Pollen Dispersal Distance Research Articles

Pollen Germinability of Asclepias exaltata: Effects of Flower Age, Drying Time, and Pollen Source

Using in vitro pollen germination experiments, we quantified the effects of flower age, drying time, and pollen source on germinability of pollinia of Asclepias exaltata. Germinability varied greatly among plants; some plants produced pollinia that germinated with twice the average success of other pollen sources. Flower age had comparatively little effect on pollen germinability, although germination rates for 3and 4-day-old flowers were slightly less than for 0-, 1-, and 2day-old flowers. Pollinia that were removed from flowers and allowed to dry lost viability relatively rapidly. After 24 hours, their germinability was reduced by more than 50%. Thus, even though the durable covering of milkweed pollinia may reduce the rate of desiccation and loss of germinability, pollen dispersal distances should still be largely restricted to distances that pollinators travel in a 24-hour period. The Asclepiadaceae and Orchidaceae are unique among dicots and monocots, respectively, in that transfer of pollen between plants involves discrete packets (pollinia). In milkweeds, this packaging of pollen appears to increase the efficiency of pollination (Wyatt 1976). Morse (1982) suggested that the durable covering of milkweed pollinia might allow a long residence time on pollinators, which would promote long-distance dispersal of pollen. This assumes that pollinia are highly resistant to desiccation and are slow to lose viability following their removal from flowers. To test this assumption, we performed in vitro pollen germination experiments. These experiments quantified the effects of flower age, drying time, and pollen source on germinability. The species selected for these tests was Asclepias exaltata L., a perennial herb commonly called poke milkweed. It occurs in moist forests and meadows in the eastern United States from Maine to Georgia and westward to Minnesota and Iowa (Woodson 1954). At maturity the plants typically produce 1-2 stems, each of which bears 3-4 umbels from the upper nodes. Each umbel is composed of about 20 flowers. Flowering occurs in June and July, and the flowers are pollinated by a variety of large flying insects, including Hymenoptera, Lepidoptera, and Diptera. We report elsewhere on nectar production, pollination, and other aspects of the reproductive biology of A. exaltata (Wyatt and Shannon 1986; Shannon and Wyatt

Pollen flow in Psiguria warscewiczii: a comparison of Heliconius butterflies and hummingbirds.

Pollen dispersal by male and female Heliconius butterflies and hermit hummingbirds was examined in a natural population of the tropical vine Psiguria warscewiczii. Fluorescent dyes were used to track flowers visited by individual pollinators. Mean distance of pollen dispersal was significantly higher by males than by female Heliconius. Butterflies dispersed pollen greater distances than did hummingbirds. There were no significant differences in dispersal distance among species of Heliconius. The number of flowers or plants to which dye was dispersed was greater for Heliconius than for hummingbirds.The home range behavior and site specificity exhibited by Heliconius, together with the preference by most Heliconius species in the study site for Psiguria led us to examine the relationship between the butterflies' home range and its daily foraging among Psiguria. A Heliconius mark-recapture program was conducted simultaneously with the dye program. This study showed that individual butterflies, for which extensive recaptures were made, visited an area of Psiguria plants on any one day that corresponded to the area in which those butterflies were caught or observed. The area over which a butterfly moved was termed its home range; within home ranges relative movement by butterflies was estimated as the mean distance between sequential captures. This estimate differed significantly among species of Heliconius; males of all species moving greater distances than females.

Leptokurtic pollen-flow, non-leptokurtic gene-flow in a wind-pollinated herb, Plantago lanceolata L.

The purpose of this study was to simultaneously measure pollen dispersal distance and actual pollen-mediated gene-flow distance in a wind-pollinated herb, Plantago lanceolata. The pollen dispersal distribution, measured as pollen deposition in a wind tunnel, is leptokurtic, as expected from previous studies of wind-pollinated plants. Gene-flow, measured as seeds produced on rows of male-sterile inflorescences in the wind tunnel, is non-leptokurtic, peaking at an intermediate distance. The difference between the two distributions results from the tendency of the pollen grains to cluster. These pollen clusters are the units of gene dispersal, with clusters of intermediate and large size contributing disproportionately to gene-flow. Since many wind-pollinated species show pollen clustering (see text), the common assumption for wind-pollinated plants that gene-flow is leptokurtic requires re-examination. Gene-flow was also measured in an artifical outdoor population of male-steriles, containing a single pollen source plant in the center of the array. The gene flow distribution is significantly platykurtic, and has the same general properties outdoors, where wind speed and turbulence are uncontrolled, as it does in the wind tunnel. I estimated genetic neighborhood size based on my measure of gene-flow in the outdoor population. The estimate shows that populations of Plantago lanceolata will vary in effective number from a few tens of plants to more than five hundred plants, depending on the density of the population in question. Thus, the measured pollen-mediated gene-flow distribution and population density will interact to produce effective population sizes ranging from those in which there is no random genetic drift to those in which random genetic drift plays an important role in determining gene frequencies within and among populations. Despite the platykurtosis in the distribution, pollen-mediated gene dispersal distances are still quite limited, and considerable within and among-population genetic differentiation is to be expected in this species.

POLLEN AND GENE DISPERSAL: THE INFLUENCES OF COMPETITION FOR POLLINATION.

Pollinators that forage indiscriminately can transfer pollen from one species to another, reducing the amount that reaches conspecific flowers. I present evidence that the presence of another plant species visited by the same pollinators can also reduce pollen dispersal distances and outcrossing. This has the potential to influence gene flow and reproductive success. Pollen carryover and movement patterns were measured for the shared insect pollinators of Stellaria pubera and Claytonia virginica in North Carolina. Bee flies deposited similar amounts of Stellaria pollen on a series of pistillate Claytonia flowers as on a series of pistillate Stellaria flowers. In arrays of potted plants, flies and solitary bees visited most flowers on a plant before leaving and then flew to a nearby plant chosen independently of species; 95% of moves were to one of 12 nearest neighbors. These measures of pollen carryover and movement patterns were used in a set of computer simulations to predict pollen dispersal distances. The simulations suggested that C. virginica substantially reduces outcrossing and pollen flow in S. pubera. These predictions were tested by tracking dye movement from anthers in populations of potted plants. Addition of C. virginica reduced the mean squared distance moved by dye to receptive S. pubera flowers by 23% and reduced the amount of dye moved by 51%. The estimated pollen component of gene flow was also much lower in a natural population of 5. pubera mixed with C. virginica than in the synthetic single-species populations.

DENSITY-DEPENDENT POLLINATOR FORAGING, FLOWERING PHENOLOGY, AND TEMPORAL POLLEN DISPERSAL PATTERNS IN LINANTHUS BICOLOR.

Gene flow in flowering plant populations occurs through the dispersal of pollen and seeds. In animal-pollinated plants, the foraging behavior of pollinators can affect the distance and frequency of pollen transfer between and within plants, and thus can have an important impact on gene dispersal and outcrossing potential (Levin, 1979a, 1979b). The distribution of gene dispersal distances, the rate of outcrossing and the density of reproductive individuals in a population determine the size of a genetic neighborhood, in the sense of Wright's (1943, 1946) isolation by distance model. In general, as neighborhood size and area increase, a population's evolutionary potential for microgeographic genetic differentiation will diminish (Wright, 1943, 1946). Several authors have estimated neighborhood size and area by using pollen dispersal distances inferred from pollinator flight patterns (Kerster and Levin, 1968; Levin and Kerster, 1968, 1969; Schaal and Levin, 1978; Beattie, 1979; Beattie and Culver, 1979; Schmitt, 1980a, 1980b; Waser, 1982; Zimmerman, 1982). Recent studies of artificial populations indicate, however, that actual gene flow may often be more extensive than that estimated from pollinator movements (Schaal, 1980; Levin, 1981; Ennos and Clegg, 1982; Handel, 1982). These findings probably result from the carryover of pollen on a pollinator's body from a given plant successively to many other plants (Thomson and Plowright, 1980; Morse, 1982; Price and Waser, 1979, 1982; Waser

Patterns of Ant-mediated Pollen Dispersal in Diamorpha smallii (Crassulaceae)

Gene flow distances in Diamorpha smallii (Crassulaceae) effected through pollen dispersal by ants (Formica schaufussi and F. subsericea) are extremely short and their frequency distributions are strongly leptokurtic in form. Increased plant density is associated with decreased pollen dispersal distances. This pattern of pollen dispersal may explain earlier electrophoretic data indicating a highly subdi- vided population structure in D. smallii. Only recently, with increased emphasis on plant-animal interactions (Percival 1965; Free 1970; Faegri and van der Pijl 1971; Proctor and Yeo 1972; Heinrich and Raven 1972; Gilbert and Raven 1975), has the phe- nomenon of pollination by ants, dismissed by classical pollination ecol- ogists (Kerner 1878; Muiller 1883; Knuth 1906), been described and documented. Hickman (1974) was the first to report effective pollination of the flowers of Polygonum cascadense Baker (Polygonaceae) by the ant Formica argentea Wheeler (Formicidae). That study did not, however, include an assessment of the quantity and quality of pollen dispersal effected by the ants. Recent studies of a diversity of flowering plants indicate that gene flow in the form of pollen or seeds is highly leptokurtic and restricted in range (Levin and Kerster 1974; Levin 1978, 1979a). We report here the pattern of pollen dispersal within populations of Diamorpha smaliji Britt. (Crassulaceae), a succulent annual endemic to granite outcrops in the southeastern United States (McCormick and Platt

Dispersal Versus Gene Flow in Plants

Dispersal Versus Gene Flow in Plants

Time and Distance of Pollen Dispersal in Grain Sorghum

The obiective of the present study was to determine the time of day when pollens were dispersed and the distance travelled by pollen in grain sorghum. Time of the active pollen dispersal was estimated from seed set on panicle of an A-line (Experiment 1). The distance for pollen dispersal was measured from seed set on panicles of A-lines which were grown at various distances and directions around pollen dispersal field fcr three years. The isolation drstance necessary to produce certified hybrid seeds of *'rain sorghum was estimated (Experiment 2). When paper envelopes were removed from panicles of the A-1ine before 0400 and/or after 1200, seed set on the panicle was not observed. However, seed set was observed between 0600 and 1000. Time of active pollen dispersal is, therefcre, from 0600 to 1000. The isolated plots in which seed set was observed were located less than 400m from the pollen dispersal field. Seed set was not observed in some isolated plots less than 400m, as wind direction and obstacles near the isolated plot prevented pollination. Pollen was dispersed into the isolated plot within 250m from the pollen dispersal field, even though the isolated plot was surrounded by houses and was located on the wind-ward side of main wind direction. Adequate isolation distance for certified hybrid seed production was approximately 400 to 500m, with modifications down to 250 to 300m permitted under certain conditions.

The Necessity of Cross-Pollination for Fruit Set of Manzanillo Olives

SummaryCross-pollination of cv Manzanillo flowers was found essential in Israel. The cv Uovo de Piccione was the most suitable pollinator for Manzanillo. The addition of Manzanillo stigma tissue to pollen germination media enhanced germination and served as a pollination efficiency indicator. The percentage of normal pollen and its germination were determined for various cultivars. The distance of pollen dispersal in the Eastern Yisre’el Valley was established.

Genetic studies in natural populations of forest trees

The present study deals with the genetic differentiation within a natural forest of Cryptomeria japonica. Needle leaves were collected from approximately 50 trees in each of three sites, called subpopulations I, II and III, located at the apices of a triangle. The distances from each other among these three subpopulations were 760 m or 1,300 m. It was found, by an electrophoretic study of peroxidase, that the mean number of isozyme bands exhibited per individual tree was 11.68 in I, 13.73 in II and 12.53 in III. The t-test revealed that the differences between I and II and between II and III were statistically significant. The frequency of occurrence of each band was compared among the subpopulations. It was found that the variation among the three subpopulations was statistically significant. Not a few papers dealing with the study of pollen dispersal in coniferous trees have been published so far. Some of the authors consider that the distance of pollen dispersal is short and gene flow among subpopulations within a forest seldom occurs, while others are of the opinion that pollen travels long distances and no effective isolation is possible within a forest. Findings from the present study support the former view. They show that subpopulations within a forest of Cryptomeria japonica are genetically differentiated from each other, i. e. Sewall Wright's neighbourhood size is small in Cryptomeria japonica.

Density-Dependent Gene Dispersal in Liatris

The effect of colony density on gene migration has been interpreted in the perennial herb, Liatris aspera. Colonies with 1, 3.2, 5, and 11 plants/square meter, respectively, were studied. The pollen component of gene migration was inferred from pollinator flight distances, 1,614 of which were analyzed. Bees were the primary pollinators. The seed component of gene migration was measured in the experimental garden, utilizing transplants as the seed source. Axial distances were determined for 2,309 wind-borne seeds. Means of pollinator flight distances displayed a close curvilinear relationship to colony density, an increase in density being accompanied by a decrease in flight distance. Flights are predominately to near neighbors, and there is strong positive correlation between the variance of flight distance and the variance of plant spacing. Thus we conclude that pollinator behavior and pollen-dispersal distance are density-dependent. Gene migration has been considered within the framework of Wright's neighborhood model. Neighborhood area is density-dependent, the relationship between density and area being an inverse one. The dependency of Na on density is somewhat obscured by the seed-dispersal variance, which is density-independent and which dominates the total gene-dispersal variance. Since gene migration is influenced by colony density, the potential for local differentiation is to some extent density-dependent, high-density arrays being most conducive to subdivision. Genetically effective density increased with colony density. However, when only the density-dependent component was considered, Ne remained relatively constant over the range of densities. This feature is traceable to the Ne equation, which includes a square function of spacing (pollen dispersal) and an inverse square-root function of spacing (density).

Comparison of the Present Vegetation with Pollen‐Spectra in Surface Samples from Brownington Pond, Vermont

Pollen-bearing sediments found in lakes or bogs provide a record of past vegetation, presumably arranged in a stratigraphic column. MVany pollen deposits contain essentially the same pollen types throughout. Thus description of a deposit depends largely upon a quantitative determination of the relative proportions of pollen types at various levels. Interpretation of pollen deposits assumes that frequencies of pollen types deposited reflect the complexion of the surrounding vegetation. Therefore, changes in frequencies of pollen types within the stratigraphic column are assumed to reflect changes in proportions of species or genera in the surrounding vegetation. These changes are often interpreted as the result of climatic change, one of the major factors affecting species composition of vegetation. For this reason, interpretations of pollen-bearing deposits are of great interest to students in such fields as glacial geology and paleoclimatology. It has long been recognized, however, that the efficiency of pollen production and dispersal in different species of plants varies widely. A reasonal)le interpretation of a pollen spectrum can be made only if the relative pollen dissemination efficiency of each species is known. Attempts to estimate pollen production and pollen dispersal distance have been made in Europe (reviewed by Faegri and Iversen 1950) and in Japan (e.g. Tsukada 1958), but research in this aspect of pollen analysis has been largely neglected in North America. Pollen analysts here have tended to rely on European estimates of pollen production, a dangerous practice because the pollen production of congeneric species has been shown to vary (Pohl 1937). In a few areas, the present vegetation. studied at least in part quantitatively, has been compared to the pollen spectra in surface moss polsters or lake muds: the Great Smoky Mountains (Carroll 1943), Oregon (Hansen 1949), Alaska (Livingstone 1955), Alberta (Erdtman 1943) and Quebec (Potzger et al. 1956). However, pollen dissemination character-