BIOLOGY OF REPRODUCFION 4, 309-326 (1971) Semiochemicals-Structure and Function F. E. REGNIER Department of Biochemistry, Purdue University, Lafayette, Indiana Downloaded from https://academic.oup.com/biolreprod/article/4/3/309/2768582 by guest on 23 September 2021 Received September 9, 1970 To appreciate the influence of the environ- they relate directly and indirectly to repro- ment on reproduction we must first consider duction. the nature of the environment and an ani- There are two major types of semiochemic mal’s response to it. The biotic community communication in nature; (1) between mem- is a dynamic system of interacting plant and bers of the same species (intraspecific com- animal populations. Few species are free munication), and (2) between different spe- from some sort of pressure from another cies (interspecific communication). Those species, whether it be in the form of preda- compounds that carry signals between mem- tion, competition for the same resources, or bers of the same species have been called parasitism. To maintain its place in an eco- “pheromones” (Karlson and Butenandt, system an organism must be able to com- 1959). Pheromones (Fig. I) are of two types, municate with its environment; i.e., it must releasers and primers. These two types of be able to sense physical and chemical stimuli pheromones are easily differentiated on the from the environment and respond to them. basis of the behavior of the receiver. When a In some cases this response consists of send- releaser pheromone is detected by an animal ing messages of its own to other organisms. an immediate behavior response occurs; or In the case of a prey, it must be able to sense behavior is said to be released. An example its predator and in some species defend it- would be a sex attractant produced by a self against the predator by the emission of a female insect which upon reception by a noxious odorant that “tells” it to desist. mature male immediately releases sexual In recent years, it has been found that behavior. The response of an animal upon organic compounds may be used to carry reception of a primer pheromone is delayed: messages between animals in nature. This is a Primer pheromones often induce physio- type of chemical communication for which I logical changes in an organism which ulti- propose the term “semiochemic communi- mately manifest themselves in a behavioral cation.” The word semiochemic is derived response sometime after initial pheromone from the Greek semeion (sign or signal) and reception. Blockage of ova implantation in chemeceia (aichemic). At the present time a female mice by the odor of an alien male is wide array of these signal-carrying com- an example of the action of a primer phero- pounds have been isolated, chemically char- mone. acterized, and synthesized. In addition, I Interspecific semiochemic communication propose the name “semiochemical” for those is of two types: (1) that which adaptively chemical signal vehicles that carry informa- favors the emitter, and (2) that which is tion between organisms. Semiochemicals are adaptively favorable to the receiver. Brown, clearly different from hormones and nucleic Eisner, and Wittaker (1970) have designated acids, since the latter two are used for chemi- those semiochemicals that evoke behavioral cal communication within an organism. It is or physiological changes in a receiving or- the purpose of this review to discuss the ganism that adaptively favor the emitter structure and function of semiochemicals as “allomones”. It is obvious that the noxious 309 310 REGNIER Semiochemicois Introspecies Interspecies Pherdmones Downloaded from https://academic.oup.com/biolreprod/article/4/3/309/2768582 by guest on 23 September 2021 ReIesers Pri’mers AIIoriones Kairdmones FIG. 1. Classification of types of chemical signal vehicles used to carry signals between animals in nature. odor of the skunk is adaptively favorable to will be chosen for discussion because they the transmitter. On the other hand, the in- are unusual or illustrate a particular point I advertent release of specific compounds by wish to emphasize. As a consequence, ex- an animal that reveals it to a predator favors clusion of many excellent pieces of work will the receiver. Brown el a!. (1970) has desig- be necessary and by no means intentional. nated these semiochemicals that favor the I. FOOD AND HOST SELECTION receiver “kairomones”. It is apparent in nature that ecology and It is well known that organisms show reproduction of a species are intimately as- feeding preferences for different types of food sociated. The survival of a species is depend- (Schoonhoven, 1968). There is often little or ent on the development of sexually mature no relationship between the nutritional value adults that are capable of reproducing at a of food and its probability of being con- rate that will maintain or increase the popu- sumed. In many cases, this specific food lation. If we divide the life cycle of an or- selection has a chemosensory basis. An or- ganism into presexual and sexual phases, it ganism is able to orient spatially to its favor- is easier to isolate the effects of the environ- ite food by diffusible compounds that ema- ment and the different kinds of semiochemic nate from it. The type of signal vehicles in- communication used by organisms. During volved range from simple amino acids and the presexual or nonsexual stage of life, the sugars in the bacteria to more specific com- primary preoccupation of the organism is pounds such as those used in host selection survival. Animals must find food and shelter, in higher animals. avoid predators, defend themselves, and in Motile bacteria are chemotactic to a num- the case of animal societies maintain colonial ber of amino acids, sugars, and TCA cycle organization. The importance of the chemo- acids (Weibull, 1960). It has been shown by sensory channel to survival through food and Adler (1969) in a series of papers that this host selection, recruitment to food sources, chemotaxis is initiated by chemoreceptors alarm signaling, defense, and aggregation on the surface of the bacteria that are not will make up the first section of this paper. triggered by metabolism of compounds to The second section of the paper will deal which the bacteria show chemotaxis. Since specifically with sex pheromones, i.e., terri- many of the attractants are not metabolized, torial marking substances and sex attract- it is clear that chemotaxis is not dependent ants. on energy produced by the metabolism of the The amount of work done on semiochemic attractant. The general basis for chemore- communication in nature is indeed vast. I ception and chemotaxis has been established will therefore not attempt a complete dis- by the isolation of a series of E. coil chemo- cussion of the subject in this limited treat- tactic mutants. These mutants showed ment of chemical communication. Examples altered chemotaxis toward serine, aspartate, SEMIOCHEMICAL STRUCTURE AND FUNCTION 311 and galactose, but normal metabolism of new prey. Observations of Hydra feeding in the compounds. This supports the idea that this situation indicate that this does not there are specific chemoreceptors that trigger happen. Under these conditions a chemically chemotaxis upon reception of a specific controlled modification in the feeding be- attractant molecule. The isolation by Arm. havior occurs. The gastrovascular cavity con- strong (1969) of 40 motile mutants that are stricts locally just below the tentacles to seal not chemotactic to any of the normal attract- off the gut from the outside medium during Downloaded from https://academic.oup.com/biolreprod/article/4/3/309/2768582 by guest on 23 September 2021 ants suggests that the mutants are defective mouth opening. As soon as the new prey is at some stage beyond chemoreception. ingested and the mouth closes, the constric- It is also possible that microorganisms use tion disappears and the food is dropped into chemotaxis for more than the selection of the gut (Fig. 2). Blanquet and Lenhoff (1968) simple nutrients. Recent studies have indi- have found that this modified feeding re- cated that microorganisms may be predatory sponse is controlled by the interaction of an and parasitic on other species of microor- ectodermal receptor for glutathione and an ganisms and that this predation and para- endodermal receptor system for tyrosine. In sitism is an important factor in maintaining the situation where the Hydra is fasted when the balance of microflora in nature (Mitchell, it encounters prey, the concentration of tyro- 1970). Prey and host selection in many cases sine at the endodermal receptors will be low appears to occur by techniques other than by while the concentration of reduced gluta- chance encounter between organisms. It is thione at the ectodermal receptors will be not unlikely that cases will be found where selection is on the basis of the emanation of kairomone(s) from a prey or host. External chemical stimuli are also impor- tant in the feeding behavior of Hydra. When prey brush against the tentacles of the Hydra, spearlike nematocysts are discharged into the body of the prey. Loomis (1955) discovered that nematocyst discharge into a suitable prey is followed by a coordinated feeding reaction in which the mouth is opened and the tentacles move to the mouth with the A B prey. It was found that this feeding reaction is controlled by reduced glutathione that is released from the body of the prey when its skin is punctured. Since Hydra only feed on C prey that release glutathione after nemato- cyst puncture, its feeding is confined to Nemathelminihes and higher phyla. Cani- balism does not occur in Hydra because they do not produce reduced glutathione. Because feeding in Hydra is discontinuous, new prey are often captured before previ- ously captured prey are digested.