Epigenetic control of sexual phenotype in a dioecious plant,Melandrium album

Melandrium album (syn.Silene latifolia) is a model dioecious species in which theY chromosome, present only in heterogametic males, plays both a male-determining and a strict female-suppressing role. We showed that treatment with 5-azacytidine (5-azaC) induces a sex change to androhermaphroditism (andromonoecy) in about 21% of male plants, while no apparent phenotypic effect was observed in females. All of these bisexual androhermaphrodites (with the standard male 24,AA +XY karyotype) were mosaics possessing both male and hermaphrodite flowers and, moreover, the hermaphrodite flowers displayed various degrees of gynoecium development and seed setting. Southern hybridization analysis with a repetitive DNA probe showed that the 5-azacytidine-treated plants were significantly hypomethylated in CG doublets, but only to a minor degree in CNG triplets. The bisexual trait was transmitted to two successive generations, but only when androhermaphrodite plants were used as pollen donors. The sex reversal was inherited with incomplete penetrance and varying expressivity. Based on the uniparental inheritance pattern of androhermaphroditism we conclude that it originated either by 5-azaC induced inhibition ofY-linked female-suppressing genes or by a heritable activation of autosomal female-determining/promoting genes which can be reversed, on passage through female meiosis, by a genomic imprinting mechanism. The data presented indicate that female sex suppression inM. album XY males is dependent on methylation of specific DNA sequences and can be heritably modified by hypomethylating drugs.     

Sex-specific physiology and source-sink relations in the dioecious plant Silene latifolia

Differences in reproductive demands between the sexes of dioecious plants could cause divergence in physiology between the sexes. We found that the reproductive effort of female Silene latifolia plants increased to more than twice that of male plants or female plants that were prevented from setting fruit by lack of pollination after 4 weeks of flowering. Whole-plant source/sink ratios of pollinated females were significantly lower than those of males or unpollinated females because of investment in fruit. We hypothesized that these differences in source/sink ratio between the sexes and within females, depending on pollination, would lead to differences in leaf photosynthetic rates. Within females, we found that photosynthetic capacity was consistent with measurement of whole-plant source/sink ratio. Females that were setting fruit had 30% higher light-saturated photosynthetic rates by 28 days after flowering than females that were not setting fruit. Males, however, had consistently higher photosynthetic rates than females from 10 days after flowering onwards. Males also had approximately twice the dark respiration rates of fruiting females. We found that female reproductive structures are longer-lived and contribute more carbon to their own support than male reproductive structures. Despite the higher rates of leaf dark respiration and lower calyx photosynthetic rates, males fix more carbon than do females. We conclude that females have a sink-regulated mechanism of photosynthesis that allows them to respond to variations in fruit set. This mechanism is not, however, sufficient to explain why male S. latifolia plants have higher rates of photosynthesis, higher source/sink ratios, and lower reproductive allocation, but fail to grow larger than female plants.     

Studies on copper and cobalt tolerance in three closely related taxa within the genusSilene L. (Caryophyllaceae) from Zaïre

Summary Experiments were carried out on the tolerance to and uptake of copper and cobalt by three members of a phylogenetic series of taxa within the genusSilene from Zaïre which were reputed to represent a progression of increasing adaptation to metalliferous soils. Plants studied were the widespread and presumably non-tolerantSilene burchelli var.angustifolia (B), a more tolerant variant of this taxon (E) designated an ecotype, and the metallophyteS. cobalticola (C). Studies on the effects of both metals, singly and in combination, on seed germination, seedling and plant performance and yield, and metal uptake from soil cultures, confirmed in general the sequence B<E<C in relation to tolerance of both copper and cobalt. The experimental evidence also supports the ecotypic status of E and points to E and C being relatively more tolerant to copper than to cobalt.     

Photoperiod and gibberellic acid – control of the cell cycle in the meristem of Silene armeria and its effects on flowering

Plants of Silene armeria L., strain S_2.1, a quantitative long-day (LD) species which is known to react to GA₃ by flowering after attaining, the'intermediate stage', were induced by two LD or by two GA₃ applications. Changes in the mitotic index and DNA content (microdensitometric estimation) of cells in the axial zone, lateral zone and rib meristem of the shoot apex were observed during the first 48 h of each treatment. Similar mitotic activation occurred in response to LD or GA₃ after a 6-8 h lag period. This was preceded by a decrease in the proportion of nuclei with a 2C DNA content, indicating that in this species the control point for the shortening of the cell cycle was essentially in G₁. A second mitotic peak was observed 16 h later in photoinduced meristems, resulting in more pronounced cellular synchronization. These further events were not seen in GA₃-treated plants where only the meristematic activity was slightly, but reproducibly higher than in the control. Thus, two successive synchronizations of cell division are a typical feature of LD induction. The data are discussed with regard to the competence of shoot apical cells to be reactivated. The essential changes for the transition to flowering depend on these differential patterns of cell reactivation.     

Life history of the long-lived gynodioecious cushion plant Silene acaulis (Caryophyllaceae), inferred from size-based population projection matrices

Alpine plants often appear to have long life-spans as an adaptation to harsh and unpredictable environmental conditions, yet many lack reliable indicators of age that would make it possible to determine their true longevity. Their extended life-spans also pose problems for measuring lifetime reproductive success, a key component of breeding system evolution in species such as the gynodioecious cushion plant Silene acaulis. For a population of S. acaulis in south-central Alaska, we applied a recently derived analytical approach using size-based population projection matrices that allowed us to estimate: (1) the relationship between cushion diameter and age; and (2) lifetime reproductive success through seed production by females relative to hermaphrodites. Because of a combination of slow growth, frequent shrinkage, and extremely high adult survival, we estimate that the largest cushions in our study population exceed 300 yr in age, and some may live substantially longer, despite the seemingly inhospitable alpine environment they inhabit. Females are estimated to produce 4.4 times as many offspring via seed production over the course of their lives as do hermaphrodites, a difference that is more than sufficient to assure the persistence of females despite their inability to transmit genes through pollen. These results highlight the utility of size-based projection matrices for studying the life histories of herbaceous perennials whose life-span and lifetime reproductive success cannot be determined easily by any other means.     

Analysis of the clinal variation pattern inSilene latifolia (Caryophyllaceae) pollen morphology

Reticulate and microechinate pollen forms withinSilene latifolia (S. alba, S. pratensis) were first described from N. America, where the species is an introduced weed. A previous study showed that the two forms also exist in Europe and intergrade along a zone roughly congruent with intergradation zones in seed morphology and flavone glycosylation genotypes. The present survey of pollen from herbarium specimens is more extensive and covers localities from nearly the whole of the species' native Eurasian and North African range. The first axis of a detrended correspondence analysis (DCA) of qualitative characterstate data for 11 pollen morphological characters shows a gradient between the two extreme exine types, reticulate and microechinate. When mapped, the co-ordinates of pollen samples on this axis indicate a broad zone of intergradation which coincides approximately with the somewhat sharper transition between low- and high-tubercle seeds in C. and N. Europe, but becomes diffuse in Italy and the Mediterranean region and diverges from the seed transition zone in SW. Asia and the Middle East. The biological significance of the pollen morphs is unknown; the variation pattern in pollen morphology, unlike that in seeds, is not consistently correlated with macroclimate.     

Substitution rates in a new Silene latifolia sex-linked gene, SlssX/Y

Dioecious white campion Silene latifolia has sex chromosomal sex determination, with homogametic (XX) females and heterogametic (XY) males. This species has become popular in studies of sex chromosome evolution. However, the lack of genes isolated from the X and Y chromosomes of this species is a major obstacle for such studies. Here, I report the isolation of a new sex-linked gene, Slss, with strong homology to spermidine synthase genes of other species. The new gene has homologous intact copies on the X and Y chromosomes (SlssX and SlssY, respectively). Synonymous divergence between the SlssX and SlssY genes is 4.7%, and nonsynonymous divergence is 1.4%. Isolation of a homologous gene from nondioecious S. vulgaris provided a root to the gene tree and allowed the estimation of the silent and replacement substitution rates along the SlssX and SlssY lineages. Interestingly, the Y-linked gene has higher synonymous and nonsynonymous substitution rates. The elevated synonymous rate in the SlssY gene, compared with SlssX, confirms our previous suggestion that the S. latifolia Y chromosome has a higher mutation rate, compared with the X chromosome. When differences in silent substitution rate are taken into account, the Y-linked gene still demonstrates significantly faster accumulation of nonsynonymous substitutions, which is consistent with the theoretical prediction of relaxed purifying selection in Y-linked genes, leading to the accumulation of nonsynonymous substitutions and genetic degeneration of the Y-linked genes.     

Genome size, quantitative genetics and the genomic basis for flower size evolution in Silene latifolia

BACKGROUND AND AIMS: The overall goal of this paper is to construct an overview of the genetic basis for flower size evolution in Silene latifolia. It aims to examine the relationship between the molecular bases for flower size and the underlying assumption of quantitative genetics theory that quantitative variation is ultimately due to the impact of a number of structural genes. SCOPE: Previous work is reviewed on the quantitative genetics and potential for response to selection on flower size, and the relationship between flower size and nuclear DNA content in S. latifolia. These earlier findings provide a framework within which to consider more recent analyses of a joint quantitative trait loci (QTL) analysis of flower size and DNA content in this species. KEY RESULTS: Flower size is a character that fits the classical quantitative genetics model of inheritance very nicely. However, an earlier finding that flower size is correlated with nuclear DNA content suggested that quantitative aspects of genome composition rather than allelic substitution at structural loci might play a major role in the evolution of flower size. The present results reported here show that QTL for flower size are correlated with QTL for DNA content, further corroborating an earlier result and providing additional support for the conclusion that localized variations in DNA content underlie evolutionary changes in flower size. CONCLUSIONS: The search image for QTL should be broadened to include overall aspects of genome regulation. As we prepare to enter the much-heralded post-genomic era, we also need to revisit our overall models of the relationship between genotype and phenotype to encompass aspects of genome structure and composition beyond structural genes.     

Climate and clinal variation in seed morphology of the white campion, Silene latifolia (Caryophyllaceae)

Silene latifolia (white campion) is widespread in W Eurasia and shows clinal variation in seed morphology. Two extensive parapatric races, within which seed morphology changes gradually with geographic distance, are separated by a transition zone where the morphological gradient is steeper. A sample of seeds from across the species' native range was scored for morphological characters and analysed using detrended correspondence analysis ordination and multiple regression, in order to simplify mapping of the variation pattern and to investigate statistical relationships with climatic variables. The transition zone, as defined by ordination scores, follows the convolutions of the 0C isotherm for January mean temperature along its entire length from N Europe to the Middle East. January temperature proved to be a highly significant predictor of seed morphology even after the removal of simple spatial trends. Although the origins of comparable large-scale clines have often been interpreted in historical terms, the result reported here supports a role for climatic selection in maintaining the existing variation pattern.