Natural selection on floral traits of female Silene dioica by a sexually transmitted disease

Floral traits endowing high reproductive fitness can also affect the probability of plants contracting sexually transmitted diseases. We explore how variations in floral traits influence the fitness of Silene dioica females in their interactions with pollinators carrying pollen or spores of the sterilizing anther-smut fungus Microbotryum violaceum. We collected healthy and infected plants in a highly diseased population and grew them under conditions that 'cure' infected individuals, and used standard regression methods to detect natural selection on floral traits. Narrow-sense heritabilities, coefficients of additive genetic variation (CV(A)) and genetic correlations among traits were estimated from paternal half-sib groups. Pollinator preferences imposed strong direct and directional selection on traits affecting female attractiveness and pollen-/spore-capturing abilities. Levels of additive genetic variance were high in these traits, suggesting that rapid responses to selection are possible. By considering our results in the light of spatial and temporal heterogeneity resulting from the colonization dynamics typical for this species, we suggest that the conflicting selective effects of pollen/spore loads lead to the maintenance of genetic variation in these traits.     

Effects of inbred/outbred crosses on progeny sex ratio in Silene latifolia (Caryophyllaceae)

Sex ratio polymorphism has been extensively studied in Silene latifolia, but it is neither known whether inbreeding (which is likely to occur under field conditions) affects it, nor which of the proposed mechanisms (Y degeneration, X-linked drive) is more important. Both mechanisms predict reduced pollen performance. In this study, females were crossed with pollen from related and unrelated males in single-donor and two-donor crosses, and the sex ratio of offspring (n = 866, 60 crosses), sons'in vitro pollen germination and sex ratios in parental families were scored. Flowers receiving only unrelated pollen produced a significant excess of sons. Sex ratios were not significantly correlated between generations. Sons'in vitro pollen germination was significantly negatively correlated with the 'sex-ratio phenotype' of maternal grandfathers, but not of fathers. This generation leap may be consistent with X-linked determinants because Y-linked determinants alone cannot explain it (grandfathers, fathers and sons share the same Y chromosome). Further work is required, but inbreeding and limited dispersal may lead to local accumulation of biasing factors, a process potentially countered by conditional shifts to produce more sons in pure outbred crosses.     

Characterization of microsatellite loci in Lychnis flos‐cuculi (Caryophyllaceae)

We cloned microsatellite repeats from ragged robin Lychnis flos‐cuculi (Caryophyllaceae) and developed 20 primer pairs for microsatellite marker analysis. We used 18 individuals of a large Swiss population to screen microsatellites. Seven loci were polymorphic. Between seven and 11 alleles were found per locus and the observed heterozygosity was between 0.308 and 0.813. Observed heterozygote deficits were significant in five of the seven loci, in line with the mixed mating system of L. flos‐cuculi.     

Consequences of inbreeding for offspring fitness and gender in Silene vulgaris,a gynodioecious plant

Abstract In gynodioecious plants, hermaphrodite and female plants co‐occur in the same population. In these systems gender typically depends on whether a maternally inherited cytoplasmic male sterility factor (CMS) is counteracted by nuclear restorer alleles. These restorer alleles are often genetically dominant. Although plants of the female morph are obligatorily outcrossing, hermaphrodites may self. This selfing increases homozygosity and may thus have two effects: (1) it may decrease fitness (i.e. result in inbreeding depression) and (ii) it may increase homozygosity of the nuclear restorer alleles and therefore increase the production of females. This, in turn, enhances outcrossing in the following generation. In order to test the latter hypothesis, experimental crosses were conducted using individuals derived from four natural populations of Silene vulgaris, a gynodioecious plant. Treatments included self‐fertilization of hermaphrodites, outcrossing of hermaphrodites and females using pollen derived from the same source population as the pollen recipients, and outcrossing hermaphrodites and females using pollen derived from different source populations. Offspring were scored for seed germination, survivorship to flowering and gender. The products of self‐fertilization had reduced survivorship at both life stages when compared with the offspring of outcrossed hermaphrodites or females. In one population the fitness of offspring produced by within‐population outcrossing of females was significantly less than the fitness of offspring produced by crossing females with hermaphrodites from other populations. Self‐fertilization of hermaphrodites produced a smaller proportion of hermaphroditic offspring than did outcrossing hermaphrodites. Outcrossing females within populations produced a smaller proportion of hermaphrodite offspring than did crossing females with hermaphrodites from other populations. These results are consistent with a cytonuclear system of sex determination with dominant nuclear restorers, and are discussed with regard to how the mating system and the genetics of sex determination interact to influence the evolution of inbreeding depression.     

Within‐ and among‐population variation in infectivity,latency and spore production in a host–pathogen system

In spatially structured populations, host–parasite coevolutionary potential depends on the distribution of genetic variation within and among populations. Inoculation experiments using the plant, Silene latifolia, and its fungal pathogen, Microbotryum violaceum, revealed little overall differentiation in infectivity/resistance, latency or spore production among host or pathogen populations. Within populations, fungal strains had similar means, but varied in performance across plant populations. Variation in resistance among seed families indicates the potential for parasite‐mediated selection, whereas there was little evidence for local pathogen genotype × plant genotype interactions assumed by most theoretical coevolution models. Lower spore production on sympatric than allopatric hosts confirmed local fungal maladaptation already observed for infectivity. Correlations between infectivity and latency or spore production suggest a common mechanism for variation in these traits. Our results suggest low variation available to this pathogen for tracking its coevolving host. This may be caused by random drift, breeding system or migration characteristic of metapopulation dynamics.     

米瓦罐的光合特性研究

采用美国LI-COR生产的LI-6400便携式光合系统研究了米瓦罐的光合特性。结果表明,米瓦罐光 合速率的日变化呈单峰曲线,上午10:00时光合速率达到最大值;在大气CO2浓度下,米瓦罐的光饱和点为 1 800μmol·m-2·s-1,光补偿点为30μmol·m-2·s-1;在光饱和点的光强下,米瓦罐的CO2饱和点为1 200 μmolCO2·mol-1,CO2补偿点为40μmolCO2·mol-1。     

Inbreeding and reintroduction: Progeny success in rare Silene populations of varied density

Genetic factors influence the populationviability of rare species, yet the fitnessconsequences of inbred and outbred progeny areseldom tested empirically in reintroductionstrategies designed for species recovery orhabitat restoration. Rare and endangeredplants of Silene (Caryophyllaceae) occuron four continents, including North America. In Oregon, inbred and outbred progeny weremonitored for three years after experimentalreintroduction of a narrow endemic, S.douglasii var. oraria, into formerlygrazed habitat within its presumed historicalrange. Survival and reproduction were comparedfor progeny that were derived from the seeds ofself- versus cross-pollinated flowersproduced in situ at Cascade Head, aUNESCO Biosphere Reserve where the largest ofthree extant populations occurs. Progeny ofcross-pollinated flowers had significantlygreater field survival in all years than didoffspring of selfed or open-pollinated flowers(P < 0.01). Outbred progeny alsosignificantly exceeded other treatment cohortsin canopy area, and produced more reproductivestems and flowers than other progeny types ofthe same maternity. For plots varying in plantdensity, mortality was greatest in thehigh-density competitive regime but thesurvivors reached significantly larger sizesand reproductive capacities than in low densityplots (P < 0.05). In all, successfulconservation plans involving reintroduction mayrequire genetically diverse progeny to offsetinbreeding depression as well as suitableplanting densities and source populations.     

Climatic control of germination in the genus Silene L

Background: Seed germination strategies depend on a number of factors such as climatic variables, habitat, life cycle, phylogenetic affiliation and morphological seed traits.

Aims: We present a novel evaluation of the ecological correlates of germination strategies focussing on plant populations drawn from a single genus and considering a number of relevant explanatory variables.

Methods: Temperature and light-controlled germination experiments were carried out on 21 seed accessions belonging to 13 closely related species of the genus Silene L. Accessions were selected to tightly control the phylogenetic effect and give broad ecological and geographic coverage of the genus in Europe, with three North American and Macaronesian outgroups from climatically extreme environments. We used principal component analysis and correlation analyses to identify the correlation structure of germination traits and include multiple explanatory variables.

Results: Three germination strategies were shown to be related to climatic control. (1) Seeds from arid regions with hot, dry summers germinated optimally at cool temperatures associated with the rainfall period under field conditions, whereas (2) the benefit of cold stratification was more marked in provenances characterised by cold, dry winters, and (3) seeds from mild climates preferred warm temperatures for germination. Moreover, (4) biennial populations of disturbed habitats showed synchronised and rapid seed germination over a wide thermal window. In agreement with previous findings, (5) habitat-related syndromes were observed only for wetland populations. Correlations with seed mass were significant when related to summer precipitation, but weak or absent in relation to germination traits, indicating that, though influenced by the local climate, seed mass is a poor predictor of germination strategies.

Conclusions: These results suggest that whilst habitat and life cycle might shape germination patterns to a certain extent, long-term climatic differences play a substantial role in selecting specific germination traits and strategies.     

石竹科植物2个中国新记录种

西藏南部地形较为复杂,植物资源丰富,物种多样性高。笔者在对西藏山南地区高山植物多样性调查过程中,经野外调查、采集标本、查阅植物志等相关资料及分类鉴定,结果发现,山南地区隆子县的无心菜属(Arenaria)球花福禄草［A. globiflora (Fenzl) Edgew. & Hook. f.］和错那县蝇子草属(Silene)的错那蝇子草(S. birgittae Bocquet)为中国新记录种。该报道一方面丰富了中国植物区系的内容,更新了国产石竹科植物资料,对中国西藏山南地区植物多样性的保护及研究具有一定意义。另一方面说明中国西藏山南地区的植物多样性调查仍存在许多调查薄弱地区,还需加强相关地区植物多样性调查工作。     

SEX-SPECIFIC COSTS OF RESISTANCE TO THE FUNGAL PATHOGEN USTILAGO VIOLACEA (MICROBOTRYUM VIOLACEUM) IN SILENE ALBA

Costs of resistance are often invoked to explain the maintenance of polymorphisms for resistance to fungal pathogens in natural plant populations. To investigate such costs, 27 half-sib families of Silene alba, collected from a single host population, were grown in experimental populations in the presence and absence of the anther-smut fungus Ustilago violacea, a host-sterilizing pathogen transmitted by insects that are both pollinators and vectors of the disease. Host families differed significantly in resistance to inoculation, indicating the presence of genetic variation for mechanisms that impede fungal growth once the disease is encountered (“biochemical” resistance) within the host population. In addition, host families differed significantly in onset of flowering and in flower production in the absence of the disease. Path analysis revealed that late onset of flowering in male host families made a direct contribution to high field resistance (P < 0.01), probably due to a reduced rate of contact between hosts and vectors carrying high spore loads (avoidance, or “phenological” resistance). The contribution of low flower production to field resistance only approached significance (P < 0.10). There was a significantly positive genetic association between biochemical and phenological resistance, suggesting that delayed flowering is either a pleiotropic effect of biochemical resistance, or that genes governing these traits are in linkage disequilibrium. Path analysis revealed that biochemical resistance made both a direct contribution to field resistance (P < 0.01) and a positive indirect contribution via its association with phenology and flower production (P < 0.05) in male hosts. Costs of resistance were sex specific. Male host families with high field resistance had significantly lower reproductive success in healthy populations, indicating a fitness cost of field resistance (P < 0.01), whereas no costs were detected for female hosts. Path analysis revealed that the biochemical component of field resistance made no direct contribution to the observed fitness cost in male hosts, whereas its indirect effect through phenology was only marginally significant (P < 0.10). This finding indicates that fitness costs were mainly due to the phenological component of field resistance. Because the host population had no known history of disease, it is not clear whether the fitness costs are responsible for maintenance of the resistance polymorphism or whether the polymorphism is present for reasons unrelated to pathogen infection. Interactions between host families and pathogen strains with respect to inoculation success were not significant. Hence, there was no evidence for indirect costs of biochemical resistance, that is, reduced resistance to alternative strains. Infection rates in experimental populations with an initially patchy distribution of the pathogen were lower than in populations with a uniform pathogen distribution, suggesting that the effective pathogen pressure and hence the relative success of susceptible and resistant individuals may, in addition to fitness costs of resistance, depend on the spatial population structure of the pathogen.