Interspecific Pollen Transfer Research Articles

Partitioning of Pollinators during Flowering in an African Acacia Community

Competition for pollination is an important factor structuring flowering in many plant communities. We examined mechanisms reducing interspecific pollen flow in a community of 10 Acacia species in a highly seasonal savannah habitat in Tanzania. Partitioning is achieved, in part, through separation of flowering in space and seasonal time, and through interspecific differences in pollinator guilds. Nevertheless, coflowering Acacia species shared several pollinators; this means that interspecific pollen transfer is possible. We analyzed daily patterns of pollinator activity and pollen release in 10 Acacia assemblages containing a total of 10 Acacia species. Pollinator activity was scored using counts at flowers over constant time intervals throughout the day. Pollen availability was assessed using a simple method which allows quantification of pollen exposed on the surface of the Acacia inflorescence. Sympatric co-flowering Acacia species each show high intra- specific synchrony but release their pollen at different times of day. Pollinators rapidly harvest available pollen and move from one Acacia species to the next, following the daily sequence of pollen release. The activity of shared pollinators is structured throughout the day as a result of temporal patterns of pollen release across Acacia species. The observed temporal structuring of pollen release is compatible with patterns predicted to result from competitive displacement. Additional support for a competition-based explanation for this patterning comes from the observation that an Acacia species flowering without competitors

Interspecific Pollen Loss by Hummingbirds Visiting Flower Mixtures: Effects of Floral Architecture

Tropical hummingbirds often forage in mixtures of several plant species. As a result, pollen is often transferred among heterospecific flowers. The "Sexual Architecture Hypothesis," a paradigm of pollination ecology and angiosperm evolution, suggests that the severity of such interspecific pollen transfer should reflect the similarity in the architecture of sexual structures (specifically, placement of stigma and anthers) among the flowers involved. We evaluated the hypothesis with six species of Costa Rican cloud—forest plants varying greatly in sexual architecture but all pollinated by the same hummingbird, Lampornis calolaema. Captive Lampornis visited five long—styled, "pollen—donor" flowers of distylous, self—incompatible Palicourea lasiorrachis (Rubiaceae), then 20 short—styled recipient flowers of Palicourea, with or without five flowers of a second species intervening. We assessed intraspecific pollen transfer from Palicourea donors by counting pollen tubes in styles of the recipients. We examined interspecific pollen transfer between Palicourea and each of the five other species by counting heterospecific grains deposited on stigmas of Palicourea recipients, and Palicourea grains deposited on stigmas and anthers of the heterospecific flowers. In a separate experiment, we examined how mixed—species flower visits modified the pollen loads actually carried by birds. Results did not uphold the Sexual Architecture Hypothesis. A single visit to any heterospecific flower greatly affected the hummingbird's prior load of Palicourea pollen, regardless of where the second species placed its own pollen on the bird. Interspecific pollen transfer occurred consistently, but (1) heterospecific grains usurped little space on stigmas of Palicourea recipients and had no discernible effect on pollen—tube growth, whereas (2) few Palicourea grains landed specifically on anthers and stigmas of heterospecific flowers. Instead, numerous Palicourea grains apparently were scraped off on other floral structures such as the corolla. Consequently, every species of intervening flowers, regardless of its sexual architecture per se, strongly reduced pollen transfer from donor to recipient Palicourea flowers, on average by 76%. In fact, the most drastic reductions resulted from the two species whose sexual architectures least resembled that of Palicourea. Results imply that "pollen scraping" by floral structures other than anthers and stigmas may be important and widespread among some animal—pollinated flowers. Thus, except in cases where no other floral structures contact pollinators, the positions of anthers and stigmas alone may have little bearing on the severity of interspecific pollen transfer and competitive interactions among plants sharing pollinators.

Bidirectional cytoplasmic and nuclear introgression in the New World cottons Gossypium barbadense and G. Hirsutum (Malvaceae)

Patterns of interspecific cytoplasmic (plastid and mitochondrial) and nuclear introgression are typically asymmetrical: cytoplasmic gene flow or “capture” is frequently observed without evidence of nuclear introgression. In contrast, nuclear introgression without concomitant cytoplasmic introgression has rarely been demonstrated. Gossypium barbadense L. and G. hirsutum L. have large indigenous ranges in the New World semiarid tropics, with an extensive area of sympatry in the Caribbean and Central America. Numerous accessions of both species were surveyed for diagnostic cpDNA restriction sites. These data, in conjunction with previous information on nuclear markers, lead to several conclusions: 1) introgression between G. hirsutum and G. barbadense is bidirectional for both nuclear and cytoplasmic genes; 2) patterns of introgression between the two species are not symmetrical—in G. barbadense, introgression of G. hirsutum alleles is largely restricted to modem cultivars and is uncommon in areas of sympatry; in contrast, introgression of G. barbadense alleles into G. hirsutum is relatively common in areas of sympatry and is rare in modem cultivars; 3) nuclear introgression is geographically more widespread and more frequently detected than cytoplasmic introgression. Several mechanisms may underlie the differential patterns of cytoplasmic and nuclear gene flow observed, including differential fitness of infraspecific and interspecific cytonuclear combinations and selection against female function in interspecific backcrosses. Possible explanations for the observed asymmetrical patterns of introgression include differences in population sizes combined with phenological differences that bias interspecific pollen transfer.

Bidirectional Cytoplasmic and Nuclear Introgression in the New World Cottons, Gossypium barbadense and G. hirsutum (Malvaceae)

Patterns of interspecific cytoplasmic (plastid and mitochondrial) and nuclear introgression are typically asymmetrical: cytoplasmic gene flow or “capture” is frequently observed without evidence of nuclear introgression. In contrast, nuclear introgression without concomitant cytoplasmic introgression has rarely been demonstrated. Gossypium barbadense L. and G. hirsutum L. have large indigenous ranges in the New World semiarid tropics, with an extensive area of sympatry in the Caribbean and Central America. Numerous accessions of both species were surveyed for diagnostic cpDNA restriction sites. These data, in conjunction with previous information on nuclear markers, lead to several conclusions: 1) introgression between G. hirsutum and G. barbadense is bidirectional for both nuclear and cytoplasmic genes; 2) patterns of introgression between the two species are not symmetrical—in G. barbadense, introgression of G. hirsutum alleles is largely restricted to modem cultivars and is uncommon in areas of sympatry; in contrast, introgression of G. barbadense alleles into G. hirsutum is relatively common in areas of sympatry and is rare in modem cultivars; 3) nuclear introgression is geographically more widespread and more frequently detected than cytoplasmic introgression. Several mechanisms may underlie the differential patterns of cytoplasmic and nuclear gene flow observed, including differential fitness of infraspecific and interspecific cytonuclear combinations and selection against female function in interspecific backcrosses. Possible explanations for the observed asymmetrical patterns of introgression include differences in population sizes combined with phenological differences that bias interspecific pollen transfer.

Unilateral incompatibility and the effects of interspecific pollination for Erythronium americanum and Erythronium albidum (Liliaceae)

To determine whether interspecific pollen transfer could reduce seed production by two sympatric lilies, Erythronium albidum and Erythronium americanum, we hand-pollinated flowers with mixtures of conspecific and heterospecific pollen. These species exhibited typical unilateral interspecific incompatibility, i.e., pollen tubes from the self-infertile species (E. americanum) grew apparently unimpeded in styles of the self-fertile species (E. albidum), whereas the reverse cross resulted in an incompatibility reaction. Because of this asymmetrical relation and faster growth by heterospecific pollen tubes in E. albidum stigmas than by conspecific tubes, pollination with abundant heterospecific pollen reduced fruit and seed production by E. albidum, but not by E. americanum, as long as the stigma also received some conspecific pollen. Unilateral incompatibility could benefit self-infertile species in reproductive interactions with closely related self-fertile species; however, this benefit remains to be demonstrated for naturally pollinated plants. Key words: Erythronium albidum, Erythronium americanum, interspecific pollen transfer, pollination, unilateral incompatibility.

Interspecific pollen transfer as a mechanism of competition: Consequences of foreign pollen contamination for seed set in the alpine wildflower, Polemonium viscosum.

Earlier studies showed that under natural conditions foreign pollen receipt is correlated with loss of seed set in populations of P. viscosum. Here, we report on experimental pollinations using foreign pollen from two co-flowering species that share pollinators with P. viscosum. Experiments were designed to ascertain whether improper pollen transfer causes reduced seed set under controlled conditions and, if so, to determine the stage at which interference occurs. In flowers pre-treated by pollination with either Mertensia or Castilleja pollen, adherence of subsequently deposited conspecific pollen grains was unaffected. However, conspecific grains had significantly lower germination and ovule fertilization success in flowers receiving foreign pollen than in controls. These results suggest that pollinator infidelity in high alpine plant communities can reduce female reproductive success of P. viscosum by inhibiting the performance of conspecific pollen grains. Negative effects of Mertensia and Castilleja pollen were comparable despite large interspecific differences in pollen grain morphology. Accordingly, susceptibility to foreign pollen interference appears to be an intrinsic property of P. viscosum that is not dependent on the unique structural properties of pollen produced by competitive species.

Pollen precedence and stigma closure: a mechanism of competition for pollination between Delphinium nelsonii and Ipomopsis aggregata.

Previous experiments showed that the sympatric herbs Delphinium nelsonii and Ipomopsis aggregata compete for hummingbird pollination and that deleterious effects of the former species on seed set of the latter involve interspecific pollen transfer. However, seed set was not reduced when pollen of both species was applied simultaneously to I. aggregata stigmas. Hence a competitive effect may require arrival of foreign pollen before conspecific pollen. To explore this possibility we subjected I. aggregata flowers to a "competition" treatment in which they received D. nelsonii pollen 6 h before I. aggregata pollen, or to a "control" in which they received only the conspecific pollen. Foreign pollen precedence decreased mean seed set by almost 50%, which is consistent with effects observed in previous experiments. Reduced seed set can be explained by the fact that foreign pollen often caused stigma lobes to close together within 1.5-6 h, reducing subsequent receptivity. Stigma closure was also elicited by conspecific pollen, but not by mechanical stimulation, and was influenced by size of the pollen load and identity of the plant being pollinated.

Processes of isolation and introgressive interplay between Platanthera bifolia (L.) Rich and P. chlorantha (Custer) Reichb. (Orchidaceae)

NILSSON, L. A., 1983. Processes of isolation and introgressive interplay between Platanthera bifolia L. Rich, and P. chlorantha (Custer; Reichb. (Orchidaceae). Platanthera bifolia and P. chlorantha are highly specialized for pollination by crepuscular and nocturnal Lepidoptera. The plants are largely sympatric, have overlapping flowering-times, and are completely interfertile. Principal differences exist in floral fragrance and column morphology. Isolation is effected by a set of integrated morphological, ethological, and seasonal mechanisms. Odour-based constancy by moths, differences in column structure, attachment of pollinaria, spurlength, nectar availability, and flowering-peaks reduce interspecific pollen transfer. The plants form a functional species pair in relation to their pollination environment and locally share the same main pollinator species. Hybrids, which infrequently occur in areas of sympatry, emit a blend of the species-specific fragrance compounds and have intermediate column structure. They are frequently visited by-moths but suffer a serious reduction in seed production due to imperfect interaction vis-a-vis the pollen vectors. Hybrid viscidia, in an intermediate position on the column, are touched by moths' scaly and hairy palps and get insufficient attachment, evidently causing hybrids to act solely as female recipient. First backcross individuals largely join the functional range of either species which promotes rapid introgression. The two gene-pools interplay, although always at a low level. Patterns of introgressive hybridization were exemplified.

The Ecology of Flowers of Chalk Grassland and their Insect Pollinators

(1) Competition for pollinators and avoidance of interspecific pollen transfer have been proposed as selective factors influencing flowering time and floral specializations. Their importance in a community of chalk grassland plants was studied. (2) The flowering periods of the important insect-pollinated herbs were determined. The most prominent flowering species were Helianthemum nummularium, Hippocrepis comosa and Onobrychis viciifolia during June and early July, and Centaurea nigra, C. scabiosa, Knautia arvensis and Scabiosa columbaria from late July to September. (3) Bumblebees, particularly Bombus lapidarius, were the most abundant visitors to most of these plant species; Apis mellifera, some Lepidoptera and Syrphidae were of lesser importance. (4) Some insect species differed in the flowers they visited most frequently. (5) In the early-flowering groups of plants there was some specialization for different pollinators and divergences in flowering time, suggesting that competition for pollinators or avoidance of interspecific pollen transfer has exerted selection pressure in the past. (6) In the late-summer group all species overlapped considerably in flowering time and their insect visitors, which suggests that there has not been any strong selection against an overlap. There appeared to be a saturation of insect visitors in late summer, with particularly large numbers of Bombus lapidarius.

Floral Convergence and Pollinator Sharing in Two Bee‐Pollinated Tropical Herbs

The hypothesis that the understory herbs Costus allenii and C. laevis (Zingiberaceae) have converged in floral characteristics to use the same pollinator was investigated in central Panama. Observations and experiments indicated that these species (1) occupy the same habitats, (2) flower synchronously, (3) are identical in flower color, morphology, and nectar secretion patterns, (4) share the same pollinator, the bee Euglossa imperialis, (5) are self—compatible, but not autogamous, and (6) have strong barriers to hybridization. Both grow in low density along streamsides and produce a single flower per day for an extended period (up to 4 mo). Flower density is depressed through extensive predation by the weevil Cholus cinctus, which damaged 31% of all C. allenii and 60% of all C. laevis inflorescences. Direct observation of foraging bees indicated that individuals regularly visit both plant species. An experimental analysis of interspecific pollen transfer using powered paint as a marker verified these results; 97% of the flowers checked had received heterospecific visits. The high probability of interspecific pollination did not affect fruiting success. I suggest that low flower density, exaggerated by extreme floral predation, has selected for floral similarity and pollinator sharing in these species. Floral convergence increases effective flower density and nectar supplies, and probably increases the regularity and rate of pollinator visitation.

Competition for Hummingbird Pollination and Sequential Flowering in Two Colorado Wildflowers

The simultaneous flowering of co—occurring plant species with similar pollinator affinities may result in interspecific pollen transfer and consequent fecundity reductions due to wastage of pollen, stigma surfaces, and effective pollinator visits. In such cases competition for pollination occurs and may lead to or maintain sequential flowering. Two common perennials in the mountains of west—central Colorado, Delphinium nelsoni and Ipomopsis aggregata, flower sequentially in the same meadows and are visited commonly by Broad—tailed Hummingbirds (Selasphorus platycercus). Hummingbirds carry pollen of both species and their exclusion from flowers leads to significant seed set reductions. During the brief period of flowering overlap between D. nelsoni and I. aggregata in natural meadows, hummingbirds visit both species, carry mixtures of their pollen, and appear to cause interspecific pollen transfer. Flowers of both species receptive during this period suffer significant seed set reductions relative to those receptive during nonoverlap periods. Interspecific pollinator flights and pollen transfer also occur in mixtures of potted plants, and seed set reductions consistently occur for both D. nelsoni and I. aggregata in such mixtures relative to single—species controls. Finally, seed set reductions occur for both species following interspecific hand pollination of potted plants. Fecundity reductions in natural and synthetic mixtures of D. nelsoni and I. aggregata indicate that the 2 species compete for hummingbird pollination and suggest that the competitive interaction involves interspecific pollen transfer. The observed reproductive effects represent a selective force sufficient to maintain divergent flowering times of D. nelsoni and I. aggregata in nature.

Interspecific pollen transfer and competition between co-occurring plant species.

Computer simulations of a pollinator foraging in a mixture of two species were used to explore how plant reproduction can be influenced by interspecific pollination movements. Interspecific pollen transfer led to strong competitive effects when availabilities of pollen, receptive stigma surfaces, or pollinator movements were limited relative to the total number of fertilizations possible in the mixed population. Results from simulations suggest that competition for pollination through interspecific pollen transfer can result in rapid exclusion of one of two species, and that such competition represents a selective force promoting stable divergence of potential competitors in habitat affinity, flowering time, or other characteristics related to pollinator sharing.

PHLOX AND COLIAS: THE EFFICIENCY OF A POLLINATION SYSTEM.

Pollination in hundreds of self-incompatible plant species is accomplished by insects and other animals which use these species as food sources. Although pollinators are adapted for exploiting these resources, they are not adapted for pollination per se. Pollination is a fortuitous by-product of food gathering, and in no way maximizes the result of this activity. Accordingly, there is no a priori reason to assume that the transfer of pollen from one plant to another will be performed with a high degree of efficiency. Very little is known about the efficiency of pollination systems, although anecdotal information suggests extensive pollen wastage. This investigation was undertaken to shed light on the efficiency of intraand interspecific pollen transfer in Phlox. The species under consideration are P. pilosa L. and P. glaberrima L. They display the floral syndrome of butterfly-pollinated plants (cf. Faegri and van der Pijl, 1966), and are pollinated almost exclusively by lepidopterans (Grant and Grant, 1965; Levin and Kerster, 1968). One pollinator which services both species is Colias eurytheme Boisduval. Two sets of questions were posed in this investigation. The first questions relate to C. eurytheme as studied in the laboratory and are as follows. (1) What is the size of the pollen load on the proboscis of C. eurytheme after it is withdrawn from the corolla tubes of the 2 phloxes? (2) What fraction of the pollen load is lost on the palps as a consequence of proboscis coiling? (3) What fraction of the pollen remaining on the proboscis is deposited on the first conspecific and heterospecific stigmas encountered? (4) How rapidly is the outcross pollen load dissipated as the pollinator moves from one flower to another on a recipient plant? Other questions deal with phenomena in natural populations. What is the size of the outcross pollen loads on stigmas of P. pilosa and P. glaberrima? What is the differential between outcross conspecific and heterospecific pollen loads in a mixed population of the aforementioned species? A discussion of the floral design of the phloxes is a necessary prelude to what follows. The corollas of both species are salverform with relatively narrow and flat rims. They are also colored pink and emit a weak fragrance. The nectaries are located at the base of a narrow corolla tube which averages 14 mm and 19 mm in length in P. pilosa and P. glaberrima, respectively. The species differ in characters of the gynoecium and androecium (Fig. 1). In P. pilosa, the style is approximately 3 mm long, and is buried deep within the corolla tube. The stamens are inserted near the middle of the tube. Pollen grain diameters average ca. 30 tu. In P. glaberrima, the style is nearly as long as the corolla tube, and carries the stigma to the orifice of the tube. Stamen insertion is just below the tube orifice. Pollen grain diameters average ca. 55 tu. Both species produce an average of 15,000 pollen grains per flower. In spite of differences in the position of anthers and stigmas, the floral design of the two species is such that most pollinators of one can, and often do, service the other. Both species are self-incompatible.