Seeds of many arid habitat plants have a water-impermeable coat and can germinate only after being scarified. Bruchine seed beetles are important parasites of legume seeds in these environments, but their effect on germination can be unpredictable. Beetles deplete seed resources and can kill the embryo but also scarify seeds. We investigated the effects of a generalist parasite, Stator limbatus, on the germination and growth of two common legumes in the Sonoran desert, catclaw acacia (Acacia greggii) and blue paloverde (Parkinsonia florida). Feeding damage from a single larva greatly increased germination of paloverde but not acacia. This benefit was reduced if seeds were attacked by multiple larvae. Beetle-damaged seeds of both hosts germinated more quickly than did control seeds. Infestation by beetles reduced seedling size, though effects were greater for paloverde than for acacia. Our results demonstrate that the effect of S. limbatus can be highly host-specific. In addition, beetle infestation may enhance or reduce seedling recruitment, depending on the availability of other scarifying agents and the number of larvae per seed. Such contingencies make it difficult to predict the net effect of seed beetles on efforts to control invasive legume hosts or establish native hosts during aridland restoration. (C) 2011 Elsevier Ltd. All rights reserved.

Sexual size dimorphism varies substantially among populations and species but we have little understanding of the sources of selection generating this variation. We used path analysis to study how oviposition host affects selection on body size in a seed-feeding beetle (Stator limbatus) in which males contribute large ejaculates (nuptial gifts) to females. Females use nutrients in these ejaculates for egg production. Male body size, which affects ejaculate size, affects female fecundity and is thus under fecundity selection similar in magnitude to the fecundity selection on female body size. We show that when eggs are laid on a host on which larval mortality is low (seeds of Acacia greggii) fecundity predicts fitness very well and fecundity selection is the major source of selection on both male and female adult size. In contrast, when eggs are laid on a host on which larval mortality is high (seeds of Parkinsonia florida) fecundity poorly predicts fitness such that fecundity selection is relaxed on both male and female size. However, because egg size affects larval mortality on this poor host (P. florida) there is selection on female size via the female size --> egg size --> fitness path; this selection via egg size offsets the reduction in fecundity selection on female, but not male, body size. Thus, differences in host suitability (due to differences in larval mortality) affect the relative importance of two sources of selection on adult body size; fecundity selection on both male and female body size is lower on the poor quality host (P. florida) relative to the high quality host (A. greggii) whereas selection on female body size via effects of egg size on offspring survival (body size --> egg size --> fitness) is greater on the poor quality host relative to the high quality host. Because selection via the egg size path affects only females the difference in larval survival between hosts shifts the relative magnitude of selection on female vs. male size. Researchers working on other study systems should be alerted to the possible importance of subtle, but consequential, indirect selection on their study organisms.

Evolutionary responses to selection can be complicated when there is substantial nonadditivity, which limits our ability to extrapolate from simple models of selection to population differentiation and speciation. Studies of Drosophila melanogaster indicate that lifespan and the rate of senescence are influenced by many genes that have environment- and sex-specific effects. These studies also demonstrate that interactions among alleles (dominance) and loci (epistasis) are common, with the degree of interaction differing between the sexes and among environments. However, little is known about the genetic architecture of lifespan or mortality rates for organisms other than D. melanogaster. We studied genetic architecture of differences in lifespan and shapes of mortality curves between two populations of the seed beetle, Callosobruchus maculatus (South India and Burkina Faso populations). These two populations differ in various traits (such as body size and adult lifespan) that have likely evolved via host-specific selection. We found that the genetic architecture of lifespan differences between populations differs substantially between males and females; there was a large maternal effect on male lifespan (but not on female lifespan), and substantial dominance of long-life alleles in females (but not males). The large maternal effect in males was genetically based (there was no significant cytoplasmic effect) likely due to population differences in maternal effects genes that influence lifespan of progeny. Rearing host did not affect the genetic architecture of lifespan, and there was no evidence that genes on the Y-chromosome influence the population differences in lifespan. Epistatic interactions among loci were detectable for the mortality rate of both males and females, but were detectable for lifespan only after controlling for body size variation among lines. The detection of epistasis, dominance, and sex-specific genetic effects on C. maculatus lifespan is consistent with results from line cross and quantitative trait locus studies of D. melanogaster.

Few studies have examined the genetic architecture of population differences in behaviour and its implications for population differentiation and adaptation. Even fewer have examined whether differences in genetic architecture depend on the environment in which organisms are reared or tested. We examined the genetic basis of differences in oviposition preference and egg dispersion between Asian (SI) and African (BF) populations of the seed beetle, Callosobruchus maculatus. We reared and tested females on each of two host legumes (cowpea and mung bean). The two populations differed in mean oviposition preference (BF females preferred cowpea seeds more strongly than did SI females) and egg dispersion (SI females distributed eggs more uniformly among seeds than did BF females). Observations of hybrid and backcross individuals indicated that only the population difference in oviposition preference could be explained by complete additivity, whereas substantial dominance and epistasis contributed to the differences in egg dispersion. Both rearing host and test host affected the relative magnitude of population differences in egg dispersion and the composite genetic effects. Our results thus demonstrate that the relative influence of epistasis and dominance on the behaviour of hybrids depends on the behaviour measured and that different aspects of insect oviposition are under different genetic control. In addition, the observed effect of rearing host and oviposition host on the relative importance of dominance and epistasis indicates that the genetic basis of population differences depends on the environment in which genes are expressed.

Hybrid plants often differ in resistance to arthropods compared to the parental species from which they are derived. To better understand the relative contribution of genetic effects in influencing plant resistance to arthropods, we examined the genetic architecture of resistance in a willow hybrid system, Salix eriocephala, S. sericea, and their interspecific hybrids. Resistance to two arthropods, a willow leaf aphid (Chaitophorus sp.: Aphididae) and an eriophyoid mite (Aculops tetanothrix: Eriophyidae), were compared because resistance to different herbivores may be controlled by different traits and influenced by different genetic effects. We found additive and nonadditive genetic effects to be important in explaining the difference between willow species in resistance to aphids and mites. F2 hybrids exhibited low resistance to aphids, suggesting breakdown of favourable epistatic interactions that confer resistance. F2 hybrids, however, exhibited high resistance to mites, suggesting either the breakdown of interactions that affect traits used by mites in host location or the creation of favourable epistatic interactions. This study demonstrates the potential role of herbivores in affecting plant genetic structure, such that selection by herbivores can potentially lead to the creation of gene interactions that influence host resistance traits or host recognition traits used by the herbivore.

In many organisms, large offspring have improved fitness over small offspring, and thus their size is under strong selection. However, due to a trade-off between offspring size and number, females producing larger offspring necessarily must produce fewer unless the total amount of reproductive effort is unlimited. Because differential gene expression among environments may affect genetic covariances among traits, it is important to consider environmental effects on the genetic relationships among traits. We compared the genetic relationships among egg size, lifetime fecundity, and female adult body mass (a trait linked to reproductive effort) in the seed beetle, Stator limbatus, between two environments (host-plant species Acacia greggii and Cercidium floridum). Genetic correlations among these traits were estimated through half-sib analysis, followed with artificial selection on egg size to observe the correlated responses of lifetime fecundity and female body mass. We found that the magnitude of the genetic trade-off between egg size and lifetime fecundity differed between environments--a strong trade-off was estimated when females laid eggs on C. floridum seeds, yet this trade-off was weak when females laid eggs on A. greggii seeds. Also differing between environments was the genetic correlation between egg size and female body mass-these traits were positively genetically correlated for egg size on A. greggii seeds, yet uncorrelated on C. floridum seeds. On A. greggii seeds, the evolution of egg size and traits linked to reproductive effort (such as female body mass) are not independent from each other as commonly assumed in life-history theory.

Plants can vary substantially among species in morphological and physiological traits that influence their resistance to herbivores and their quality for herbivore growth and development. Seeds of the desert trees Cercidium floridum and C. microphyllum vary within and among populations in their resistance to, and suitability for, growth and development of larvae of the seed beetle, Stator limbatus. We examined how this variation influences the magnitude of natural selection on the size of eggs laid by beetles and tested whether female beetles can distinguish between seeds of more vs. less suitable trees (within species). We found that selection favoring large eggs differed substantially between the two tree species (greater on C. floridum) and varied substantially among populations of C. floridum and among trees (within populations) of both C. floridum and C. microphyllum. In general, the magnitude of selection favoring large eggs correlated positively with seed coat resistance between tree species, among populations within species, and among trees within populations. Female S. limbatus are capable of distinguishing seeds of the two host species and respond to species differences in selection by laying larger eggs on seeds of C. floridum than on seeds of C. microphyllum. However, despite large variation in seed suitability within populations of trees, females did not preferentially lay eggs on seeds of less resistant trees, nor did they adjust egg size in response to variation in resistance among trees.

Ejaculate size in seed beetles (Coleoptera: Bruchidae) is subject to both sexual and fecundity selection. We examined interpopulation variation and inheritance of ejaculate size in the seed beetle Callosobruchus maculatus (F.). There was significant variation among three populations in both body mass and the proportion of a male's body mass that was transferred to females during mating. The seed upon which beetles were raised had a small effect on male body size but not the size of their ejaculates. To investigate the inheritance of ejaculate size, we performed inter- and intrapopulation crosses with two of these populations. The progeny of interpopulation crosses were intermediate between the intrapopulation (parental) crosses, suggesting additive genetic autosomal inheritance. This result differs from an earlier study in which ejaculate size of a different population was maternally inherited. This study demonstrates that there is indeed genetic variation in ejaculate size, but that the loci exhibiting within-population variation may be different than the loci producing among-population variation.

Most models of optimal progeny size assume that there is a trade-off between progeny size and number, and that progeny fitness increases with increasing investment per young. We find that both assumptions are supported by empirical studies but that the trade-off is less apparent when organisms are iteroparous, use adult-acquired resources for reproduction, or provide parental care. We then review patterns of variation in progeny size among species, among populations within species, among individuals within populations, and among progeny produced by a single female. We argue that much of the variation in progeny size among species, and among populations within species, is likely due to variation in natural selection. However, few studies have manipulated progeny environments and demonstrated that the relationship between progeny size and fitness actually differs among environments, and fewer still have demonstrated why selection favors different sized progeny in different environments. We argue that much of the variation in progeny size among females within populations, and among progeny produced by a single female, is probably nonadaptive. However, some species of arthropods exhibit plasticity in progeny size in response to several environmental factors, and much of this plasticity is likely adaptive. We conclude that advances in theory have substantially outpaced empirical data. We hope that this review will stimulate researchers to examine the specific factors that result in variation in selection on progeny size within and among populations, and how this variation in selection influences the evolution of the patterns we observe.

In response to food limitation, many insects have evolved developmental plasticity in which larvae mature at a smaller body size. Here we investigate the consequences of this smaller body size for development of individuals in the next generation, after resource competition has been relaxed. Families of the seed beetle Stator pruininus (Horn) reared at high density (≈20 eggs per seed) matured at a substantially smaller adult body size than families reared at 1 egg per seed. Females emerging from high density seeds also laid smaller eggs than did females reared at low density (7–14% lighter). Progeny developing from these smaller eggs eventually attained the same adult body size as progeny developing from eggs laid by low-density mothers when all progeny were reared at low density (i.e., resource competition was relaxed). They achieved this by developing on average ≈0.5 d longer. Reciprocal crosses between high and low-density lines demonstrated that the differences among lines in development time was maternally inherited; only maternal lineage explained some of the variance in development time. Thus, larvae appear to compensate for the small eggs laid by their mothers by extending development time to mature at the same size as progeny from larger eggs.