Pollination and seed predation by moths on Silene and allied Caryophyllaceae: evaluating a model system to study the evolution of mutualisms

Nursery pollinators, and the plants they use as hosts for offspring development, function as exemplary models of coevolutionary mutualism. The two pre-eminent examples--fig wasps and yucca moths--show little variation in the interaction: the primary pollinator is an obligate mutualist. By contrast, nursery pollination of certain Caryophyllaceae, including Silene spp., by two nocturnal moth genera, Hadena and Perizoma, ranges from antagonistic to potentially mutualistic, offering an opportunity to test hypotheses about the factors that promote or discourage the evolution of mutualism. Here, we review nursery pollination and host-plant interactions in over 30 caryophyllaceous plants, based on published studies and a survey of researchers investigating pollination, seed predation, and moth morphology and behavior. We detected little direct evidence of mutualism in these moth-plant interactions, but found traits and patterns in both that are nonetheless consistent with the evolution of mutualism and merit further attention.     

Relative effects of nocturnal vs diurnal pollinators and distance on gene flow in small Silene alba populations

Silene alba exists in natural metapopulations throughout its range and is visited by a suite of both diurnal and nocturnal pollinators. Pollen-mediated gene flow may help reduce genetic isolation of subpopulations. Here, we compared the relative effects of nocturnal vs diurnal pollinators on pollen-mediated gene flow in subpopulations separated by two distance treatments. We established populations consisting of genetically marked individuals in an old field in Tennessee (USA). Electrophoretic examination of seedlings produced by plants exposed to nocturnal, diurnal and control pollinator treatments and separated by either 20 or 80 m allowed us to directly measure pollen-mediated gene flow. Gene flow was more common between populations separated by only 20 m. Nocturnal pollinators were responsible for most gene flow between populations, regardless of distance. Diurnal pollinators played only a small role in pollen-mediated gene flow. The results suggest that nocturnal pollinators are better than diurnal pollinators at moving pollen between small S. alba subpopulations. However, their effectiveness declines as the distance between subpopulations increases, making them relatively ineffective at moving genes between isolated subpopulations.     

Accumulation of chloroplast DNA sequences on the Y chromosome of Silene latifolia

Silene latifolia is a model dioecious plant with heteromorphic sex chromosomes. The Y chromosome is the largest in this species. Theoretical models propose an accumulation of repetitive DNA sequences in non-recombining parts of the Y chromosome. In this study, we isolated a BAC7H5 clone preferentially hybridizing to the Y chromosome of S. latifolia. Sequence analysis revealed that this BAC7H5 contains part of the chloroplast genome, indicating that these chloroplast sequences have accumulated on the Y chromosome and also may contribute to its large size. We constructed Y chromosome- and X chromosome-specific libraries and screened them to find Y- and/or X-linked copies of chloroplast sequences. Sequence analysis revealed higher divergence of a non-genic region of the chloroplast sequences located on the Y chromosome while genic regions tested showed only very low (max 0.9%) divergence from their chloroplast homologues.     

Unravelling genetics at the top: mountain islands or isolated belts?

Background and Aims

In mountain plant populations, local adaptation has been described as one of the main responses to climate warming, allowing plants to persist under stressful conditions. This is especially the case for marginal populations at their lowest elevation, as they are highly vulnerable. Adequate levels of genetic diversity are required for selection to take place, while high levels of altitudinal gene flow are seen as a major limiting factor potentially precluding local adaptation processes. Thus, a compromise between genetic diversity and gene flow seems necessary to guarantee persistence under oncoming conditions. It is therefore critical to determine if gene flow occurs preferentially between mountains at similar altitudinal belts, promoting local adaptation at the lowest populations, or conversely along altitude within each mountain.

Methods

Microsatellite markers were used to unravel genetic diversity and population structure, inbreeding and gene flow of populations at two nearby altitudinal gradients of Silene ciliata, a Mediterranean high-mountain cushion plant.

Key Results

Genetic diversity and inbreeding coefficients were similar in all populations. Substantial gene flow was found both along altitudinal gradients and horizontally within each elevation belt, although greater values were obtained along altitudinal gradients. Gene flow may be responsible for the homogeneous levels of genetic diversity found among populations. Bayesian cluster analyses also suggested that shifts along altitudinal gradients are the most plausible scenario.

Conclusions

Past population shifts associated with glaciations and interglacial periods in temperate mountains may partially explain current distributions of genetic diversity and population structure. In spite of the predominance of gene flow along the altitudinal gradients, local genetic differentiation of one of the lower populations together with the detection of one outlier locus might support the existence of different selection forces at low altitudes.     

Sibling competition arena: selfing and a competition arena can combine to constitute a barrier to gene flow in sympatry

Closely related species coexisting in sympatry provide critical insight into the mechanisms underlying speciation and the maintenance of genetic divergence. Selfing may promote reproductive isolation by facilitating local adaptation, causing reduced hybrid fitness in parental environments. Here, we propose a novel mechanism by which selfing can further impair interspecific gene flow: selfing may act to ensure that nonhybrid progeny systematically co-occur whenever hybrid genotypes are produced. Under a competition arena, the fitness differentials between nonhybrid and hybrid progeny are then magnified, preventing development of interspecific hybrids. We investigate whether this "sibling competition arena" can explain the coexistence in sympatry of closely related species of the plant fungal pathogens (Microbotryum) causing anther-smut disease. The probabilities of intrapromycelial mating (automixis), outcrossing, and sibling competition were manipulated in artificial inoculations to evaluate their contribution to reproductive isolation. We report that both intrapromycelial selfing and sibling competition significantly reduced rates of hybrid infection beyond that expected based solely upon selfing rates and noncompetitive fitness differentials between hybrid and nonhybrid progeny. Our results thus suggest that selfing and a sibling competition arena can combine to constitute a barrier to gene flow and diminish selection for additional barriers to gene flow in sympatry.     

Characterization of microsatellite loci in the Hawaiian endemic shrub Schiedea adamantis (Caryophyllaceae) and amplification in related species and genera

Schiedea adamantis is a rare, perennial shrub endemic to the Hawaiian island of Oahu where it consists of a single population. Using a nonradioactive protocol, 12 microsatellite primers were developed that consisted of di-, tri-, penta- and hexanucleotide repeats. Using multiplexed reactions, all but two primers exhibited polymorphism with an average of 3.67 alleles per primer. Most primers also amplified in 28 additional Schiedea species, revealing wide applicability across the genus; eight and nine primers also amplified in Honckenya peploides and Silene lanceolata, respectively, related genera in the Caryophyllaceae. This is the first known report of microsatellite primers developed in Schiedea.     

Structural studies on hairy region of pectic polysaccharide from campion Silene vulgaris (Oberna behen)

Using enzymic digestion with pectinase, controlled Smith degradation and NMR-spectroscopy, some structural features of the hairy region of pectic polysaccharide termed silenan SV from the aerial part of campion Silene vulgaris (Moench) Garke (Oberna behen (L.) Ikonn) were elucidated.

Silenan was subjected to enzymic digestion with pectinase to furnish the polysaccharide fraction (SVP). The contained residues of -galacturonic acid (43%), arabinose, galactose and rhamnose as main constituents. The backbone of the hairy region of silenan was found to consist of -1,4-galactopyranosyl uronic acid and 2-O-glycosylated rhamnopyranose residues. The side chains contained linear regions of residues of -1,5-linked arabinofuranose and β-1,3-, β-1,4-linked galactopyranose. Silenan SV and its fragment SVP were subjected to Smith degradation to give fractions SVS and SVPS. These contain the residues of terminal and 2-substituted -arabinofuranose as well as residues of terminal, 3-, and 2,3-substituted β-galactopyranose. In addition, NMR-spectral data confirmed that the residues of -rhamnopyranose 2-O-glycosylated with the residues of -1,4-galactopyranosyl uronic acid of the backbone occurred in the core of SVPS and, therefore, in the backbone of silenan SV.

On the basis of data obtained, the hairy regions of silenan were suggested to contain mainly the linear chains of β-1,3-, β-1,4-galactopyranan and -1,5-arabinofuranan. The chains of -1,5-linked arabinofuranose, β-1,3- and β-1,4-linked galactopyranose were shown to be involved in the side chains of the hairy region having branching points at 2,3-substituted β-galactopyranose residues.     

麦瓶草种群中的花药黑粉菌感染：花的形态学变异、菌孢子散布式样及病原菌—传媒调节性选择

花药黑粉菌（Microbotryum violaceum)可系统侵染其寄主植物麦瓶草（Silene dioica),使其不育而代之以在植物花中布满菌孢子,这些菌孢子主要由花传媒昆虫带到健康植株,花传媒昆虫对花部特征变异的行为反应不仅可能影响花粉的输入/输出率,而且影响菌孢子的着落率和植株的发病与否,为研究S.dioica雌株花部性状特征与花粉传布,菌孢子着落及由此而导致的植株染病之间的相关性,用采自4个不同S.dioica种群的植物进行了移栽实验,讠4个种群均位于瑞典北部的Skeppsvik群岛,代表了从健康（岛1,种群1）,低度发病（岛2,种群2）到高度发病（岛3、4,种群3、4）的发病梯度,从上述4个不同种群中各采50个植株移栽至岛4的中部,来自健康种群的植株较来自高发病种群的植株具有较大的花,较长的花柱,研究发现,着落在花上的花粉粒数和菌孢子数呈强正相关,来自健康种群的植株每朵花上着落的花粉粒和菌孢子数分别是高发病种群（种群4）植株的4倍和9倍,导致来自4个不同种群的植株的发病率存在着显差异。种群1、2、3、4的发病率分别为37%,20%,18%,0。在涉及10个种群的田间调查研究中,发现种群中健康植株的平均花柱长度（与花冠大小,胚珠数目正相关）与植株的发病率显负相关,讨论了病原体-传媒调节的植物花部性状特征选择潜势及其导致的麦瓶草S.dioica种群音质 基因漂移。