Between-sex genetic covariance constrains the evolution of sexual dimorphism in Drosophila melanogaster.

Males and females share much of their genome, and as a result, intralocus sexual conflict is generated when selection on a shared trait differs between the sexes. This conflict can be partially or entirely resolved via the evolution of sex-specific genetic variation that allows each sex to approach, or possibly achieve, its optimum phenotype, thereby generating sexual dimorphism. However, shared genetic variation between the sexes can impose constraints on the independent expression of a shared trait in males and females, hindering the evolution of sexual dimorphism. Here, we examine genetic constraints on the evolution of sexual dimorphism in Drosophila melanogaster cuticular hydrocarbon (CHC) expression. We use the extended G matrix, which includes the between-sex genetic covariances that constitute the B matrix, to compare genetic constraints on two sets of CHC traits that differ in the extent of their sexual dimorphism. We find significant genetic constraints on the evolution of further dimorphism in the least dimorphic traits, but no such constraints for the most dimorphic traits. We also show that the genetic constraints on the least dimorphic CHCs are asymmetrical between the sexes. Our results suggest that there is evidence both for resolved and ongoing sexual conflict in D. melanogaster CHC profiles.

Immunogenic males: a genome-wide analysis of reproduction and the cost of mating in Drosophila melanogaster females.

In Drosophila melanogaster, mating radically transforms female physiology and behaviour. Post-mating responses include an increase in the oviposition rate, a reduction in female receptivity and an activation of the immune system. The fitness consequences of mating are similarly dramatic--females must mate once in order to produce fertile eggs, but additional matings have a clear negative effect. Previously, microarrays have been used to examine gene expression of females differing in their reproductive status with the aim of identifying genes influenced by mating. However, as only virgin and single mated females were compared, transcriptional changes associated with reproduction (under natural selection) and male-induced effects (possibly under sexually antagonistic selection) cannot be disentangled. We partitioned these fundamentally different effects by instead examining the expression profiles of virgin, single mated and double mated females. We found substantial effects relating to reproduction and further effects that are only attributable to mating itself. Immune response genes dominate this male-induced effect indicating that the cost of mating may be due partly to this system's activation. We propose that both sexually antagonistic and natural selection have been important in the evolution of the innate immunity genes, thereby contributing to the sexual dimorphism and rapid evolution at these loci.

Hemiclonal analysis reveals significant genetic, environmental and genotype x environment effects on sperm size in Drosophila melanogaster.

Spermatozoa are the most diverse of all animal cells. Variation in size alone is enormous and yet there are still no clear evolutionary explanations that can account for such diversity. The basic genetics of sperm form is also poorly understood, although sperm size is known to have a strong genetic component. Here, using hemiclonal analysis of Drosophila melanogaster, we demonstrate that there is not only a significant additive genetic component contributing to phenotypic variation in sperm length but also a significant environmental effect. Furthermore, the plasticity of sperm size has a significant genetic component to it (a genotype x environment interaction). A genotype x environment interaction could contribute to the maintenance of the substantial genetic variation in this trait and thereby explain the persistent inter-male differences in sperm size seen in numerous taxa. We suggest that the low conditional dependence and high heritability but low evolvability (the coefficient of additive genetic variation) of sperm length is more consistent with a history of stabilizing selection rather than either sexual selection or strong directional selection.

Assessing the extent of genome-wide intralocus sexual conflict via experimentally enforced gender-limited selection.

Intralocus sexual conflict, which occurs when a trait is selected in opposite directions in the two sexes, is a taxonomically widespread phenomenon. The strongest genetic evidence for a gender load due to intralocus sexual conflict comes from the Drosophila melanogaster laboratory model system, in which a negative genetic correlation between male and female lifetime fitness has been observed. Here, using a D. melanogaster model system, we utilize a novel modification of the 'middle class neighbourhood' design to relax selection in one sex, while maintaining selection in the other. After 26 generations of asymmetrical selection, we observed the expected drop in fitness of the non-selected sex compared to that of the selected sex, consistent with previous studies of intralocus sexual conflict in this species. However, the fitness of the selected sex also dropped compared to the base population. The overall decline in fitness of both the selected and the unselected sex indicates that most new mutations are harmful to both sexes, causing recurrent mutation to build a positive genetic correlation for fitness between the sexes. However, the steeper decay in the fitness of the unselected sex indicates that a substantial number of mutations are gender-limited in expression or sexually antagonistic. Our experiment cannot definitively resolve these two possibilities, but we use recent genomic data and results from previous studies to argue that sexually antagonistic alleles are the more likely explanation.

The origin of interlocus sexual conflict: is sex-linkage important?

Sexual conflict has been proposed as a potential selective agent in the evolution of a variety of traits. Here, we present a simple model that investigates the initial conditions under which sex-linked and sex-limited harming alleles can invade a population. In this paper, we expand previous threshold models to study how sex-linkage and sex determination mechanisms affect the spreading conditions of a harming allele. Our models provide new insights into how sexual conflict could originate, showing that in diploid organisms the probability of a new harming allele spreading is independent of both the genetic sex determination system and the dominance relationships. However, the incidence of interlocus sexual conflicts in the initial steps of the invasion critically depends on the inheritance system.

Artificial selection and heritability of sperm length in Gryllus bimaculatus.

The adaptive significance of variation in sperm size remains poorly understood but there has been even less attention focused on the genetic mechanisms controlling spermatozoal traits (only three species have been studied). Here we explore heritability and artificial selection of sperm length in the cricket Gryllus bimaculatus. Using a within-family selection design we selected sperm with increasing and decreasing sperm size and bred divergence across five generations. Although we recorded no significant parent-offspring heritability, realized heritability from a within-family analysis was significant at 0.52 +/- 0.06 SE (P = 0.01) and we successfully achieved significant divergence. However, we only achieved a response when the maternal line was incorporated into selection (from the F1 onwards) and our findings therefore suggest that sperm length in G. bimaculatus is a sex-linked trait that is influenced by genes which are active on the female chromosome. Accordingly, sperm length heritability in G. bimaculatus can only be measured using a within-family design because the spermatozoal phenotype is sex-limited (to males) while the genotype is sex-linked (to females). The evolutionary significance of the heritability of sperm length is discussed with reference to sex-linkage of this important sex-limited trait.

Sperm competition experiments between lines of crickets producing different sperm lengths.

Sperm numbers can be important determinants of fertilization success in sperm competition. However, the importance of variation in sperm size is less well understood. Sperm size varies significantly both between and within species and comparative studies have suggested that some of this variance can be explained by sperm competition. In this study we examine whether variation in sperm length has consequences for fertilization precedence using controlled sperm competition experiments in the field cricket Gryllus bimaculatus. This species is an ideal model for such investigations because the mechanism of sperm competition generates complete mixing of different males' spermatozoa in the female (thereby allowing individual sperm to express their own competitive abilities). We successfully bred lines of crickets, the males of which produced short, medium and long sperm types with narrow and non-overlapping distributions. Males of different lines were then sequentially mated with control females in order to create two-male sperm competitions. The paternity outcomes of these competitions were measured after matings using an irradiated male technique (with a full reciprocal design that controls for natural fertility and any irradiation effects on gamete competitiveness) over a 12 day oviposition period. However, having successfully bred diverging sperm length lines and competing males that differed in sperm length, we found no evidence that a male's sperm size explained any of the variation in their relative fertilization success. Males from lines producing longer sperm showed no fertilization advantage over males producing shorter sperm across 97 double matings. There was also no advantage for males producing a sperm length close to the population mean over those competitors whose sperm length had been selectively diverged across 63 matings.

The evolution of sperm length in moths.

Sperm form and function remain poorly understood despite being of fundamental biological importance. An instructive approach has been to examine evolutionary associations across comparable taxa between sperm characters and other, potentially selective reproductive traits. We adopt this approach here in a comparative study examining how sperm lengths are associated with male and female reproductive characters across moths. Primary data have revealed Lepidoptera to be an ideal order for examination: there is profound variation in the dimensions (but not organization) of the reproductive traits between closely related species which all share a monophyletic ancestry, for example, eupyrene sperm length varies from 110 to 12,675 microm. Eupyrene (normal fertilizing) and apyrene (anucleate and non-fertile) sperm lengths are positively correlated across taxa and both sperm types show positive associations with mating pattern (as measured by the residual testis size). At fertilization, eupyrene sperm must migrate down the often elongated female spermathecal duct from storage to unite with the ovum. Across taxa, the elongation of this duct is associated with increased eupyrene sperm length, suggesting a positive female influence on sperm size since longer, more powerful sperm may be selected to migrate and/or compete successfully down greater ductal lengths. Apyrene sperm length is not associated with female reproductive tract dimensions. However, we found a positive relationship between the residual testis volume and spermathecal volume, suggesting coevolution between male investment in spermatogenesis and the extent of the female sperm storage capacity. Within males, there is a positive association between the two organs which form the ejaculate-containing spermatophore: the testes and the accessory gland. The 'trade-up' in investment to these components is discussed in relation to paternal investment and mating patterns.

Dose-response in double-blind, placebo-controlled oral food challenges in children with atopic dermatitis.

BACKGROUND: Double-blind, placebo-controlled oral food challenges (DBPCFCs) are considered the "gold standard" for diagnosing food hypersensitivity, but the dose that elicits positive challenges, or determinants that may predict dose-response relationships, have not been reported. OBJECTIVE: Our purpose was to determine the quantity of food that elicits reactions during DBPCFCs and to evaluate parameters that may predict the provocative dose and severity of reaction. METHODS: We reviewed challenge data for all positive challenges to 6 common allergenic foods in children with atopic dermatitis evaluated for food allergy over a 13-year period. Challenge food was generally administered in 6 doses at 10- to 15-minute intervals beginning with 400 to 500 mg and completing with a total of 8 to 10 g of food. An open feeding of a larger portion followed negative challenges. At the physician's discretion, a lower starting dose was occasionally used (100 mg, 250 mg). Food-specific IgE antibody concentrations (radioallergosorbent test [RAST]) were determined on stored sera of 20% of the challenges selected randomly and 99.6% had prick skin tests (PSTs) performed to the challenged food. RESULTS: A total of 196 children (45% male; median age 5 y 9 mo; atopic dermatitis 98%, asthma 62%) had 513 positive challenges distributed as follows: egg 267, milk 117, soy 53, wheat 40, peanut 24, fish 12. The percentage of children reacting at the first dose (500 mg or less) was as follows: egg 49%, milk 55%, soy 28%, wheat 25%, peanut 26%, and fish 17%. Twenty-six milk challenges and 22 egg challenges were positive at a first dose of 250 mg; 3 milk challenges and 7 egg challenges were positive at a first dose of 100 mg. Eleven percent of the reactions that occurred on the first dose were severe. The percentage reacting after the final dose of the DBPCFC (or during open challenge) were egg 11%, milk 12%, soy 19%, wheat 12.5%, peanut 8.7%, and fish 25%. There was not a strong correlation between PST absolute wheal size or score (adjusted for histamine controls) and dose at reaction or severity of reaction (R(s) range -0.22 to 0.39 for particular foods). Serum concentration of food-specific IgE did not correlate well with the dose causing a reaction or with severity (R(s) range -0.40 to 0.55 for particular foods). CONCLUSIONS: This food-allergic population may react to as little as 100 mg of food, possibly less, and the dose causing a reaction and the severity of reaction is not predicted by PST or RAST. Lower doses (100 mg or less) should be investigated for their appropriateness in initiating DBPCFCs.