Semiochemical activity

This chapter reviews the literature of semiochemical (mostly pheromone) identification in Hymenoptera published since 1990. For this review, we separate the order Hymenoptera into the following three, somewhat overlapping, classes to reflect their differences in biology and semiochemistry solitary, parasitic, and social (Table 1). Although there is considerable literature on the semiochemical activity of specific glandular extracts and the chemical composition of specific glands, only those chemicals with demonstrated pheromonal (or semiochemical) activity will be specifically discussed here. The earlier literature of pheromones in social hymenoptera has previously been reviewed [4-6]. There have been more recent reviews of pheromones in social hymenoptera [7-10], parasitic wasps [11,12], sawflies and seed wasps [13,14], and mating pheromones across Hymenoptera [15]. 

The overall picture of the secretions of the dwarf antelope seems to suggest that secretions that are produced slowly are more complex. This could be explained in terms of microbiological action, which has more time to contribute to the complexity of a secretion, the slower it is produced. If this is indeed the reason for the complexity of secretions that are produced very slowly, it is possible that, in these animals, with exception of the klipspringer, the long-chain lipid constituents of the secretions could be controlled-release carrier materials rather than semiochemicals. If these heavy compounds were semiochemicals, it could be asked why is it necessary for an animal to spend so much energy to regularly renew its territorial marks. In retrospect, it is possible that up to now too much attention could have been devoted to the heavy constituents of the secretions, while the semiochemically active constituents could have been overlooked because they could be present in such low concentrations that they were not detected by the methods that were employed. 

Although quite a detailed picture of the compound types that are generally found in the exocrine secretions of the artiodactyls is slowly emerging, attempts to demonstrate the semiochemical activity of these compounds in field tests were not met with unqualified success, probably because it is mostly not feasible to apply the so-called response-guided strategy in work on the artiodactyls. Nevertheless, an enormous volume of chemical information has been made available to zoologists that are interested in studying these animals. 

The vertebrates produce, send and detect information which is conveyed by one or more molecular types. Chemical information of biological value (semiochemicals) which partly or wholly activates the accessory olfactory system (AOS) is transferred during intra- and inter-species communication. The compounds involved convey messages of social importance originating from the need to co-ordinate gamete release. It seems quite likely that gradual improvements by selection of semiochemical molecules and their receptors eventually enhanced the reproductive benefits both for the sender and for the receiver (Sorensen, 1996). The dual olfactory systems interpret chemical input to allow the discrimination of odour... 

Behavioural testing of protein fractions has not kept pace with semiochemical studies. Belcher et al. (1990) found that the mixed scent marks of the Saddle-backed Tamarin (S. fusicollis) comprise urine and genital/suprapubic gland secretions. Both sexes deposit mixtures of pheromonally active large molecules at, for example, exudate feeding... 

The prophecy on semiochemical systematics in the headline quotation at the start of this chapter remains just that — an intriguing speculation. Some single-component chemosignals do turn up as apparently the main active compound in a complex secretion. In male gerbils (Meriones) one volatile, phenylacetic acid, appears to represent the dominant male state (Thiesen, 1974). Individuality must be added by further components — dietary or variable sebum constituents in this case. Indeed, amongst mammals and some reptiles, complex mixtures seem to be the norm very few taxonomically relevant examples have emerged. 

Field and laboratory bioassay of chemosignals from related sympatric and allopatric species (overlapping and discrete distributions) are essential to an understanding of the relatedness or otherwise of functionally active compounds. The semiochemicals involved in speciation surely utilise the main and vomeronasal senses, but their relative contributions cannot be predicted at present. 

One well-analysed chemosignal system is that of the Red-bellied Newt (Cynops pyrrhogaster). Males of this species produce from the abdominal gland a semiochemical protein (sodefrin) with marked VNO activity as a female attractant (Kikuyama et al., 1997). Courtship displays of newts often contain tail-waving bouts, which direct cloacal or other secretions from the male toward female recipients (Fig. 3.1). A large stable molecule like sodefrin, alone or as part of a VNPr complex, is presumably suitable for such local transference. 

There are three reviews emphasizing the importance of synthesis in pheromone science [4],in semiochemicals research [5],and in chemical ecology [6]. Stereochemistry-pheromone activity relationships are also discussed in the above three reviews, and more thoroughly in three other reviews [7-9]. 

Much as in other mammals, human females concentrate in their mammary regions features potentially related with infant-directed communication. The potential semiochemical significance of the human breast will be examined here in terms of 1) morphology and secretory activity of areolar skin glands 2) infant responses when exposed to the effluvium of their mother s breast and areolae, and to the odour of related secretions 3) volatile compounds present in pure areolar secretions as compared with those of milk 4) relations of maternal areolar gland endowment to adaptive outcomes in the infant and the mother. 

Fig-1 Schematic view of the overall olfactory processing in insects. Pheromones and other semiochemicals are detected by specialized sensilla on the antennae, where the chemical signal is transduced into nervous activity. The olfactory receptor neurons in the semiochemi-cal-detecting sensilla are connected directly to the antennal lobe. Here the semiochemical-derived electrical signals are processed and sent out (through projection neurons) to the protocerebrum. Olfactory information is then integrated with other stimulus modalities, a decision is made, and the motor system is told what to do... 

In this model, OBPs participate in the selective transport of pheromone and other semiochemicals to their olfactory receptors. The selectivity of the system is likely to be achieved by layers of filters [ 16], i.e., by the participation of compartmentalized OBPs and olfactory receptors. It seems that OBPs transport only a subset of compounds that reach the pore tubules. Some of these compounds may not bind to the receptors compartmentalized in the particular sensilla. The odorant receptors, on the other hand, are activated by a subset of compounds, as indicated by studies in Drosophila, showing that a single OR is activated by multiple compounds [66]. If some potential receptor ligand reaches the pore tubules but are not transported by OBPs, receptor firing is prevented because the receptors are protected by the sensillar lymph. In other words, even if neither OBPs nor odorant receptors (ORs) are extremely specific, the detectors (olfactory system) can show remarkable selectivity if they function in a two-step filter. 

Syntheses of pheromones have been comprehensively treated by Mori [10-14]. The role of synthesis in the research on semiochemicals, the importance of stereochemistry in chemical signalling, and the significant relations between enantiomeric composition and biological activity of chiral semiochemicals have been thoroughly discussed by Mori [15-17]. In the present context, presentation of pheromone synthesis plays a minor role syntheses... 

Techniques in isolation and structure elucidation of (volatile) semiochemicals from beetles are the same as in other insects. Problems are mainly due to the often very small amounts of target compounds, embedded in large amounts of non-active substances which form a kind of cosmetic formulation for the biologically active principle. Comprehensive reviews of analytical approaches have been published [18-20]. 

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