WOLBACHIA INFECTION IN DROSOPHILA SIMULANS: DOES THE FEMALE HOST BEAR A PHYSIOLOGICAL COST?

Fitness traits of three Drosophila simulans strains infected by endocellular bacteria belonging to the genus Wolbachia have been compared with those of replicate stocks previously cured from the infection by an antibiotic treatment. The traits measured were development time, egg-to-adult viability, egg hatch, productivity, fecundity, and the number of functional ovarioles. Individuals of the first strain were bi-infected by two Wolbachia variants, wHa and wNo. The second strain was infected by wHa, the third one by wNo. The Wolbachia studied here cause cytoplasmic incompatibility (CI), a high embryonic mortality (70% to > 90%) when an infected male is crossed with an uninfected female. Three generations after antibiotic treatment, we observed in all strains a significant drop in productivity in the cured stocks. This drop was not due to antibiotic toxicity and was associated with the loss of the Wolbachia. However the effect had disappeared in two of the three strains five generations after treatment, and could not be found in the third strain in a third measurement carried out 14 generations after treatment. The temporary nature of the productivity difference indicates that Wolbachia do not enhance productivity in infected strains. On the other hand, in all traits measured, our results show the absence of any negative effects of the Wolbachia on their host. This could be explained when considering Wolbachia evolution, as maternally transmitted parasites bear a strong selective pressure not to harm their female host. However, CI would allow the bacteria to be maintained even when harming the female. The apparent absence of deleterious effects caused by these Wolbachia might result from a trade-off, where a relatively low bacteria density would advantage the Wolbachia by suppressing any deleterious effects on the female host, at the cost of a weaker maternal transmission rate of the infection.     

DOES MULTIPLE PATERNITY IMPROVE FITNESS OF THE FROG CRINIA GEORGIANA?

Abstract.— In the Australian myobatrachid frog Crinia georgiana simultaneous polyandry occurs in about half of all matings, which leads to multiple paternity, but reduced fertilization success and occasional female mortality. Multiple paternity may provide benefits to females that compensate for these costs, for example, through enhanced genetic diversity of a clutch. In nature, embryos and tadpoles of C. georgiana develop in shallow, temporary pools and may be exposed to fluctuating water levels and the risk of desiccation between rain events. Fertilization by genetically diverse sires may act as a bet hedge against these conditions. To evaluate this hypothesis, females were artificially mated with one or two males in the field and eggs and larvae reared in the laboratory under constant or fluctuating developmental conditions. Experiment 1 exposed embryos from single- and multiple-paternity clutches to conditions where eggs were completely covered during development or eggs sat in air on a moist substrate. Experiment 2 exposed freshly hatched larvae from single- and multiple-paternity clutches to constant wet conditions, where larvae were completely covered, or fluctuating wet conditions, where larvae ranged from being completely submersed to partially exposed over a 13- day cycle. We measured mean performance and best performance as alternate measures of genetic benefits. There were no effects of paternity on percent survival to hatching, time to hatching, body size at hatching, percent survival to metamorphosis, time to metamorphosis, or body size at metamorphosis. We also analyzed variance within clutches as a measure of genetic diversity. Again there were no predictable effects of multiple paternity. Polyandry does not appear to provide any genetic benefits that compensate for the high costs of polyandry in this species.     

DOES VARIATION IN SELECTION IMPOSED BY BEARS DRIVE DIVERGENCE AMONG POPULATIONS IN THE SIZE AND SHAPE OF SOCKEYE SALMON?

Few studies have determined whether formal estimates of selection explain patterns of trait divergence among populations, yet this is one approach for evaluating whether the populations are in equilibria. If adaptive divergence is complete, directional selection should be absent and stabilizing selection should prevail. We estimated natural selection, due to bear predation, acting on the body size and shape of male salmon in three breeding populations that experience differing predation regimes. Our approach was to (1) estimate selection acting within each population on each trait based on an empirical estimate of reproductive activity, (2) test for trait divergence among populations, and (3) test whether selection coefficients were correlated with trait divergence among populations. Stabilizing selection was never significant, indicating that these populations have yet to attain equilibria. Directional selection varied among populations in a manner consistent with trait divergence, indicating ongoing population differentiation. Specifically, the rank order of the creeks in terms of patterns of selection paralleled the rank order in terms of size and shape. The shortest and least deep-bodied males had the highest reproductive activity in the creek with the most intense predation and longer and deeper-bodied males were favored in the creeks with lower predation risk.     

DOES LINKAGE DISEQUILIBRIUM GENERATE HETEROZYGOSITY‐FITNESS CORRELATIONS IN GREAT REED WARBLERS?

Heterozygosity-fitness correlations (HFCs) at noncoding genetic markers are commonly assumed to reflect fitness effects of heterozygosity at genomewide distributed genes in partially inbred populations. However, in populations with much linkage disequilibrium (LD), HFCs may arise also as a consequence of selection on fitness loci in the local chromosomal vicinity of the markers. Recent data suggest that relatively high levels of LD may prevail in many ecological situations. Consequently, LD may be an important factor, together with partial inbreeding, in causing HFCs in natural populations. In the present study, we evaluate whether LD can generate HFCs in a small and newly founded population of great reed warblers (Acrocephalus arundinaceus). For this purpose dyads of full siblings of which only one individual survived to adult age (i.e., returned to breed at the study area) were scored at 19 microsatellite loci, and at a gene region of hypothesized importance for survival, the major histocompatibility complex (MHC). By examining siblings, we controlled for variation in the inbreeding coefficient and thus excluded genome-wide fitness effects in our analyses. We found that recruited individuals had significantly higher multilocus heterozygosity (MLH), and mean d2 (a microsatellite-specific variable), than their nonrecruited siblings. There was a tendency for the survivors to have a more diverse MHC than the nonsurvivors. Single-locus analyses showed that the strength of the genotype-survival association was especially pronounced at four microsatellite loci. By using genotype data from the entire breeding population, we detected significant LD between five of 162 pairs of microsatellite loci after accounting for multiple tests. Our present finding of a significant within-family multilocus heterozygosity-survival association in a nonequilibrium population supports the view that LD generates HFCs in natural populations.     

DOES HYBRIDIZATION DRIVE THE TRANSITION TO ASEXUALITY IN DIPLOID BOECHERA?

Gametophytic apomixis is a common form of asexual reproduction in plants. Virtually all gametophytic apomicts are polyploids, and some view polyploidy as a prerequisite for the transition to apomixis. However, any causal link between apomixis and polyploidy is complicated by the fact that most apomictic polyploids are allopolyploids, leading some to speculate that hybridization, rather than polyploidy, enables apomixis. Diploid apomixis presents a rare opportunity to isolate the role of hybridization, and a number of diploid apomicts have been documented in the genus Boechera (Brassicaceae). Here, we present the results of a microsatellite study of 1393 morphologically and geographically diverse diploid individuals, evaluating the hypothesis that diploid Boechera apomicts are hybrids. This genus‐wide dataset was made possible by the applicability of a core set of microsatellite loci in 69 of the 70 diploid Boechera species and by our ability to successfully genotype herbarium specimens of widely varying ages. With few exceptions, diploid apomicts exhibited markedly high levels of heterozygosity resulting from the combination of disparate genomes. This strongly suggests that most apomictic diploid Boechera lineages are of hybrid origin, and that the genomic consequences of hybridization allow for the transition to gametophytic apomixis in this genus.     

CHEMOTAXIS IN THE NUDIBRANCH HERMISSENDA CRASSICORNIS: DOES INGESTIVE CONDITIONING INFLUENCE ITS BEHAVIOUR IN A Y-MAZE?

The nudibranch mollusc Hermissenda crassicornis can be culturedeasily in the laboratory and has been used as a biomedical modelfor learning and memory studies. Cultured animals whose onlyprey was Tubu-laria crocea were tested for responses to otherprey. Naive slugs were conditioned with six different dietsand tested for behaviour in a Y-maze. H. crassicornis was ableto detect chemotactically food items which it had never beenin contact with (such as Pennaria), but it did not detect someof the conditioning diets (e.g. Metridium). At least three hydroidspecies induce a chemotactic behaviour in the slug. It is shownthat ingestive conditioning does affect their response in single-choiceand double-choice experiments. As food items other than cnidariansare also detected, it is likely that different kairomones areresponsible for the nudibranch's behaviours. ^*Present address: Centre d'Estudis Avançats de Blanes,C.S.I.C., Cami de Sta. Bárbara s/n. 17300 Blances (Girona)Spain. (Received 10 March 1997; accepted 25 June 1997)     

Z LINKAGE OF FEMALE PROMISCUITY GENES IN THE MOTH UTETHEISA ORNATRIX: SUPPORT FOR THE SEXY‐SPERM HYPOTHESIS?

Female preference genes for large males in the highly promiscuous moth Utetheisa ornatrix (Lepidoptera: Arctiidae) have previously been shown to be mostly Z‐linked, in accordance with the hypothesis that ZZ–ZW sex chromosome systems should facilitate Fisherian sexual selection. We determined the heritability of both female and male promiscuity in the highly promiscuous moth U. ornatrix (Lepidoptera: Arctiidae) through parent–offspring and grandparent–offspring regression analyses. Our data show that male promiscuity is not sex‐limited and either autosomal or sex‐linked whereas female promiscuity is primarily determined by sex‐limited, Z‐linked genes. These data are consistent with the “sexy‐sperm hypothesis,” which posits that multiple‐mating and sperm competitiveness coevolve through a Fisherian‐like process in which female promiscuity is a kind of mate choice in which sperm‐competitiveness is the trait favored in males. Such a Fisherian process should also be more potent when female preferences are Z‐linked and sex‐limited than when autosomal or not limited.     

POPULATION ON THE VERGE OF A MUTATIONAL MELTDOWN? FITNESS COSTS OF GENETIC LOAD FOR AN AMPHIBIAN IN THE WILD

Abstract.— The fitness costs of high genetic load in wild populations have rarely been assessed under natural conditions. Such costs are expected to be greatest in small, bottlenecked populations, including those occurring near range edges. Britain is at the northwesterly range limit of the natterjack toad Bufo calamita . We compared fitness attributes in two populations of this amphibian with very different recent histories. Key larval fitness attributes in B. calamita , notably growth rate and metamorph production, were substantially higher in the large outbreeding population (Ainsdale) than in the small and isolated one (Saltfleetby). These differences were manifest under seminatural conditions, when larvae were reared in mesh cages within breeding ponds at the site of the small population, and were exacerbated by high stress treatments. The results indicate that genetic load effects can be sufficiently severe enough to predispose extinction over relatively short time frames, as predicted by extinction vortex models.     

DOES FREQUENCY‐DEPENDENT SELECTION WITH COMPLEX DOMINANCE INTERACTIONS ACCURATELY PREDICT ALLELIC FREQUENCIES AT THE SELF‐INCOMPATIBILITY LOCUS IN ARABIDOPSIS HALLERI?

Frequency-dependent selection is a major force determining the evolutionary dynamics of alleles at the self-incompatibility locus (S-locus) in flowering plants. We introduce a general method using numerical simulations to test several alternative models of frequency-dependent selection on S-locus data from sporophytic systems, taking into account both genetic drift and observed patterns of dominance interactions among S-locus haplotypes (S-haplotypes). Using a molecular typing method, we estimated S-haplotype frequencies in a sample of 322 adult plants and of 245 offspring obtained from seeds sampled on 22 maternal plants, collected in a single population of Arabidopsis halleri (Brassicaceae). We found eight different S-haplotypes and characterized their dominance interactions by controlled pollinations. We then compared the likelihood of different models of frequency-dependent selection: we found that the observed haplotype frequencies and observed frequency changes in one generation best fitted a model with (1) the observed dominance interactions and (2) no pollen limitation. Overall, our population genetic models of frequency-dependent selection, including patterns of dominance interactions among S-haplotypes and genetic drift, can reliably predict polymorphism at the S-locus. We discuss how these approaches allow detecting additional processes influencing the evolutionary dynamics of the S-locus, such as purifying selection on linked loci.     

INTERSPECIFIC X‐CHROMOSOME AND MITOCHONDRIAL DNA INTROGRESSION IN THE IBERIAN HARE: SELECTION OR ALLELE SURFING?

Introgression from a resident species into an invading one is predicted to occur through the demographic process of "allele surfing," and to particularly affect genomic regions transmitted by the lower migrating sex, such as mtDNA. This could explain that northern Iberian populations of Lepus granatensis harbor high frequencies of mtDNA from L. timidus, an arctic hare it replaced there after deglaciation. We report that variation of introgressed timidus-like mtDNA reflects several predicted effects of this process: increasing frequency and diversity in the direction of expansion, strong perpendicular phylogeographic structure and signs of postglacial demographic growth. However, demographic inferences for the granatensis and timidus-like mtDNA lineages suggest the latter may have outcompeted the former in northern Iberia. Autosomal introgression occurs at low frequencies and species-wide rather than only in the north. If this difference with mtDNA resulted from sex-biased migration, an intermediate pattern should prevail for the X-chromosome, but we report species-wide and high-frequency introgression of an X-fragment. Either selection favored this ubiquitous X-introgression, or more complex postglacial expansion patterns prevailed, with different consequences depending on the genomic and geographic region. This illustrates the difficulty of distinguishing demographic and selective effects and the need for genome and species-wide based demographic models.     

SELECTION ON MOTHERS AND OFFSPRING: WHOSE PHENOTYPE IS IT AND DOES IT MATTER?

Reproductive and early life-history traits can be considered aspects of either offspring or maternal phenotype, and their evolution will therefore depend on selection operating through offspring and maternal components of fitness. Furthermore, selection at these levels may be antagonistic, with optimal offspring and maternal fitness occurring at different phenotypic values. We examined selection regimes on the correlated traits of birth weight, birth date, and litter size in Soay sheep (Ovis aries) using data from a long-term study of a free-living population on the archipelago of St. Kilda, Scotland. We tested the hypothesis that selective constraints on the evolution of the multivariate phenotype arise through antagonistic selection, either acting at offspring and maternal levels, or on correlated aspects of phenotype. All three traits were found to be under selection through variance in short-term and lifetime measures of fitness. Analysis of lifetime fitness revealed strong positive directional selection on birth weight and weaker selection for increased birth date at both levels. However, there was also evidence for stabilizing selection on these traits at the maternal level, with reduced fitness at high phenotypic values indicating lower phenotypic optima for mothers than for offspring. Additionally, antagonistic selection was found on litter size. From the offspring's point of view it is better to be born a singleton, whereas maternal fitness increases with average litter size. The decreased fitness of twins is caused by their reduced birth weight; therefore, this antagonistic selection likely results from trade-offs between litter size and birth weight that have different optimal resolutions with respect to offspring and maternal fitness. Our results highlight how selection regimes may vary depending on the assignment of reproductive and early life-history traits to either offspring or maternal phenotype.