Beyond the pollination syndrome: nectar ecology and the role of diurnal and nocturnal pollinators in the reproductive success of Inga sessilis (Fabaceae)

Inga species present brush‐type flower morphology allowing them to be visited by distinct groups of pollinators. Nectar features in relation to the main pollinators have seldom been studied in this genus. To test the hypothesis of floral adaptation to both diurnal and nocturnal pollinators, we studied the pollination ecology of Inga sessilis, with emphasis on the nectar secretion patterns, effects of sequential removals on nectar production, sugar composition and the role of diurnal and nocturnal pollinators in its reproductive success. Inga sessilis is self‐incompatible and pollinated by hummingbirds, hawkmoths and bats. Fruit set under natural conditions is very low despite the fact that most stigmas receive polyads with sufficient pollen to fertilise all ovules in a flower. Nectar secretion starts in the bud stage and flowers continually secreting nectar for a period of 8 h. Flowers actively reabsorbed the nectar a few hours before senescence. Sugar production increased after nectar removal, especially when flowers were drained during the night. Nectar sugar composition changed over flower life span, from sucrose‐dominant (just after flower opening, when hummingbirds were the main visitors) to hexose‐rich (throughout the night, when bats and hawkmoths were the main visitors). Diurnal pollinators contributed less than nocturnal ones to fruit production, but the former were more constant and reliable visitors through time. Our results indicate I. sessilis has floral adaptations, beyond the morphology, that encompass both diurnal and nocturnal pollinator requirements, suggesting a complementary and mixed pollination system.     

Pollination in Lilium sargentiae (Liliaceae) and the first confirmation of long‐tongued hawkmoths as a pollinator niche in Asia

The geographical distribution of pollinators is an important factor determining the biogeographical pattern of floral evolution and diversification. In Africa, a guild of plant species has converged in a floral syndrome for pollination by long‐tongued hawkmoths (predominantly Agrius convolvuli). It is hypothesized that such floral convergence could track the geographical distribution of long‐tongued hawkmoths, so it may not be confined to Africa. We investigated the pollination biology of Lilium sargentiae E. H. Wilson, which is endemic to China and exhibits traits suggestive of long‐tongued hawkmoth pollination. Lilium sargentiae was visited by A. convolvuli as well as small beetles (Nitidulidae) but pollination was mostly or totally effected by the moth. It was consistent with other hawkmoth‐pollinated plants in terms of floral tube length, nectar traits, tepal reflectance, and scent composition. We present the first experimental evidence for the hypothesis proposed above and for hawkmoth pollination in the widespread and ornamentally and economically important genus Lilium L. in a natural habitat. Our findings imply that long‐tongued hawkmoths (especially the extremely widespread A. convolvuli) as a distinctive pollinator niche may have underlain the evolution of long‐tubed (>8 cm) flowers across different continents.     

Variable selection in Platanthera bifolia (Orchidaceae): phenotypic selection differed between sex functions in a drought year

We estimated selection on three morphological characters in the hermaphroditic, hawkmoth-pollinated orchid Platanthera bifolia and explored selection surfaces through male and female function. The work was carried out in northern Sweden during two flowering seasons (1994 and 1995) in one natural population and one season (1995) in another natural population. Fitness was estimated as number of pollinia removed (male function) and number of fruits produced (female function). We detected directional selection towards larger inflorescence size (flower number) through both sex functions in both populations in 1995. In 1994, with an unusually dry growing season, 78% of the individuals failed to set any fruit, and there was selection for larger inflorescences only through male function. In this year, there was selection towards longer flower spurs, which could be a direct or indirect effect of spurs being shortened by drought. The results demonstrate that selection patterns may vary temporally and spatially, and that the 'male function hypothesis' may be applicable as female function is more resource dependent than male function.     

EVOLUTION AND COEXISTENCE OF POLLINATION ECOTYPES IN AN AFRICAN GLADIOLUS (IRIDACEAE)

Pollinator‐mediated selection has been suggested as a key driver of speciation in plants. We examined the potential role of hawkmoth pollinators in driving allopatric divergence and maintaining sympatric coexistence of morphotypes in the African iris Gladiolus longicollis. Floral tube length in this species varies from 35 mm to 130 mm across its geographic range and reflects the prevailing tongue lengths of local hawkmoth assemblages. The distribution of floral tube lengths is bimodal with two relatively discrete categories—long (about 90 mm) or short (about 50 mm)—that match the bimodal distribution of hawkmoth tongue lengths in eastern South Africa. At a contact site between these two floral morphs, we found few individuals of intermediate length, suggesting limited gene flow between morphs despite their interfertility. A difference in flowering phenology appears to be the main isolating barrier between morphs at this site. Long‐ and short‐tubed morphs differed markedly in the chemical composition of their floral fragrance, a trait that could be used as a cue for morph‐specific foraging by hawkmoths. Positive directional selection on tube length was found to occur in both morphs.     

Specialized use of pollen vectors by Caesalpinia gilliesii, a legume species with brush-type flowers

The pollination of Caesalpinia gilliesii (Fabaceae) , a legume species with long, brush-type flowers, was studied in wild populations in Argentina. A total of 341 hawkmoths (Sphingidae), belonging to 10 species, with tongues ranging from 1.5 to 13.5 cm in length, were captured by means of light traps. Hawkmoths could be classified in four significantly different tongue-length classes or guilds: short (15.35 ± 1.11 mm), medium (30.23 ± 2.24 mm), long (55.85 ± 4.98 mm) and very long (95.61 ± 13.63 mm). A total of 133 of the hawkmoths, belonging to seven species, carried pollen of Cae. gilliesii , mainly on their venter. Successful contact with anthers while drinking nectar depended upon hawkmoth tongue length. The relative frequency of pollen-carriage and the mean tongue length of each species were positively and significantly correlated. Considering a null hypothesis of nonspecialization, in which hawkmoths carrying pollen are distributed at random among individuals of the four guilds, long-tongued hawkmoths carried Cae. gilliesii pollen more frequently than would be expected by chance. In contrast, short- and medium-tongued guilds carried pollen less frequently than is predicted by random expectation. Individuals with short tongues accessed nectar from below without touching the anthers, whereas very long-tongued hawkmoths, with tongues longer than the length range of the flower reproductive organs, can probably reach nectar without touching the fertile organs. Results show that a pterotribic pattern of pollen deposition on the body of long-tongued hawkmoths is operating in this species. This suggests that brush-type blossoms are not necessarily unspecialized, despite popular belief.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 579–592.     

Flexible responses to visual and olfactory stimuli by foraging Manduca sexta: larval nutrition affects adult behaviour

Here, we show that the consequences of deficient micronutrient (β-carotene) intake during larval stages of Manduca sexta are carried across metamorphosis, affecting adult behaviour. Our manipulation of larval diet allowed us to examine how developmental plasticity impacts the interplay between visual and olfactory inputs on adult foraging behaviour. Larvae of M. sexta were reared on natural (Nicotiana tabacum) and artificial laboratory diets containing different concentrations of β-carotene (standard diet, low β-carotene, high β-carotene and cornmeal). This vitamin-A precursor has been shown to be crucial for photoreception sensitivity in the retina of M. sexta. After completing development, post-metamorphosis, starved adults were presented with artificial feeders that could be either scented or unscented. Regardless of their larval diet, adult moths fed with relatively high probabilities on scented feeders. When feeders were unscented, moths reared on tobacco were more responsive than moths reared on β-carotene-deficient artificial diets. Strikingly, moths reared on artificial diets supplemented with increasing amounts of β-carotene (low β and high β) showed increasing probabilities of response to scentless feeders. We discuss these results in relationship to the use of complex, multi-modal sensory information by foraging animals.     

Learning to feed in the dark: how light level influences feeding in the hawkmoth Manduca sexta

Nocturnal insects like moths are essential for pollination, providing resilience to the diurnal pollination networks. Moths use both vision and mechanosensation to locate the nectary opening in the flowers with their proboscis. However, increased light levels due to artificial light at night (ALAN) pose a serious threat to nocturnal insects. Here, we examined how light levels influence the efficacy by which the crepuscular hawkmoth Manduca sexta locates the nectary. We used three-dimensional-printed artificial flowers fitted with motion sensors in the nectary and machine vision to track the motion of hovering moths under two light levels: 0.1 lux (moonlight) and 50 lux (dawn/dusk). We found that moths in higher light conditions took significantly longer to find the nectary, even with repeated visits to the same flower. In addition to taking longer, moths in higher light conditions hovered further from the flower during feeding. Increased light levels adversely affect learning and motor control in these animals.     

Speciation genes in the genus Petunia

A major innovation in angiosperms is the recruitment of animal pollinators as a means to enhance the efficiency and specificity of pollen transfer. The implementation of this reproductive strategy involved the rapid and presumably coordinated evolution of multiple floral traits. A major question concerns the molecular identity of the genetic polymorphisms that specify the phenotypic differences between distinct pollination syndromes. Here, we report on our work with Petunia, an attractive model system for quantitative plant genetics and genomics. From interspecific crosses, we obtained F2 plants that differed in the length of the floral tube or the size of the limb. We used these plants to study the behaviour of the hawkmoth pollinator, Manduca sexta. Plants with larger limbs were preferentially visited, consistent with the notion that flower size affects visibility under low light conditions. The moths also displayed an innate preference for shorter tubes. However, in those cases that flowers with long tubes were chosen, the animals fed for equal time. Thus, the perception of tube length may help the moths, early on, to avoid those plants that are more difficult to handle.     

Foodplant effects on colour morphs of Eumorpha fasciata caterpillars (Lepidoptera: Sphingidae)

Caterpillars of the hawkmoth Eumorpha fasciata are highly polymorphic for colour, with green, pink, and pink-and-yellow forms in the second through fourth instars, and green and multicoloured forms in the fifth instar. Four years of field censuses on four foodplant species determined that all morphs were found on all plant species; morph frequencies were homogeneous on each plant species over time; and morph frequencies differed consistently among plant species. When larvae were reared from eggs on three of the hostplant species in the laboratory, differences in morph frequencies paralleled the census results. Thus foodplant quality is one factor affecting larval colour in E. fasciata. A literature survey reveals that foodplant effects on larval colouration may be widespread in the family Sphingidae, but most reports are anecdotal rather than experimental. The implications of this mechanism of colour determination are discussed.     

Camouflage and selective predation in caterpillars of the poplar and eyed hawkmoths (Laothoe populi and Smerinthus ocellata)

Final instar caterpillars of the poplar hawkmoth ( Laothoe populi ) are usually yellow-green when they have been feeding on Salix fragilis but white when on Populus alba. Similarly final instar caterpillars of the eyed hawkmoth (Smerinthus ocellata) vary in colour from yellow-green when fed on Salix cinerea or S. fragilis to grey-green when fed on Salix ripens or S. alba. Most caterpillars of these two moths are thus very cryptic to the human eye. It has already been shown that colour matching is brought about by young caterpillars changing colour depending on the colour of the substrate perceived by their eyes. Evidence is presented here for selective predation, probably by birds. Yellow-green poplar hawk caterpillars disappeared from P. alba more rapidly than did white caterpillars, but there was no significant difference in the survival of the two colour forms on S. fragilis. Yellow-green caterpillars survived better on S. fragilis than on P. alba , but there was no significant difference in the survival of white caterpillars on the two plants. Reasons why selective predation did not occur in all of the experiments are discussed.