EFFECT OF DENSITY ON SECONDARY SEX CHARACTERISTICS AND SEX RATIO IN SILENE ALBA (CARYOPHYLLACEAE)

A field survey of plant and flower sex ratio and secondary sex characteristics was made in Silene alba. Female-biased plant sex ratios were found, as seems typical for the species. Sex ratio distribution correlated with a gradient of soil moisture (with the more moist area having a more female-biased ratio) and with changes in the density of Silene (intermediate and higher density areas having greater female bias). The floral sex ratio was significantly female-biased only at the site that was most female-biased in terms of plant sex ratio. Otherwise the population of flowers was significantly male-biased. Male and female plants harvested from the field differed in secondary sexual characteristics. Males had more flowers and invested proportionately more biomass in leaf, but less in root, stem and reproductive tissue than did females. Although both males and females were larger in terms of total dry weight at the moist site, males produced more flowers at the driest (high density) site. Here the female bias in plant sex ratio was intermediate, but the floral sex ratio was significantly male-biased. A glasshouse experiment was performed in which plants were grown at four densities. Density significantly influenced plant survivorship and the probability of flowering, and increased female bias in the pots, but it did not affect patterns of biomass allocation in flowering plants. Patterns of male and female biomass allocation did not differ in the experiment, except in terms of reproductive allocation (greater in females) and allocation to leaf, greater in males, but only at the lowest density. This work urges caution in interpreting differences between males and females in the field as secondary sex characteristics, since we find such properties to be overlapping under experimental conditions. It supports the idea that males and females of a species may sustain different reproductive output under differing conditions.     

THE QUANTITATIVE GENETICS OF SEXUAL DIMORPHISM IN SILENE LATIFOLIA (CARYOPHYLLACEAE). II. RESPONSE TO SEX-SPECIFIC SELECTION

A well-established theoretical relationship exists between genetic correlations between the sexes and the dynamics of response to sex-specific selection. The present study investigates the response to sex-specific selection for two sexually dimorphic traits that have been documented to be genetically variable, calyx diameter and flower number, in Silene latifolia. Following the establishment of a base generation with a known genetic background, selection lines were established and two generations of sex-specific selection were imposed. Calyx diameter responded directly to sex-specific selection, and the positive genetic correlation between the sexes was reflected in correlated responses in the sex that was not the basis for selection within a particular line. Flower number showed a more erratic response to sex-specific selection in that selection in some lines was initially in the wrong direction, that is, selection for a decrease in flower number resulted in an increase. These erratic responses were attributable to genotype-environment interaction as reflected in significant heteroscedasticity in variance among families. Correlated responses to selection in the sex that was not the immediate basis for selection indicated the possible existence of a negative genetic correlation between the sexes for this trait. These results test for the first time the impact of genetic correlations between the sexes on the evolutionary dynamics of sexually dimorphic traits in a plant species.     

PROGENY SEX RATIOS IN DIOECIOUS SILENE LATIFOLIA (CARYOPHYLLACEAE)

Most sex ratios reported for Silene latifolia are female biased. As a result of experiments performed by Correns in the early 1900s, pollen tube competition has generally been accepted as the primary cause of these skewed ratios. We did four sets of hand pollinations in which we varied the size of pollen loads and placement of pollen along the filamentous stigma. The effect of pollen load size on progeny sex ratios was not statistically significant. Of 32 maternal families, 17 contained more females than males (one ratio deviated statistically from 1:1), and 13 contained more males than females. Paternal families exhibited a greater range of sex ratios, including three with a significant female bias and one with a significant male bias. Within experiments, neither the maternal parent nor where pollen was placed had a statistically significant effect on progeny sex ratios; the paternal effect was significant in one experiment. We suggest that sex ratios in Silene latifolia are not necessarily affected by the level of pollen competition. Other factors, including variation among males and sex-linked mortality, may help explain the skewed sex ratios that characterize populations of this species. Further, Correns' observations of excess females may have resulted from his use of interspecific hybrids.     

POLLEN TUBE COMPETITION AND SELECTION FOR METAL TOLERANCE IN SILENE DIOICA (CARYOPHYLLACEAE) AND MIMULUS GUTTATUS (SCROPHULARIACEAE)

An experiment was conducted to test whether metal tolerance expressed in pollen would provide a competitive advantage during pollen tube growth and fertilization. Copper or zinc was introduced into the pistil by growing metal tolerant plants of Silene dioica or Mimulus guttatus in a nutrient solution to which metals were added. Flowers from treated and control plants were pollinated with either metal tolerant or metal sensitive pollen. The rate of pollen tube growth, the number of seeds set, and the number of viable seeds produced were measured. In general, no effects were found on the rates of pollen tube growth. However, the number of fertilizations and viable seeds were affected. When toxic metals were present, the relative success of pollen from the nontolerant parent was reduced.     

THE EFFECT OF SERPENTINE ON THE POPULATION STRUCTURE OF SILENE DIOICA (CARYOPHYLLACEAE)

Serpentine soils are rich in heavy metals and have a distinctive flora. Silene dioica is a member of the Scandinavian serpentine plant community but is also widespread outside serpentine soils. To study the population genetic consequences of serpentine stress and the origin and evolution of serpentine populations we analyzed the isozyme genetic structure of S. dioica. Seventeen populations located in the mountains of Västerbotten and Jämtland, central Sweden, were investigated by starch gel enzyme electrophoresis. About one half of the populations grow in serpentine soils and the rest on adjacent non-serpentine sites. Analyses of allele frequencies show that both serpentine and non-serpentine populations in the northern part of the studied area (Västerbotten) are genetically similar. Evidently serpentine does not exert strong selection acting upon isozyme loci. In the south (Jämtland), however, the serpentine populations exhibit genetic differentiation. This allozyme divergence is probably not due to direct selection but rather represents the effects of isolation and genetic drift. The results suggest that S. dioica has colonized serpentine repeatedly and that the tolerant populations have a multiple origin.     

ECOLOGICAL ASPECTS OF SEX EXPRESSION IN SUBDIOECIOUS SCHIEDEA GLOBOSA (CARYOPHYLLACEAE)

The importance of ecological factors such as sex lability, spatial segregation, and resource allocation in the evolution of dioecy were examined in Schiedea globosa. S. globosa is a subdioecious species with equal numbers of plants possessing strictly male or female function and a small proportion of hermaphrodites. The propensity for labile sex expression was under both environmental and genetic control; some plants with male function became hermaphroditic (by producing female flowers) under better growing conditions in the field and in the greenhouse. There was some spatial segregation of the sexes. Because of sex lability, more hermaphrodites than males occurred on moister slopes. Although there were not measurable sex-related differences in mortality within or between two flowering seasons, more females than males and hermaphrodites occurred at the bottom of slopes. Males and females produced the same number of ramets and inflorescences, but females had a greater number of flowers per inflorescence. Males and females had the same number of ovules (vestigial in males), but females had larger ovules and longer stigmas. Hermaphrodites and males had the same amount of pollen per flower despite the production of fruit by the hermaphrodites. In hermaphrodites, there was no apparent tradeoff within flowers between pollen production and ovule production. These results indicate that spatial segregation, sex lability, and environmental conditions influence allocation patterns of S. globosa, and in combination with high inbreeding depression and selling rates, may promote the further evolution of dioecy in S. globosa.     

SEX RATIO SELECTION AND THE EVOLUTION OF ENVIRONMENTAL SEX DETERMINATION IN LABORATORY POPULATIONS OF MENIDIA MENIDIA

What happens when a population with environmental sex determination (ESD) experiences a change to an extreme environment that causes a highly unbalanced sex ratio? Theory predicts that frequency-dependent selection would increase the proportion of the minority sex and decrease the level of ESD in subsequent generations. We empirically modeled this process by maintaining five laboratory populations of a fish with temperature-dependent sex determination (the Atlantic silverside, Menidia menidia) in extreme constant temperature environments that caused highly skewed sex ratios to occur initially. Increases in the minority sex consistently occurred from one generation to the next across all five populations, first establishing and then maintaining a balanced sex ratio until termination of the experiment at 8 to 10 generations. The extent to which the level of ESD changed as balanced sex ratios evolved, however, was not consistent. Two populations that experienced high temperatures each generation displayed a loss of ESD, and in one of these ESD was virtually eliminated. This suggests that temperature-insensitive, sex-determining genes were being selected. In populations maintained in low temperature environments, however, the level of ESD did not decline. Instead, the response of sex ratio to temperature was adjusted upward or downward, perhaps by selection of sex-determining genes sensitive to higher (or lower) temperatures. The two different outcomes at low versus high temperatures occurred independent of the geographic origin of the founding population. Our results demonstrate that ESD is capable of evolving in response to selection.     

ADVANTAGES OF BEING A SPECIALIST FEMALE IN NODIOECIOUS SILENE VULGARIS S.L. (CARYOPHYLLACEAE)

According to sex allocation theory, to maintain a mutant male-sterile plant in a population of hermaphrodites such a plant must compensate its loss of fitness caused by inhibition of pollen production with a higher reproductive success through its female function. In the present study of a gynodioecious population of Silene vulgaris (Caryophyllaceae) I show that hermaphrodites not only benefit from outcrossing, in that progeny from outcrossed flowers are more vigorous than those from selfed flowers within an individual plant, but they also suffer heavily from self-pollination between different flowers of the same individuals, which could be demonstrated in experimentally made male-sterile (emasculated) individuals. Seeds from the emasculation period were heavier and germinated better than when the same individual was an intact hermaphrodite. Naturally male-sterile (female) individuals produced more fruits due to flowers staying open longer for pollen to arrive via some vector. However, the higher seed number alone could not provide the fitness advantage needed for females to be maintained in the population, but females also produced heavier seeds as compared to the hermaphrodites. Differences in seed survival and seedling establishment in the field are expected to add the advantages necessary for female plants to be selectively plausible.