Trapped for a noble cause

October 3, 2018

Harold Suarez Baron

Trapped for a noble cause

by Harold Suárez-Baron, Arnold Arboretum Deland Fellow

Harold Suarez Baron

Harold Suárez-Baron collects samples of Aristolochia macrophylla (Dutchman’s pipe) growing near the Hunnewell Building.

Flowering plants represent nearly ninety-five percent of all described species in the plant kingdom and display extraordinary diversity in their floral anatomy. This astonishing variation is paralleled by the sophisticated ways in which flowers are pollinated. For many plants pollination occurs through an interplay of visual, olfactory, and sensorial stimuli that arise between insects and flowers. Such interactions are spawned by a plethora of floral features, such as shape and symmetry, surface texture, hairs, scents and nectar production, and the existence of a broad palette of colors. This dynamic and complex process ensures the transfer of pollen from one flower to another, allowing for reproduction and seed formation.

Pollination in Dutchman’s pipevines (Aristolochia) is truly one of the most astonishing systems in flowering plants. Flies are its reproductive agent, attracted by the flower´s rank odor, which simulates decaying organic materials, carrion, or even fungi. This scent attraction can occur in combination with fly pheromone chemicals produced by the flower. Dull purple and maroon colors, and/or hairs called trichomes inside the flower can also increase the chance of a successful pollination event. The flower itself is pipe-shaped, forming a convoluted and tubular structure consisting of a balloon-like portion at its base called the utricle, followed by a narrow tube, and ending in an expanded, flattened portion called the limb. This atypical flower temporarily traps insects and releases them afterwards to increase the chances of successful pollen transfer.

Floral development in Aristolochia

The pipe-like flowers of A. macrophylla emerge in bud at less than a half of a centimeter in length from the axils of the leaves (above, far left). The bud develops into a flower in about 10 days, forming a balloon-like portion called the utricle, which houses the fused style and stamens; a narrow zone called the tube; and an expanded portion called the limb with a brownish-purple opening. When fully mature (above, far right) the flower is approximately 3.5 centimeters in diameter, and emits a pungent odor to attract the flies that serve as the vine’s pollinators. Flies enter through the limb and get trapped temporarily inside the floral tube, leaving pollen on the stigmas and becoming covered in pollen themselves as they seek an exit.

Pollination in Aristolochia takes two to three days, and begins with potential pollinators arriving on the limb, likely attracted by the colors and scent, often loaded with pollen from another flower. Small flies begin their journey by passing through the tube and reaching the utricle. This a road with no return as the tube is densely covered by downward-pointing trichomes covered in slippery wax that trap the flies and make their journey out more difficult. Thus, position, number, and density of trichomes in the tube and the utricle are an essential feature of the flower’s trapping mechanism. Once flies arrive at the utricle and begin their search for an alternative way out, the flowers provide a window effect—thinner areas beneath the pollen sacs and the receptive portion of the stigmas allow light to enter the inside of the shady utricle. The deception triggers the expected escape behavior and flies move around the reproductive structures, first leaving the pollen on the stigmas and afterwards gathering a fresh pollen load from the anther sacs. The pollen discharge and reload takes one to two days, during which time the flower nourishes the fly with nectar and water likely produced in the long multicellular trichomes that cover the inner utricle. Finally, once pollination is complete, the flower triggers its own decay, leaving the fertilized ovules and withering the rest of the structure, disarming the trichomes so that the insects can fly off to the next flower.

The genetics of trichome development has been studied in model plant species, but research is sparse across flowering plants and nearly unexplored in floral organs. Thus, the genes responsible for forming trichomes, as well as the timing of their activation, have been minimally compared across flowering plants. In that sense, Aristolochia flowers provide a spectacular atypical flower in which to assess these mechanisms. So far in my home university in Colombia, we have been able to sample neotropical as well as South American native taxa like Aristolochia fimbriata (white veined pipevine) as representatives of the herbaceous trichome-bearing species. However, some species like A. macrophylla, a deciduous, woody, climbing vine native to eastern North America, lack trichomes in the flower, allowing us to better understand why trichomes are present in some pipevines and absent in others.

As a Deland Fellow at the Arnold Arboretum, my research focuses on the genetic mechanisms underlying trichome development in Aristolochia flowers. This research comprises not only a wide-ranging comparative study of anatomical and functional traits in the flowers, but also a detailed evaluation of the genetic information that might determine the formation of these structures in the flower. Receiving the Deland award has offered me an exceptional opportunity to enrich my research as a graduate student, taking full advantage of the resources, outstanding living collections, and state-of-the-art laboratories at the Arnold Arboretum to carry out cutting edge research.

Together, this study will provide the first comparative and comprehensive data of its kind on the genus, which may allow us to build a more complete understanding of the genetic basis of trichome formation in Aristolochia flowers. With all this new knowledge, I hope to construct a clearer picture of how these ancient and unusual plants have successfully evolved to adapt and persist in different environments across our planet.

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