Primer Pheromones

Primer pheromones have long-term effects on the physiology of the recipient, mainly by stimulating sensory neurons that send signals to the brain to release hormones of the endocrine system. Often it is difficult to study the effect of such primer pheromones because the long time-scales on which they act make it difficult to relate gradual

Figure 2 Semiochemicals. (a) Pheromones with high (ethyl acetate), medium ((Z)-9-tetradecenyl acetate), and low (nonacosane) volatility. Numbers indicate the vapor pressure of the shown compounds at room temperature. C. capitata = Mediterranean fruit fly, A. nigroaenea = solitary bee. (b) Structures of compounds and composition of pheromone mixtures of the two noctuid butterflies, corn earworm (Helicoverpa zea) and the tobacco budworm (H. virescens). (c) Compounds which can elicit the full behavioral repertoire at very low, biologically meaningful concentrations. Examples are the sex pheromone of the American cockroach (P. americana) and that of the brownbanded cockroach (S. longipalpa). (d) Different species of the same genus use different enantiomers: bark beetles.

Ethyl acetate Ceratitis capitata

(Z)-9-Tetradecenyl acetate >50 lepidopteran species

Nonacosane, Andrena nigroaenea 1.7 x 10 5 kPa

Compound

Compound

Abbreviation

16:AL Z 7-16:Al Z 9-16:Al Z11-16:Ol OH Z 11-16:Ol O Z 9-14:AI 14:AI

Composition (%) H. zea H. virescens 9

Periplanone B Supellapyrone

Periplaneta americana Supella longipalpa

H OH

HO H

(S)-(+)-Ipsdienol Ips parconfusus

(R)-(-)-Ipsdienol Ips calligraphus

physiological changes to the initial contact with the chemical signal.

In many different types of organisms, primer pheromones are instrumental in coordinating reproduction. Especially in social mammals that breed cooperatively, interactions mediated by primer pheromones have reached great complexity (Figure 3). Cyclic hormone changes in females leading to sexual receptivity during discrete intervals known as estrus, for example, are affected by the social environment. In female mice (Mus musculus), the presence of an adult male and his pheromones induces estrus (Whitten effect) and accelerates puberty in young females (Vandenbergh effect). These two effects are caused by the same pheromones, which are released with the urine of dominant males. Two compounds which are both active in accelerating puberty and inducting estrus are 2-sec-butyl-dihydrothiazole (SBT)

and dehydro-exo-brevicomin (DHB) (Figure 3). On the other hand, female mice that are kept crowded together without males have suppressed estrus cycles and delayed puberty in juvenile females. Again, urinary odors are responsible for this so-called Lee-Boot effect. The key pheromone for puberty delay and estrus inhibition seems to be 2,5-dimethylpyrazine (Figure 3).

In many mammalian species, females living together in groups synchronize their estrus, by using primer pheromones. Evolutionary explanations for synchronous estrus may be the possibility for communal rearing or the reduced opportunity for males to select multiple mates. Observations on female college students suggest that similar physiological mechanisms operate also in humans: Groups of women who resided together in dormitories had synchronized and suppressed menstrual cycles.

Producer: dominant males

2-sec-Butyl-4,5-dihydrothiazole

(Estrus synchronization puberty acceleration) N

2,5-Dimethylpyrazine (puberty delay)

Figure 3 Structure and functions of some urinary primer pheromones of mice (Mus musculus) involved in coordinating reproduction among social groups.

Beyond their role for reproduction, primer pheromones can be pivotal for regulating major developmental changes in animals. In social organisms such as ants, bees, or termites, the long-term effects of priming pheromones are particularly well understood. In all three groups, reproductively dominant queens use primer pheromones to suppress the fecundity of other sexuals, inhibit reproduction by worker castes, and regulate worker ontogeny. Convergently evolved mechanisms mediate these physiological changes by altering the level of the juvenile hormone in the receiver. In the honeybee (Apis mellifera), the so-called queen retinue pheromone (QRP) is produced in the queen's head and consists of at least nine synergistically acting components (Figure 4). The relatively nonvolatile pheromone is spread by messenger bees through the rest of the colony. Removal of the queen and hence loss of the QRP causes worker bees to start to rear new queens within 24 h. Thus the workers have to respond quickly, because developmental paths of larvae are fixed 6 days after egg laying, and without an egg-laying queen, the colony will die.

Beyond, primer pheromones are involved in recognition learning of olfactory cues. In contrast to associative learning or unlearned reactions to odors, recognition learning is a form of imprinting that occurs during a sensitive period coinciding with a particular developmental stage or physiological state. Imprinting of odor cues can be important for kin recognition and mate choice.

For many mammal species including humans, olfaction plays a crucial role for mother-infant recognition. This phenomenon has been well studied using sheep as a

Figure 4 The queen honeybee (Apis mellifera) secretes the so-called queen retinue pheromone that is spread by messenger bees through the colony. This primer pheromone is responsible for inhibiting ovary development and queen rearing behavior that underlies reproductive division of labor. Photograph by Stephan Hartel, with permission.

Figure 4 The queen honeybee (Apis mellifera) secretes the so-called queen retinue pheromone that is spread by messenger bees through the colony. This primer pheromone is responsible for inhibiting ovary development and queen rearing behavior that underlies reproductive division of labor. Photograph by Stephan Hartel, with permission.

model system. Many lambs are born to the flock in a short period of time, so each mother (ewe) needs to recognize her offspring to avoid the risk of erroneously distributing limited resources to alien neonates. An enduring bond between a mother and her lamb is usually established within 2 h after giving birth (parturition). The sensitive period for learning lasts 4-12 h after giving birth, and if ewes are deprived of their lamb during this period, the bond fails to develop. Perceiving olfactory cues of the lamb during this period triggers a cascade of neurochemi-cal and hormonal mechanisms, causing the mother to lick the amniotic fluid and learn the individual odors of her lamb. Afterward, she will exclusively nurse her own lambs, which she selectively recognizes by their smell. The lamb's odor phenotype, which acts as priming pher-omone, is influenced by a combination of genetic processes and environmental factors such as cues from the mother's saliva or milk.

Also mating preferences are often learned through recognition learning of olfactory cues during early ontogeny. Even though the exact mechanisms of this chemosensory imprinting are not known in detail, exposure to specific odors during early life stages has been shown to alter the development ofthe main olfactory bulb (i.e., the structure of the vertebrate forebrain involved in olfaction) and the neural centers that process the information received by olfactory receptors. Mammals including humans tend to select their mating partners based on the major histocompatibility complex (MHC), which they detect by smell. Genes of the MHC are the most

Producer: crowded females

2,3-Dehydro-exo-brevicomin

(Estrus synchronization, puberty acceleration)

2-Heptanone (estrus extension)

polymorphic loci known among vertebrates and play a central role in immunodefense. Preferring odors of MHC-dissimilar individuals, as is the case in mice and humans, may function to produce disease-resistant offspring (i.e., MHC heterozygotes) and/or to reduce inbreeding. House mice, for example, show disassortative mating by referring to the MHC odors of their nestmates early in life.

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