Anne Atlan

Sex allocation in an hermaphroditic plant: the case of gynodioecy in Thymus vulgaris L.

Resource allocation to male and female functions was investigated in Thymus vulgaris L. (thyme), a gynodioecious species, in which females produce twice as many seeds as hermaphrodites. Negative correlations were found between male and female fertility of hermaphrodites, providing evidence of a trade‐off. There was a high variability in sexual investment, some of the hermaphrodites functioning almost as males, and others almost as females. Estimation of the relative cost of male and female gametes showed that the female advantage in seed production was mainly due to reallocation of the resources not allocated to male function into female function. The determination of sex allocation was shown to have a genetic component, and there were some evidence that an interaction between nuclear and cytoplasmic genomes was involved.

Sex‐ratio distortion in Drosophila simulans: co‐occurence of a meiotic drive and a suppressor of drive

A sex‐ratio distortion factor was found at high frequency in D. simulans strains from Seychelles and New Caledonia. This factor is poorly or not expressed within those strains which are resistant to it. Its presence was detected by crossing females from New Caledonia or the Seychelles with males from a different geographic origin. Most of the F1 males obtained produced an excess of females (up to 99%) in their progeny. The two strains are infected with Wolbachia, but these micro‐organisms are not involved in the sex‐ratio distortion. The sex‐ratio factor is shown to be an X‐linked meiotic driver; nuclear resistance factor(s) act by suppressing the drive. It is likely that the same X‐located driver invaded the two populations, which subsequently developed resistance factor(s) against it.

Analytic and simulation models predicting positive genetic correlations between traits linked by trade-offs

SummaryUsing a two-loci multiplicative model of resource allocation, we show how the existence of several levels of resource allocation may affect the sign of the genetic correlations between traits linked by trade-offs. Positive genetic correlations between components of fitness affected by genetic trade-offs may result from different amounts of genetic variability at the pleiotropic loci determining the allocation of resources. Thus positive genetic correlations may be obtained in the absence both of environmental variation and of differences between individuals in resource acquisition. Nevertheless, positive correlations between all components of fitness at the same time cannot be obtained without variability in the acquisition of resources.

THE SEX-RATIO TRAIT IN DROSOPHILA SIMULANS : GEOGRAPHICAL DISTRIBUTION OF DISTORTION AND RESISTANCE

The sex‐ratio trait we describe here in Drosophila simulans results from X‐linked meiotic drive. Males bearing a driving X chromosome can produce a large excess of females (about 90%) in their progeny. This is, however, rarely the case in the wild, where resistance factors, including autosomal suppressors and insensitive Y chromosomes, prevent the expression of the driver. In this study, we searched for drive and resistance factors in strains of Drosophila simulans collected all over the world. Driving X chromosomes were found in all populations whenever a good sample size was available. Their frequency may reach up to 60%. However, the presence of driving X chromosomes never results in an excess of females, due to the systematic co‐occurrence of resistance factors. The highest frequencies of driving X chromosomes were observed in islands, while populations from East and Central Africa (the supposed center of origin of the species) showed the highest level of resistance. The geographical pattern of drive and resistance factors, as well as the results of crosses between strains from different geographical areas, suggest that the sex‐ratio system described here has a unique and ancient origin in the species.

Flowering phenology of Ulex europaeus: ecological consequences of variation within and among populations

Reproductive phenology of gorse (Ulex europaeus L., Genisteae, Fabaceae) is unusual in that the onset and duration of flowering vary greatly among individuals within populations: some plants initiate flowering in autumn or winter and continue flowering through spring, others initiate flowering in early spring. To understand the origin of this diversity and its ecological consequences, we investigated flowering phenology of randomly sampled individuals from five different natural populations in Brittany (France). Reproductive success was evaluated for individuals with contrasting flowering patterns, from 16 natural populations. Flower production, pod production, seed production and seed predation were estimated. Plants initiating flowering in spring produced larger numbers of flowers and pods over a shorter period than plants flowering from winter to spring, which produced few flowers and pods at a time but over a longer period. Pod production of long-flowering plants did not differ significantly between winter and spring, but their pods were more intensively attacked by seed predators in spring than in winter. We discuss our results in relation to biotic and abiotic parameters. We postulate that long-flowering can be interpreted as a bet-hedging strategy, spreading the risk of pod failure (rotting or freezing) in winter and of seed predation in spring.

Founder effects and sex ratio in the gynodioecious Thymus vulgaris L.

Thymus vulgaris is a gynodioecious species (in which females and hermaphrodites coexist) with a highly variable frequency of females among natural populations (5–95%) and a high average female frequency (60%). Sex determination involves both cytoplasmic genes responsible for male sterility, i.e. the female phenotype, and specific nuclear factors responsible for the restoration of male fertility, and thus a hermaphrodite phenotype. In this study, molecular markers of the mitochondrial genome have been used to quantify the cytoplasmic diversity in 11 clumps of individuals observed in four recently founded populations. The very low diversity within patches in conjunction with the strong diversity among patches strongly suggests that clumps of individuals are the result of single matrilinear families. In clumps that contain mainly females, all the analysed females showed the same cytoplasmic pattern. This pattern differed from that shown by neighbouring hermaphrodites, indicating that the determination of sex is locally cytoplasmic. A comparison of genetic diversity before and after fire in one population showed that disturbances may cause a reduction in genetic diversity and a concurrent induction of local cytoplasmic determination of sex. Such cytoplasmic determination of sex in colonizing populations, together with the greater seed set of females, may largely improve the colonizing ability of the species.

Mitochondrial genome of Thymus vulgaris L. (Labiate) is highly polymorphic between and among natural populations

Analysis of mitochondrial DNA polymorphism in natural populations of Thymus vulgaris revealed the existence of high variability between and among populations. Using southern hybridization with two restriction enzymes analysis and one heterologous probe, 13 mitotypes were detected among three natural populations. All the mitotypes except two were specific to a single population. This high polymorphism was also detected with three other heterologous probes. To analyse this variability further, we studied restriction patterns of purified mtDNA from single individuals sampled from each of three natural populations. The three individuals showed completely different EcoRI mtDNA restriction fragment patterns, while EcoRI cpDNA restriction fragment patterns were identical for the three individuals. To test the possibility of paternal mtDNA as a potential source of the polymorphism observed, the mode of inheritance of mtDNA was studied and found to be maternal.

Sex-ratio distorter of Drosophila simulans reduces male productivity and sperm competition ability

The aim of the present study was to determine whether the effects of sex‐ratio segregation distorters on the fertility of male Drosophila simulans can explain the contrasting success of these X‐linked meiotic drivers in different populations of the species. We compared the fertility of sex‐ratio and wild‐type males under different mating conditions. Both types were found to be equally fertile when mating was allowed, with two females per male, during the whole period of egg laying. By contrast sex‐ratio males suffered a strong fertility disadvantage when they were offered multiple mates for a limited time, or in sperm competition conditions. In the latter case only, the toll on male fertility exceeded the segregation advantage of the distorters. These results indicate that sex‐ratio distorters can either spread or disappear from populations, depending on the mating rate. Population density is therefore expected to play a major role in the evolution of sex‐ratio distorters in this Drosophila species.

THE Y CHROMOSOMES OF DROSOPHILA SIMULANS ARE HIGHLY POLYMORPHIC FOR THEIR ABILITY TO SUPPRESS SEX-RATIO DRIVE

Abstract The sex‐ratio trait, known in several species of Drosophila including D. simulans, results from meiotic drive of the X chromosome against the Y. Males that carry a sex‐ratio X chromosome produce strongly female‐biased progeny. In D. simulans, drive suppressors have evolved on the Y chromosome and on the autosomes. Both the frequency of sex‐ratio X and the strength of the total drive suppression (Y‐linked and autosomal) vary widely among geographic populations of this worldwide species. We have investigated the pattern of Y‐linked drive suppression in six natural populations representative of this variability. Y‐linked suppressors were found to be a regular component of the suppression, with large differences between populations in the mean level of suppression. These variations did not correspond to differences in frequency of discrete types of Y chromosomes, but to a more or less wide continuum of phenotypes, from nonsuppressor to partial or total suppressor. We concluded that a large diversity of Y‐linked suppressor alleles exists in D. simulans and that some populations are highly polymorphic. Our results support the hypothesis that a Y‐chromosome polymorphism can be easily maintained by a balance between meiotic drive and the cost of drive suppression.

Invasive Plants and Enemy Release: Evolution of Trait Means and Trait Correlations in Ulex europaeus

Several hypotheses that attempt to explain invasive processes are based on the fact that plants have been introduced without their natural enemies. Among them, the EICA (Evolution of Increased Competitive Ability) hypothesis is the most influential. It states that, due to enemy release, exotic plants evolve a shift in resource allocation from defence to reproduction or growth. In the native range of the invasive species Ulex europaeus, traits involved in reproduction and growth have been shown to be highly variable and genetically correlated. Thus, in order to explore the joint evolution of life history traits and susceptibility to seed predation in this species, we investigated changes in both trait means and trait correlations. To do so, we compared plants from native and invaded regions grown in a common garden. According to the expectations of the EICA hypothesis, we observed an increase in seedling height. However, there was little change in other trait means. By contrast, correlations exhibited a clear pattern: the correlations between life history traits and infestation rate by seed predators were always weaker in the invaded range than in the native range. In U. europaeus, the role of enemy release in shaping life history traits thus appeared to imply trait correlations rather than trait means. In the invaded regions studied, the correlations involving infestation rates and key life history traits such as flowering phenology, growth and pod density were reduced, enabling more independent evolution of these key traits and potentially facilitating local adaptation to a wide range of environments. These results led us to hypothesise that a relaxation of genetic correlations may be implied in the expansion of invasive species.