Marcela Moré

Extreme variation in floral characters and its consequences for pollinator attraction among populations of an Andean cactus

BACKGROUND AND AIMS A South American cactus species, Echinopsis ancistrophora (Cactaceae), with dramatic among-population variation in floral traits is presented. METHODS Eleven populations of E. ancistrophora were studied in their habitats in northern Argentina, and comparisons were made of relevant floral traits such as depth, stigma position, nectar volume and sugar concentration, and anthesis time. Diurnal and nocturnal pollinator assemblages were evaluated for populations with different floral trait combinations. KEY RESULTS Remarkable geographical variations in floral traits were recorded among the 11 populations throughout the distribution range of E. ancistrophora, with flower lengths ranging from 4.5 to 24.1 cm. Other floral traits associated with pollinator attraction also varied in a population-specific manner, in concert with floral depth. Populations with the shortest flowers showed morning anthesis and those with the longest flowers opened at dusk, whereas those with flowers of intermediate length opened at unusual times (2300-0600 h). Nectar production varied non-linearly with floral length; it was absent to low (population means up to 15 microL) in short- to intermediate-length flowers, but was high (population means up to 170 microL) in the longest tubed flowers. Evidence from light-trapping of moths, pollen carriage on their bodies and moth scale deposition on stigmas suggests that sphingid pollination is prevalent only in the four populations with the longest flowers, in which floral morphological traits and nectar volumes match the classic expectations for the hawkmoth pollination syndrome. All other populations, with flowers 4.5-15 cm long, were pollinated exclusively by solitary bees. CONCLUSIONS The results suggest incipient differentiation at the population level and local adaptation to either bee or hawkmoth (potentially plus bee) pollination.

RESTRICTION OF POLLINATOR ASSEMBLAGE THROUGH FLOWER LENGTH AND WIDTH IN THREE LONG-TONGUED HAWKMOTH–POLLINATED SPECIES OF MANDEVILLA (APOCYNACEAE, APOCYNOIDEAE)1

Abstract Apocynaceae have a precise pollination mechanism. However, broad pollinator assemblages, including several insect orders, have often been recorded. We test how the corolla tube length and operative width required for effective release of the pollination mechanism could restrict the pollinator assemblage in putatively hawkmoth-pollinated Apocynaceae that differ in flower depth. If corolla tube length were effective in filtering pollinators, a broader pollen vector assemblage would be expected in short-flowered species. For this to occur, the operative width of the pollinating apparatus would be additionally expected not to be specific to any particular pollinator. Alternatively, if the pollination mechanism were to have any effect in the exclusion of pollinators, access to short flowers would be expected to be limited by proboscis width rather than length. Pollination of three species in the Apocynaceae was studied in seven populations through direct observations and palynological analysis of hawkmoths. Proboscis widths of hawkmoths carrying and not carrying pollen were compared with operative floral width as measured by proboscis analogues of different diameters that could successfully release the pollination mechanism. Species studied were exclusively hawkmoth pollinated. Pollen was always attached to pollinator proboscides, either near the base or on the apical half. The long-tongued hawkmoth species Manduca sexta (L.) was the major pollinator of Mandevilla longiflora (Desf.) Pichon and Mandevilla petraea (A. St.-Hil.) Pichon. Surprisingly, another long-tongued species, Manduca tucumana (Rothschild & Jordan), was the main pollinator of the short-flowered Mandevilla laxa (Ruiz & Pav.) Woodson. Here, the operative flower width was a decisive factor restricting the pollinator spectrum to hawkmoths with proboscides narrow enough to release the pollination apparatus. Short-tongued hawkmoths, which also have wider proboscides, cannot release the pollination mechanism. In M. petraea, the operative length, and not the operative width, restricts the pollinator assemblage. Thus, two different plant strategies were observed to restrict the pollinator spectrum: floral tube length and the operative width of the pollination mechanism.

Armament Imbalances: Match and Mismatch in Plant-Pollinator Traits of Highly Specialized Long-Spurred Orchids

Background Some species of long-spurred orchids achieve pollination by a close association with long-tongued hawkmoths. Among them, several Habenaria species present specialized mechanisms, where pollination success depends on the attachment of pollinaria onto the heads of hawkmoths with very long proboscises. However, in the Neotropical region such moths are less abundant than their shorter-tongued relatives and are also prone to population fluctuations. Both factors may give rise to differences in pollinator-mediated selection on floral traits through time and space. Methodology/Principal Findings We characterized hawkmoth assemblages and estimated phenotypic selection gradients on orchid spur lengths in populations of three South American Habenaria species. We examined the match between hawkmoth proboscis and flower spur lengths to determine whether pollinators may act as selective agents on flower morphology. We found significant directional selection on spur length only in Habenaria gourlieana, where most pollinators had proboscises longer than the mean of orchid spur length. Conclusions/Significance Phenotypic selection is dependent on the mutual match between pollinator and flower morphologies. However, our findings indicate that pollinator-mediated selection may vary through time and space according to local variations in pollinator assemblages.

THE IMPORTANCE OF OLIGOSULFIDES IN THE ATTRACTION OF FLY POLLINATORS TO THE BROOD-SITE DECEPTIVE SPECIES JABOROSA ROTACEA (SOLANACEAE)

Premise of research. Brood-site deceptive flowers use dishonest signals—especially floral odors that mimic oviposition substrates—to attract and deceive saprophilous insects to pollinate them. In this work, we recorded the pollinators of the sapromyiophilous species Jaborosa rotacea (Solanaceae) endemic to southern South America. Then, we characterized the floral volatiles of this species, and finally, we carried out field experiments to decouple the effects of scent and color as attractants for saprophilous flies. Methodology. We made direct observations of pollinators in a natural population of J. rotacea. We characterized floral volatiles by means of gas chromatography–mass spectrometry. Subsequently, we used a mixture of 2 oligosulfides (dimethyl disulfide and dimethyl trisulfide), which our analyses revealed were the main constituents of the floral scent of J. rotacea, as baits to determine the attractiveness of this olfactory signal to flies in a geographical region where J. rotacea is not present. Finally, we used the same foul-scented baits in arrays of artificial flowers resembling those of J. rotacea to assess the dual importance of olfactory and visual cues in fly attraction. Pivotal results. Pollination of J. rotacea occurs when saprophilous flies belonging to the families Calliphoridae, Muscidae, and Sarcophagidae—with similar body dimensions to the anther-stigma distance in these flowers—acquire and deposit pollen in the flowers in a nototribic mode. Our chemical analyses revealed that J. rotacea floral scent is chemically simple and features 2 oligosulfide compounds (dimethyl disulfide and dimethyl trisulfide) commonly found in carrion-mimicking flowers. We found that saprophilous flies belonging to the same families that we recorded as pollinators of J. rotacea in its native South American habitat were attracted to foul-scented baits in temperate North America. The flies’ visitation frequencies (recorded as approaches and landings on the artificial flowers) depended significantly on the presence of the foul-scented baits. Conclusions. These results support the hypothesis that oligosulfides are universally effective signals by which deceptive flowers may effect pollen dispersal by attracting flies that use carrion or carnivore feces as brood sites.

Possible tobacco progenitors share long-tongued hawkmoths as pollen vectors

The putative ancestors of the allopolyploid hybrid Nicotiana tabacum have distinct flower features, apparently suited either for hawkmoth or bat pollination. This suggests that progenitors were reproductively isolated by mechanical and ethological barriers. However, the present data show that in natural populations pollen vectors could be shared by two of the possible progenitors. Pollen vectors of one of the possible male progenitors (N. otophora) were short- and long-tongued hawkmoths and a nectar-feeding bat, while those of the female ancestor (N. sylvestris) were only long-tongued hawkmoths. The latter are then the most likely vectors responsible for the presumed spontaneous hybridization. These data also suggest that interspecific pollen transfer occurred more likely in one direction.

The role of fetid olfactory signals in the shift to saprophilous fly pollination in Jaborosa (Solanaceae)

Floral scents can act as important contributing factors to plant reproductive isolation mediated by pollinators. Plants may utilize fetid floral odors that specifically lure saprophilous flies seeking high protein content substrates, such as dung or carrion, to reach sexual maturity or as food sources for their larvae. In this work, we used baits with fetid volatile organic compounds (VOCs) that are produced during the decay of animal protein substrates (oligosulfides and a fermented bone-meal blend) to evaluate the role that olfactory signals may have played in the shift to saprophilous fly pollination in species of the nightshade genus Jaborosa Juss. Traps with the fetid baits attracted the same assemblage of saprophilous fly species that were recorded pollinating the flowers in different populations of the Andean-distributed species J. laciniata, whereas no flies were attracted to the control traps using mineral oil. Furthermore, the addition of oligosulfides to flowers of J. integrifolia, a lowland distributed species pollinated by nocturnal hawkmoths, resulted in the nearly immediate attraction of saprophilous flies (mainly Calliphoridae) to the flowers. These results provide evidence that the emission of fetid floral VOCs is sufficient to attract flies to flowers irrespective of other flower features and geographic region. This suggests that the evolutionary shift to saprophilous fly pollination in the genus Jaborosa could have been initiated with novel floral visitors attracted by the emission of fetid VOCs and then followed by major changes in other flower traits such as corolla color and morphology to optimize pollen export and placement.