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Dissertation/Thesis Abstract

Pollination biology and inter-island geographical variation in the mutualistic Heliconia (Heliconiaceae)-hummingbird (Trochilidae) interaction of the Eastern Caribbean Islands
by Gowda, Vinita, Ph.D., The George Washington University, 2009, 209; 3366548
Abstract (Summary)

Plants from the family Heliconiaceae (predominantly Neotropical family; 1 genus, ∼200 species) are known to be exclusively pollinated by hummingbirds (Trochilidae, Stiles 1975; Kress 1983a).

In this study I first examined the heliconias in their natural habitats and investigated their breeding system and pollination biology. Although presence of pollen tubes serves only as a more proximate yet still indirect measure of pollination success, it was observed that although self-compatibility is present (53-59%) in both heliconia species, the mean number of pollen tubes observed per style was significantly smaller in self versus out-crossed treatments (self = 0.52-1; outcross = 2.98-3.98). Outcrossing treatments also showed the highest pollination success (100%) in both heliconia species and autonomous pollination was generally rare (30-33%).

I next investigated the effectiveness and importance of the two sexes of Purple-throated Carib hummingbirds as pollen vectors on H. bihai and H. caribaea. Pollinator effectiveness measures the total number of pollen grains deposited by a pollinator in a single visit while pollinator importance represents a simplified estimate of total pollen delivering capacity of the pollinator. Pollinator importance is a simple product of visitation frequency and pollinator effectiveness. As female Caribs were the sole pollinators of H. bihai, their pollinator effectiveness and importance estimates were indisputable. However, in H. caribaea male and female Caribs showed similar estimates for pollinator effectiveness (PE Male = 11.33 compare with PE Male = 11.67). However, male Caribs had higher values of pollinator importance than female Caribs due to their significantly higher visitation rates (PI male = 2.97 compare with PI female= 1.4).

The second suite of factors that can affect a mutualistic plant-pollinator interaction includes the foraging behavior of the pollinator. Our results of the genetic population structure of the two heliconias show that H. bihai indeed has a lower inbreeding rate than H. caribaea (FIS, bihai Vs. caribaea = 0.07 Vs. 0.15). This study therefore provides one of the first examples where foraging patterns of pollinators indeed produce the predicted effects on the gene flow patterns of the plants. In summary, the results show that territorial male Caribs do promote inbreeding within H. caribaea, while traplining female Caribs promote outcrossing within H. bihai .

The third factor that can influence a plant-pollinator mutualism is the flowering phenology of the plant. Flowering phenologies of plants are suggested to have coevolved as a response to their pollinators and have been shown to affect Heliconia-hummingbird interactions. Our results on flowering phenologies show that H. bihai has a circum-annual flowering season with a distinct peak from March to June. This coincides with the peak but strictly-seasonal flowering pattern of H. caribaea, which flowers only from March to June. I further investigated the relationship between flowering phenologies and visitation rates of the two heliconia species. Despite contrasting flowering phenologies it can be concluded that both Heliconia species show specialization with their primary pollinators, male Purple-throated Caribs for H. caribaea and female PTC for H. bihai. However, temporal specialization patterns differ in the two heliconia species: H. bihai shows a phenologically-correlated specialization where a positive increase in flower availability is reinforced with higher visitation rate by the traplining female Caribs; in H. caribaea, increase in flower availability does not show a positive increase in visitation rates of the territorial male Caribs. However, the male Caribs also do not abandon their territories in times of least flower output and thus maintain almost a steady state of visitation rate through the flowering season of the plants.

Inter-island comparisons of plant-pollinator interactions suggest that on islands where H. caribaea are present, sex-specific pollinator specialization in H. bihai is present. On islands where H. caribaea is absent (such as St. Vincent), however, sexual specialization of H. bihai is lost and adaptation in nectar traits and pollinator response (in terms of visitation rates) can be observed. Results from this study suggests that hummingbirds are important selection agents for the presence of high sugar and volume of nectar in St. Vincent H. bihai. Although sexual specialization was not found in H. caribaea, species-specific specialization is prevalent. However, on St. Kitts and Dominica, variable H. caribaea-pollinator interactions were observed. In St. Kitts, the most generalized plant-pollinator interaction was observed where two additional species of hummingbirds (Green-throated Carib, Antillean Crested) visited H. caribaea across its temporal flowering season. In contrast, Dominica had the most species-specific plant-pollinator interaction where male and female Caribs were the dominant pollinators and Green-Throated and Antillean Crested hummingbirds could be occasionally observed only on non-defended clumps of H. caribaea. (Abstract shortened by UMI.)

Indexing (document details)
Advisor: Herendeen, Patrick S., Kress, Walter John
Commitee: Church, Sheri A., Gill, Douglas E., Lill, John T., Weiss, Martha R.
School: The George Washington University
Department: Biological Sciences
School Location: United States -- District of Columbia
Source: DAI-B 70/07, Dissertation Abstracts International
Subjects: Biology, Plant biology, Ecology
Keywords: Breeding system, Coevolution, Heliconia, Hummingbirds, Microsatellite, Plant phenology, Pollination, Population genetics, Tropical biology
Publication Number: 3366548
ISBN: 978-1-109-27990-0
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