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Alula Images From Palomar College Hawaiian Garden
© Updated W.P. Armstrong 8 June 2019
  Polynesian Garden At Palomar College Arboretum  

Who Pollinated The Rare Hawaiian Alula?
References Used On This Page:

Some Information on this page is sumarized from Wayne's Word (waynesword.palomar.edu),  
including Botanical Record-Breakers, Arboretum Newsletters and Flower Terminology.

Johnson, S.D., & R.A. Raguso. 2016. "The Long-Tongued Hawkmoth Pollinator Niche For
Native and Invasive Plants in Africa.
" Ann. Bot. 117 (1): 15-36. doi: 10.1093/aob/mcv137.

Lammers, T.G. 1989. "Revision of Brighamia (Campanulaceae: Lobelioideae) a Caudiciform
Succulent Endemic to the Hawaiian Islands.
" Systematic Botany 4 (1): pp 133-138.

Wagner, W.L., D.R. Herbst, and S.H. Sohmer. 1999. Manual of the Flowering Plants
of Hawaii.
Volume 1. University of Hawaii Press, Honolulu, Hawaii. 988 p.

Walch, Seana. 2015. Floral Biology, Breeding System, Pollination Ecology, and Ex Situ Genetic
Diversity of the Endangered Hawaiian Species, Brighamia insignis A. Gray (Campanulaceae).
A
Thesis Submitted to the Graduate Division of the University of Hawai'i at Manoa in Partial
Fulfillment of the Requirements for the Degree of Master of Science In Botany. 102 p.

  Go To Section On Hawkmoth Pollination  
Alula (Brighamia insignis) is a rare member of the lobelia family (Campanulaceae) endemic to steep sea cliffs on the island of Kauai. As of the year 2000, fewer than 100 of these remarkable plants grew in the wild. Alula can no longer produce seeds in the wild because its native pollinator moth is now extinct. Some authors have suggested that its pollinator is the very rare & endangered green sphinx moth (Tinostoma smaragditis), rediscovered in eastern Kauai in 1998. In the wild alula was apparently pollinated by long-tongued hawkmoths through outcrossing (cross pollination). Luckily alula responds well to hand pollination. Like the California condor, this unique species has been brought back from the brink of extinction through breeding programs at botanical gardens. A remarkable phenomenon occurred in the Polynesian Garden at Palomar College. An alula plant produced seed capsules with viable seed and it was not hand pollinated by horticulturist extraordinaire & Arboretum president Tony Rangel! The seeds germinated and the plants are currently growing in the campus greenhouse. Its pollination mechanism remains a mystery--could it be another local long-tongued hawkmoth? Palomar College Professor Beth Pearson and two of her general botany students are attempting to solve this mystery during the fall & spring semesters of 2018-2019.

Alula is perfectly adapted for living on vertical volcanic cliffs. A single rosette of leaves arises from the top of a thick, succulent stem, like a cabbage head on a baseball bat. The rosette varies in size, depending on the availability of moisture. Roots penetrate the cliffs horizontally, and the base of the plant is rounded, permitting the plant to rock slightly in the wind. Water stored in the stem enables the plant to survive periods of drought which may last days or weeks.

Alula plants in the Palomar College Polynesian Garden showing their remarkable growth form. The simulated volcanic lava (faux rock) was created by horticulturist Tony Rangel.

Alula (Brighamia insignis), a rare member of the lobelia family (Campanulaceae) endemic to steep sea cliffs on the island of Kauai. In 1994 the United States Fish and Wildlife Service reported five populations totaling 45 to 65 individuals, and listed the plant as an endangered species. The IUCN lists only 7 mature plants in the wild. According to the U.S. Botanic Garden, its only pollinator was a certain type of now-extinct hawkmoth. This has made it impossible for B. insignis to reproduce on its own because individuals only produce seed when artificially pollinated by humans.

The flower is very different from members of the lobelia family on the mainland of North America. Another rare species with white flowers (B. rockii) grows on sea cliffs along the windward coast of Molokai. Like Hawaii's endemic silver sword alliance that evolved from an ancestral tarweed (Asteraceae), the alula is another example of adaptive radiation. According to Sherwin Carlquist (Hawaii: A Natural History, 1980), the Hawaiian lobeliads evolved from several ancestral introductions rather than a single original colonization; however, molecular data from Thomas J. Givnish of the University of Wisconsin (Evolution on Islands, 1998) indicate that they are monophyletic in origin and represent the product of a single introduction.

Alula In The Polynesian Garden At Palomar College

This alula (Brighamia insignis) produced fruit capsules with viable seed and it was not hand pollinated. Its pollination is a mystery--could it be another local long-tongued hawkmoth (family Sphingidae) or possibly a hummingbird? According to Lammers (1989), the nectar of alula is dominated by sucrose sugar, a characteristic of hawkmoth-pollinated (sphingophilous) flowers. Perching bird (passerine-pollinated) flowers produce hexose-dominated nectars. Argentine ants (Linepithema humile) have been observed on the flowers; however, ants are generally not considered major plant pollinators, with the exception of low-growing plants like spurges (Chamaesyce & Euphorbia). In fact, some ants even give off chemicals, such as antibiotics to protect their nests, that are harmful to pollen. This is explained in the classic book The Ants by Bert Hölldobler & Edward O. Wilson (1990). In fact, Hölldobler & Wilson state "Given the abundance and antiquity of ants, it is puzzling to find that they play a relatively minor role as pollinators." It would be interesting to observe our Argentine ants more closely to see if their bodies contain alula pollen.


Two alula flowers showing immature stigma (left) and mature, receptive stigma (right). Pollen is present on the base of several petals (corolla lobes). Assuming this pollen is from the included anther column, is it viable and could it be used to pollinate this receptive stigma?


Tricolporate pollen from alula flower with receptive stigma. The grains have a triangular shape with 3 furrows or grooves in the exine (outer layer of grain). Presumably the stigma matured and was receptive after the anthers shed pollen. Most of the grains in this image look viable, but I haven't tested them. There are stains to test pollen viability. I have used acetocarmine on Brodiaea pollen. In Brodiaea, viable grains turn blue and smaller, abortive grains remain empty and colorless. Photographed through 100x objective on compound microscope and zoomed up to at least 200x magnification with Sony W300 digital camera.


The tuft of white hairs at apex of anther column is visible in the corolla throat. Pollen from anthers below the apical tuft is evident on the bases of petals.


Close-up view of the tuft of white hairs at apex of anther column.


Although anther column is not exerted, pollen still gets on the bases of petals of this flower.


Floral tube is about 10.5 cm (over 4 inches) on this flower.


This dissected flower shows the very long style and staminal column that is attached below the middle of floral tube. According to Lammers (1989) the corolla tube (floral tube) is entire at anthesis but later splits dorsally for about 1/3 its length.


Hawkmoth Pollination

We Have Native Long-Tongued Hawkmoths In San Diego County!

Pollination Of South African Gardenia thunbergia In Twin Oaks Valley

The striking blossom of Gardenia thunbergia is quite different from the common gardenias (G. jasminoides) cultivated in southern California gardens. It consists of a rosette of eight or nine white petals at the end of a slender floral tube five inches (12 cm) long. Nectar glands at the base of the long floral tube produce copious amounts of sweet fragrant nectar, but it is inaccessible to most insects and birds. A tough green calyx sheath at the base prevents large bees, such as carpenter bees (Xylocopa) and bumble bees (Bombus), from chewing through the delicate inner corolla tube. The opening of the corolla tube is lined by a ring of eight anthers which are heavily laden with pollen.

Left: The scented flower of Gardenia thunbergia consists of eight or nine petals at the end of a slender floral tube. Only long-tongued hawkmoths can reach the sweet nectar deep inside the floral tube. Right: Longitudinal section of a flower showing the long, slender floral tube. The entrance to the floral tube is surrounded by a ring of eight anthers that shed copious pollen.

Seed capsule presumably resulting from pollinated blossom of Gardenia thunbergia during a previous summer flowering season. Seed capsule photographed 5 June 2019 at my home in Twin Oaks Valley, San Marcos, CA. Note: This woody capsule requires a South African kudu to chew it open and release the seeds.

In its native habitat, the blossoms of G. thunbergia are visited by nocturnal hawkmoths of the genus Agrius (family Sphingidae), similar to our western North American Manduca moths that pollinate jimsonweeds (Datura wrightii). They are called sphinx moths because the alarm posture of some larvae resemble the form of the Egyptian sphinx. Some hawkmoths have tongues (proboscides) up to 5.5 inches (14 cm) long (or longer), long enough to reach the plentiful supply of nectar deep within the floral tube. The proboscis roles up into a neat, compact coil when the moth is not feeding. When the proboscis is inserted into the floral tube, it passes through a ring of eight stamens which are heavily laden with pollen. Pollen adheres to the sticky proboscis and other furry body parts and is transferred to other flowers as the moths feed. Anthers of individual flowers mature and shed pollen before the stigma is receptive, a maturation sequence conducive to cross pollination. This maturation sequence is called protandry (1st + male).

The African hawkmoth (Agrius convolvuli) pollinates Gardenia thunbergia and at least 13 additional species with long corolla tubes, including Crinum, Gladiolus, Rangaeris, and Harveya (Johnson & Raguso, 2016). It seems entirely possible that our local hawkmoths (Hyles lineata and/or the longer-tongued Manduca sexta) could pollinate Gardenia thunbergia in San Diego County, although I suppose there could be other pollinators during daytime hours. It is interesting to note that the African moth Agrius has a Hawaiian relative, the pink-spotted hawkmoth (Agrius cingulata).

There is an extremely rare gardenia shrub (Gardenia brighamii) endemic to the Hawaiian Islands. According to the Hawaiian Native Plant Propagation Database, this species is self-compatible and viable seed is produced by self pollination. Another African species (G. volkensii) has night-blooming flowers and woody fruits like G. thunbergia and is presumably moth pollinated.

Johnson, S.D., & R.A. Raguso. 2016. "The Long-Tongued Hawkmoth Pollinator Niche For Native and Invasive Plants in Africa." Ann. Bot. 117 (1): 15-36. https://doi.org/10.1093/aob/mcv137

The beautiful "pink-spotted hawkmoth" (Agrius cingulata) on the island of Maui. It pollinates tubular flowers at night with its long proboscis, and is related to our tomato hornworm (Manduca sexta). It is native to the New World tropics, Galapagos and Hawaiian Islands. As a migrant, it flies north to Canada and south to Patagonia and the Falkland Islands. Why couldn't this moth pollinate alulas in cultivation? It is also called sweet potato hornworm because the larvae feed on sweet potatoes (Ipomoea batatas). Both A. cingulata & A. convolvuli are known to visit morning glory and petunia flowers and I am reasonably certain our local hawkmoths also visit these flowers (especially Hyles lineata). By the way, the campus Polynesian Garden contains the ornamental sweet potato. In addition, Seana Walch of the National Tropical Botanical Garden, Kauai reported a video of Agrius cingulata visiting a potted alula plant on Maui (personal communication, 2019).

White-Lined
Sphinx Moth
(Hyles lineata)

White-lined sphinx moth hovering while feeding on the nectar of native wildflower (Penstemon spectabilis). Note the long proboscis that extends into corolla tube. This is a very common hawkmoth in the desert and coastal areas of San Diego County.

 More Images Of Hyles 
lineata & Penstemon

  Native Long-Tongued Hawkmoths In San Diego County  
Pollination Of The Night-Blooming Gardenia thunbergia

Conclusions & Remarks

1. According to T.G. Lammers (1989), the flowers of alula are protandrous. This means that the anthers mature and release pollen before the stigma is receptive, a plant strategy favoring cross pollination.
  Flower Terminology: Strategies For Insuring Cross Pollination  

2. According to the Hawaiian Native Plant Propagation Database, when the flowers are a couple of days old, the pollen will begin being shed. Use a small paint brush to pick up pollen that has fallen onto the floral tube and apply the pollen to the stigma of "another" flower. The stigma is ready to receive the pollen when it appears shiny and sticky. By "another" flower, I assume they mean a different plant; however, I would like to try the same flower. I have seen ample pollen on the same flower with a receptive stigma.

3. On some of the above images there is abundant pollen on the bases of petals in flowers with receptive stigmas. Is this viable pollen? According to Tony Rangel, our alula plants came from seed from the San Diego Zoo (circa 2007-2009). The Zoo acquired their seed from NTBG (National Tropical Botanical Garden). Since the original plants on the cliffs of Kauai are so few in number, the genetic variability in the seed from hand pollinated plants is undoubtedly low. I originally thought our plants required cross pollination because they are protandrous (anthers release pollen before stigma is receptive); however, I am puzzled by pollen in flowers with receptive stigmas. Ideally, we should try hand pollination on our plants using pollen from same flower as receptive stigma.

4. Based on this dissected flower, slits (splits) in floral tube of alula flowers do not extend into the style. After pollination at the stigma, the pollen grain forms a pollen tube that grows all the way down the style into the ovary where the ovules are located. The pollen tube penetrates a tiny receptive ovule and injects 2 sperm cells. If double fertilization is successful this tiny ovule will develop into a seed. Each ovule must receive a separate pollen tube in order to develop into a seed. Unfertilized ovules simply abort.

Longitudinal view of alula ovary.

Flowering Plant Double Fertilization
  Plant Sexuality & Political Correctness  

5. When a long-tongued hawkmoth inserts its long proboscis deep into the floral tube, it is rewarded with sweet, nourishing nectar produced by nectaries deep in the floral tube. The moth will undoubtedy get pollen on its hairy face in the process. On the arrival at another flower, possibly on a different plant, some of this pollen may be transferred to the sticky, receptive stigma exerted from the flower. This is cross pollination. Whether this event can occur on alula plants with native hawkmoths of San Diego County is unknown to me, but it certainly seems plausible. The tomato hornworm moth Manduca sexta certainly has a long enough tongue (proboscis).

6. The bottom line here is how does an alula flower set seed in the Polynesian Garden without hand pollination? In other words, who is pollinating it? Is it self-pollinated by some unknown mechanism? Seana Walch of the National Tropical Botanical Garden reported a self-pollinated alula flower (bagged in bud and left unmanipulated) that set fruit with viable seeds (personal communication, 2019).

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