The mechanism of delayed selfing in Collinsia verna (Scrophulariaceae)

Collinsia verna, blue-eyed Mary, has floral attributes of an outcrossing species, yet most flowers readily self-pollinate under greenhouse conditions. Here we describe the mechanism of self-pollination in C. verna via changes in relative positions of the stigma and anthers and late timing of receptivity, resulting in delayed selfing. Each flower contains four anthers that dehisce sequentially over ∼1 wk. Pollen that is not collected by pollinators accumulates in the keel petal and retains high viability (>80% pollen germination) up to the time of corolla abscission. The stigmatic surface does not become receptive until after the third anther dehisces. This overlap in the sexual phases is concurrent with a change in herkogamy during floral development. In most flowers (70%), the stigma has moved to the front of the keel and is positioned near the anthers when the third anther dehisces. Under field conditions, fruiting success of plants within pollinator exclosures was ∼75% of the fruiting success in open-pollinated plants (33% fruiting success via autogamy vs. 44% fruiting success, respectively). Collinsia verna plants in pollinator exclosures exhibit variation in autogamy rates within natural populations (range 0–80%). In addition, only half of naturally pollinated, receptive flowers examined had pollen tubes growing in their styles. In contrast, shortly after corolla abscission, nearly all flowers examined (96%) had pollen tubes in their styles. Thus we find that in C. verna, autogamy occurs late in floral development, which has the potential to provide substantial reproductive assurance, and that individuals vary in their ability to set fruit through this mechanism. We suggest that delayed selfing mechanisms may be overlooked in other species and that variable pollinator availability may play a significant role in the maintenance of mixed mating in species with delayed selfing, such as C. verna.     

Wind-dragged corolla enhances self-pollination: a new mechanism of delayed self-pollination

BACKGROUND AND AIMS: Delayed self-pollination is a mechanism that allows animal-pollinated plants to outcross while ensuring seed production in the absence of pollinators. This study aims to explore a new mechanism of delayed self-pollination facilitated by wind-driven corolla abscission in Incarvillea sinensis var. sinensis. METHODS: Floral morphology and development, and the process of delayed self-pollination were surveyed. Experiments dealing with pollinator and wind exclusion, pollination manipulations, and pollinator observations were conducted in the field. KEY RESULTS: Delayed self-pollination occurs when the abscising corolla driven by wind drags the adherent epipetalous stamens, thus leading to contact of anthers with stigma in late anthesis. There is no dichogamy and self-incompatibility in this species. The significantly higher proportion of abscised corolla under natural conditions as compared with that in wind-excluding tents indicates the importance of wind in corolla abscission. When pollinators were excluded, corolla abscission significantly increased the number of pollen grains deposited on the stigma and, as a result, the fruit and seed set. Only half of the flowers in plots were visited by pollinators, and the fruit set of emasculated flowers was significantly lower than that of untreated flowers in open pollination. This species has a sensitive stigma, and its two open stigmatic lobes closed soon after being touched by a pollinator, but always reopened if no or only little pollen was deposited. CONCLUSIONS: This delayed self-pollination, which involved the movement of floral parts, the active participation of the wind and sensitive stigma, is quite different from that reported previously. This mechanism provides reproductive assurance for this species. The sensitive stigma contributes to ensuring seed production and reducing the interference of selfing with outcrossing. The pollination pattern, which combines actions by bees with indirect participation by wind, is also a new addition to ambophily.     

The reproductive biology ofJaborosa integrifolia (Solanaceae): Why its fruits are so rare?

Jaborosa integrifolia exhibits stigma-height polymorphism. There are individuals with flowers where anthers and stigma are at the same height but the rule is variable herkogamy, the most common type (75%) being that with an exerted stigma. Self- and cross-tubes did not differ in their capability to reach the ovary (t = –0.67,P < 0.53); they had a high growth rate (6.95 ± 2.28 mm h^–1). There is not autogamy but mostly self-incompatibility. Fruits from controlled cross-pollination showed the highest seed set and seed viability. The nectar sugar is characterized by a similar amount of glucose and fructose, and by the absence of sucrose. Although nectar secretion was continuous throughout the life of the flower, most nectar was secreted during the first 24 h after flower opening. Nectar production costs appear to be lower than in other species since nectar secretion is neither inhibited after a removal (i.e. a pollinator visit) nor reabsorbed as the flower ages. Sphingids visit the flowers mainly after midnight. They insert their proboscis down to the base of the corolla tube to reach the nectar. The upper limit to fruit production is set by pollinator visits. Fruits produced from open-pollinated flowers are often predated by numerous larvae (mainly lepidopteran ones). Considering that this species is mostly self-incompatible and pollination is limited, that each plant displays only a low number of flowers throughout the flowering season, and that there is a high rate of fruit predation, it is not surprising that fruits ofJ. integrifolia are so rare.     

FLORAL PARAMETERS,BREEDING SYSTEM AND POLLINATOR TYPE IN TRICHOSTEMA (LABIATAE)

Diversity in flower size and degree of exsertion of anthers and stigma from the corolla in the California species of Trichostema appear to be correlated with breeding system and pollinator type (bee vs. bird). Autogamous (self-pollinating) species unlike xenogamous (cross-pollinating) species lack spatially separate anthers and stigma and nototribic pollination. The outcrossing species have significantly larger flowers, significantly larger nectar volumes and significantly higher pollen-ovule ratios than do selfing species. Because autogamous species are less reliant on flower visitors to facilitate pollination, there may be relaxed selection for large nectar producing flowers. Pollen-ovule ratios are correlated with breeding system and reflect the efficiency of pollen transport. Data on floral parameters suggest xenogamous species expend more energy per floral unit and are less efficient seed producers than closely related autogamous species.     

How Does <Emphasis Type="Italic">Mimulus verbenaceus</Emphasis> (Phrymaceae) Set Seed in the Absence of Pollinators?

Mimulus verbenaceus has successfully invaded and/or competed for the very specialized habitats of desert seeps and springs characterized by scarce or no pollinators (personal observation). It has done so by evolving an ingenious mechanism of reproductive assurance in which the senescing epipetalous corolla bends down, abscisses and slides down its style dragging its anthers over the stigma lobes, resulting in self-pollination. This dragging mechanism both depends on the plants ability to self-pollinate and presumably promotes the evolution of self-pollination by providing an advantage—invasion of new habitats—to counter-balance the disadvantages of selfing. M. verbenaceus has evolved a second means of reproductive assurance that depends on the nearness of the anthers to the stigma lobes. They are close, even touching, in the red flowers, leading to much self-pollination and are well separated in the yellow flowers leading to little self-pollination. This advantage of the red-flowered plants likely explains the relative abundance of the red morph and the rarity of the yellow morph in nature as well as the greater seed set of the red-flowered morph than that of the yellow-flowered morph.     

Notes on the floral morphology in Vivianiaceae (Geraniales)

The functional floral morphology of the three genera of Vivianiaceae (= Ledocarpaceae, Geraniales), Rhynchotheca, Viviania and Balbisia, is compared. Likely pollination mechanisms are inferred from morphology and field observations. The flowers of Viviania are nectariferous and apparently zoophilous with nectar as the (primary) pollinator reward. Balbisia has pollen flowers without nectaries, its showy corolla indicates that it is also zoophilous with pollen as sole pollinator reward; bees were observed as flower visitors. One taxon (B. gracilis) may be anemophilous. Rhynchotheca has flowers without petals, with large, pendulous anthers and lacks nectaries. It shows synchronous mass flowering in its natural populations and is evidently anemophilous. A comparison with other Geraniales shows that nectar flowers with small anthers are likely the ancestral condition in Vivianiaceae. This suggests that the pollen flowers with larger anthers of Balbisia and Rhynchotheca may represent an apomorphic condition. The documentation of pollen flowers and anemophily in Vivianiaceae expands the range of known floral and pollination syndromes in Geraniales.     

Delayed autonomous self-pollination in the colonizer Crotalaria micans (Fabaceae: Papilionoideae): structural and functional aspects

We investigated the reproductive biology of Crotalaria micans, a colonizing species that occurs in disturbed sites. The flowers have two whorls of stamens with differential growth and dimorphic anthers. The inner anthers serve two functions: 1) to cooperate with the stylar brush in the pollen presentation, and 2) to push the remaining pollen within the keel onto the stigma at the end of the receptivity period. Flowers pass through three phases: a) male phase, when outer anthers release their pollen, but stigmas are not receptive; b) female phase, when stigmas are receptive, but still separated from flowers' own pollen, and c) an autogamous phase, when the round anthers grow towards the stigma, leading to delayed autonomous self-pollination. Because C. micans is completely self-compatible, delayed self-pollination and geitonogamy result in approximately 76% of seeds being self-fertilized.     

FLORAL DISPLAY IN PHYLA INCISA: CONSEQUENCES FOR MALE AND FEMALE REPRODUCTIVE SUCCESS

The inflorescences of Phyla incisa consist of flowers in two phases: younger, nectar-containing flowers that have yellow corolla throats and older, nectar-lacking flowers that have dark purple corolla throats. Observations of pollinator visitation patterns to both natural and manipulated inflorescences were made to determine the role of each flower phase in pollinator attraction. The effect of older-phase flowers on male and female reproductive success was determined by comparing stigmatic pollen loads and estimates of pollen removal from inflorescences having different numbers of these flowers. The pollinators of Phyla selected larger inflorescences more often than expected based upon the size distribution of inflorescences available to them. Both younger- and older-phase flowers contributed to the attraction of pollinators, but the latter were less effective in this function. The presence of older-phase flowers significantly increased the visitation rate to inflorescences and the amount of pollen removed but had little effect on pollen deposition on stigmas. The lack of correspondence between pollen deposition and pollinator-visitation rate was not due to stigma saturation, since stigma loads varied greatly. The data indicate that the deposition of pollen on stigmas in this species is a relatively stochastic process, whereas pollen removal from inflorescences occurs at a much more regular rate. Old-phase flower retention appears to contribute to reproductive success through increased pollen donation when pollinator activity is high and may also increase the probability of seed set when pollinators are rare.     

MULTIPLE PATERNITY AND SELF-FERTILIZATION IN RELATION TO FLORAL AGE IN MIMULUS GUTTATUS (SCROPHULARIACEAE)

To gain some understanding of the mechanisms responsible for the intermediate levels of both multiple paternity and selfing observed previously in the common monkeyflower, Mimulus guttatus, we performed a field experiment to assess the role of 1) multiple paternity through sequential pollinator visits and 2) selfing through corolla abscission. In M. guttatus, flowers remain open for several days; then, in the process of corolla abscission anthers drag past the stigma. We predicted that multiple visitation by pollinators over this period should increase the degree of multiple paternity of sibling arrays, and that corolla dragging may be the primary cause of the observed selfing. In three northern California populations corollas were removed after being open for 1 day (preventing further mating), and the type of matings resulting was compared to matings from adjacent flowers with undisturbed corollas. On average, seed set was twice as great in flowers with unmanipulated corollas, indicating that about half of all matings in the lifespan of a M. guttatus flower occur after 1 day. Pollen supplementation revealed that pollen was limiting in one population. Electrophoretic assay of progeny showed multiple paternity was greater in unmanipulated flowers in the two populations that were not pollen limited, suggesting that sequential pollination may be an important source of multiple paternity. However, no difference was detected in the selfing rate between the two treatments, suggesting that corolla dragging was not a source of selfing even under pollen-limited conditions.     

Movement herkogamy in Kosteletzkya virginica: Effect on reproductive success and contribution to pollen receipt and reproductive assurance

Herkogamy may act as a mechanism to reduce interference between the reproductive functions of female and male organs, but too great a distance between stigmas and anthers may also decrease the outcross pollen transfer and male and female fitness. Our goal was to quantify the effect of movement herkogamy in Kosteletzkya virginica on reproductive success and its contribution to pollen receipt and reproductive assurance. Field investigations and hand-pollination experiments were performed to test: (1) the relationship between herkogamy before curvature of un-pollinated lobes and reproductive success under pollinator exclusion; (2) the contribution of movement herkogamy to pollen receipt; and (3) the relationship between pollen load and seed set. We noted that variation in herkogamy before curvature of un-pollinated lobes did not affect the outcome of autogamous self-pollination measured by fruit set and mean number of seeds per fruit. Under natural conditions, 39% of pollen grains deposited per flower were from autogamous self-pollination via movement herkogamy resulting in contact between anthers and stigmas. The relationship between the amount of pollen grains and the number of seeds per capsule was highly significant. Flowers producing four or five seeds received more pollen grains than those producing only one seed. Our data provides experimental evidence that movement herkogamy, which decreases the separation between anthers and stigmas and results in delayed self-pollination, contributes to pollen receipt and further enhances seed set when pollinators are scarce or absent.     

橙花瑞香的繁殖特性研究

瑞香属植物具有重要的药用和观赏价值,在中国资源丰富,但自然条件下低坐果率限制了该属植物的进一步开发和利用。该研究以橙花瑞香为对象,通过对其有性繁殖及传粉特性的研究,探索其自然坐果率低的原因,内容包括花部特征的测量分析,MTT染色法测定花粉活性,联苯胺-过氧化氢法测定柱头可授性,扫描电镜观察柱头、花粉的形态,传粉者观察,通过花粉胚珠比(P/O)和人工授粉实验推测橙花瑞香的繁育系统类型。结果表明:橙花瑞香的花部结构特殊,管状小花,花药两轮,雌雄蕊分离。花开后的花粉具有活性,柱头具有可授性,扫描电镜下,柱头和花粉的结构没有发育异常,且柱头上有花粉落置。橙花瑞香的传粉者主要是夜间访花的蛾类,访花频率低。P/O及人工授粉实验表明橙花瑞香的繁育系统为专性异交。橙花瑞香的坐果率非常低,自然坐果率为1.4%,人工异花授粉为23.3%,低坐果率可能是受其开花量大、异花花粉限制、资源限制以及花部结构等因素的影响。     

珍稀濒危植物陕西羽叶报春的开花及传粉生物学研究

陕西羽叶报春为中国珍稀濒危植物,研究其开花物候、传粉者及访花行为、花粉活力、柱头可授性、繁育系统,以明确陕西羽叶报春的传粉特性及制濒机制。以勉县陕西羽叶报春野生居群为研究对象,通过野外观察和人工授粉实验首次对陕西羽叶报春的传粉生物学进行研究。结果表明:(1)陕西羽叶报春种群花期为2-4月中旬,单花花期一般为14~17 d;单花开花进程中花部形态有6个明显发育变化:全部被萼片包裹、花苞伸出并为深粉色、花朵打开为淡粉色、花冠为白色、花朵枯萎、花冠脱落花萼宿存。(2)陕西羽叶报春为异花授粉植物,蜜蜂属为其主要传粉者。(3)柱头在花粉未开裂前就具有可授性,可授性逐渐增强;花药在花瓣打开后的第3天才开始开裂、散粉,在第4天活力最高,为98.18%,此后逐渐减弱。(4)繁育系统检测显示,长、短柱型花的花粉/胚珠比分别为902.26和831.48,杂交指数为4;套袋实验显示,陕西羽叶报春异型交配可育,同型交配不可育。由此表明,陕西羽叶报春的繁育系统为异交型,自交不亲和,需要传粉者。综上所述,陕西羽叶报春传粉者种类少、自交不亲和、胚珠败育现象可能是导致陕西羽叶报春濒危的重要原因。     

Lack of Control by Early Pistillate Ethylene of the Accelerated Wilting of Petunia hybrida Flowers

Well before pollen tube penetration, ethylene has begun to disseminate from pollinated styles of Petunia hybrida flowers. Previous stigmatic application of aminoethoxyvinylglycine (AVG) completely prevented this ethylene synthesis, indicating that the endogenous 1-aminocyclopropane-1-carboxylic acid (ACC) in pollen is not readily converted on the stigma. Compared to other flower parts, the capacity of the ethylene forming enzyme was largest in the stigma. When applied to the stigma, ACC caused ethylene synthesis, but did not accelerate wilting, unless high concentrations (20 nanomols) were used. Upon pollination or stigma wounding, the early ethylene evolved exclusively from the gynoecium, much later followed by the synthesis of corolla ethylene. Employing wideneck Erlenmeyer flasks, the competitive inhibitor of ethylene action, norbornadiene, was applied to entire flowers in situ, with delaying effects on wound-induced wilting. In contrast, norbornadiene treatment of styles alone, using capillaries, could not postpone wilting. Pollination with foreign pollen species did not lead to accelerated corolla wilting, notwithstanding considerable synthesis of ethylene during the first 5 hours. In situ treatment of the stigma with AVG considerably delayed wound- and pollination-induced wilting. Removal of the entire AVG-treated style 6 hours after stigma wounding still allowed for the postponement of the accelerated wilting, even at very low concentrations of AVG. It is concluded that early stylar ethylene does not play a role in the acceleration of wilting but that, much later, corolla ethylene does, induced by a mobile wilting factor from the stigma, which is ACC.     

Evaluation of pollinator effectiveness based on pollen deposition and seed production in a gynodieocious alpine plant,Cyananthus delavayi

Examining variations in pollinator effectiveness can enhance our understanding of how pollinators and plants interact. Pollen deposition and seed production after a single visit by a pollinator are often used to estimate pollinator effectiveness. However, seed production is not always directly related to pollen deposition because not all pollen grains that are deposited on a stigma are compatible or conspecific. In the field, we tested pollinator effectiveness based on pollen deposition and the resulting seed production after single visits by different pollinator groups in a gynodieocious alpine plant Cyananthus delavayi (Campanulaceae). Our results showed that mean pollen deposition was generally inconsistent with mean seed production when comparisons were performed among different pollinator groups and sexes. In general, the correlations were not significant between pollen deposition and seed production in both perfect and female flowers after single visits by halictid bees, bumble bees, and hoverflies. We suggest seed set of virgin flowers after single visits is a more reliable indicator of pollinator effectiveness than pollen deposition and would be a better indicator of pollinator effectiveness for future studies.     

Does the morphological fit between flowers and pollinators affect pollen deposition? An experimental test in a buzz‐pollinated species with anther dimorphism

Some pollination systems, such as buzz‐pollination, are associated with floral morphologies that require a close physical interaction between floral sexual organs and insect visitors. In these systems, a pollinator's size relative to the flower may be an important feature determining whether the visitor touches both male and female sexual organs and thus transfers pollen between plants efficiently. To date, few studies have addressed whether in fact the “fit” between flower and pollinator influences pollen transfer, particularly among buzz‐pollinated species. Here we use Solanum rostratum, a buzz‐pollinated plant with dimorphic anthers and mirror‐image flowers, to investigate whether the morphological fit between the pollinator's body and floral morphology influences pollen deposition. We hypothesized that when the size of the pollinator matches the separation between the sexual organs in a flower, more pollen should be transferred to the stigma than when the visitor is either too small or too big relative to the flower. To test this hypothesis, we exposed flowers of S. rostratum with varying levels of separation between sexual organs, to bumblebees (Bombus terrestris) of different sizes. We recorded the number of visits received, pollen deposition, and fruit and seed production. We found higher pollen deposition when bees were the same size or bigger than the separation between anther and stigma within a flower. We found a similar, but not statistically significant pattern for fruit set. In contrast, seed set was more likely to occur when the size of the flower exceeded the size of the bee, suggesting that other postpollination processes may be important in translating pollen receipt to seed set. Our results suggest that the fit between flower and pollinator significantly influences pollen deposition in this buzz‐pollinated species. We speculate that in buzz‐pollinated species where floral morphology and pollinators interact closely, variation in the visitor's size may determine whether it acts mainly as a pollinator or as a pollen thief (i.e., removing pollen rewards but contributing little to pollen deposition and fertilization).     

The effect of wild radish floral morphology on pollination efficiency by four taxa of pollinators

The effects of floral morphology on rates of pollen removal and deposition by different pollinators in generalist plant species are not well known. We studied pollination dynamics in wild radish, Raphanus raphanistrum, a plant visited by four groups of pollinators: honey bees, small native bees, butterflies, and syrphyd flies. The effects of anther position and other factors on pollen removal during single visits by all four pollinator taxa were measured. Flowers with high anther exsertion (i.e., anthers placed higher above the opening of the corolla tube) tended to have the highest numbers of pollen grains removed, but this effect was strongest for honey bees and butterflies. For all pollinator taxa, pollen removal increased with the number of pollen grains available on a flower and whowed a positive, decelerating relationship with the duration of the visit. The effects of stigma position and other factors on pollen deposition during single visits by honey bees and butterflies were also studied. The nectar-feeding butterflies had a higher pollination efficiency (percentage of pollen grains removed from anthers that were subsequently deposited on a stigma) than the nectar- and pollen-feeding honey bees. Flowers with intermediate stigma exsertion had the highest numbers of pollen grains deposited on their stigmas by butterflies, but stigma exsertion had no effect on deposition by honey bees. For both butterflies and honey bees, pollen deposition on the recipient flower increased with the amount of pollen removed from the donor flower, and there was a positive, decelerating relationship between deposition and time spent at the flower; these results are analogous to those for pollen removal. The effects of anther and stigma exsertion on pollen removal and denosition did not fit predictions based on patterns of floral correlations, but results for morphology, pollen availability, time spent per visit, and pollinator efficiency are in broad agreement with previous studies, suggesting the possible emergence of some general rules of pollen transfer.     

Floral ecology of Oreocharis pumila (Gesneriaceae): a novel case of sigmoid corolla

Attraction of pollinators and successful pollen transfer represent the primary targets of selection during flower evolution, leading to repeated evolutionary shifts between pollinators and consequently to the diversification of floral forms. However, most studies in floral evolution focus on the characteristics of flowers with straight corolla tube. Here, we report on an unusual form of sigmoid corolla combined with protandry and herkogamy in a Chinese species of Gesneriaceae, Oreocharis pumila (formerly Opithandra pumila). Contrary to species with sigmoid corollas studied previously, the base of the corolla tube of this species is inclined at an oblique angle downwards before the tube bends forward, and the stigma and anthers are included in the upper part of the corolla tube. The plants were found to be self‐compatible but incapable of autonomous selfing. Successful pollination was found to depend fully on the presence of insect pollinators (Nomia sp.) and pollen grains are the greatest reward for the visitors. Different from the other sigmoid flowers, the sigmoid corolla of O. pumila was not found to favor insect pollinators with long flexible proboscises. A mechanical fit between floral morphology and pollinator was found, in which only small insect visitors with specialized visiting behavior are legitimate pollinators. The protandry combined with herkogamy in the sigmoid corolla tube strongly ensures pollination efficiencies. Oreocharis pumila is the only species with sigmoid corolla in the genus Oreocharis. We hypothesize that such a corolla has arisen through selection due to inadequate pollination in early spring in the mountainous habitat that O. pumila occupies.     

Reproductive Ecology of Ocimum americanum L. and O. basilicum L. (Lamiaceae) in India

Abstract Ocimum americanum and O. basilicum bloom once a year. They produce flowers over a long period, a few flowers being produced each day. The flowers of both are short-lived (3-4 hours), bisexual, zygomorphic, and chasmogamic with anthers dehiscing in the bud stage. The open flowers offer nectar and pollen as rewards and are visited and sternotribically pollinated by day-flying animals. The blossoms are flag-shaped, and the reproductive organs are close to the lower corolla lip. The stamens and stigma show movements immediately after anthesis and remain for 20–30 minutes and this may cause self-pollination. Both species reproduce primarily through autogamy. Flowers are open during 5:30–13:30 hr in O. americanum and during 7:00–13:00 hr in O. basilicum . Certain bee species like Apis florea, A. cerana indica, Amegilla sp., and Pseudapis oxybeloies , and the butterfly Surandra queretroum are the most frequent and consistent visitors and can be pollinators for both plant species.     

POLLEN TRANSFER BY NATURAL HYBRIDS AND PARENTAL SPECIES IN AN IPOMOPSIS HYBRID ZONE

Models of hybrid zones differ in their assumptions about the relative fitnesses of hybrids and the parental species. These fitness relationships determine the form of selection across the hybrid zone and, along with gene flow, the evolutionary dynamics and eventual outcome of natural hybridization. We measured a component of fitness, export and receipt of pollen in single pollinator visits, for hybrids between the herbaceous plants Ipomopsis aggregata and I. tenuituba and for both parental species. In aviary experiments with captive hummingbirds, hybrid flowers outperformed flowers of both parental species by receiving more pollen on the stigma. Although hummingbirds were more effective at removing pollen from anthers of I. aggregata, hybrid flowers matched both parental species in the amount of pollen exported to stigmas of other flowers. These patterns of pollen transfer led to phenotypic stabilizing selection, during that stage of the life cycle, for a stigma position intermediate between that of the two species and to directional selection for exserted anthers. Pollen transfer between the species was high, with flowers of I. aggregata exporting pollen equally successfully to conspecific and I. tenuituba flowers. Although this study showed that natural hybrids enjoy the highest quality of pollinator visits, a previous study found that I. aggregata receives the highest quantity of pollinator visits. Thus, the relative fitness of hybrids changes over the life cycle. By combining the results of both studies, pollinator-mediated selection in this hybrid zone is predicted to be strong and directional, with hybrid fitness intermediate between that of the parental species.     

Critical studies on the sexuality of southern Argentinian Ericaceae

Summary The family Ericaceae is considered to be mostly hermaphroditic and only in a few cases unisexual or dioecious. Five Argentinian species (Gaultheria antarctica Hook, f., G. caespitosa P. et E., G. phillyreaefolia (Pers.) Sleumer, G. tenuifolia (Phil.) Sleumer and Pernettya insana (Molina) Gunckel), previously described as hermaphroditic, were found by us to have two types of flowers: female and male. Female flowers have both a rudimentary androecium with stamens, in which the anthers are small, collapsed and without pollen grains or are reduced to filaments only, and a perfect gynoecium with well-developed ovules filling the carpel locules. The stigma is expanded and of the wet, papillate type, with copious to slight secretion. The stigma protrudes above the anthers. Male flowers have a well-developed androecium included within the corolla and a reduced gynoecium. Anthers are as long as the filaments, except in G. caespitosa. Pollen is shed as tetrads which are 100% viable. Ovule development ranges from total absence, passing through aborted ovules of various sizes to apparently normal ones. Stigmata are neither expanded nor papillate, except in Pernettya insana. Only in two species do the style and stigma extend beyond the level of the anthers. Heterostyly does not occur in any of the five species studied. Functional dioecy thus characterizes the five species considered and is reported for the first time in the genus Gaultheria.