CONNECTING QTLS TO THE G-MATRIX OF EVOLUTIONARY QUANTITATIVE GENETICS

Evolutionary quantitative genetics has recently advanced in two distinct streams. Many biologists address evolutionary questions by estimating phenotypic selection and genetic (co)variances ( G matrices). Simultaneously, an increasing number of studies have applied quantitative trait locus (QTL) mapping methods to dissect variation. Both conceptual and practical difficulties have isolated these two foci of quantitative genetics. A conceptual integration follows from the recognition that QTL allele frequencies are the essential variables relating the G -matrix to marker-based mapping experiments. Breeding designs initiated from randomly selected parental genotypes can be used to estimate QTL-specific genetic (co)variances. These statistics appropriately distill allelic variation and provide an explicit population context for QTL mapping estimates. Within this framework, one can parse the G -matrix into a set of mutually exclusive genomic components and ask whether these parts are similar or dissimilar in their respective features, for example the magnitude of phenotypic effects and the extent and nature of pleiotropy. As these features are critical determinants of sustained response to selection, the integration of QTL mapping methods into G -matrix estimation can provide a concrete, genetically based experimental program to investigate the evolutionary potential of natural populations.     

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.     

EVOLUTIONARY GENETICS OF ALLORECOGNITION IN THE COLONIAL HYDROID HYDRACTINIA SYMBIOLONGICARPUS

Many sedentary, clonal marine invertebrates compete intensively with conspecifics for habitable space. Allorecognition systems mediate the nature and outcome of these intraspecific competitive interactions, such that the initiation of agonistic behavior and the potential for intergenotypic fusion depend strongly on the relatedness of the contestants. The dependence of these behaviors on relatedness, along with the extraordinary precision with which self can be discriminated from nonself, suggest that allorecognition systems are highly polymorphic genetically. However, allotypic specificity of this sort could be produced by any number of genetic scenarios, ranging from relatively few loci with abundant allelic variation to numerous loci with relatively few alleles per locus. At this point, virtually nothing is known of the formal genetics of allorecognition in marine invertebrates; consequently, the evolutionary dynamics of such systems remain poorly understood. In this paper, we characterize the formal genetics of allorecognition in the marine hydrozoan Hydractinia symbiolongicarpus. Hydractinia symbiolongicarpus colonizes gastropod shells occupied by hermit crabs. When two or more individuals grow into contact, one of three outcomes ensues: fusion (compatibility), transitory fusion (a temporary state of compatibility), and rejection (incompatibility, often accompanied by the production of agonistic structures termed hyperplastic stolons). Observed patterns of compatibility between unrelated, half-sib pairs, and full-sib pairs show that unrelated and half-sib pairs under laboratory culture have a very low probability of being fusible, whereas full sibs have a roughly 30% rate of fusion in experimental pairings. The genetic simulations indicate that roughly five loci, with 5–7 alleles per locus, confer specificity in this species. In ecological terms, the reproductive ecology of H. symbiolongicarpus should promote the cosettlement of kin, some of which should be full sibs, and some half sibs. Thus, there is potential for kin selection to play a major role in the evolution of the H. symbiolongicarpus allorecognition system. In genetic terms, this system conforms to theoretical predictions for a recognition system selected to distinguish among classes of kin, in addition to self from nonself.     

EVOLUTIONARY GENETICS OF OVIPOSITION PREFERENCE IN SWALLOWTAIL BUTTERFLIES

Models of the evolution of host shifts and speciation in phytophagous insects are often built upon the assumption that host selection is under simple genetic control, perhaps even a single locus. The genetic basis for differences in host-plant preference by ovipositing insects was investigated using two closely related species of swallowtail butterfly, Papilio oregonius and P. zelicaon, which differ in the plant families on which females oviposit. Both species had been shown previously to vary within populations in host selection. A combination of analyses using reciprocal interspecific crosses and isofemale strains within populations indicated that oviposition preference in these species is determined significantly by one or more loci on the X chromosome, which female Lepidoptera inherit only paternally. Hence, preferences in hybrid females tended toward the paternal species. This is the first insect group for which partial control of oviposition preference has been localized onto a particular chromosome. In addition, one or more loci on another chromosome(s) appear to contribute to preference, as indicated by the partially intermediate preferences of some hybrid crosses. The overall differences in preference in the reciprocal interspecific hybrids were restricted to differences in the distribution of eggs laid among the local host plants of these two Papilio species; the reciprocal crosses did not differ in the small percentage of eggs laid on a novel potential host species. The variation in host selection found among the isofemale strains reinforces earlier results for these strains, indicating that there is genetic variation in host selection within these populations. Overall, the results indicate that the evolution of oviposition preference in these species involves genetic changes at two or more chromosomes with the X chromosome playing an important role in determining preference.     

THE EVOLUTIONARY GENETICS AND DEVELOPMENTAL BASIS OF WING PATTERN VARIATION IN THE BUTTERFLY BICYCLUS ANYNANA

We have studied interactions between developmental processes and genetic variation for the eyespot color pattern on the adult dorsal forewing of the nymphalid butterfly, Bicyclus anynana. Truncation selection was applied in both an upward and a downward direction to the size of a single eyespot consisting of rings with wing scales of differing color pigments. High heritabilities resulted in rapid responses to selection yielding divergent lines with very large or very small eyespots. Strong correlated responses occurred in most of the other eyespots on each wing surface. The cells at the center of a presumptive eyespot (the “focus”) act in the early pupal stage to establish the adult wing pattern. The developmental fate of the scale cells within an eyespot is specified by the “signaling” properties of the focus and the “response” thresholds of the epidermis. The individual eyespots can be envisaged as developmental homologues. Grafting experiments performed with the eyespot foci of the selected lines showed that additive genetic variance exists for both the response and, in particular, the signaling components of the developmental system. The results are discussed in the context of how constraints on the evolution of this wing pattern may be related to the developmental organization.     

ON THE EVOLUTIONARY CONSEQUENCES OF SEXUAL IMPRINTING

The idea that sexual imprinting may generate sexual selection and possibly lead to speciation has been much discussed in the ethological literature. Here the feasibility of three such hypotheses is investigated using mathematical models of sexual selection in which mating preferences are acquired through imprinting and hence dependent upon the parental phenotypes. The principal findings are the following. (1) Sexual imprinting reduces the likelihood of novel adaptive traits spreading through a population, except in some circumstances in which there is heterozygote advantage. (2) Asymmetrical mating preferences, acquired through imprinting, can generate sexual selection for traits that impair survival. (3) The conditions under which sexual imprinting will maintain a genetic polymorphism in a population are fairly restricted. (4) Sexual imprinting can act as a barrier to gene flow minimizing the impact of migration and preserving and accentuating genetic differences between populations. The findings suggest that sexual imprinting may be of considerable evolutionary significance.     

我国植物遗传学的溯源

一在古代为了利用野生植物作为食品和药物,对于千百种植物进行了采集、识别、分类和品尝,人们逐渐形成了物种或品类的概念,这是植物遗传学建立的准备阶段。以后在将一些植物驯化种植和逐步选择改良的漫长历史中,人们积累了丰富的感性     

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

In many organisms, a female's 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 (r_G > 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.