Gynodioecy in the common spindle tree (Euonymus europaeus L.) involves differences in the asymmetry of corolla shapes between sexually differentiated flowers

Gynodioecy is typically associated with a smaller perianth size in purely pistillate flowers than in hermaphrodite flowers. However, it is unclear whether this size differentiation is associated with any differences in flower shape between the two sexual groups. A geometric morphometric analysis of the symmetry of tetrameric corolla shapes was used in the study of Euonymus europaeus L., Darwin’s classical system of floral sexual differentiation. I investigated whether there are any shape differences between the female and bisexual flowers, with respect to both purely symmetric variation involving coordinated shape changes of the four petals and asymmetry among petals within flowers. The corolla shapes of the female and bisexual flowers and the variability among flowers within each sexual group were very similar in the purely symmetric components of shape variation. However, the female flowers were significantly more asymmetric with respect to both the lateral and transversal asymmetry of their corolla shapes. This is the first study to apply geometric morphometrics in the analysis of morphological patterns in a sexually differentiated gynodioecious plant system. The results showed that subtle shape differences in corolla asymmetry differ between the sexual groups and indicate diverging developmental or selection signals between the sexes.

Desert-like badlands and surrounding (semi-)dry grasslands of Central Germany promote small-scale phenotypic and genetic differentiation in Thymus praecox

Environmental heterogeneity among sites can generate phenotypic and genetic variation facilitating differentiation and microevolution of plant populations. Badlands are desert-like, predominantly vegetation-poor habitats often embedded in (semi-)dry grasslands. The desert-like conditions of badlands demand extreme adaptation of plants, that is, phenotypic modifications in short-term and/or natural adaptation in long-term. However, detailed knowledge is missing about both plant phenotypic and genetic differentiation in this unique and widely occurring habitat type. The present study focused on the largest known badlands systems in Central Europe located in the "Drei Gleichen" region, a designated nature conservation area in Central Germany. Locations were suitable for this study in terms of having co-occurring badlands and (semi-)dry grassland habitats (sites) occupied by the pioneer plant Thymus praecox. Here, we studied the environmental preferences, morphological and functional trait variation, and genetic variation using microsatellite markers of T. praecox. Results revealed significant, mainly site-dependent environmental, phenotypic, and genetic differentiation. In general, individuals in badlands are shorter in height and have lower patch sizes (length × width), relative growth rates, and smaller stomata. The PCA additionally unveiled slightly increased leaf robustness, trichome density, decreased stomatal conductance, fewer females, and earlier phenology in badlands. We interpret differentiation patterns as adaptive responses to light, temperature, drought, and nutrient stress conditions supported by reviewed literature. Genetic differentiation was strongest between local badlands and grassland sites, and clearly weaker among locations and between sites (in total) as indicated by G ST, AMOVA, PCoA, and population structure. Our study supports the importance of small-scale microhabitat conditions as a driver of microevolutionary processes, and the population's need for sufficient phenotypic variation and genetic resources to deal with environmental changes. We demonstrated that badlands are an appropriate model system for testing plant response to extreme habitats and that more research is needed on these fascinating landscapes.

Sex-differential reproduction success and selection on floral traits in gynodioecious Salvia pratensis

BACKGROUND

Gynodioecy, a sexual system with hermaphrodite and female individuals in a population, raises the question how the two sexual morphs are maintained. Salvia pratensis is a gynodioecious species featured by its modified stamens that act as a lever mechanism in pollination. Given sexual dimorphism in floral traits of the species, it is predictable that two sexual morphs differ in their interplay with pollinators and thus in their fitness. In this study, we investigated sex-specific reproduction success and floral adaptation in a population of S. pratensis.

RESULTS

We found that two sexual morphs in S. pratensis distinctly differed in their floral proportions. Female flowers fitted better to the pollinators than hermaphrodites in terms of touching the stigmas when being probed, and hence were more efficient in pollen deposition. Floral traits overall underwent stronger selection in the population, with stigma position and corolla length subject to disruptive selection mediated by different body-sized bumble bees; some selections on floral traits were significantly different in the strength, even opposite in the direction between two morphs. Flower production tended to be under correlational selection with floral structural traits, implying that a large plant with many flowers did not show an advantage in fitness unless its flower construction mechanically matched the pollinators well.

CONCLUSIONS

In conclusion, the pollinator-mediated selection likely played an important role in the evolution and maintenance of sexual dimorphism in the gynodioecious S. pratensis; and sex-divergent mechanical interaction with pollinators served as a critical mechanism by which female individuals were maintained in the population with a female advantage in pollen deposition efficiency (i.e. receiving pollen).

Variable mycorrhizal benefits on the reproductive output of Geranium sylvaticum, with special emphasis on the intermediate phenotype

In several gynodioecious species, intermediate sex between female and hermaphrodite has been reported, but few studies have investigated fitness parameters of this intermediate phenotype. Here, we examined the interactions between plant sex and arbuscular mycorrhizal (AM) fungal species affecting the reproductive output of Geranium sylvaticum, a sexually polymorphic plant species with frequent intermediate sexes between females and hermaphrodites, using a common garden experiment. Flowering phenology, AM colonisation levels and several plant vegetative and reproductive parameters, including seed and pollen production, were measured. Differences among sexes were detected in flowering, fruit set, pollen production and floral size. The two AM species used in the present work had different effects on plant fitness parameters. One AM species increased female fitness through increasing seed number and seed mass, while the other species reduced seed mass in all sexes investigated. AM fungi did not affect intermediate and hermaphrodite pollen content in anthers. The three sexes in G. sylvaticum did not differ in their reproductive output in terms of total seed production, but hermaphrodites had potentially larger fathering ability than intermediates due to higher anther number. The ultimate female function--seed production--did not differ among the sexes, but one of the AM fungi used potentially decreased host plant fitness. In addition, in the intermediate sex, mycorrhizal symbiosis functioned similarly in females as in hermaphrodites.

Cytoplasmic male sterility in Mimulus hybrids has pleiotropic effects on corolla and pistil traits

The mechanisms underlying genetic associations have important consequences for evolutionary outcomes, but distinguishing linkage from pleiotropy is often difficult. Here, we use a fine mapping approach to determine the genetic basis of association between cytonuclear male sterility and other floral traits in Mimulus hybrids. Previous work has shown that male sterility in hybrids between Mimulus guttatus and Mimulus nasutus is due to interactions between a mitochondrial gene from M. guttatus and two tightly linked nuclear restorer alleles on Linkage Group 7, and that male sterility is associated with reduced corolla size. In the present study, we generated a set of nearly isogenic lines segregating for the restorer region and male sterility, but with unique flanking introgressions. Male-sterile flowers had significantly smaller corollas, longer styles and greater stigmatic exsertion than fertile flowers. Because these effects were significant regardless of the genotypic composition of introgressions flanking the restorer region, they suggest that these floral differences are a direct byproduct of the genetic incompatibility causing anther abortion. In addition, we found a non-significant but intriguing trend for male-sterile plants to produce more seeds per flower than fertile siblings after supplemental pollination. Such pleiotropic effects may underlie the corolla dimorphism frequently observed in gynodioecious taxa and may affect selection on cytoplasmic male sterility genes when they initially arise.

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.

Pollination and reproductive success in the gynodioecious endemic Thymus loscosii (Lamiaceae)

Sexual dimorphism can have implications in the reproductive biology of gynodioecious species, affecting sex fitness. We explored the effects of flower sex on pollination visitation rates and pollinator efficiency in terms of stigmatic pollen loads, as well as on quantitative and qualitative aspects of reproductive success in two populations of Thymus loscosii Willk. (Lamiaceae) endemic to the northeastern Iberian Peninsula. We also assessed the dependence of T. loscosii on insect pollination in both hermaphrodite and female plants by performing an insect exclusion test. Apis mellifera and different species of Bombylidae were the most frequent pollinators (68% of approaches to studied plots and 93% of total visited flowers). Hermaphrodite plants received more visits than female ones, possibly as a response to visual attraction, since flowers of the former are larger. Conspecific pollen deposition was higher on stigmas of hermaphrodite flowers than on those of females; in contrast, female stigmas received more heterospecific pollen loads, notably higher in one population. Despite these differences, seed set from open-pollinated flowers was similar in both sexes and in both populations, and relatively low (around 0.5–1 nutlet per fruiting calyx, on average). Thymus loscosii is self-compatible as it was able to produce seeds by spontaneous selfing, but at very low rates, indicating that it is insect-dependent for pollination. In addition, bagged female flowers also set seeds formed by apomitic mechanisms. In general, seeds from females were slightly heavier and began to germinate earlier and at higher rates than those from hermaphrodites, even those formed by apomixis. These results suggest that female plants enjoy a resource allocation advantage that allows increased seed quality and contributes to the maintenance of gynodioecy.Key words: gynodioecy, pollinator visitation, pollen loads, seed set, sex fitness, Thymus loscosii.

Constraints on the evolution of males and sexual dimorphism: field estimates of genetic architecture of reproductive traits in three populations of gynodioecious Fragaria virginiana

To understand how genetic constraints may limit the evolution of males and sexual dimorphism in a gynodioecious species, I conducted a quantitative genetic experiment in a gynodioecious wild strawberry, Fragaria virginiana. I estimated and compared genetic parameters (narrow-sense heritabilities, between-trait and between-sex genetic correlations, as well as phenotypic and genetic variance-covariance matrices) in the two sex morphs from three populations grown in a common field garden. I measured pollen and ovule production per flower, petal size, fruit set, and flower number. My major findings are as follows. (1) The presence of a phenotypic trade-off between pollen production and fruit set in hermaphrodites reflects a negative genetic correlation in the narrow sense that is statistically significant when pooled across populations. (2) The main constraints on the evolution of males are low genetic variation for pollen per flower and strong positive correlations associated with ovule number (e.g., between pollen and ovules in hermaphrodites, and between ovules in hermaphrodites and females). (3) Traits with the lowest levels of sexual dimorphism (ovule number and flower number) have the highest between-sex genetic correlations suggesting that overlap in the expression of genes in the sex morphs constrains their independent evolution. (4) There are significant differences in G matrices between sex morphs but not among populations. However, evidence that male-female trait correlations in hermaphrodites were lower in populations with higher frequencies of females may indicate subtle changes in genetic architecture.