Sandrine Maurice

Evolution of Reproductive Systems in the Genus Silene

The genus Silene contains both hermaphrodite, gynodioecious and dioecious species, dioecy being represented in three sections of the genus. To locate the events of change of reproductive systems, we compared ITS sequences of 22 species of Silene chosen throughout the whole genus, and four putative outgroup species. Gynodioecy, which is the most common reproductive system within the genus Silene and in closely related genera such as Saponaria and Dianthus, is proposed to be ancestral in the genus. Dioecy has evolved at least twice: once in the section containing S. latifolia, and once in the clade containing S. otites and S. acaulis ssp. bryoides. Evolution towards hermaphroditism, associated with evolution of selfing, has also occurred at least twice, in S. gallica and S. conica.

The effect of pollen limitation on plant reproductive systems and the maintenance of sexual polymorphisms

Insufficient pollination can affect the reproductive output and the rate of outcrossing of individual plants. We use a phenotypic model to explore the effect of pollen limitation on the evolution of plant reproductive systems. Compared to situations without pollen limitation, we show that conditions for the stability of different reproductive systems can change under pollen limitation : hermaphrodites are maintained under a larger set of conditions at the expense of unisexual types, especially males. We also show that trioecy, i.e., coexistence of hermaphrodites, males and females, can be evolutionarily stable, which is not the case in the absence of pollen limitation.

Ploidy level and origin of the European invasive weed Senecio inaequidens (Asteraceae)

Native to South-Africa, species of the Senecio inaequidens complex are presently invasive in Europe, Australia and South-America. Previously, different ploidy levels have been found in these different areas, with only tetraploid individuals reported in Europe, and only diploids in South-Africa and Australia. In the present study chromosome counts and flow cytometry were used to survey DNA ploidy levels in a large sample of 66 native and 21 European invasive populations. One Mexican individual was also added to the study. We found only tetraploid individuals occurring in Europe, whereas both ploidy levels, diploid and tetraploid, were found in South-Africa. Moreover, based on genome size, we suggest that two largely allopatric varieties of diploids exist in South-Africa. The Mexican individual was diploid. We suggest that European tetraploid individuals come from South-Africa and hypothesize that a hybridization event between the two DNA types of diploids occurred in the Lesotho area. The taxonomic difficulties surrounding species of theS. inaequidens complex are briefly discussed.

An experimental evaluation of self-interference in Narcissus assoanus: functional and evolutionary implications.

Floral traits that reduce self‐pollination in hermaphroditic plants have usually been interpreted as mechanisms that limit the genetic consequences of self‐fertilization. However, the avoidance of sexual conflict between female and male function (self‐interference) may also represent an important selection pressure for the evolution of floral traits, particularly in self‐incompatible species. Here, we use experimental manipulations to investigate self‐interference in Narcissus assoanus, a self‐incompatible species with a stigma‐height dimorphism in which the degree of spatial separation between sex organs (herkogamy) differs strikingly between the long‐ and short‐styled morphs (hereafter L‐ and S‐morphs). We predicted that weak herkogamy in the L‐morph would cause greater self‐pollination and hence self‐interference. Experimental self‐pollination reduced seed set when it occurred prior to, or simultaneously with, cross‐pollination in the L‐morph, but only if it occurred prior to cross‐pollination in the S‐morph. In the field, autonomous self‐pollination was greater in the L‐morph than the S‐morph, but we found no evidence that self‐interference reduced maternal or paternal fitness in either morph. One‐day‐old flowers of the L‐morph have reduced stigma receptivity and hence exhibit protandry, whereas stigma receptivity and anther dehiscence are concurrent in the S‐morph. This suggests that the two style morphs have alternative strategies for reducing self‐interference: dichogamy in the L‐morph and herkogamy in the S‐morph. These results provide insight into the mechanisms that reduce sexual conflict in hermaphrodite plants and are of significance for understanding the evolution and maintenance of sexual polymorphisms.

Consequences of Low Mate Availability in the Rare Self-Incompatible Species Brassica insularis

Self-incompatibility systems prevent self-fertilization in angiosperms. Although numerous S alleles are usually maintained by negative frequency-dependent selection, the number of S alleles can be low in small populations, which limits mate availability and reduces fecundity in endangered populations of self-incompatible plants. Despite the increasing evidence of the negative effect of self-incompatibility in small populations, the direct link between the number and the distribution of S alleles and their reproductive consequences has been rarely reported. Brassica insularis is a rare self-incompatible species with medium to very small populations. Results of a previous study showed that the smallest population has very few S alleles. We investigated whether reduced mate availability affects reproduction in this species. We compared the pollination success and the fruit set in 4 populations differing in population size and number of S alleles. Our results suggest that reproduction may be negatively affected by the low S-allele diversity in the smallest population. Nevertheless, other populations also had reduced fruit set that could not be attributed to self-incompatibility alone.

Geographic variation in the breeding system and the evolutionary stability of trioecy in Pachycereus pringlei (Cactaceae)

The Sonoran Desert columnar cactus Pachycereus pringlei has a geographically variable, non-hermaphroditic breeding system. It is trioecious (separate males, females and hermaphrodites) in the northern two-thirds of its range in Sonora, Mexico, and in the southern three-quarters of its range in Baja California, Mexico, and is gynodioecious (separate females and hermaphrodites) elsewhere. Trioecy occurs near known maternity roosts of its major pollinator, the nectar-feeding bat Leptonycteris curasoae; gynodioecy occurs>50km from known bat roosts. The observed geographic patterns cannot be explained by limited gene flow or by the geographic distributions of diurnal avian pollinators. Our field observations plus a theoretical analysis suggest that the abundance of chiropteran pollinators plays an important role in the maintenance of trioecy in this plant. Under pollinator limitation, trioecy can be a stable breeding system in this species.

Altitudinal variation in fertility and vegetative growth in the invasive plant Rubus alceifolius Poiret (Rosaceae), on Réunion island.

Rubus alceifolius Poiret (Rosaceae) was introduced to the island of Réunion in the southeastern Indian Ocean about 1850 and is now highly invasive. This bramble, native from southeastern Asia and Malaysia, has invaded a wide variety of habitats (lowland rainforest, mountain and submountain rainforest, Acacia heterophylla rainforest) from sea level to 1700 m. It is suspected to be monoclonal so, its remarkable success may be due in part to great phenotypic plasticity. On Réunion, bud, flower, fruit and seed production, the duration of the flowering period and the importance of the seed bank were found to be negatively correlated with elevation (50-1500 m a.s.l.). At a lowland site, fruit production in mature stands averaged between 30 and 80 fruits/m2 during 1999 and 2000. No fruit set occurred above 1100 m. This fruit production pattern was similar over two years. Although the number of leaves per unit area is similar along the whole gradient studied, decrease of fruit set in upland areas might be compensated for by an increase in vegetative growth. Temperature variation is very sharp along the elevation gradient and may control the fruit and the seed production. Fruit production allows establishment of new populations all around the island via bird dissemination. Once established, R. alceifolius maintains dense patches that can grow vegetatively. Our results may be relevant for eradication programs that should take into account variation in reproductive strategy in lowland vs. highland habitats.

Sex inheritance in gynodioecious species: a polygenic view

Gynodioecy is defined as the coexistence of two different sexual morphs in a population: females and hermaphrodites. This breeding system is found among many different families of angiosperms and is usually under nucleo-cytoplasmic inheritance, with maternally inherited genes causing male sterility and nuclear factors restoring male fertility. Numerous theoretical models have investigated the conditions for the stable coexistence of females and hermaphrodites. To date, all models rest on the assumption that restoration of a given male sterile genotype is controlled by a single Mendelian factor. Here, we review data bearing on the genetic determinism of sex inheritance in three gynodiecious plant species. We suggest that restoration of male fertility is probably best viewed as a quantitative trait controlled by many loci. We develop a threshold model that accommodates an underlying polygenic trait, which is resolved at the phenotypic level in discrete sexual morphs. We use this model to reanalyse data in Thymus vulgaris, Silene vulgaris and Plantago coronopus. A simple Mendelian inheritance of sex determinism is unlikely in all three species. We discuss how our model can shed additional light on the genetics of restoration and point towards future efforts in the modelling of gynodioecy.

The Evolution of Dimorphism and Separate Sexes in Schistosomes

The fixation of gonochorism (separate sexes) from an hermaphrodite ancestor in the Schistosomatidae may have been the result of selection for dimorphism, under the particular constraint of having to expel eggs from the vertebrate circulatory system. The hermaphroditic system, which exists in blood parasites closely related to Schistosomatidae, e. g. Spirorchidae and Sanguinicolidae, may result from a weaker constraint caused by the different pathways of egg output, and a weaker aggregation of parasites in the host population that will act against the fixation of gonochorism. The importance of these two factors relative to two others (inbreeding depression and allocation to female function in hermaphrodites) in the evolution of gonochorism in blood parasites is evaluated using an analytical model.

Interaction of climate, demography and genetics: a ten-year study of Brassica insularis, a narrow endemic Mediterranean species

Long-term demographic surveys, needed to obtain accurate information on population dynamics and efficiently manage rare species, are still very scarce. Matrix population models are useful tools to identify key demographic transitions and thus help setting up conservation actions. Furthermore, the combination of ecological, demographic and genetic data is likely to improve the identification of the threats acting upon populations and help conservation decisions. In this paper we illustrate the power of this approach on Brassica insularis, a Mediterranean endemic plant species, rare and endangered in Corsica (France). In four populations of this species, a long-term demographic survey (2000–2009), genetic analyses (in 2000 and 2009) and survey of ecological variables (climatic variables, competition and herbivory) were performed. By using both deterministic and stochastic matrix model analyses, we assessed the viability of each population and tested for both spatial and temporal variations in demographic vital rates. Populations exhibited differing demographic behaviours and environmental stochasticity occurred in populations. Significant correlations between climatic variables and vital rates were detected. Stochastic simulations suggested that three out of the four populations studied might present a high risk of extinction on the short-term and should actively be managed, or at least surveyed. It could be, however, that two of these populations are experiencing density-dependent regulation, rather than being declining. Microsatellite diversity was slightly reduced in a single population and similar in the three others, consistently with expectations based on population census size and geographic area, as well as with diversity at the S-locus observed in 2000. The combination of all data led to specific recommendations for managing each population. We discuss the implications for conservation of such a general approach.