Sex Expression and the Evolutionary Advantages of Male Flowers in an Andromonoecious Species, Sagittaria guyanensis subsp. lappula (Alismataceae)

Sex expression in Sagittaria guyanensis H.B.K. subsp. lappula (D. Don) Bojin is typically andromonoecious while the other species in the same genus are basically monoecious. The evolutionary advantages of male flowers and hermaphrodite flowers in S. guyanensis subsp. lappula were assessed by measuring sex allocation and pollen movements in two wild populations of the species. Two cultivated populations served as controls. The percentage of male flowers was very low in the two wild populations in Dongxiang, Jiangxi Province and Wuyishan, Fujian Province, viz., 2.48% and 0.96% respectively. In the two cultivated populations, male flower percentage significantly increased when the soil was of higher nutrient content. This indicates that the allocation to male versus female reproduction might change in response to environmental factors. Pollen production per male flower was 4.1 times higher than that of a hermaphrodite flower. The floral shape and size of male and hermaphrodite flowers were similar. No difference was observed between these dimorphological flowers in pollen germination rate in vitro and in the speed of pollen tube growth in vivo . Anthesis was only 4-5 h. Male flowers usually opened 0.5 h earlier than hermaphrodite flowers. An unexpected finding was that no pollen from the male flowers was found on the stigmas of the hermaphrodite flowers, in spite of the occasional visits by insects to both types of flowers in both wild and control populations. A consistent pattern of fruit development was found to exist in open pollinated flowers as well as in flowers that had been bagged. The sex ratios did not have significant influence on fruit set. Approximately 25% of the pistils in a gynoecium failed to develop into fruits because no pollen was deposited on them, indicating that the fruit set of this andromonoecious plant is mainly affected by pollen limitation rather than resource limitation. Reproduction in S. guyanensis subsp. lappula in the habitats was dependent on self_pollination in hermaphrodite flowers. The male flowers in this species might be a potential source of additional pollination and may facilitate cross_pollination. The fact that the flowers of monoecious species in Sagittaria pollinated by a wide diversity of insect visitors may contribute enormously to the diversification of sex expression in this genus.

An Examination of the Function of Male Flowers in an Andromonoecious Shrub Capparis spinosa

The pollen donor and pollinator attractor hypotheses are explanations for the functions of the male flowers of andromonoecious plants. We tested these two hypotheses in the andromonoecious shrub Capparis spinosa L. （Capparaceae） and confirmed that pollen production and cumulative volume and sugar concentration of nectar do not differ between male and perfect flowers. However, male flowers produced larger anthers, larger pollen grains and smaller ovaries than perfect flowers. Observations on pollinators indicated that two major pollinators （Xylocopa valga Gerst and Proxylocopa sinensis Wu） did not discriminate between flower morphs and that they transferred pollen grains a similar distance. However, there were more seeds per fruit following hand pollination with pollen from male flowers than from perfect flowers. Individuals of C. spinosa with a larger floral display （i.e. bearing more flowers） received more pollen grains on the stigma of perfect flowers. Female reproductive success probably is not limited by pollen. These results indicate that male flowers of C. spinosa save resources for female function and that they primarily serve to attract pollinators as pollen donors.

Pollen and Resource Limitation in Veratrum nigrum L. （Liliaceae）, an Andromonoecious Herb

Pollen limitation and resource limitation were invoked to account for the pattern that flowering plants produce more flowers and ovules than fruits and seeds. This study aimed to determine their relative importance in Veratrum nigrum, a self-compatible, perennial, andromonoecious herb. In order to determine whether female production was limited by pollen grains on stigmas or by available resources, we performed supplemental hand pollination in three populations, male-flower-bud removal in three other populations, and emasculation of hermaphroditic flowers in still another population, resulting in a total of seven populations experimentally manipulated. Across the three populations, supplemental hand pollination did not significantly increase fruit set, seed number per fruit, and total seed production per individual, nor did emasculation of hermaphroditic flowers. Taken together, our results suggest that pollen grains deposited on stigmas were abundant enough to fertilize all the ovules. Male-flower-bud removal significantly increased the mean size of hermaphroditic flowers in all three populations. Female reproductive success was increased in one population, but not in the other two populations possibly due to heavy flower/seed predation. We concluded that the female reproductive success of V. nigrum was not limited by pollen grains but by available resources, which is consistent with Bateman＇s principle. Furthermore, the female reproduction increase of male-flower-bud removal individuals might suggest a trade-off between male and female sexual functions.

Increased Maleness at Flowering Stage and Femaleness at Fruiting Stage with Size in an Andromonoecious Perennial, Veratrum nigrum

Theory predicts that cosexual plants should adjust their resource investment in male and female functions according to their size if female and male fitness are differentially affected by size. However, few empirical studies have been carried out at both the flowering and fruiting stages to adequately address size-dependent sex allocation in cosexual plants. In this paper, we investigated resource investment between female and male reproduction, and their size-dependence in a perennial andromonoecious herb, Veratrum nigrum L. We sampled 192 flowering plants, estimated their standardized phenotypic gender, and assessed the resource investment in male and female functions in terms of absolute dry biomass. At the flowering stage, male investment increased with plant size more rapidly than female investment, and the standardized phenotypic femaleness （ranging from 0.267 to 0.776） was negatively correlated with plant size. By contrast, female biased allocation was found at the fruiting stage, although both flower biomass and fruit biomass were positively correlated with plant size. We propose that increased maleness with plant size at the flowering stage may represent an adaptive strategy for andromonoecious plants, because male flowers promote both male and female fertility by increasing pollinator attraction without aggravating pollen discounting.

Both small and large plants are likely to produce staminate(male)flowers in a hermaphrodite lily

Resource allocation to female and male function may vary among individual plants in species with variable sex expression.Size-dependent sex allocation has been proposed in hermaphrodites,in which female-biased allocation may increase with plant size.In many hermaphrodites with large floral displays,however,later-produced flowers tend to be functionally male.This paradoxical relationship between female and male function and plant size remains poorly understood.The subalpine lily Lilium lankongense has individuals of three sexual types:males with only staminate flowers,hermaphrodites with only perfect flowers,and andromonoecious plants with both perfect and staminate flowers.Here we tested theoretical predictions of size-dependent sex allocation in L.lankongense by measuring plant height and flower number of individuals of each sex at five field sites in the mountainous region of Shangri-La,southwestern China.To investigate variation in phenotypic gender,we identified sex expression of 457 individuals one year later.Our investigation showed that male plants,which usually produced one flower,were significantly smaller than and ro monoecious and hermaphrodite plants.In addition,the total flower numbers of andromonoecious and hermaphrodite plants increased significantly with plant size.Large individuals were more likely to produce terminal staminate flowers,as there were more flowers in andromonoecious than in hermaphrodite individuals.Non-flowered plants were significantly smaller than flowering ones.Perfect flowers had significantly larger petals and pistils than staminate flowers,but they did not differ in dry weight of stamens.Our findings indicate that when plants are small,the less costly sex is favored,consistent with the‘size-advantage hypothesis’.When plants are large,both female and male investments change isometrically,as later-produced flowers tend to be functionally male.

Phenology and Reproductive Biology of <i>Acacia karroo</i>Hayne (Leguminosae: Mimosoideae)

The architectural development of Acacia karroo conforms to Troll’s model. Growth of the branches is modular and sympodial with heteroblastic leaves on all long shoots of the tree, including the seedling. Axillary buds tend to proliferate especially on flowering shoots where they form fascicles consisting of up to 10 inflorescences arranged in two parallel serial rows per leaf axil. Most axillary buds are sylleptic and basal buds which give rise to short shoots, each producing two to five cataphylls each season, but no flowers. Inflorescences are only produced on long shoots (modules) of the current season. After flowering the terminal part of the module aborts, trees are usually andromonoecious with capitate inflorescences containing 40 to 100 flowers each, with some male and some hermaphrodite. Some trees produce only male flowers. Anthesis in the same inflorescence, the same tree as well as amongst trees of the same community are synchronised and occur at intermittent intervals, each lasting three or more days at a time. Flowers are protogynous and pollen is produced in polyads, each consisting of 16 pollen grains. Ovaries contain 10 to 14 ovules each. The concave stigma has space for only one polyad which can fertilise all ovules in the ovary after a single pollination event. Fruit set is low with 0 to 10 fruits (pods) per inflorescence.

Maintenance of andromonoecy in an autogamous species:Superior male function in male flowers of the endangered Sagittaria guayanensis

Andromonoecy is a rare sexual system in plants.The function of additional male flowers in andromonoecious species has been widely discussed;however,few studies have taken offspring fitness into account.In addition,little is known about the mechanisms that maintain andromonoecy in autogamous species.In this study,we compared morphology,pollinator preference,pollen production and export,siring ability,natural siring success,hundred seed dry weight,and seed germination rates between male and hermaphroditic flowers in an endangered autogamous andromonoecious species,Sagittaria guayanensis.Male flowers,which are larger than hermaphroditic flowers,required fewer resources to produce.Pollinators visited male flowers more frequently than they visited hermaphroditic flowers.In addition,pollen production and export were higher in male flowers.Hand pollination demonstrated that siring ability did not differ between flower type.However,the natural siring success of male flowers was triple that of hermaphroditic flowers.The seeds sired by male flowers performed better than those sired by hermaphroditic flowers,with greater dry weight and higher germination rate.In conclusion,male flowers may be superior pollen donors for outcrossing.The maintenance of andromonoecy in S.guayanensis may result from the better performance of male flowers in male function compared to that of hermaphroditic flowers.