabolic byproducts becoming attractants and defensive chemicals becoming either alarms or attractants, depending on the situation [1]. Exaptations of defensive chemicals is common in insects, and these chemicals often end up functioning as sex, aggregation, alarm, or trail pheromones [1]. The chosen evolutionary route depends heavily on the receiver of the signal and the information that it actually receives. Defensive chemicals can evolve to serve as alarms, as organisms may sense the release of a defensive chemical and use it as a warning of danger [1]. Exaptations can also be seen in the amplification of a pre-existing chemical release system, and the type of exaptation depends largely on the response the receiver evokes as well as how the organism releasing the chemical tries to enhance the reception of its signal [1].
Enhancement of Signal Reception
Enhancing the reception of a signal is often accomplished by merely increasing the amount of the signal released [1]. Various organisms have evolved glands that allow for easier storage of chemicals and therefore release of larger quantities [1]. This demonstrates convergent evolution, which is likely a response to selection for release of larger amounts of chemical signals [1]. If the released chemical is connected to particular characteristics of the organism sending it, it can also be selected for.
Pheromones that indicate characteristics of the sending organism like sex, age, or reproductive state are often more difficult for receivers to sense. This is due to the nature of the quality-based chemicals as non-volatile and higher molecular weight, which in turn diminishes their ability to be easily dispersed and therefore received [1]. For this reason, selection often results in the addition of more effective signaling to those chemicals that indicate quality of the organism [1]. If the chemicals themselves were altered, it would negatively impact their effectiveness [1]. Changing the location in which signals are released is another way to change efficiency without affecting the success of the chemical [1].
Organisms typically give off blends of chemical cues and signals, made up of components that serve two purposes: volatile components attract the attention of the receiving organism, while less volatile compounds inform the receiving organism of the sender’s characteristics and quality [1]. Attention-grabbing components tend to be signals, while quality-related chemicals are often cues [1]. Such attention-grabbing components are likely intentional because in part, they enable organisms to find mates. Selection more often improves the ability of a chemical to be detected instead of affecting the actual information being sent [1]. Traits improving mating are selected for because they result in increased potential for reproductive success, which leads to improved fitness. An example of such diversifying selection is seen in burying beetles, which have two important components of signaling to attract mates. Long-chain hydrocarbons identify the beetle’s reproductive state, while esters work to attract mates from farther away [1]. Selection for improved communication often leaves the original cue unaffected and instead adds signaling or behavioral components [1].
However, it is also possible that receivers select cues that only inform them of the sender’s location and nothing else. In this case, selection can turn the original cue into a more conspicuous one that will better attract other organisms [1]. Depending on the communicatory needs, the more conspicuous signal, one of a lower molecular weight that is easily dispersed, would be selected for [1].
Plastic vs. Conserved Evolution
Disagreement over the evolutionary model of pheromones in different species of bark beetles involves two different ideas: plastic and conserved evolution [2,3]. Plastic evolution is caused by selection of predators, and it results in a distribution of pheromone components that does not match the species’ phylogenetic tree [2]. Conserved evolution, tied closely to allopatry, demonstrates a distribution of pheromone components that mirrors the phylogeny of the species and maintains the geographic separation of closely related species [2]. To test for the relationship between pheromone distribution and phylogeny, pheromones were extracted from different species of bark beetles and compared to pre-existing data on the aggregation pheromones of these insects [2,3].
Cane, Stock, Wood, and Gast (1990) studied 7 species of Ips beetles and found that the system of their pheromone communication matched their phylogenetic relationships [7]. Francke et al. (1995) determined from their study that little connection existed between the pheromones and the phylogeny of the beetles [6]. Cognato, Seybold, Teale, and Wood (1997) found, through a slightly more extensive study, that convergent evolution had occurred among the pheromones, indicating that the evolution of the pheromones was plastic and bore little connection to the phylogeny [3]. The last of these studies is the most recent and likely the most conclusive of the three. Phylogenetic analysis and homoplasy, or convergent evolution, were used to investigate how conserved or plastic the pheromone components were [2]. Volatiles of both male and female beetles were collected and extracted to separate out the components of the pheromones [2]. After matching the components to a phylogeny, those congruent with the phylogeny were expected to exhibit homoplasy similar to other data sets that were known to be tied closely to a phylogeny; however, it was determined that the pheromone components did not match the phylogenies and that the evolution of the pheromones was not conserved [2]. The pheromone components were determined to be plastic, and therefore unrelated to their evolutionary history [2].
An even more recent study of a similar design included more species of Ips and also introduced the genera Dendroctonus in order to better see the connection between pheromone evolution and species phylogeny both within a singular genus and between genera [3]. Chemicals were also fitted with updated phylogenies to determine the relationships [3]. Number of chemical-character changes along phylogeny, degree of homoplasy, and degree of phylogenetic independence were used to evaluate the genera separately [3]. To test the relationship between pheromone blend and phylogeny, phenotypic differences in blends were counted and evaluated against different models of evolution: minor change, intermediate change, and major change [3].
Species of Dendroctonus demonstrated little relation to phylogeny; the distribution of pheromones in this genus was similar to what would have been expected by chance [3]. Species of the Ips genus demonstrated a stronger connection to the phylogeny [3]. While some of the chemicals lacked such congruence and appeared to be distributed more randomly, most matched the phylogeny closely [3]. Overall, the intermediate and major change models fit the data most closely [3]. This study observed results that contradicted the findings of Cognato et al., as the distribution of Ips pheromones seemed to match the phylogeny [2,3]. It is possible this resulted from the fact that more species were used. The earlier, less expansive studies could have failed to observe such connections because of the particular species they investigated. For future research, an even larger data set should be used to more accurately determine the relationship between pheromone evolution and phylogeny.
Timing of Evolutionary Changes
Initially, it was thought that pheromone blends evolved in Darwinian fashion, with their chemical structures changing gradually over time [3]. However, because pheromones are so species specific and would therefore resist even small changes in the signals, it is unlikely that gradual changes are responsible for the diversity of pheromone blends [3]. It has been found that pheromones likely evolve through larger phenotypic changes at speciation events of the species, and saltational shifts, or drastically sudden major shifts, are the most probable method of pheromone evolution [3]. No evidence was found that aggregation pheromones evolve through slow changes, nor do the pheromones undergo simple changes to their chemical makeup [3]. For both of Ips and Dendroctonus it was shown that more closely related species exhibited larger differences in phenotype of their pheromones [3].
Effects of Species Overlap
It has also been observed that the location of the different species’ habitats affects the composition and evolution of their pheromones. While it was expected that signals of spatially overlapping species would differ more than non-overlapping species, there was no evidence of this [4]. Instead, pheromones of species whose habitats overlap are more similar to each other than those of non-overlapping species [4]. This demonstrates the effect of host trees on the pheromones blends, as the species that used the same hosts tended to be more closely related [4]. This implies that location plays a larger role in pheromone diversity than the prevention of interspecific mating. While smaller studies have used character displacement to explain why greater differences were observed when species overlapped in habitat, sympatric speciation may not be as important in the pheromone evolution of these bark beetles.
Further evidence of the effect of host trees on pheromone composition is provided by the fact that pheromones synthesized de novo have an energetic cost, while pheromones that are oxidation products of their host tree are must less costly to the organism [5]. This potentially explains why host trees play a larger role in pheromone evolution. Organisms taxed by a more energetically costly process would potentially experience lower fitness through some kind of tradeoff with pheromone production, and the production of pheromones linked to host trees would therefore be favored. The relationship between pheromone production and habitat helps to explain the lack of connection between pheromone blend and phylogeny that was observed in so many studies.
Conclusion
It had previously been difficult to determine the true relationships between pheromone evolution and phylogeny, because so few chemical signals had been analyzed. To continue this research, more work needs to be done on more species to produce larger data sets and therefore more information from which conclusions can be drawn. Researchers should use more genera as well, as that would enable them to reach stronger conclusions regarding phenotypic differences between species of the same genera. In regards to the research that was conducted, researchers failed to evaluate the chirality of many of the pheromone components [3]. Chiral consideration would increase the number of compounds and could therefore strengthen the data and findings.
The study of pheromone evolution is important, as it has several implications for the future. Pest control is incredibly important to agriculture and the forestry industry, so evaluating the aggregation pheromones of certain pest species could be very commercially valuable [3]. Understanding the mode through which pheromones become attractive or repellent could benefit those in agriculture, as they could find ways to control the insects, good or bad, that affect the growth of crops and trees. Similar to antibiotic resistance seen in bacteria, it is also possible that insects could become resistant to certain pheromones [3]. This could be useful in agriculture or foresting as well, as pheromone resistance could potentially be used by farmers or foresters to prevent aggregation of harmful species in a way that does not rely on pesticides. Exploring the pheromone evolution of a larger range of species would tell even more about how pheromones work and shed light on the potential for resistance. While a lot of research has been done on pheromone evolution, expanding the research to include a greater number of organisms would provide a much stronger story of the history.
Works Cited
[1] Steiger, S., Schmitt, T. & and Schaefer, H. (2010). The origin and dynamic evolution of chemical information transfer. Proceedings of the Royal Society Biological Sciences, 278, 970-979.
[2] Cognato, A., Seybold, S., Teale, S., & Wood, D. (1997). A cladistic analysis of pheromone evolution in Ips bark beetles (Coleoptera: scolytidae). Evolution, 51.1, 313.
[3] Symonds, M. & Elgar, M. (2004). The mode of pheromone evolution: evidence from bark beetles. Proceedings of the Royal Society Biological Sciences, 271, 839-846.
[4] Symonds, M. & Elgar, M. (2004). Species overlap, speciation and the evolution of aggregation pheromones in bark beetles. Ecology Letters, 7, 202-212.
[5] Pureswaran, D., Sullivan, B., & Ayres, M. (2006). Fitness consequences of pheromone production and host selection strategies in a tree-killing bark beetle (Coleoptera: Curculionidae: Scolytinae). Oecologia, 148, 720-728.
[6] Francke, W., Bartels, J., Meyer, H., Schroder, F., Kohnle, U., Baader, E., & Vite, J. (1995). Semiochemicals from bark beetles: new results, remarks, and reflections. Journal of Chemical Ecology, 21, 1043-1063.
[7] Cane, J., Stock, M., Wood, D., & Gast, S. (1990). Phylogenetic relationships of Ips bark beetles (Coleoptera: scolytidae): Electrophoretic and morphometric analyses of the grandicollis group. Biochemical Systematics and Ecology, 18, 359-368.