Quantitative genetics of signal evolution: a comparison of the pheromonal signal in two populations of the cabbage looper, Trichoplusia ni

Trade-off between sensitivity and specificity in the cabbage looper moth response to sex pheromone

The evolution of male moth responses to pheromone blends may be constrained by a trade-off between two response traits: sensitivity and breadth of response. Population genetic simulations predict that if sensitivity and breadth of response are negatively correlated (i.e., a trade-off exists), then selection will favor males with narrow response phenotypes and high sensitivity. Although sensitivity-breadth of response trade-offs are generally assumed to exist and are implicit to the shape of male preference function, this study is the first to provide empirical support measuring behavior at the level of the individual. Previous studies with the cabbage looper, Trichoplusia ni, have documented the existence of a mutant pheromone strain. While mutant females produce a pheromone blend significantly different from wild-type females, mutant males respond equally to the wild-type and mutant pheromone blends. This study used wind tunnel bioassays to document that relative to wild-type males, mutant males had broader response profiles but lower pheromone sensitivity. While wild-type male responses were highest to the wild-type pheromone blend, mutant males did not discriminate among pheromone blends. These results are consistent with a trade-off between breadth of response and sensitivity. Pure wild-type and mutant lines were crossed and hybrid males assayed. Both hybrid types (maternal wild-type and maternal mutant hybrids) responded similarly. Hybrid males had response profiles similar to wild-type males and the reduced sensitivity observed in mutant males. These results suggest a possible hybrid disadvantage and a putative mechanism for reinforcement of male pheromone response traits.

The role of chemical communication in mate choice

Chemical signals are omnipresent in sexual communication in the vast majority of living organisms. The traditional paradigm was that their main purpose in sexual behaviour was to coordinate mate and species recognition and thus pheromones were conserved in structure and function. In recent years, this view has been challenged by theoretical analyses on the evolution of pheromones and empirical reports of mate choice based on chemical signals. The ability to measure precisely the quantity and quality of chemicals emitted by single individuals has also revealed considerable individual variation in chemical composition and release rates, and there is mounting evidence that prospecting mates respond to this variation. Here, we review the evidence for pheromones as indicators of mate quality and examine the extent of their use in individual mate assessment. We begin by briefly defining the levels of mate choice--species recognition, mate recognition and mate assessment. We then explore the degree to which pheromones satisfy the key criteria necessary for their evolution and maintenance as cues in mate assessment; that is, they should exhibit variation across individuals within a sex and species; they should honestly reflect an individual's quality and thus be costly to produce and/or maintain; they should display relatively high levels of heritability. There is now substantial empirical evidence that pheromones can satisfy all these criteria and, while measurements of the actual metabolic cost of pheromone production remain to some degree lacking, trade-offs between pheromone production and various fitness-related characters such as growth rate, immunocompetence and longevity have been reported for a range of species. In the penultimate section, we outline the growing number of studies where the consequences of chemical-based mate assessment have been investigated, specifically focussing on the reported direct and genetic benefits accrued by the receiver. Finally, we highlight potential areas for future research and in particular emphasise the need for interdisciplinary research that combines exploration of chemical, physiological and behavioural processes to further our understanding of the role of chemical cues in mate assessment.

Genetics of sex pheromone blend differences between Heliothis virescens and Heliothis subflexa: a chromosome mapping approach

Males of the noctuid moths, Heliothis virescens and H. subflexa locate mates based on species-specific responses to female-emitted pheromones that are composed of distinct blends of volatile compounds. We conducted genetic crosses between these two species and used AFLP marker-based mapping of backcross families (H. subflexa direction) to determine which of the 30 autosomes in these moths contained quantitative trait loci (QTL) controlling the proportion of specific chemical components in the pheromone blends. Presence/absence of single H. virescens chromosomes accounted for 7-34% of the phenotypic variation among backcross females in seven pheromone components. For a set of three similar 16-carbon acetates, two H. virescens chromosomes interacted in determining their relative amounts within the pheromone gland and together accounted for 53% of the phenotypic variance. Our results are discussed relative to theories about population genetic processes and biochemical mechanisms involved in the evolution of new sexual communication systems.

Heritable variation in the sex pheromone of the almond moth, Cadra cautella

The sex pheromone of the female almond moth, Cadra cautella, constitutes a highly variable signal. Age and period of the L/D cycle had significant effects on the variation in titers of (Z,E)-9,12-tetradecadienyl acetate (Z9,E12-14:Ac) and (Z)-9-tetradecenyl acetate (Z9-14:Ac) and the ratio of Z9,E12-14:Ac to Z9-14:Ac. Across age classes, minimum and maximum mean titers (ng +/- SE) of Z9,E12-14:Ac and Z9-14:Ac ranged from 8.7 +/- 0.46 to 21 +/- 1.0 and from 2.0 +/- 0.14 to 2.9 +/- 0.15, respectively; the mean ratio of Z9,E12-14:Ac to Z9-14:Ac ranged from 5.2 +/- 0.21 to 11 +/- 0.40. The titers of Z9,E12-14:Ac and Z9-14:Ac and the ratio of Z9,E12-14:Ac to Z9-14:Ac were highest at the onset of the scotophase and lowest at the onset of the photophase. Similarly, the titer of Z9,E12-14:Ac and the ratio of Z9,E12-14:Ac to Z9-14:Ac were highest in females sampled 1 d postemergence and declined over all age classes. The titer of Z9-14:Ac increased from day 1 to day 2, and then declined to levels equivalent to day 1. Analysis of pupal and adult mass demonstrated a positive correlation (r = 0.874, P < 0.001); however, the titer of neither Z9,E12-14:Ac or Z9-14:Ac nor the ratio of Z9,E12-14:Ac to Z9-14:Ac were significantly correlated with pupal mass. Age-related variations in pheromone titer and ratios were heritable. The narrow-sense heritability of the observed variation in 1-d-old females, 1 hr into the scotophase is 1.2 +/- 0.32, 0.75 +/- 0.24, and 0.46 +/- 0.17 for the titer of Z9,E12-14:Ac and Z9-14:Ac and the ratio of Z9,E12-14:Ac to Z9-14:Ac, respectively. A significant additive genetic correlation was observed between that of Z9,E12-14:Ac and Z9-14:Ac, but not between that of either Z9,E12-14:Ac or Z9-14:Ac and the ratio of Z9,E12-14:Ac to Z9-14:Ac. The results of the genetic analyses suggest that the traits of titer of Z9,E12-14:Ac and Z9-14:Ac have a greater potential to respond to selection than the trait of ratio of Z9,E12-14:Ac to Z9-14:Ac.

Chemical signal mediated premating reproductive isolation in a marine polychaete, Neanthes acuminata (arenaceodentata)

Neanthes acuminata Ehlers (1868) is a monogamous coastal polychaete with male parental care and a high level of sexual selection. We measured the level of prezygotic isolation among allopatric populations of N. acuminata; from the East and West Coast of the USA, a population from Hawaii, and a laboratory culture originating from Los Angeles, CA. All populations were found to preferably mate with members of their own population. Individuals from populations from Atlantic vs. Pacific Ocean failed to pair and to mate, either during the 10 min or 48 hr experiments. Instead, individuals showed high levels of aggressive behavior. Experiments measuring the levels of interpopulation aggression, established that individuals can recognize and discriminate among different populations of N. acuminata on the basis of olfactory cues. Aggressive behavior was induced by exposure of animals to seawater "conditioned" by individuals from the other populations, thus demonstrating the role of olfaction in the detection of "home" populations. The aggressive display was stronger upon exposure to seawater conditioned with "unrelated" populations and especially between Pacific and Atlantic populations.

Heritable variation of sex pheromone composition and the potential for evolution of resistance to pheromone-based control of the Indian meal moth, Plodia interpunctella

The short-term evolutionary effect of pheromone-based mating disruption on the mating ability of the Indian meal moth, Plodia interpunctella, was investigated. Three independent selection lines were established, and the mating ability of moths in plastic tents treated with high doses of pheromone and in control tents was compared for two consecutive generations. In addition, the heritability of the sex pheromone blend, measured as the ratio of two major pheromone components (Z,E)-9,12-tetradecadienyl acetate and (Z,E)-9,12-tetradecadienol, was estimated. Based on a mother-daughter regression analysis including 21 families, the heritability of the pheromone blend was 0.65 +/- 0.14, indicating a potential for evolutionary change of the character. However, no increase in mating ability of females in pheromone-treated tents or alteration of the pheromone blend was observed in any selection line when compared with control lines, indicating no or weak selection on the pheromone blend as well as other traits influencing mating ability of this species under the created mating disruption conditions. Factors contributing to the lack of selection effects are discussed.