Potential Autonomous Selfing in Gesneria citrina (Gesneriaceae), a Specialized Hummingbird Pollinated Species with Variable Expression of Herkogamy

Species with mixed mating systems often demonstrate variable expression of breeding system characteristics and thus represent the opportunity to understand the factors and mechanisms that promote both outcrossed and selfed seed production. Here, we investigate variation in levels of herkogamy (variation in stigma-anther separation distance) in a Puerto Rican population of hummingbird pollinated Gesneria citrina Urban. There is significant variation in herkogamy levels among individuals of this species and stigma-anther separation is negatively associated with the ability to set fruits and seeds in the absence of pollinators. The variation in levels of herkogamy may represent a mechanism to ensure the production of some self-fertilized progeny in the absence of hummingbird pollinators. We also describe a novel breeding system in G. citrina , where stamens elongate over time to reach stigma height, but stamen elongation is accelerated by pollination. These results suggest that once the flowers are pollinated, stamen elongation may favor increased pollen removal and siring success, while the reduction in stigma-anther distance no longer imposes the risk of interference between male and female functions. We discuss our findings of breeding system variation in the context of pollination system evolution in an island setting (Antillean islands).     

A Nectar Reward: Is More Better?1

Characters involved in pollinator attraction are likely maintained by selection. Plants that invest more in floral displays and/or rewards are expected to attract more pollinators than those that do not. A large number of plants, however, are severely pollen-limited yet either produce small rewards or none at all. The orchid, Comparettia falcata, is a pollinator-dependent, self-compatible epiphyte distributed throughout the Greater Antilles, Central and South America. In Puerto Rico where it is pollinated by the hummingbird Chlorostilbon maugaeus, C. falcata presents a smaller nectar reward than most other plants pollinated by the same species. To determine whether or not selection would favor the production of higher nectar levels, we enhanced the quantity of nectar offered by flowers in a Puerto Rican population for two flowering seasons. We monitored visitation frequencies, pollen movement, and reproductive success at three sites with different canopy coverages. Daily censuses of hummingbirds provided estimates of relative pollinator abundance. A multiway contingency test employing Wal?s statistic showed no overall differences in reproductive success between plants with enhanced rewards and unmanipulated controls. Site differences, however, were clear. Plants of the mid- and high-light sites had greater success than those of the low-light site, and the differences were usually at least two-fold. There was a significant site-treatment interaction in reproductive success that could be attributed to the overall trend whereby controls of both mid- and high-light sites did better than the nectar-enhanced plants. Most of the observed pollinations (85%) with stained pollinia resulted in self-pollinations that did not differ among treatments. Seed crops from self- and cross-pollinations revealed no differences in the number of viable seeds. Because we found little evidence of selection for increasing nectar reward via inbreeding depression or male and female reproductive success, and previous studies have indicated that meager natural levels of reward are better than none at all, we suspect that reward production in C. falcata may be driven by a combination of pollination-limitation and resource constraints.     

Potential Autonomous Selfing in Gesneria citrina （Gesneriaceae）, a Specialized Hummingbird Pollinated Species with Variable Expression of Herkogamy

Species with mixed mating systems often demonstrate variable expression of breeding system characteristics and thus represent the opportunity to understand the factors and mechanisms that promote both outcrossed and selfed seed production. Here, we investigate variation in levels of herkogamy （variation in stigma-anther separation distance） in a Puerto Rican population of hummingbird pollinated Gesneria citrina Urban. There is significant variation in herkogamy levels among individuals of this species and stigma-anther separation is negatively associated with the ability to set fruits and seeds in the absence of pollinators. The variation in levels of herkogamy may represent a mechanism to ensure the production of some self-fertilized progeny in the absence of hummingbird pollinators. We also describe a novel breeding system in G. citrina, where stamens elongate over time to reach stigma height, but stamen elongation is accelerated by pollination. These results suggest that once the flowers are pollinated, stamen elongation may favor increased pollen removal and siring success, while the reduction in stigma-anther distance no longer imposes the risk of interference between male and female functions. We discuss our findings of breeding system variation in the context of pollination system evolution in an island setting （Antillean islands）.     

Is Echeveria gibbiflora (Crassulaceae) fecundity limited by pollen availability? An experimental study

1. An experimental approach, manually pollinating all the flowers of individual plants, was used to measure the effect of pollen limitation on female fecundity of the hummingbird-pollinated perennial shrub Echeveria gibbiflora in the ecological preserve of Pedregal de San Angel around México City, México. Eleven randomly selected plants were manually over-pollinated in all their flowers and another 11 were left to be freely visited by natural pollinators.
2. Manually pollinated plants produced significantly more fruit and seeds than control plants (1·38 and 1·74 times, respectively). There was no change in average mass of fruits.
3. Considering individual fruit production per plant sampled three times in one season, decreases in fruit mass and average seeds per fruit were observed within the same reproductive season for both treatments. For the manually pollinated plants, from the start to the end of the reproductive season, seed set decreased 55·9%; while in control plants seed set decreased 33·4% in the same period. For both treatments, average fruit mass decreased 26%.
4. Vegetative growth was not significantly different between control and experimental plants but hand-pollinated plants showed a smaller reproduction probability for the following year.
5. It is concluded that female fecundity in E. gibbiflora is limited by pollen early in the reproductive season and by resources in the middle and the end of the season.     

Crane flies and microlepidoptera also function as pollinators in Epidendrum (Orchidaceae: Laeliinae): the reproductive biology of E. avicula

Crane flies and microlepidoptera have been recorded as pollinators in unrelated orchid groups, but these insects have never been recorded in Epidendroideae, the most species‐rich orchid subfamily, which includes one of the most diverse genera among Orchidaceae, Epidendrum. Based on data on phenology, floral morpho‐anatomy, pollinators, pollination mechanisms and breeding system, the reproductive biology of E. avicula was studied in south‐eastern Brazil. Epidendrum avicula possess osmophores that produce a citric fragrance at night. The flowers attract Tipulidae flies and several families of microlepidoptera that drink the nectar produced in a tube formed by the adnation of the labellum and column. As is common in Epidendrum, after removing the pollinarium, both crane flies and micro‐moths get trapped by the proboscis, which frightens the insects and inhibits any possible intent to immediately visit another flower. The behavior of the pollinators on flowers, plus the retention of the anther cap by the pollinarium, results in a reduction in the occurrence of geitonogamy. Because E. avicula is self‐incompatible, the consequence of pollinator behavior and the floral mechanisms tend to reduce the pollen loss. As far as we know, this is the first study to report the reproductive biology of a species of Epidendroideae pollinated by crane flies and microlepidoptera. Based on more recent concepts of plant–pollinator interactions, although E. avicula is pollinated by several species belonging to two distinct orders, suggesting an unspecialized pollination system is involved, nectar‐seeking microlepidoptera and Tipulidae flies can be recognized as a single functional group.     

Floral morph composition and pollen limitation in the seed set of <Emphasis Type="Italic">Nymphoides indica</Emphasis> populations

We investigated the floral morph composition and seed set of the populations in a heterostylous aquatic plant, Nymphoides indica O. Kuntze (Menyanthaceae). The bias of floral morph ratio in the populations was negatively correlated with the seed set. In the populations with biased floral morph ratio, the pollen on the stigma of the common morphs were dominated by pollen of the same morph, resulting in reduced seed set. The addition of pollen of the opposite morph on the stigmas of the common morph resulted in an increase of fruit and seed set rates. The intermorph pollination rate and the fruit and seed set rates of Nymphoides indica were highest when two floral morphs were close in the populations. Various insect species including Lepidoptera, Diptera and Hymenoptera visited the flowers. However, geitonogamous pollination was common because of the short flying distance of the pollinators. These results indicate that the seed set of Nymphoides indica is limited by the shortage of compatible pollens and that the success of intermorph pollination is influenced by spatial distribution of the two floral morphs. As Nymphoides indica is endangered, conservation of the populations with high fecundity is proposed by us.     

Pollination Ecology and Effect of Nectar Removal in Macleania bullata (Ericaceae)1

The floral syndrome of Macleania bullataYeo (Ericaceae) reflects its adaptation to hummingbird pollination. Its flowers, however, are subject to high levels of nectar robbing. I examined the floral visitor assemblage of M. bullata in a tropical montane wet forest in southwestern Colombia, focusing on the behavior of the visitors. I also tested for the presence of nocturnal pollination and the effects of nectar removal on new nectar production. The principal floral visitors were the nectar robbing hummingbirds Ocreatus underwoodii (19.1% of visits) and Chlorostilbon mellisugus (18.9%). Only two species of long–billed hummingbirds visited the flowers of M. bullata as “legitimate” pollinators: Coeligena torquata (14.7% of visits) and Doryfera ludoviciae (14.3%). The remaining visits constituted nectar robbing by bees, butterflies, and other species of hummingbirds. Nocturnal pollination took place, although fruit set levels were 2.4 times higher when only diurnal pollination was allowed as opposed to exclusively nocturnal pollination. Nectar robbers removed floral nectar without pollinating the flower. Treatments of experimental nectar removal were carried out to examine if flowers synthesize more nectar after nectar removal. Nectar removal increased the total volume of nectar produced by each flower without affecting sugar concentration. Thus, nectar robbing can impose a high cost to the plants by forcing them to replace lost nectar.     

Pollination limitation,costs of capsule production and the capsule-to-flower ratio in Dendrobium monophyllum F. Muell. (Orchidaceae)

Abstract In perennial plants, life-history theory suggests that natural selection should result in the optimization of fruit-to-flower ratios within the limits imposed by the trade-offs between resource allocation for present reproduction and future growth and reproduction. The tropical orchid Dendrobium monophyllum F. Muell., an epiphyte or lithophyte, offers no nectar rewards, is self-incompatible and has a capsule-to-flower ratio of about 1:14. The influence of pollination limitation and the costs of capsule production on capsule-to-flower ratios were assessed using experimental and field studies in which individual plants were observed for 3 years. Pollinators visited about 80% of flowers, and capsule production was significantly related to inflorescence size and pollinaria removal. About nine pollinator visits occurred per capsule. Pollinator visitation and capsule production did not vary significantly between years. The inflorescence size classes most successful in capsule production were also the most frequent in natural populations. The experimental supplementation of outcross pollen to flowers increased capsule set over controls by 45% within a year, but was limited to about 53%. A capsule-to-flower ratio of 1:2 in experimental plants significantly decreased the subsequent growth and flowering of individuals relative to controls. A capsule-to-flower ratio above 1:10 in naturally pollinated plants decreased flowering in the subsequent year. Thus, it is suggested than an increase in capsule production above 10% would not necessarily correlate with greater reproductive fitness because of the increased cost of capsule production. The capsule-to-flower ratio recorded in this study could be evolutionarily stable because of trade-offs between selection for pollinator attraction and the cost of capsule production. The production of surplus flowers appears to function in pollinator attraction and increases fitness through male function.     

马先蒿属植物花冠分化与繁殖适应的研究进展

结合已有的研究报道和作者近年来的工作,对马先蒿属（Pedicularis）植物的花冠多样化成因与繁殖适应特性进行了总结和探讨。通过对该属4种进化花冠型的花器官发生和分化的研究发现,花部各器官在发生和发育初期基本一致,后期上唇形态的分化是导致成熟花形态结构产生较大差异的重要阶段。孢粉学研究认为,花冠类型与花粉萌发孔类型之间具有显著相关性;萌发沟的演化可能与繁殖适应有一定的关系。分子系统学研究表明,多样化的花冠类型在不同的谱系内经过若干次的独立进化而表现出了高度的平行演化（parallelism）。传粉生物学研究证实,该属植物花冠多样化与其主要传粉者熊蜂属（Bombus）昆虫的传粉行为存在较为密切的关系。具有相同（似）花冠类型的马先蒿可能被同种或不同种的熊蜂以相同的方式访问,但在花粉落置位置上存在显著差异,这可能有助于同域分布重叠的物种间在生殖上的机械隔离,而花冠的分化在一定程度上促进了新的物种形成。     

Are Nectar Sugar Composition and Corolla Tube Length Related to the Diversity of Insects that Visit Asteraceae Flowers?

Abstract: In this work, we analysed interspecific variation in nectar sugar composition, corolla tube length, and the diversity of floral visitors of 35 Asteraceae species. The potential correlations between these variables could arise either as a result of selection to improve pollinator attractiveness or simply as a consequence of phylogenetic constraints. Samples of nectar and flowers, and data on floral visitors, were obtained from living plants in natural populations from Argentina. Asteraceae species showed a large variability in corolla tube length. Nectar of most species presented a larger proportion of hexoses than sucrose. All species were visited by numerous insects belonging to 2 different orders. Results showed that floral traits are not significantly correlated with the diversity of floral visitors. These characters seem to be linked to the phylogeny of the species. Early branching species (species phylogenetically close to the root of the Asteraceae tree) tend to have longer corollas, higher sucrose proportions and lesser diversity of floral visitors than late branching species. Considering that longer corolla tubes and higher nectar sucrose percentages may indicate some specialization in the pollination system, we suggest that there is an evolutionary tendency toward generalist pollination systems within the family.     

Reproductive Biology of the Distylous Manettia luteo-rubra (Rubiaceae)

The reproductive biology of Manettia luteo-rubra was studied in the coastal montane Atlantic rain forest of southeastern Brazil. This Rubiaceae is a perennial vine that flowers all the year round, but has a flowering peak during the dry season. It presents reciprocal herkogamy, thus the plants are morphologically distylous. The morphs occur in a 1:1 ratio, and pollen diameter and corolla length vary between short and long-styled flowers. Manettia luteo-rubra displays typical heterostylous self-incompatibility and sets almost no fruits from self- or intramorph pollinations. The flowers are tubular, red with yellow lobes, and odourless. Anthesis is asynchronous, and the flowers last about four days. Concentration of sugars in nectar is similar in both morphs, ca. 24%, this concentration being typical for hummingbird flowers. Three species of hummingbirds are the major pollinators of the flowers of M. luteo-rubra at the study site: the hermits Phaethornis eurynome and P. squalidus, and the trochiline Thalurania glaucopis. Three species of Heliconius butterflies act as minor pollinators. Both morphs of M. luteo-rubra exhibit natural fruit-set of about 80%, this reproductive output being maintained throughout the year by the pollinators' constancy to the flowers.