Showing papers in "Journal of Evolutionary Biology in 1991"

Colony‐level sex ratio selection in the eusocial Hymenoptera

Abstract: We present an inclusive fitness model on worker-controlled sex investments in eusocial Hymenoptera which expands the existing theory for random mating populations as formulated by Trivers and Hare (1976) and Benford (1978). We assume that relatedness asymmetry is variable among colonies — owing to multiple mating, worker reproduction and polygyny — and that workers are able to assess the relatedness asymmetry in their own colony. A simple marginal value argument shows that “assessing” workers maximize their inclusive fitness by specializing on the production of the sex to which they are relatively more related than the average worker in the population is related to that sex. The model confirms our earlier verbal argument on this matter (Boomsma and Grafen, 1990) and gives further quantitative predictions of the optimal sex ratio of relatedness-asymmetry classes for both infinite and finite, random mating populations. It is shown that in large populations all but one of the relatedness-asymmetry classes should specialize on the production of one sex only. The remaining, balancing class is selected to compensate any bias induced by the other class(es) such that the population sex ratio reflects the relatedness asymmetry of that balancing class. In the absence of worker-reproduction, the sex ratio compensation by the balancing-class is generally close to 100%, unless the population is very small. In the Discussion we address explicitly the likelihood of our relatedness-assessment hypothesis and other assumptions made in the model. The relationship of our model with previous theory on sex allocation in eusocial Hymenoptera is worked out in the Appendix.

The genetics of phenotypic plasticity. II. Response to selection

Abstract: We selected on phenotypic plasticity of thorax size in response to temperature in Drosophila melanogaster using a family selection scheme. The results were compared to those of lines selected directly on thorax size. We found that the plasticity of a character does respond to selection and this response is partially independent of the response to selection on the mean of the character. One puzzling result was that a selection limit of zero plasticity was reached in the lines selected for decreased plasticity yet additive genetic variation for plasticity still existed in the lines. We tested the predictions of three models of the genetic basis of phenotypic plasticity: overdominance, pleiotropy, and epistasis. The results mostly support the epistasis model, that the plasticity of a character is determined by separate loci from those determining the mean of the character.

Establishment of a tetraploid cytotype in a diploid population: Effect of relative fitness of the cytotypes

Abstract: The conditions for the establishment of a tetraploid in a diploid population were investigated by means of a deterministic model, on the assumption that the diploid cytotype produces some 2n gametes. If the fertility and viability of both cytotypes were the same and the initial population was diploid, then a mixed population would occur if the production of 2n gametes was below 17.16%. The tetraploid excluded the diploid above this limit. By modifying the fertility and the viability of the polyploid this threshold varied, dropping to 10% when one of the two parameters was twice that of the diploid, and falling to as low as 6% if both fertility and viability were double that of the diploid. The conditions for the establishment of a polyploid are therefore quite restrictive under the assumptions of this model. In nature, such processes would probably allow the spread of the polyploid only if the immigration of polyploids considerably enhanced the frequency of tetraploids, or if genetical or environmental changes, or chance processes in small populations caused a substantial increase in the frequency of 2n gametes produced by the diploid.

The genetics of phenotypic plasticity. III. Genetic correlations and fluctuating asymmetries

Abstract: We examined the relationship of three aspects of development, phenotypic plasticity, genetic correlations among traits, and developmental noise, for thorax length, wing length, and number of sternopleural bristles in Drosophila melanogaster. We used 14 lines which had previously been selected on either thorax length or plasticity of thorax length in response to temperature. A half-sib mating design was used and offspring were raised at 19” C or 25” C. We found that genetic correlations were stable across temperatures despite the large levels of plasticity of these traits. Plasticities were correlated among developmentally related traits, thorax and wing length, but not among unrelated traits, lengths and bristle counts. Amount of developmental noise, measured as fluctuating asymmetry and withinenvironmental variation, was positively correlated with amount of plasticity only for some traits, thorax length and bristle number, and only at one temperature, 25” c.

Nestling growth in the Great Tit I. Heritability estimates under different environmental conditions

Abstract: Evolutionary change requires natural selection in the presence of heritable variation for the trait(s) under selection. Since heritabilities and selection pressures are known to vary with environmental conditions, it is crucial to know how much genetic variation is expressed under which conditions. This study addresses the question of how the expression of genetic variation for fledgling body size of Great Tits varies with the environment. Different environmental conditions were created experimentally by manipulating brood sizes. The treatment affected body size, measured as either fledging weight or tarsus length, and interacted with natural temporal variation in food availability. Both measurements show stabilizing selection. A cross-fostering design was carried out to separate genetic and environmental causes of variation. Heritabilities as measured from offspring-midparent regressions and from full-sib analyses were substantial for both traits, except that no heritability was found for weight under poor conditions. Instead, fledging weights were significantly correlated with the weights of their unrelated guardians’ ( = fosterparents’) weight under poor conditions. We propose that under poor conditions, when selection on fledging weights is expected to be directional and strong, only little genetic variance is expressed. Any evolutionary response to this selection on fledging weight might therefore be slow, if the increase in selection pressure is not greater than the decrease in heritability.

Sex differences in recombination

Abstract: One of the stronger empirical generalizations to emerge from the study of genetic systems is that achiasmate meiosis, which has evolved 25–30 times, is always restricted to the heterogametic sex in dioecious species, usually the male. Here we collate data on quantitative sex differences in chiasma frequency from 54 species (4 hermaphroditic flatworms, 18 dioecious insects and vertebrates and 32 hermaphroditic plants) to test whether similar trends hold. Though significant sex differences have been observed within many species, only the Liliaceae show a significant sexual dimorphism in chiasma frequency across species, with more crossing over in embryo mother cells than in pollen mother cells; chiasma frequencies are unrelated to sex and gamety in all other higher taxa studied. Further, the magnitude of sexual dimorphism, independent of sign, does not differ among the three main ecological groups (dioecious animals, plants, and hermaphroditic animals), contrary to what would be expected if it reflected sex-specific selection on recombination. These results indicate that the strong trends for achiasmate meiosis do not apply to quantitative sex differences in recombination, and contradict theories of sex-specific costs and benefits. An alternative hypothesis suggests that sex differences may be more-or-less neutral, selection determining only the mean rate of recombination. While male and female chiasma frequencies are more similar than would be expected under complete neutrality, a less absolute form of the hypothesis is more difficult to falsify. In female mice the sex bivalent has more chiasmata for its length than the autosomes, perhaps compensating for the absence of recombination in males. Finally, we observe that chiasma frequencies in males and females are positively correlated across species, validating the use of only one sex in comparative studies of recombination.

Environmentally induced morphological variation in the Barnacle Goose, Branta leucopsis

Abstract: The environmental conditions to which juvenile barnacle geese (Branta leucopsis) were exposed during growth were found to affect their body size at fledging as well as their final adult body size. Small juveniles showed compensatory growth from the time of fledging up to one year of age, but this did not fully compensate the differences in body size that were established before fledging. The variation in protein content in plants eaten during growth could probably explain the observed body size differences, sometimes of more than 10%, between different categories of adult geese. Our results imply that one cannot infer selection on morphological characters from differences between samples of adult birds from different localities or from different cohorts within a population, without first showing that environmental conditions during growth do not affect the development of the characters under study.

Homology and hierarchies : problems solved and unresolved

Abstract: Homology as a topic in phylogenetic analysis has to do with what is conserved in evolution. The problem of homology in systematics — to find homologues, and in so doing, to identify taxa — is distinct from the problem of identifying what kinds of features tend to be conserved, how and why. The two sets of issues are fundamentally interdependent at the point that one selects the appropriate taxonomic units, identifies the characters one wishes to study, or decides what constitutes a single character. Homology as a phenomenon is a manifestation of replication and of continuity of biological information. Replication occurs at many levels in the biological hierarchy: from the DNA replication that accompanies cell replication, to the replication of gross phenotypic characteristics within individual organisms that results in iterative homologues, to the replication of individuals to form a population that persists (in replication through successive generations) in evolutionary time. In replication, biological information may persist unchanged, or it may be disrupted or transformed. Different patterns of change may be expressed at different levels of the biological hierarchy. Here a concept developed in arguments on levels of selection becomes useful: change at one level of the hierarchy — e.g., genes or gross phenotype — may be screened off from changes at other levels. Understanding the manner in which phenotypic features develop or are replicated, the mechanisms of screening off, and the evolutionary origin and transformation of these mechanisms is a major challenge for understanding the biological basis of homology. The recognition or coding of characters for a phylogenetic analysis calls for decisions on what level of description and how complex a unit character is to be recognized. Here an additional point of comparison within the biological hierarchy — the relationship between organisms and taxa — becomes important. We use characters of organisms to trace phylogenies of taxa; yet because traits can arise and subsequently become fixed in different segments of a population linage, phylogenies of organismal characters can conflict with the phylogeny of the taxa that compairse them. For these reasons, a full understanding of evolutionary changes undergone in lineages will require us to combine phylogenetic analyses with analyses of development, studies of developmental and population genetics, and comparisons of gross phenotype.

Wormy mice in a hybrid zone: A genetic control of susceptibility to parasite infection

Abstract: ‘Laboratoire de Parasitologie Cornparke, (URA 698, CNRS). UniversitP de Montpellier II. PI. E Bataillon; 34095 Montpellier cedex 5; FRANCE. ‘Institut des Sciences de I’Ecolution, “GPnome et Population”, (URA 327 CNRS). UnitlersitP de Montpellier II. PI. E. Bataillon; 34095 Montpeflier cedex 5; FRANCE. ‘Department of Molecular Biology and Plant Physiology. UnicersitJt of Aarhus. C. F. M&en AlIP 130. DK-8000; Aarhus, DENMARK

Parental effects in Lychnis flos‐cuculi. I: Seed size, germination and seedling performance in a controlled environment

Abstract: Selection responses in natural plant populations depend on how the phenotypic variation of traits is composed. The contributions of nuclear genetic, maternal, paternal, environmental and inbreeding effects to variation in time to germination, germination percentage, and seed- and seedling size were studied in a population of Lychnis flos-cuculi. It was found that: (1) Maternal effects predominated in the determination of progeny seed size and germination characteristics; (2) Maternal environment during seed development was less important than maternal genotype; (3) Small but significant variation within maternal families could be observed among individuals sired by different fathers; (4) Additive genetic variance was significant for seedling size 4 weeks after germination. In conclusion, selection shortly after emergence will mainly favour particular maternal genotypes, while selection later in the life cycle may act upon zygotic genotypes. Inbreeding depression was significant, especially for vegetative growth. Consistent differences were found among maternal genotypes in the degree of variation in the time to germination, suggesting that selection could operate to favour polymorphic or uniform germination behaviour.

Multi‐residue gaps, a class of molecular characters with exceptional reliability for phylogenetic analyses

Abstract: Qualitative methods of obtaining phylogenies seek distinct classes of characters that collectively are common, yet individually are insufficiently improbable that they provide reliable indications of monophyly. Multi-residue gaps (insertions or deletions) in DNA and protein sequences are easily recognised and are available in diverse organisms and loci. They are likely to experience few homoplasies, because these require two separate events to be matched in both starting position and length. Two tests of the ability of multi-residue gaps to recognise clades are conducted. (1) Among the 10.76 kb of noncoding DNA sequences from the ψη-globin region of 5 primates, there are 7 analysable multi-nucleotide gaps. These provide a corroborated and noncontradictory, fully resolved gene tree of hominoids. (2) Among the 35 amino acid sequences of globin chains available in 1978, there are 9 nonoverlapping multi-amino acid gaps. These indicate 8 noncontradictory divisions that agree with recognised taxa and gene categories. In the 412 realigned globin sequences available in 1989, 8 of the gaps are still analysable. These show a maximum of two homoplasies among the 8 × 412 = 3296 species-characters. Multi-residue gaps have a high potential to indicate clades. They could be used to reduce the computing labour required in a quantitative study or pooled from different data sets to examine particularly important or difficult phylogenetic questions.

Parental effects in Lychnis flos‐cuculi. II: Selection on time of emergence and seedling performance in the field

Abstract: Selection on the timing of seedling emergence was investigated in an experimental population of Lychnis Jos-cuculi, a perennial hay-meadow species. Seeds obtained from a full dial14 cross of 8 genotypes from a field population were sown along an environment gradient that included the parental site. Significant directional selection for early emergence was found and the intensity of selection varied among sites. Emergence time varied significantly among progeny families of different maternal and paternal genotypes. These differences could be attributed to parental effects whereas narrow-sense heritabilities were close to zero. Survivorship until autumn differed among progeny of paternal families. Survivorship of maternal progeny varied among sites. Whereas differences in survival and plant size among individuals from different emergence cohorts persisted over the winter, the significance of these differences among progeny from different parental genotypes disappeared. It is suggested that a response to selection on emergence time might be low since (1) the narrow sense heritability was low, (2) parental genotypes differed in their effect on offspring emergence time when used as female parent or as pollen donor and (3) there was a family x site interaction for survival. Families with relatively early emerging seedlings also had a significantly higher seed weight, emergence percentage, and plant weight although the strength of these among-family correlations varied among sites. It is therefore not likely that simultaneous selection on emergence time and either of these traits would retard a response to selection on emergence time.

Phylogenetic analysis and the evolution of queen number in eusocial Hymenoptera

Abstract: Analyses of the evolution of colony queen number in eusocial insects have generally been conducted without specific reference to phylogenetic relationships, leading to incomplete evolutionary explanations for this key attribute of social organization. Consideration of queen number in a phylogenetic context in the highly eusocial Hymenoptera reveals that its evolution has been very conservative in the bees but that it is a highly labile character in most ants. The wasps appear intermediate in this respect, with some large and widespread clades characterized by little or no phylogenetic variability in queen number. This hierarchy of phylogenetic lability suggests that while ant populations may often be responsive to selection on colony queen number linked with local ecology, bees and wasps appear less responsive in this regard, with a significant element of phylogenetic conservatism involved in the expression of this social trait in the latter two groups.

The evolution of queen-rearing nepotism in social Hymenoptera : effects of discrimination costs in swarming species.

Abstract: Polyandry by social hymenopteran queens leads to a potential worker reproductive strategy of rearing full-sister queens in preference to half-sister queens. If there is no cost to discrimination, discriminatory queen rearing will be the ESS. However, if discrimination has a cost this conclusion is weakened. This study examines the effect of a “worker efficiency cost” (i.e. discriminatory, nepotistic workers are less effective than non-nepotistic workers in working to increase total colony reproductive output) on the invasion of rare “nepotist” and “non-nepotist” alleles in a population in which the other allele is near fixation. Efficiency costs are considered of special relevance to species with swarm founded colonies (e.g., army ants, honey bees). Two analyses are presented: 1) A general model exploring the effects of efficiency costs, the queen-rearing-biasing ability of nepotists, and queen mating frequency on invasion of nepotists and non-nepotists; 2) a discriminatory removal model, in which the actual amount of biasing is dependent on recognition abilities and the discriminatory intensity of nepotists, queen mating frequency, and whether or not removed immature queens are replaced. The results of the removal model indicate that when queen mating frequency is close to one (i.e., because of double mating with unequal sperm use or mixed double and single mating) non-nepotist is likely to be the ESS. At a higher mating frequency nepotist will become the ESS. At even higher mating frequencies, non-nepotist may reinvade. This latter conclusion is robust across all models. In particular, non-nepotists may reinvade at the high mating frequencies found in the honey bee (10–20) if their work efficiency is only a few percent higher than nepotists (the exact amount depends on recognition errors and the intensity of discrimination), resulting in a genetic polymorphism of both nepotists and non-nepotists. Results of a recent study of patriline discrimination in honey bees (Page et al., 1989) are consistent with such a polymorphism.

Cytoplasmic incompatibilities in the mosquito Culex pipiens : how to explain a cytotype polymorphism?

Abstract: Although cytoplasmic incompatibilities have been used as a means of eradicating the mosquito Culex pipiens, the population dynamics of these sterilities in relation to the coexistence of multiple incompatible cytotypes in a single area has not been investigated, except in the case of two unidirectionally incompatible cytotypes. An analytical model of the evolution of n cytotypes in an infinite panmictic population has been developed in order to investigate polymorphic equilibrium. A necessary criterion for the stability of such an equilibrium is established; it is shown that a stable polymorphism cannot exist between incompatible cytotypes. This result is discussed in the light of population dynamics and genetics of Culex pipiens, and of our present knowledge on incompatibilities. The consequences of a geographic structuring and of homogamy are considered. A careful reconsideration of previous experimental results disclosed probable nuclear effects and a serious experimental weakness: with the common procedure of backcrossing hybrid females to males of constant genotype it is not possible to rule out probable nuclear effects with paternal expression. It is concluded that incompatibilities in Culex pipiens may have a nuclear-cytoplasmic determinism.

Temperature‐related divergence in experimental populations of Drosophila melanogaster. III. Fourier and centroid analysis of wing shape and relationship between shape variation and fitness.

Abstract: From a laboratory stock of Drosophila melanogaster (Oregon), reared for more than 20 years at 18°C, two new populations were derived and maintained at 25° and 28°C for 8 years. The chromosomal and cytoplasmic contribution to genetic divergence between the two more extreme populations was estimated at 18°C and 28°C. Wing shape and two fitness components (fecundity and fertility) were taken into account. Fourier descriptors and the position of the centroid were taken as indicators either of wing shape variation, determined by a different response of the two wing compartments to temperature selection, or of wing shape variation determined by both compartments. The descriptors appear to be good characters: they show a variability which is genetically controlled and ascribable to genes located on specific chromosomes. The third chromosome is responsible for the adaptive difference to temperature. The genes which control wing shape are located on the second and third chromosome, although the contribution of each chromosome depends on the environment in which the flies develop. Cytoplasmic genes display an effect as large as that of chromosomes, and nucleus × cytoplasm interaction is present. The correlation between the genetic contributions to compartment-dependent wing shape variation and the contributions to fitness is highly significant, especially at 28°C. Wing shape has, therefore, an adaptive significance in relation to temperature, which is particularly expressed in the environment where selection occurred.

The heritability of sexually dimorphic traits in the yellow dung fly Scathophaga stercoraria (L.)

Abstract: A precise method was used for estimating the proportion of heritable variation in two life history parameters of the yellow dung fly, whereby environmental components of variance were minimized. Significant heritable variation for body size was revealed for father to son and mother to daughter relationships, Variation in development time was not significantly heritable. There is a marked sexual dimorphism in body size in this species which is discussed in the light of the observed sex-genotype interaction in heritabilities and low genetic correlation for size between the sexes. It is suggested that opposing pressures of sexual and natural selection and/or genetic pleotropy may be responsible for the maintenance of heritable variation, and the evolution of sexual dimorphism in these two traits.

Resource allocation to growth, reproduction and survival in Gladiolus : The cost of male function

Abstract: Theoretical models of life-history evolution assume trade-offs between present and future reproduction and/or survival. Models of the evolution of sex assume trade-offs between male function and female function. Generally, experiments designed to evaluate the cost of reproduction on other functions tend to ignore male function. The present work on Gladiolus takes into account simultaneously the different primary functions of the plant and separates sexual reproduction into one male component (pollen production) and one female component (seed production). The study of environmental (within-clone), between-clone and genetic correlations using strains of Gladiolus and principal component analysis show that trade-offs exist between male function, female function and survival, including both characters of plant vigour, perennation (corm production) and vegetative propagation (cormel production). Phenotypic correlations, using different species and species-hybrids, have been obtained which confirm these results. In particular, these results underline the importance of the impact of pollen production on the other functions.

Variation in female mate preference across a grasshopper hybrid zone

Abstract: A method for studying variation in female preference among populations free from the confounding effects of variation in male characters is described. This method is applied to sixteen populations in a transect across a hybrid zone in the grasshopper, Chorthippus parallelus. Significant variation in preference is revealed among populations. The pattern of this variation is analysed in the context of the reinforcement model of speciation. While the data do indicate an increased homogamic preference on one side of the zone relative to more distant populations, this pattern cannot be distinguished statistically from an abrupt transition in preference over a distance of less than 1 km. We argue that either of these patterns implies that selection operates on female preferences in the hybrid zone.

SE‐ and SW‐migrating Blackcap (Sylvia atricapilla) populations in Central Europe: Orientation of birds in the contact zone

Abstract: In the Blackcap (Aves: Sylvia atricapilla), a widespread passerine noctural migrant, a “migratory divide” between SE- and SW-migrating populations exists in Central Europe at about 14° E and south of 52° N. The autumn migratory directions are known to have a genetic basis and are expressed in orientation cages in captivity. Migratory directions of birds in the contact zone between the two populations were studied by analysing ringing data and by testing three groups of hand-raised individuals in orientation cages. Available ringing data are insufficient to establish migratory directions in the contact zone north of the Alps. Hand-raised birds from south-west Germany and the most eastern part of Austria oriented SW and SE, respectively, confirming directions known from ringing recoveries. A sample of birds from the contact zone near Linz (Austria) oriented SW to NW (mean = 268°) and was significantly different from both adjoining populations. This contrasts with results of a cross-breeding experiment with mixed pairs of SW- and SE-migrants bred in captivity: The F1-offspring chose southerly directions, intermediate between both parental populations (Helbig, 1991). It is suggested, therefore, that a distinct subpopulation with a large fraction of birds wintering in the British Isles has established itself in the contact zone. Differences in directional choices between groups of siblings from this area indicate that intrapopulation genetic variability is present. This may have led to a rapid spread of the novel W-NW migratory direction, because north of the Alps strong selection seems to be acting against mixing of SE- with SW-migrating populations.

Microgeographic variation in Anolis oculatus, on the island of Dominica, West Indies

Abstract: The current study provides a quantitative description of the pattern of geographic variation in three morphological character systems of Anofis oculatus, on the ecologically diverse island of Dominica. Causal relationships were investigated by comparing observed patterns of variation to hypothesised patterns (derived from six ecological variables), using Mantel tests, partial correlations and canonical correlations. The similarity between generalised morphological and ecological matrices was highly significant. Partial correlations indicated which of the ecological factors had the most significant influence on individual characters, while canonical correlations showed that characters formed constellations which were influenced by constellations of ecological factors. This analysis also illustrates the incongruence of patterns of variation which result from ecogenetic adaptation. The conventional subspecific categories cannot adequately reflect this variation and consequently, in such cases, the use of subspecies is inappropriate.

Limits to runaway sexual selection: the wallflower paradox

Abstract: In his mathematical treatment of Fisher’s ideas on sexual selection (so-called runaway selection) Lande (1981) predicted that males may evolve increasingly elaborate sexual characters despite opposing viability selection as a consequence of the associated costs. Lande thereby assumed that female mate preferences are not subject to selection since (1) females are all inseminated and (2) the quantity and quality of their offspring are independent of the female’s mate preferences. Kirkpatrick (1985) removed the latter assumption and investigated the consequences for the mean phenotype with respect to both female and male traits. He also explored the dynamics of the (co)-variance matrix by numerical methods. In this paper we consider a simpler model with just two multi-allelic loci. This enables us to derive explicit expressions for (co)-variances under steady state conditions. Rather than assume natural selection through differential fertility (as in Kirkpatrick, 1985), we take sexual selection on females into account by modelling the preference-dependent risk that females remain unmated. We argue that this wallflower effect is a realistic feature of any mating system, since it merely depends on the existence of (1) variation in mating preferences and (2) a finite mating season. Our approach provided an insight into the dynamic behaviour of the means of the phenotypes. This is because the dynamics of the means depend on the steady state (co)-variance matrix. Thus, an insight into the former requires explicit expressions for the latter.

Developmental pathways, homology and homonomy in metameric animals

Abstract: Following Wagner's (1989) distinction between historical and biological concepts of homology, we analyze homology problems of metameric animals in the light of a biological concept. In identifying homology, we refer to the common informational background which two structures share. Therefore, homology relationships are matters of degree; they are ‘perfect’ only when there is full identity of informational background between the structures under comparison. Homonomy (serial homology) is not fundamentally different from other kinds of homology. We regard the differences between epimorphically and anamorphically developed segments as minor; therefore, the two kinds of segments are largely homologous. The morphogenetic processes giving rise to segmental structures are regarded as not necessarily hierarchical. We contrast the phylogenetic pattern of hierarchically nested homologies with a largely non-hierarchical pattern of homologous structures within the individual organism. This topological difference adds to heterochrony in generating the widespread mismatch of ontogeny and phylogeny.

Coming of age in fringillid birds: heterochrony in the ontogeny of secondary sexual characters

Abstract: The great variation in the timing of development of adult plumages in North American male passerine birds has in recent years provoked much theoretical interest. It has commonly been assumed that the heterochronic process involved is invariably a retardation of the development of adult plumages. On the basis of a phylogenetic analysis of the nine-primaried oscines (Fringillidae), I suggest that the heterochronic process that has operated most frequently in North American taxa is not a retardation, but an acceleration. I also suggest that sexual dimorphism in plumage, and late plumage maturation in males, may arise as a secondary result of selection for neoteny in females with a correlated response in males, and not necessarily, as traditionally thought, as a result of selection for bright plumages in males.

Genetic differentiation and habitat partition of Robertsonian house mouse populations (Mus musculus domesticus) of Tunisia

Abstract: The karyological analysis of Tunisian populations of house mice revealed the existence of a Robertsonian (Rb) chromosomal race carrying nine pairs of metacentric chromosomes in central Tunisia. Divergence estimates showed that they are genetically differentiated from local all-acrocentric populations and have a reduced level of genic variability. The Rb populations are restricted to urban habitats, whereas all-acrocentric populations occur in rural areas. Contact zones between these two types of habitat yield chromosomally polymorphic populations. Analysis of gene flow indicates that it is reduced and limited to populations bordering the contact areas. The reduced genic variability and patchy distribution exhibited by the Tunisian Rb mice do not agree with results from previous studies of the European Rb populations. Two hypotheses are presented to account for this discrepancy based on local differentiation versus introduction of the Tunisian Rb race.

Experimental study of temperature effects on the sex ratio of broods in Terrestrial Crustacea Armadillidium vulgare Latr. Possible implications in natural populations

Abstract: The woodlouse Armadillidium vulgare is characterized by female heterogamety (ZW) and male homogamety (ZZ). However, in several populations, sex determination is influenced by cytoplasmic sex factors (endosymbiotic bacteria = F). At 20 °C these maternally transmitted bacteria reverse genetic males into functional neo-females (ZZ + F) producing highly female broods. When these neo-females were reared at 30° C, the sex ratio of their broods became male-biased. The major process involved in this heat-induced sex ratio inversion was the disappearance of bacteria in embryos in the course of their development, which allowed the young to express a phenotype that conforms with their genotype (i.e. male ZZ). No heat-sensitive stage of development was observed, but at least 35 days at 30° C seem to be necessary to induce F-degradation. The presence of F at 30° C (before its degradation) also induced mortality during vitellogenesis. Daily thermoperiods including a thermophase at 30° C had effects on F similar to that of a constant temperature of 30° C. A. vulgare can live in climates having such thermoperiods (at least during one period of the year), temperature appears to be capable of limiting the presence of F-bacteria in natural populations, and then modifying the evolution of sex-determining mechanisms in such populations.

The evolutionary history of Drosophila buzzatii. XVI. Fitness component analysis in an original natural population from Argentina

Abstract: Summary The adaptive significance of the chromosomal polymorphism of Drosophila buzzati has been studied by means of fitness component analysis in an original population from Argentina. The results show evidence of selection acting through pupal viability, longevity (adult viability) and fecundity on the second chromosome polymorphism, and through pupal viability and virility on the fourth chromosome polymorphism. Changes in chromosomal inversion frequencies throughout the life-cycle suggested an endocyclic pattern of directional selection, which at first seems to be the only detectable mechanism responsible for the maintenance of the polymorphism. However, slow, long-term frequency changes cannot be ruled out. The way in which endocyclic selection acts on this population is different from that in a colonized population previously studied; that is, different fitness components are involved in the maintenance of chromosomal polymorphism. The possible factors that may explain these differences are discussed.

Within-island microgeographic variation in the colour pattern of the skink, Chalcides sexlineatus : pattern and cause

Abstract: The colour pattern of the Gran Canarian skink is described with eight independent colour pattern characters. Significant geographic variation occurs in each character. There are generally high levels of congruence between the patterns of geographic variation in each character although some differences exist. In canonical variate analyses, the first canonical variate expresses most of the among-locality variation in colour pattern. indicating a largely unidimensional pattern. Patterns of geographic variation in the colour pattern are portrayed by contouring. This reveals north-east/south-west clines for seven of the individual characters and the generalized pattern (CVl). Four causal hypotheses were erected which predicted four different unidimensional patterns of geographic variation. Mantel tests and partial correlation analyses were used to compare the observed patterns of microgeographic variation with the four hypothesized patterns. This method suggests differential selection occurring between ecotones as the cause of the microgeographic variation. Microgeographic variation in some aspects of the colour pattern can be explained by selection for different anti-predator strategies in the hot, arid southern areas vs the cooler, lusher northern areas.

Allozyme analysis and phyletic relationships of two new stick‐insects from north‐west Sicily: Bacillus grandii benazzii and B. rossius‐grandii benazzii (Insecta Phasmatodea)

Abstract: Two new Sicilian stick‐insects have been discovered within the genus Bacillus. Evidence concerning the subspecific differentiation of Bacillus grandii benazzii and the hybrid constitution of B. rossius‐grandii benazzii by means of allozyme analysis is given, and is consistent with morphological and karyological data on the differentiation of these two north‐west Sicilian morphs from south‐eastern B. g. grandii and B. whitei. B. g. benazzii is more polymorphic than B. g. grandii and B. rossius‐g. benazzii embodies its genetic variability, thus differing sharply from the south‐eastern parallel hybrid B. whitei ( = B. rossius × B. g. grandii). Reproductive mechanisms also appear to be different in the two interspecific hybrids, involving parthenogenesis in the latter and hybridogenesis in the former. Finally, the phyletic relationships among all Sicilian Bacillus taxa are summarized in a revised scheme, which also takes into account new evidence that B. atticus has contributed to hybrid constitution of the triploid B. lynceorum.

Inference of selection and recombination from nucleotide sequence data

Abstract: Two techniques for obtaining information about population structure from nucleotide sequences in DNA are summarized. The first focuses on the selection or neutrality of enzyme polymorphisms, the second on the detection of recombination. Neither method requires phylogeny estimation.