When do trade-offs occur? The roles of energy constraints and trait flexibility in bushcricket populations.

In many animal species, the expression of sexually selected traits is negatively correlated with traits associated with survival such as immune function, a relationship termed a 'trade-off'. But an alternative in which sexually selected traits are positively correlated with survival traits is also widespread. We propose that the nature of intertrait relationships is largely determined by overall energy expenditure, energy availability and trait flexibility, with trade-offs expected when individuals are subject to energy constraints. We tested this hypothesis in Ephippiger diurnus, a European bushcricket in which males are distinguished by two prominent sexually selected traits, acoustic calls and a large spermatophore transferred to the female at mating, and where immune function may be critical in survival. Ephippiger diurnus are distributed as small, isolated populations that are differentiated genetically and behaviourally. We analysed songs, spermatophores and the immune function in male individuals from eight populations spanning a range of song types. As predicted, we only found trade-offs in those populations that expended more energy on song and were less flexible in their ability to adjust that expenditure. Ultimately, energy constraints and resulting trade-offs may limit the evolution of song exaggeration in E. diurnus populations broadcasting long calls comprised of multiple 'syllables'.

Female preference functions drive interpopulation divergence in male signalling: call diversity in the bushcricket Ephippiger diurnus.

Female preferences play a major role in the elaboration and diversification of male traits: as a selective pressure on males, variation in female preferences can generate population divergence and ultimately, speciation. We studied how interpopulation differences in the shape of female mate preference functions may have shaped male advertisement signals in the bushcricket Ephippiger diurnus. This species is distributed as geographically isolated populations with striking interpopulation variation in male acoustic signals, most notably in the number of syllables per call. Here, we asked whether differences in the shape of preference functions exist among populations and whether those differences may have driven male signal evolution resulting in the observed differences in syllable numbers. Our results reveal fundamental differences in female preferences among populations, with differences in the overall preference function shape corresponding to differences in male signals. These differences in female preferences best explain the major differences in male signals among populations. The interpopulation variation in signals and preferences potentially reflects the evolutionary history of the species and may contribute to further divergence among populations and subsequent speciation.

Genetic architecture of sensory exploitation: QTL mapping of female and male receiver traits in an acoustic moth.

The evolution of extravagant sexual traits by sensory exploitation occurs if males incidentally evolve features that stimulate females owing to a pre-existing environmental response that arose in the context of natural selection. The sensory exploitation process is thus expected to leave a specific genetic imprint, a pleiotropic control of the original environmental response and the novel sexual response in females. However, females may be subsequently selected to improve their discrimination of environmental and sexual stimuli. Accordingly, responses may have diverged and the original genetic architecture may have been modified. These possibilities may be considered by studying the genetic architecture of responses to male signals and to the environmental stimuli that were purportedly 'exploited' by those signals. However, no previous study has addressed the genetic control of sensory exploitation. We investigated this question in an acoustic pyralid moth, Achroia grisella, in which a male ultrasonic song attracts females and perception of ultrasound likely arose in the context of detecting predatory bats. We examined the genetic architecture of female response to bat echolocation signals and to male song via a cartographic study of quantitative trait loci (QTL) influencing these receiver traits. We found several QTL for both traits, but none of them were colocalized on the same chromosomes. These results indicate that - to the extent to which male A. grisella song originated by the process of sensory exploitation - some modification of the female responses occurred since the origin of the male signal.

Evaluation of amplitude in male song: female waxmoths respond to fortissimo notes.

Female evaluation of male signals in the context of sexual selection is often made on the basis of signal energy. Particularly in acoustic species, females may prefer male song that is broadcast at greater amplitude or power. However, song amplitude may be represented by various parameters, and the specific one(s) that are evaluated are not clear. We addressed this problem in an acoustic moth, Achroia grisella (Lepidoptera: Pyralidae), where males attract females with trains of paired ultrasonic pulses. Previous studies showed that females prefer songs that include pulse pairs that have greater mean peak amplitude and that are delivered with greater power (= mean peak amplitude x pulse-pair rate). Here, we report that given male songs of equal acoustic power, females prefer songs in which some pulses attain peak amplitudes that exceed the mean value and that this preference depends largely on the magnitude of amplitude fluctuation. We measured significant variation among males in their degree of amplitude fluctuation, and we note that males that broadcast with lower acoustic power typically show greater relative fluctuations and attain relatively higher amplitude maxima. We discuss the potential role of multiple integration time constants in female evaluation of mean song amplitude and amplitude maxima. We then consider the possibility that the variation observed in the male population is a response to female choice, but we also indicate that mechanical factors constraining song production may be responsible for such variation.

Genotype x environment interaction for male attractiveness in an acoustic moth: evidence for plasticity and canalization.

The lek paradox arises when choosy females deplete the genetic variance for male display traits from a population, yet substantial additive genetic variation (V(A)) in male traits persists. Thus, the lek paradox can be more generally stated as one of the most fundamental evolutionary questions: What maintains genetic variation in natural populations? One solution to this problem may be found in the condition-dependent nature of many sexually selected traits. Genotype x environment (G x E) interactions can maintain V(A) under conditions of environmental heterogeneity provided certain restrictions are met, although antagonistic pleiotropy has also been proposed as a mechanism. Here, we provide evidence for G x E interactions and against the role of antagonistic pleiotropy in the maintenance of V(A) for sexually selected traits. Using inbred lines of the lesser waxmoth Achroia grisella, we measured V(A) for song attractiveness, condition and development rate under different competitive environments and found that genotypes differed in their plasticity. We argue that variation persists in natural populations because G x E interactions prevent any one variant from producing the optimal phenotype across all environments.

Condition-dependent traits and the capture of genetic variance in male advertisement song.

The occurrence of additive genetic variance (VA) for male sexual traits remains a major problem in evolutionary biology. Directional selection normally imposed by female choice is expected to reduce VA greatly, yet recent surveys indicate that a substantial amount remains in many species. We addressed this problem, also known as the 'lek paradox', in Achroia grisella (Lepidoptera: Pyralidae), an acoustic moth in which males advertise to females with a pulsed ultrasonic song. Using a standard half-sib/full-sib breeding design, we generated F1 progeny from whom we determined VA and genetic covariance (COVA) among seven traits: three song characters, an overall index of song attractiveness, nightly singing period, adult lifespan, and body mass at adult eclosion. Because A. grisella neither feed nor drink as adults, the last trait, eclosion body mass, is considered a measure of 'condition'. We found significant levels of VA and narrow-sense heritabilities (h2) for all seven traits and significant genetic correlations (= COVAi,j / radical (VA i x VA j)) between most pairs of traits (i, j). Male attractiveness was positively correlated with body mass (condition), adult lifespan, and nightly singing period, which we interpret as an energy constraint preventing males in poor condition from singing attractively, from singing many hours per night, and from surviving an extended lifespan. The positive genetic correlation (r = 0.79) between condition and attractiveness, combined with significant levels of VA for both traits, indicates that much of the variation in male song can be explained by VA for condition. Finally, we discuss the morphological and physiological links between condition and song attractiveness, and the ultimate factors that may maintain VA for condition.

Missing link in firefly bioluminescence revealed: NO regulation of photocyte respiration.

Sexual communication in most species of fireflies is a male-female dialogue of precisely timed flashes of bioluminescent light. The biochemical reactions underlying firefly bioluminescence have been known for 30 years and are now exploited in biomedical assays and other commercial applications. Several aspects of flash regulation are also understood: flash rhythm is controlled by a central pattern generator, and individual flashes are neurally triggered, with octopamine serving as the transmitter. The molecular oxygen needed by the biochemical reactants is delivered by a network of tracheal arborizations extending throughout the light organ (lantern). However, the actual means by which oxygen quickly reaches the reactants packaged within specialized photocytes and the specific event(s) triggered by neural action have not been identified; termination of axons away from the photocytes has exacerbated the latter problem. A recent paper by a consortium of cell and evolutionary biologists, however, reports that nitric oxide (NO), manufactured and released in response to neuronal discharge, is the missing link by which neural action in the firefly lantern yields a sudden flash of light.

Genetic variance of sexually selected traits in waxmoths: maintenance by genotype x environment interaction.

When traits experience directional selection, such as that imposed by sexual selection, their genetic variance is expected to diminish. Nonetheless, theory and findings from sexual selection predict and demonstrate that male traits favored by female choice retain substantial amounts of additive genetic variance. We explored this dilemma through an ecological genetic approach and focused on the potential contributions of genotype x environment interaction (GEI) to maintenance of additive genetic variance for male signal characters in the lesser waxmoth, Achroia grisella (Lepidoptera: Pyralidae). We artificially selected genetic variants for two male signal characters, signal rate (SR) and peak amplitude (PA), that influence female attraction and then examined the phenotypic plasticity of these variants (high- and low-SR and high- and low-PA lines) under a range of environmental conditions expected in natural populations. Our split-family breeding experiments indicated that two signal characters, SR and PA, and several developmental characters in both high- and low-SR and high- and low-PA lines displayed considerable phenotypic plasticity among the environments tested. Moreover, strong GEIs leading to crossover between high- and low-SR lines were found for SR and developmental period. Therefore, neither high- nor low-SR genetic variants would achieve maximum attractiveness and fitness in every environment, and those variants producing unattractive signals with low SRs under normal conditions may remain in populations provided that gene flow across environments or generation overlap are sufficiently high. We speculate that the phenotypic plasticity for SR and developmental period is adaptive in A. grisella populations experiencing a range of temperature and density conditions. Females mating with attractive (high-SR) males may be assured of obtaining good genes because these males sire offspring that develop more rapidly and a crossover for developmental period may parallel that for SR. Such parallel crossovers may be expected wherever good-genes sexual selection mechanisms operate.

Quantitative genetics of female choice in an ultrasonic pyralid moth, Achroia grisella: variation and evolvability of preference along multiple dimensions of the male advertisement signal.

The mating system of Achroia grisella (Lepidoptera: Pyralidae) is characterized by male ultrasonic advertisement signalling to which females orientate. Although males provide no direct, somatic benefits to their mates, females prefer males whose signal characters are more exaggerated than the population means. Previous studies showed that the signal characters influencing mate attraction are highly repeatable and heritable. We measured the phenotypic and additive genetic variances (heritability) of female preference in A. grisella, as this additive genetic variance is one of the genetic assumptions of indirect models of sexual selection. We determined the preference index of female A. grisella by repeated phonotaxis trials in which a choice of simulated male signals was presented. These playback experiments showed that female preference indices varied but were repeatable within individuals. Specifically, females differ in the relative importance of the several signal characters during mate assessment. A subsequent half-sib breeding design revealed an amount of additive genetic variance for the female preference index (h(s)2 = 0.212, SE = 0.1347, P = 0.0611; CVA = 0.1826). Our study highlights the importance of careful preparation of test signals and experimental design for quantifying individual variation in (female) preference along multiple signal dimensions.

Quantitative genetics of ultrasonic advertisement signalling in the lesser waxmoth Achroia grisella (Lepidoptera: pyralidae).

Males of the lesser waxmoth Achroia grisella (Lepidoptera: Pyralidae) produce ultrasonic advertisement signals attractive to females within several metres. Previous studies showed that females prefer male signals that are louder, delivered at a faster rate, and have a greater asynchrony between pulses produced by the right and left wings. These three signal characters vary considerably within populations but are repeatable within individuals. Breeding experiments employing half-sib designs were conducted on both collectively and individually reared moths to determine genetic variance within and covariance among these signal characters. Heritabilities of all signal characters were significant among collectively reared moths. Heritabilities for signal rate and right-left wing asynchrony interval were not significant, however, among individually reared moths, suggesting the presence of significant nonadditive genetic variance or common environmental variation. Development time was also significantly heritable, but only under individual rearing. The only significant genetic correlation was between signal rate and length of the right-left wing asynchrony and this was negative. Our findings on heritability of signal characters are consistent with a coevolutionary sexual selection mechanism, but the absence of signal x development genetic correlation fails to support specifically a good-genes mechanism. The variation in heritability among conditions suggests that environmental variance may be high, and may render selection on signal characters by female choice ineffective. Thus, additive genetic variance for these characters may be maintained in the presence of directional female choice.

The use of acridine orange as a rapid method for the quantitation of bacteremia in laboratory animals.

A rapid technique has been developed to quantitate the degree of bacteremia in laboratory animals. Direct staining of blood smears with acridine orange and enumeration using fluorescent microscopy allowed quantitation of Haemophilus influenzae in blood at densities from 10(5) to 10(8) cfu/ml. This technique will facilitate the accuracy with which therapeutic trials are conducted in laboratory models of infection.

Experimental meningitis in the rat: Haemophilus influenzae.

Leptomeningitis due to type b Haemophilus influenzae can be produced in infant rats (up to three weeks of age) by intranasal inoculation, and in animals up to three months of age by intraperitoneal inoculation. In infant animals, the pathogenesis appears to mimic the disease in human infants. Immunologic experiments indicate that antibody directed against the type b capsule (actively or passively acquired) will protect against bacteremia (by any route of inoculation) and the subsequent development of meningitis. However, antibody directed against other surface structures of H. influenzae b (alone or with anticapsular antibody) will protect against sustained bacteremia after any route of inoculation. Evaluation of antibiotic activity against this infection in rats is unreliable due to a marked age-dependent increase in antibiotic clearance. A means of mimicking human pharmacokinetics in rats is proposed. The rat model is useful for the study of H. influenzae meningitis provided certain limitations are recognized.