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Male-sterility mutations and inbreeding depression in Mimulus guttatus, common yellow monkey flower, in a large annual wild Oregon population in the Western Cascades are discussed. Findings of the study, which measured six fitness components, indicate most of the inbreeding depression for male fertility would not be purged in a short time with moderate inbreeding.

Bumble bee pollinated Chamaecrista fasciculata provides pollen as the sole reward to its pollinators. Male sterility, expressed as an absence or nearly complete absence of pollen production, occurs in low frequency in populations of C. fasciculata. Here we describe experiments, using C. fasciculata, to examine frequently cited determinants of the spread and maintenance of male sterility: compensation and the genetic basis of male sterility. In addition, we examine the role the pollination system plays in determining the reproductive success of the male steriles. Seventeen populations in Maryland, Illinois, and Kansas were surveyed and found to range from 0 to 6% male sterility per population. An artificial population of male-sterile simulants and hermaphrodites was created to examine how the local frequency of nonrewarding male steriles might affect male-sterile female reproductive success. Male steriles performed equally poorly, with respect to seed production, whether surrounded by other male-sterile simulants or hermaphrodites. Compensation was examined by comparison of male steriles and hermaphrodites with respect to several reproductive and nonreproductive characters. Male steriles outperformed hermaphrodites in terms of nonreproductive biomass, but performed equally in terms of ovule number and produced many fewer flowers. The genetic basis of male sterility was examined by performing both intra- and interpopulational crosses of male steriles to hermaphrodites and indicate that male sterility is not purely cytoplasmic. The low frequency of male sterility in C. fasciculata populations may reflect reduced female reproductive success because of pollinator avoidance, lack of reproductive compensation, and a mode of inheritance that is not purely cytoplasmic. Key words: Chamaecrista; compensation; dioecy; Fabaceae; frequency-dependent selection; gynodioecy; male sterility; pollinator behavior.

Evolution and maintenance of male sterility in seed plants can be explained by the maternal inheritance of mitochondria, which encode the trait, and by adaptive functions that enhance female fecundity in male-sterile compared to hermaphrodite individuals. Protogyny and male sterility can independently decrease the negative effect of pollen-pistil interference in self-incompatible species. In Plantago maritime, which possesses both traits, protogyny increases seed set in hermaphrodite individuals. This is shown both by a significantly positive association between seed set and retarded dehiscence of the anthers and by a more than 50% reduction in seed set following self-pollination. Male sterility does not seem to increase seed set further, as female and hermaphrodite plants do not differ significantly in mean seed set per capsule. Bagging experiments demonstrate strong self-incompatibility in the study populations. Hence, in P. maritime male sterility seems neither to prevent selfing nor to reduce the effect of pollen-pistil interference. Females had significantly larger stigmas than hermaphrodites, but seed set varied negatively with stigma length among females, indicating that the evolution of unisexuality in P. maritime is not due to prefertilization sex allocation. I therefore conclude that the genetical system of nucleocytoplasmic determination of gender is the main cause for maintenance of male sterility in P. maritima. Key words: gynodioecy; Plantago maritime; Plantaginaceae; pollen-pistil interference; protogyny; self-incompatibility; wind-pollination.

Gynodioecy is a breeding system in which hermaphrodites coexist with male steriles. Theoretical models predict that without any compensation in female fitness male steriles will disappear from a population due to their reproductive disadvantage. In the present study I investigated whether male-sterile (MS), partially male-sterile (IN), and hermaphroditic (H) plants of Plantago lanceolata differed in reproductive growth and allocation. Offspring of three interpopulation crosses segregating all three sex morphs were grown under nitrogen-limited conditions in a growth chamber. Independent of the genetic background MS plants attained a higher vegetative and reproductive dry mass and a higher reproductive output than H plants, whereas IN plants had intermediate values. When corrected for the mass of the pollen, the dry mass differences between the sex morphs were much reduced but still present. However, when whole-plant allocation was expressed on the basis of nitrogen, the differences between the sex morphs disappeared. Thus the sex morphs took up similar amounts of nitrogen but distributed them differently. The MS and IN plants used the nitrogen saved by not producing pollen for additional vegetative as well as reproductive growth. The data presented in this study suggest that resource compensation is one of the main mechanisms responsible for the maintenance of MS and IN plants in gynodioecious P. lanceolata. Key words: biomass allocation; compensation; gynodioecy; male sterility; Plantaginaceae; Plantago lanceolata; reproductive growth.

This study explores whether ecological factors, such as pollinators and pollen flow, or variation in pollen and ovule development account for the observed differences (approximately twofold) in the reproductive output of pin and thrum individuals of Erythroxylum havanense. The importance of ecological factors was assessed by means of comparison of the identity of pollinators and the rates of flower visitation, and by performing controlled hand pollinations and measurements of fruit set. In addition, we described the pollen and ovule development of thrum and pin individuals. Our results indicate that pollinators of E. havanense do not distinguish between floral morphs. The differences in fruit set between pin and thrum plants held even after hand pollination and, therefore, the observed differences in reproductive output between floral morphs of E. havanense cannot be explained in terms of asymmetrical pollen flow. There were no differences in the pattern of gynoecium development between the pin and thrum morphs, however androecium development showed marked differences between the morphs, and there was a resemblance between the developmental pathways leading to male sterility of the thrum morph of E. havanense with that of species with cytoplasmic male sterility (CMS). Key words: cytoplasmic male sterility; dioecy; Erythroxylaceae; heterostyly; tropical dry forests.

The evolution of gynodioecy from hermaphroditism involves modifications of floral structure such that male or female fitness is enhanced in hermaphrodites and females, respectively. We present an analysis of structural specialization of flowers of Ocotea tenera, in order to evaluate gender system evolution in this tropical tree species. Significant morphological and anatomical variation was found between high fruiting and low or nonfruiting trees. Female flowers were significantly smaller than hermaphroditic flowers, produced no viable pollen, and made relatively greater allocation to structures that increase female fitness. Hermaphroditic flowers were significantly larger than female flowers, produced copious quantities of pollen, and made relatively greater allocation to male structures. Analyses indicated that changes in allometries between whole-flower growth and growth of reproductive structures may have occurred, which enhance function of the flower and plant as a male or female. Efficiency of nutrient allocation for reproduction is argued to be a factor driving gender system evolution in Ocotea tenera. Key words: Costa Rica; floral evolution; gender system; gynodioecy; Lauraceae; male sterility; Ocotea tenera.

Worldwide production of cultivated hybrid sunflower (Helianthus annuus L.) presently utilizes only the PET1 source of cytoplasmic male sterility. The use of one sterile cytoplasm creates a high degree of genetic vulnerability. The objective of this study was to investigate the stability of a new cytoplasmic male sterile source, PEF1, derived from Helianthus petiolaris sp. fallax Heiser and to determine the inheritance of restoration of this cytoplasm. Crosses were made between the PEF1 cytoplasmic male sterile source and several restorer lines known to restore the PET1 cytoplasm. None of the PET1 restorer lines had effective restoration genes for the PEF1 cytoplasm. Two genotypes, 'Zaria' (PI 371935) and HA 60 (Ames 4035), restored male fertility to the PEF1 cytoplasm. Fully fertile [F.sub.5] plants obtained from these crosses were used to pollinate the PEF1 cytoplasm. The fertile [F.sub.1] plants were self-pollinated and backcrossed to the PEF1 source to obtain [F.sub.2] and B[C.sub.1][F.sub.1] seeds, respectively. These generations were planted in the field nursery at Fargo, ND, in 1994. Data were recorded for pollen restoration. The restoration of the PEF1 cytoplasmic male sterile was determined to be controlled by two complementary dominant genes with cumulative gene action. The stability and effective restoration of the PEF1 cytoplasm indicated that this source could be utilized to decrease the genetic vulnerability in hybrid sunflower.

Deterministic and stochastic models based on the population-genetics recursion theory were created to analyze nucleo-cytoplasmic male sterility and the conversion of systems of gynodioecy to dioecy. Results revealed that cytoplasmic polymorphism has no effect on the threshold of pollen fertility necessary for the spread of males into a balanced gynodioecious system. They also indicated that presence of females accelerates the conversion of males to dioecy when the thresholds for dioecy are similar. However, presence of male-sterile cytotype immediately converts the males to purely cytoplasmic gynodioecy.

Chalcone synthase (CHS) catalyzes the first reaction in flavonoid biosynthesis, and mutation in this CHS gene causes failure to produce functional pollen tubes. A pollen-germination rescue assay was developed to identify the specific class of flavonoid involved in the formation of functional pollen tubes. The analysis showed that flavonols could restore germination and tube-growth capacity in CHS-deficient mutants. These results have implications in the development of a reversible male sterile system for the production of hybrid seeds.

Extracellular invertase mediates phloem unloading via an apoplastic pathway. The gene encoding isoenzyme Nln88 from tobacco was cloned and shown to be characterized by a specific spatial and temporal expression pattern. Tissue-specific antisense repression of Nin88 under control of the corresponding promoter in tobacco results in a block during early stages of pollen development, thus, causing male sterility. This result demonstrates a critical role of extracellular invertase in pollen development and strongly supports the essential function of extracellular sucrose cleavage for supplying carbohydrates to sink tissues via the apoplast. The specific interference with phloem unloading, the sugar status, and metabolic signaling during pollen formation will be a potentially valuable approach to induce male sterility in various crop species for hybrid seed production.