THE COVARIANCE STRUCTURE OF LIFE-HISTORY CHARACTERS IN DAPHNIA PULEX

The genetic covariance structure for life-history characters in two populations of cyclically parthenogenetic Daphnia pulex indicates considerable positive correlation among important fitness components, apparently at odds with the expectation if antagonistic pleiotropy is the dominant cause of the maintanence of genetic variation. Although there is no genetic correlation between offspring size and offspring number, present growth and present reproduction are both strongly positively correlated genetically with future reproduction, and early maturity is genetically correlated with larger clutch size. Although the ubiquity of antagonistic pleiotropy has been recently questioned, there are peculiarities of cyclical parthenogenesis that could lead to positive life-history covariance even when negative covariance would be expected in a similar sexual species. These include the influence of nonadditive gene action on evolution in clonally reproducing organisms, and the periodic release of hidden genetic variance within populations of cyclical parthenogens. Examination of matrix similarity, using the bootstrap for distribution-free hypothesis testing, reveals no evidence to suggest that the genetic covariance matrices differ between the populations. However, there is considerable evidence that the phenotypic and environmental covariance matrices differ between populations. These results indicate approximate stability of the genetic covariance matrix within species, an important assumption of many phenotypic evolution models, but should caution against the use of phenotypic in place of genetic covariance matrices.     

THE ORIGIN AND GENETIC BASIS OF OBLIGATE PARTHENOGENESIS IN DAPHNIA PULEX

Sex in Daphnia is environmentally determined, and some obligately parthenogenetic clones of D. pulex have retained the ability to produce males. In the present study, males from 13 such clones were crossed to sexual females from closely related cyclical parthenogens both to determine whether the males were capable of producing viable hybrids and to determine the mode of reproduction of the hybrids. A total of 178 genetically confirmed hybrids were produced, with each of the 19 attempted crosses resulting in some viable hybrids. On average, only 34% of the hybrid eggs that initiated development survived to the reproductive stage, suggesting some incompatibility between the parents. The absence of any association between survivorship and parental or hybrid genotype indicated, however, that there is no specific genetic incompatibility associated with the marker loci used. The inability of most hybrids to produce normal resting eggs is further evidence of a general genomic incompatibility between the parents. Ten of the hybrids produced viable resting eggs, permitting tests to determine their mode of reproduction. Six of the 10 hybrids reproduced by cyclical parthenogenesis, like their maternal parent. The remaining four hybrids reproduced by obligate parthenogenesis, like their paternal parent, demonstrating that the genes suppressing meiosis can be transmitted by the male parent. These results support a model for the generation of new clones that involves the spread of genes suppressing meiosis and provide evidence that the high genotypic diversity observed in obligately parthenogenetic populations of D. pulex is a result of the multiple origin of new clones from the cyclical parthenogens. Evidence was also obtained suggesting that the obligately parthenogenetic clones carry a load of recessive deleterious genes.     

THE EVOLUTIONARY GENETICS OF MALE LIFE-HISTORY CHARACTERS IN DROSOPHILA MELANOGASTER

Alternative models of the maintenance of genetic variability, theories of life-history evolution, and theories of sexual selection and mate choice can be tested by measuring additive and nonadditive genetic variances of components of fitness. A quantitative genetic breeding design was used to produce estimates of genetic variances for male life-history traits in Drosophila melanogaster. Additive genetic covariances and correlations between traits were also estimated. Flies from a large, outbred, laboratory population were assayed for age-specific competitive mating ability, age-specific survivorship, body mass, and fertility. Variance-component analysis then allowed the decomposition of phenotypic variation into components associated with additive genetic, nonadditive genetic, and environmental variability. A comparison of dominance and additive components of genetic variation provides little support for an important role for balancing selection in maintaining genetic variance in this suite of traits. The results provide support for the mutation-accumulation theory, but not the antagonistic-pleiotropy theory of senescence. No evidence is found for the positive genetic correlations between mating success and offspring quality or quantity that are predicted by “good genes” models of sexual selection. Additive genetic coefficients of variation for life-history characters are larger than those for body weight. Finally, this set of male life-history characters exhibits a very low correspondence between estimates of genetic and phenotypic correlations.     

VARIATION IN SEED CHARACTERS IN NEMOPHILA MENZIESII: EVIDENCE OF A GENETIC BASIS FOR MATERNAL EFFECT

A growing body of evidence indicates that phenotypic selection on juvenile traits of both plants and animals may be considerable. Because juvenile traits are typically subject to maternal effects and often have low heritabilities, adaptive responses to natural selection on these traits may seem unlikely. To determine the potential for evolutionary response to selection on juvenile traits of Nemophila menziesii (Hydrophyllaceae), we conducted two quantitative genetic studies. A reciprocal factorial cross, involving 16 parents and 1960 progeny, demonstrated a significant maternal component of variance in seed mass and additive genetic component of variance in germination time. This experiment also suggested that interaction between parents, though small, provides highly significant contributions to the variance of both traits. Such a parental interaction could arise by diverse mechanisms, including dependence of nuclear gene expression on cytoplasmic genotype, but the design of this experiment could not distinguish this from other possible causes, such as effects on progeny phenotype of interaction between the environmental conditions of both parents. The second experiment, spanning three generations with over 11,000 observations, was designed for investigation of the additive genetic variance in maternal effect, assessment of paternal effects, as well as further partitioning of the parental interaction identified in the reciprocal factorial experiment. It yielded no consistent evidence of paternal effects on seed mass, nor of parental interactions. Our inference of such interaction effects from the first experiment was evidently an artifact of failing to account for the substantial variance among fruits within crosses. The maternal effect was found to have a large additive genetic component, accounting for at least 20% of the variation in individual seed mass. This result suggests that there is appreciable potential for response to selection on seed mass through evolution of the maternal effect. We discuss aspects that may nevertheless limit response to individual selection on seed mass, including trade-offs between the size of individual seeds and germination time and between the number of seeds a maternal plant can mature and their mean size.     

THE ORIGIN OF MORPHOLOGICAL CHARACTERS AND THE BIOLOGICAL BASIS OF HOMOLOGY

A homolog is a part of the phenotype that is homologous to equivalent parts in other species. A biological homology concept is expected to explain three properties of homologs: 1) the conservation of those features that are used to define a homolog, 2) the individualization of the homolog with regard to the rest of the body, and 3) the uniqueness of homologs, i.e., their specificity for monophyletic groups. The main obstacle to describing a mechanistic basis for homology is the variability of the developmental pathways of undoubtedly homologous characters. However, not all aspects of the developmental pathway are of equal importance. The only organizational features of the developmental system that matter are those that have been historically acquired and cause developmental constraints on the further evolutionary modification of the characters. Two main factors contribute to historically acquired developmental constraints: generative rules of pattern formation and ontogenetic networks. In particular, hierarchical and cyclical inductive networks have the required properties to explain homology. How common such networks are is an open empirical question. The development and variation of pectoral fin hooks in blenniid fishes is presented as a model for the study of a simple ontogenetic network.     

EVOLUTION OF ADULT MORPHOLOGY AND LIFE-HISTORY CHARACTERS IN CAVERNICOLOUS PTOMAPHAGUS BEETLES

Morphological and life-history characters were determined for a series of six increasingly cavernicolous species of eastern United States Ptomaphagus beetles. Contrary to expectations, dimensions of only some adult structural characters uniformly covary as an overall measure of evolutionary adaptation for cave life. Adult reproductive characters that show adaptation for cave life are loss of reproductive seasonality and production of fewer and larger eggs. Significant change in pre-imaginal life-cycle stages was not found. This is in contrast to cavernicolous bathysciine beetles of Europe which show remarkable adaptive trends in pre-imaginal stages, but larval adaptations are not strongly coevolved with adult cave-adaptive characters. This suggests that evolution of cave adaptation in adult endopterygote insects occurs before and independently from that in larvae.     

PATTERNS OF GENETIC ARCHITECTURE FOR LIFE-HISTORY TRAITS AND MOLECULAR MARKERS IN A SUBDIVIDED SPECIES

Abstract Understanding the utility and limitations of molecular markers for predicting the evolutionary potential of natural populations is important for both evolutionary and conservation genetics. To address this issue, the distribution of genetic variation for quantitative traits and molecular markers is estimated within and among 14 permanent lake populations of Daphnia pulicaria representing two regional groups from Oregon. Estimates of population subdivision for molecular and quantitative traits are concordant, with Q _ST generally exceeding G _ST. There is no evidence that microsatellites loci are less informative about subdivision for quantitative traits than are allozyme loci. Character-specific comparison of Q _ST and G _ST support divergent selection pressures among populations for the majority of life-history traits in both coast and mountain regions. The level of within-population variation for molecular markers is uninformative as to the genetic variation maintained for quantitative traits. In D. pulicaria , regional differences in the frequency of sex may contribute to variation in the maintenance of expressed within-population quantitative-genetic variation without substantially impacting diversity at the genic level. These data are compared to an identical dataset for 17 populations of the temporary-pond species, D. pulex .     

THE GENETIC BASIS OF A COMPLEX FUNCTIONAL SYSTEM

The relationship between form and function can have profound effects on evolutionary dynamics and such effects may differ for simple versus complex systems. In particular, functions produced by multiple structural configurations (many‐to‐one mapping, MTOM) may dampen constituent trade‐offs and promote diversification. Unfortunately, we lack information about the genetic architecture of MTOM functional systems. The skulls of teleost fishes contain both simple (lower jaw levers) as well as more complex (jaws modeled as 4‐bar linkages) functional systems within the same craniofacial unit. We examined the mapping of form to function and the genetic basis of these systems by identifying quantitative trait loci (QTL) in hybrids of two Lake Malawi cichlid species. Hybrid individuals exhibited novelty (transgressive segregation) in morphological components and function of the simple and complex jaw systems. Functional novelty was proportional to the prevalence of extreme morphologies in the simple levers; by contrast, recombination of parental morphologies produced transgression in the MTOM 4‐bar linkage. We found multiple loci of moderate effect and epistasis controlling jaw phenotypes in both the simple and complex systems, with less phenotypic variance explained by QTL for the 4‐bar. Genetic linkage between components of the simple and complex systems partly explains phenotypic correlations and may constrain functional evolution.     

THE EVOLUTION OF THRESHOLD TRAITS: A QUANTITATIVE GENETIC ANALYSIS OF THE PHYSIOLOGICAL AND LIFE-HISTORY CORRELATES OF WING DIMORPHISM IN THE SAND CRICKET

Many traits are phenotypically discrete but polygenically determined. Such traits can be understood using the threshold model of quantitative genetics that posits a continuously distributed underlying trait, called the liability, and a threshold of response, individuals above the threshold displaying one morph and individuals below the threshold displaying the alternate morph. For many threshold traits the liability probably consists of a hormone or a suite of hormones. Previous experiments have implicated juvenile hormone esterase (JHE), a degratory enzyme of juvenile hormone, as a physiological determinant of wing dimorphism in the crickets Gryllus rubens and G. firmus. The present study uses a half-sib experiment to measure the heritability of JHE in the last nymphal stadium of G. firmus and its genetic correlation with fecundity, a trait that is itself genetically correlated with wing morph. The phenotypic and genetic parameters are consistent with the hypothesis that JHE is a significant component of the liability. Comparison of sire and dam estimates suggest that nonadditive effects may be important. Two models have been proposed to account for the fitness differences between morphs: the dichotomy model, which assumes that each morph can be characterized by a particular suite of traits, and the continuous model, which assumes that the associated fitness traits are correlated with the liability rather than the morphs themselves. The latter model predicts that the fitness differences will not be constant but change with the morph frequencies. Variation in fecundity and flight muscle histolysis are shown to be more consistent with the continuous model. Data from the present experiment on JHE are inconclusive, but results from a previous selection experiment also suggest that variation in JHE is consistent only with the continuous model.     

EARLY EVOLUTION OF THE GENETIC BASIS FOR SOMA IN THE VOLVOCACEAE

To understand the hierarchy of life in evolutionary terms, we must explain why groups of one kind of individual, say cells, evolve into a new higher level individual, a multicellular organism. A fundamental step in this process is the division of labor into nonreproductive altruistic soma. The regA gene is critical for somatic differentiation in Volvox carteri, a multicellular species of volvocine algae. We report the sequence of regA‐like genes and several syntenic markers from divergent species of Volvox. We show that regA evolved early in the volvocines and predict that lineages with and without soma descended from a regA‐containing ancestor. We hypothesize an alternate evolutionary history of regA than the prevailing “proto‐regA” hypothesis. The variation in presence of soma may be explained by multiple lineages independently evolving soma utilizing regA or alternate genetic pathways. Our prediction that the genetic basis for soma exists in species without somatic cells raises a number of questions, most fundamentally, under what conditions would species with the genetic potential for soma, and hence greater individuality, not evolve these traits. We conclude that the evolution of individuality in the volvocine algae is more complicated and labile than previously appreciated on theoretical grounds.     

THE EFFECT OF CYTOPLASM ON NUCLEAR ACTIVITY AND GENETIC CHARACTERS IN ANIMAL DEVELOPMENT

The present report is a summary of our works, published or unpublished, on the nucleo-cytoplasmic inter-reaction, performed in the past several years. As is well known, there is a tremendous amount of data from both basic and applied research showing nuclear control of development and genetics. On this matter, there is no argument at all. However, it would be a mistake if we belittle the contribution of cytoplasm and insist on that only the nucleus is involved in the control of     

GENETICS OF BRASSICA CAMPESTRIS. 1. GENETIC CONSTRAINTS ON EVOLUTION OF LIFE-HISTORY CHARACTERS

Energy allocation arguments suggest a possible tradeoff between timing and magnitude of reproduction: plants that postpone reproduction may accumulate greater resources and consequently produce more offspring. However, early reproduction may be favored when adult mortality is high. Tradeoffs among life-history characters may be a consequence of constraints imposed by genetic and environmental covariation among traits. In this paper we examine the genetic basis of the relationship between timing and magnitude of reproduction in an annual plant, Brassica campestris, by selecting to change flowering date and plant size in each of four directions (early and large, late and large, early and small, or late and small). There is a strong positive relationship between flowering date and flowering height. The response to selection was greatest along the axis of positive genetic covariation. Populations may evolve to become early flowering and small or late flowering and tall, but there is little response for the alternative combinations of characters. In this instance, the constraints imposed by quantitative genetics are in striking accord with predictions that might be made on physiological, energetic, or ecological grounds.     

THE GENETIC BASIS OF THE TRADE-OFF BETWEEN CALLING AND WING MORPH IN MALES OF THE CRICKET GRYLLUS FIRMUS

Wing dimorphisms exist in a wide range of insects. In wing-dimorphic species one morph is winged has functional flight muscles (LW), and is flight-capable, whereas the other has reduced wings (SW) and cannot fly The evolution and maintenance of wing dimorphisms is believed to be due to trade-offs between flight capability and fitness-related traits. Although there are well-established phenotypic trade-offs associated with wing dimorphism in female insects, there only exist two studies that have established a genetic basis to these trade-offs. The present study provides the first evidence for a genetically based trade-off in male insects, specifically in the sand cricket Gryllus firmus. Because they have to expend energy to maintain the flight apparatus (especially flight muscles), LW males are predicted to call less and therefore to attract fewer females. To be of evolutionary significance, call duration wing morph, and wing muscle condition (size and functionality) should all have measurable heritabilities and all be genetically correlated. Differences between morphs in male G. firmus in the likelihood of attracting a female were tested in the laboratory using a T-maze where females chose between a LW male and a SW male. Call duration for each male was recorded on the sixth day of adult life. A significant difference in call duration was found between SW and LW males (SW = 0.86 ± 0.01, LW = 0.64 ± 0.01 h). SW males attracted significantly more females than did LW males (63% vs. to 37%). All the traits involved in the trade-off had significant heritabilities (call = 0 75 ± 0 33; wing morph = 0.22 ± 007; muscle weight = 0.38 ± 0.09) and genetic correlations (call and wing morph = -0.46 ± 0.20 for SW, -0.68 ± 0.16 for LW; LW call and muscle weight = -0.80 ± 0.14). These results provide the first documented evidence that trade-offs between a dimorphic trait and a fitness-related character in males has a genetic basis and hence can be of evolutionary significance.     

LOCAL VARIATION IN THE GENETIC BASIS OF PAEDOMORPHOSIS IN THE SALAMANDER AMBYSTOMA TALPOIDEUM

The hypothesis that local isolated populations differed in the genetic basis for life-history traits was tested in the salamander Ambystoma talpoideum. Genetic basis was defined as the specific genetic architecture (additive and nonadditive) that contributes, along with maternal and environmental factors, to the phenotype. All crosses within and between three populations were made to produce nine F₁ populations. Nine within-population crosses produced the F₂ generation. This design does not permit an estimation of the exact nature of the genetic basis (e.g., additive, nonadditive) for any trait within populations. However, hybrid dissimilarity in the F₂ generation was taken as evidence of a different genetic basis for a trait in each population. The genetic basis of life-history pathway (metamorphosis vs. paedomorphosis) and per capita fecundity differed between two populations. The genetic basis of life-history pathway, per capita fecundity, survival, and growth rate was similar between the remaining sets of populations. This study and related ones (Semlitsch and Wilbur, 1989; Semlitsch et al., 1990) suggest that a heterochronic shift that causes rapid morphological evolution between metamorphosis and paedomorphosis (a macroevolutionary pattern) can evolve independently and does not require a macromutation or other nonmicroevolutionary mechanisms.     

GENETIC AND ENVIRONMENTAL VARIATION IN LIFE-HISTORY TRAITS OF A MONOCARPIC PERENNIAL: A DECADE-LONG FIELD EXPERIMENT

Directional and stabilizing selection tend to deplete additive genetic variance. On the other hand, genetic variance in traits related to fitness could be retained through polygenic mutation, spatially varying selection, genotype-environment interaction, or antagonistic pleiotropy. Most estimates of genetic variance in fitness-related traits have come from laboratory studies, with few estimates of heritability made under natural conditions, particularly for longer lived organisms. Here I estimated additive genetic variance in life-history characters of a monocarpic herb, Ipomopsis aggregata, that lives for up to a decade. Experimental crosses yielded 229 full-sibships nested within 32 paternal half-sibships. More than 5000 offspring were planted as seeds into natural field sites and were followed in most cases through their entire life cycle. Survival showed substantial additive genetic variance (genetic coefficient of variation ≈ 54%). Small differences at seedling emergence were magnified over time, such that the genetic variability in survival was only detectable by tracking the success of offspring for several years starting from seed. In contrast to survival, reproductive traits such as flower number, seeds per flower, and age at flowering showed little or no genetic variability. Despite relatively high levels of additive genetic variation for some life-history characters, high environmental variance in survival resulted in very low heritabilities (0–9%) for all of these characters. Maternal effects were evident in seed mass and remained strong throughout the lengthy vegetative period. No negative genetic correlations between major components of female fitness were detected. Mean corolla width for a paternal family was, however, negatively correlated with the finite rate of increase based on female fitness. That negative correlation could help to maintain additive genetic variance in the face of strong selection through male function for wide corollas.