Timothy A. Mousseau

Natural selection and the heritability of fitness components.

The hypothesis that traits closely associated with fitness will generally possess lower heritabilities than traits more loosely connected with fitness is tested using 1120 narrow sense heritability estimates for wild, outbred animal populations, collected from the published record. Our results indicate that life history traits generally possess lower heritabilities than morphological traits, and that the means, medians, and cumulative frequency distributions of behavioural and physiological traits are intermediate between life history and morphological traits. These findings are consistent with popular interpretations of Fishers (1930, 1958) Fundamental Theorem of Natural Selection, and Falconer (1960, 1981), but also indicate that high heritabilities are maintained within natural populations even for traits believed to be under strong selection. It is also found that the heritability of morphological traits is significantly lower for ectotherms than it is for endotherms which may in part be a result of the strong correlation between life history and body size for many ectotherms.

Quantitative genetics and fitness: lessons from Drosophila

This paper examines patterns of heritability and genetic covariance between traits in the genus Drosophila. Traits are divided into the categories, morphology, behaviour, physiology and life history. Early theoretical analyses suggested that life history traits should have heritabilities that are lower than those in other categories. Variable pleiotrophy, environmental variation, mutation and niche variation may, however, maintain high heritabilities. In Drosophila the heritabilities of life history traits are lower than morphological or physiological traits but may exceed 20 per cent. The pattern of variation in the heritability of behavioural traits is similar to that of life history traits. Genetic covariance between morphological traits and between morphological and life history traits are all positive but those between life history traits have variable sign. Negative covariance between traits supports the variable pleiotropy hypothesis but other factors such as environmental heterogeneity, or mutation cannot be excluded.

Egg Size Plasticity in a Seed Beetle: An Adaptive Maternal Effect

In the seed beetle, Stator limbatus, the fitness consequences of egg size vary substantially among host plants. There is intense selection for laying large eggs when larvae will develop on seeds of Cercidium floridum (caused by high mortality penetrating the seed coat) but selection for laying small eggs when larvae will develop on seeds of Acacia greggii (caused by very low mortality penetrating the seed coat and an egg size/egg number trade-off). We test the hypothesis that host-associated variation in egg size within populations of S. limbatus represents an adaptive maternal effect in which females adjust egg size in response to host species. In laboratory experiments, S. limbatus females laid significantly larger and fewer eggs on C. floridum than on A. greggii. When switched between hosts, females readjusted egg size, producing progressively larger eggs on C. floridum and smaller eggs on A. greggii. When conditioned to lay either small eggs (on A. greggii) or large eggs (on C. floridum), and then forced to lay on C. floridum, females conditioned on C. floridum laid eggs that had substantially higher survivorship than eggs laid by females conditioned on A. greggii. These experiments demonstrate that egg size is an adaptively plastic character in S. limbatus.

Adaptation to seasonality in a cricket: patterns of phenotypic and genotypic variation in body size and diapause expression along a cline in season length

This paper investigates patterns of phenotypic variation in the striped ground cricket (Allonemobious fasciatus) along a cline in season length and tests the hypothesis that variation in body size and diapause propensity is the result of diversifying selection due to different local conditions. We examined 83 populations and found that A. fasciatus produced a single generation per year (univoltine) north of 37°N latitude and was bivoltine south of 35°30′N. Body size generally increased with increasing season length, with a sudden drop in the region corresponding to the transition from univoltine to bivoltine life cycles, reflecting the division of total season length in two within‐bivoltine populations. We reared ten populations in our laboratory and found that much of the interpopulation variance in body size observed in the field could be attributed to genetic differences. Diapause expression also varied significantly among populations and was strongly correlated with season length. The heritability of body size did not differ between populations, but full‐sib estimates greatly exceeded parent–offspring estimates (h  po2=0.15±0.05; h fs2=0.45±0.04) suggesting that there are important nonadditive genetic effects. The heritability of diapause expression, determined from analysis of full sibs, varied significantly between populations (0.33 ± 0.10 to 1.31 ± 0.21) with an average of 0.74 ± 0.16. Body size and diapause expression were genetically correlated in transition‐zone populations, but not in univoltine or bivoltine populations. Our findings support the suggestion that clinal variation in body size and diapause expression in the striped ground cricket reflect adaptation to season length.

Female mating bias results in conflicting sex-specific offspring fitness.

Indirect-benefit models of sexual selection assert that females gain heritable offspring advantages through a mating bias for males of superior genetic quality. This has generally been tested by associating a simple morphological quality indicator (for example, bird tail length) with offspring viability. However, selection acts simultaneously on many characters, limiting the ability to detect significant associations, especially if the simple indicator is weakly correlated to male fitness. Furthermore, recent conceptual developments suggest that the benefits gained from such mating biases may be sex-specific because of sexually antagonistic genes that differentially influence male and female reproductive ability. A more suitable test of the indirect-benefit model would examine associations between an aggregate quality indicator (such as male mating success) and gender-specific adult fitness components, under the expectation that these components may trade off. Here, we show that a fathers mating success in the cricket, Allonemobius socius, is positively genetically correlated with his sons mating success but negatively with his daughters reproductive success. This provides empirical evidence that a female mating bias can result in sexually antagonistic offspring fitness.Indirect-benefit models of sexual selection assert that females gain heritable offspring advantages through a mating bias for males of superior genetic quality. This has generally been tested by associating a simple morphological quality indicator (for example, bird tail length) with offspring viability1. However, selection acts simultaneously on many characters, limiting the ability to detect significant associations, especially if the simple indicator is weakly correlated to male fitness2,3. Furthermore, recent conceptual developments suggest that the benefits gained from such mating biases may be sex-specific because of sexually antagonistic genes that differentially influence male and female reproductive ability4. A more suitable test of the indirect-benefit model would examine associations between an aggregate quality indicator1,3 (such as male mating success) and gender-specific adult fitness components, under the expectation that these components may trade off1. Here, we show that a fathers mating success in the cricket, Allonemobius socius, is positively genetically correlated with his sons mating success but negatively with his daughters reproductive success. This provides empirical evidence that a female mating bias can result in sexually antagonistic offspring fitness.

IMMUNE SUPPRESSION AND THE COST OF REPRODUCTION IN THE GROUND CRICKET, ALLONEMOBIUS SOCIUS

Abstract One of the most common life history trade‐offs in animals is the reduction in survivorship with increasing reproductive effort. Despite the prevalence of this pattern, its underlying physiological mechanisms are not well understood. Here we test the hypothesis that immune suppression mediates this phenotypic trade‐off by manipulating reproductive effort and measuring immune function and mortality rates in the striped ground cricket, Allonemobius socius. Because A. socius males provide females with a hemolymph‐based nuptial gift during copulation, and many structural components of immunity reside in the hemolymph, we also predicted that sexual selection may differentially affect how disease resistance evolves in males and females. We found that an increased mating effort resulted in a reduced immune ability, coupled with an increased rate in age‐specific mortality for both sexes. Thus, immune suppression appears to be a link between reproductive effort and cost in this system. In addition, males and females appeared to differentially invest in several aspects of immunity prior to mating, with males exhibiting a higher concentration of circulating hemocytes and a superior bacterial defense capability. This pattern may be the result of previously established positive selection on gift size due to its affect on female fecundity. In short, female choice for larger gifts may lead to a sexually dimorphic immune ability.

A novel method for estimating heritability using molecular markers

Heritability is usually estimated with individuals of known relatedness generated using a controlled breeding programme or through response to selection. In this paper, we use two single-locus VNTR DNA fingerprint markers in conjunction with a maximum likelihood method to infer relatedness among pairs of individuals in a captive population of Pacific chinook salmon (Oncorhynchus tshawytscha). Patterns of relatedness inferred from the two DNA fingerprint markers were used to estimate heritability for, and genetic correlations among, several economically and ecologically important traits (weight, length, flesh colour and precocious male maturation). Heritabilities ranged from 0.20 for weight, 0.38 for length, 0.67 for precocious male maturation (‘jacking’) to 0.76 for flesh colour, which are in good agreement with estimates for salmonids generated using classical quantitative genetic methods. This molecular marker-based method allows for the estimation of heritability in wild, long-lived species not easily manipulated for study using controlled breeding programmes.

THE EVOLUTIONARY GENETICS OF AN ADAPTIVE MATERNAL EFFECT: EGG SIZE PLASTICITY IN A SEED BEETLE

In many organisms, a females environment provides a reliable indicator of the environmental conditions that her progeny will encounter. In such cases, maternal effects may evolve as mechanisms for transgenerational phenotypic plasticity whereby, in response to a predictive environmental cue, a mother can change the type of eggs that she makes or can program a developmental switch in her offspring, which produces offspring prepared for the environmental conditions predicted by the cue. One potentially common mechanism by which females manipulate the phenotype of their progeny is egg size plasticity, in which females vary egg size in response to environmental cues. We describe an experiment in which we quantify genetic variation in egg size and egg size plasticity in a seed beetle, Stator limbatus, and measure the genetic constraints on the evolution of egg size plasticity, quantified as the genetic correlation between the size of eggs laid across host plants. We found that genetic variation is present within populations for the size of eggs laid on seeds of two host plants (Acacia greggii and Cercidium floridum; h2 ranged between 0.217 and 0.908), and that the heritability of egg size differed between populations and hosts (higher on A. greggii than on C. floridum). We also found that the evolution of egg size plasticity (the maternal effect) is in part constrained by a high genetic correlation across host plants (rG > 0.6). However, the cross‐environment genetic correlation is less than 1.0, which indicates that the size of eggs laid on these two hosts can diverge in response to natural selection and that egg size plasticity is thus capable of evolving in response to natural selection.

Evolution of maternal effects: Past and present

It has been said that many original ideas reflect a convergence of related thought that coalesces into a unified representation of what many have been thinking. This is certainly true for the subfield of maternal effects evolution. The study of maternal effects has a long history. The first two

Altitudinal variation in life cycle syndromes of California populations of the grasshopper, Melanoplus sanguinipes (F.)

SummaryLife cycles of California populations of the grasshopper, Melanoplus sanguinipes, varied along an altitudinal gradient. Temperature records indicate a longer season at low altitude on the coast, based on computation of degree days available for development, even though summer air temperatures are cooler than at high altitude; this is a result of warm soil temperatures. At high and low altitudes there was a high proportion of diapause eggs oviposited, while intermediate proportions of diapause eggs occurred at mid altitudes. The low altitude, and especially sea level, populations diapaused at all stages of embryonic development, while at high altitudes most diapause occurred in the late stages just before hatch. Diapause was more intense at high altitudes. One result of diapause differences was delayed hatching in the sea level population. Nymphal development and development of adults to age at first reproduction were both accelerated at high altitude relative to sea level. At lower temperatures (27° C) there was a tendency for short days to accelerate development of sea level nymphs, but not high altitude nymphs. In both sea level and high altitude grasshoppers, short days accelerated maturation of adults to onset of oviposition at warm temperature (33° C); there was little reproduction at 27° C. Population differences for all traits studied appear to be largely genetic with some maternal effects possible. We interpret diapause variation at low and mid altitudes to be responses to environmental uncertainty and variations in development rates to be adaptations to prevailing season lengths.