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Identification of Floral Visitors of Iris nelsonii
Sunni J. Taylor, Kendall J. AuBuchon, and Noland H. Martin

Southeastern Naturalist, Volume 11, Issue 1 (2012): 141–144

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Identification of Floral Visitors of Iris nelsonii Sunni J. Taylor1,*, Kendall J. AuBuchon1, and Noland H. Martin1 Abstract - Floral visitors of the homoploid hybrid species, Iris nelsonii (Abbeville Red Iris), were observed and identified in order to determine whether Abbeville Red Iris flowers are visited by similar floral visitors as its progenitor Iris species. The most common floral visitors to Abbeville Red Iris flowers were Archilochus colubris (Ruby-throated Hummingbird), which were also largely successful in transferring a pollen dye-analogue between Abbeville Red Iris flowers. Other floral visitors included butterflies, wasps, and bee species. The pollinators of the Abbeville Red Iris have not been previously documented. These results suggest that pollinator isolation may be important in preventing hybridization between the Abbeville Red Iris and its geographically closest progenitor species, I. hexagona (Dixie Iris). Many closely related plant species express divergent floral characteristics (e.g., color, flower size, inflorescence size, nectar concentration and quantity) and are visited by different pollinator functional groups (Fenster et al. 2004). Natural hybridization between these species may result in hybrids that exhibit parental, intermediate, or extreme floral trait values. In many cases, the hybrid flowers are less attractive to potential pollinators than pure-species flowers (e.g., Campbell et al. 1997). However, some hybrid flowers may be equally or more attractive to pollinators as the pure species flowers (e.g., Emms and Arnold 2000, Martin et al. 2008, Sutherland and Vickery 1993, Wesselingh and Arnold 2000), or may even potentially attract a new suite of pollinators (Straw 1955). If the hybrid lineage is pollinated by a different suite of pollinators, ethological pollinator isolation may act to prevent gene flow between the hybrid lineage and the originally hybridizing species (Chase et al. 2010, Straw 1955). Here, we observed pollinators of the homoploid hybrid species, Iris nelsonii Rand (Abbeville Red Iris), to determine whether ethological pollinator isolation may operate in this system. The Abbeville Red Iris is a homoploid hybrid species derived from hybridization between Iris brevicaulis Raf. (Zigzag Iris), Iris hexagona Walter (Dixie Iris), and Iris fulva Ker Gawl. (Copper Iris) (Arnold 1993, Arnold et al. 1990, Randolph 1966). Zigzag Iris and Dixie Iris flowers are blue with prominent nectar guides and stiff sepals and are primarily pollinated by bumblebees (Emms and Arnold 2000, Martin et al. 2008). Copper Iris flowers are red with reflexed sepals and no nectar guides, and are primarily visited by hummingbirds (Emms and Arnold 2000, Martin et al. 2008, Wesselingh and Arnold 2000). Abbeville Red Iris flowers are red (Fig. 1), typical of a hummingbird pollination syndrome, but the primary floral visitors of the Abbeville Red Iris have not been previously documented. In order to determine the floral visitors of the Abbeville Red Iris, we observed floral visitation in two localities within the restricted range of the Abbeville Red Iris in Vermillion Parish, LA. Visitation was recorded on 8 April and 15–17 April 2011. Iris flowers have three pollination units, each of which is composed of a sepal and a stylar branch subtended by a single anther and the nectary (Fig. 1). When a pollinator attempts to access the nectar, pollen is deposited on the head or body of the pollinator. When the pollinator visits the next flower, pollen is deposited onto the stigmatic surface that folds down in front of the anther. The shape of the flower is such that visitors can access the reward “legitimately” through 1Texas State University-San Marcos, Department of Biology, 601 University Drive, San Marcos, TX 78666. *Corresponding author - Notes of the Southeastern Nat u ral ist, Issue 11/1, 2012 141 142 Southeastern Naturalist Vol. 11, No. 1 the pollination unit (Fig. 2a) or “illegitimately” by accessing the nectary directly from the side or top of the flower (Fig. 2b). Visitors were described as “legitimate” if they visited the pollination unit of flowers in such a way that pollen transfer was possible (Fig. 2a). Visitors were described as “illegitimate” if the pollinator attempted to access the nectar or pollen reward without visiting the pollination unit (Fig. 2b). In order for a floral visitor, even a “legitimate” visitor, to act as an effective pollinator, it must first pick up pollen from the anthers and transfer that pollen to the stigma of another flower. On 17 April 2011, for a portion of the floral visitors to the Abbeville Red Iris flowers, pollen transfer success was examined by applying a powder fluorescent dye to the anthers of focal flowers with a paintbrush and noting the success of dye transfer to flowers visited by the potential pollinator once it visited the focal flower. If dye was observed on or very near the stigmatic surface upon initial visual inspection, the visit was considered a successful transfer. If dye was not visible upon initial inspection, the flower Figure 1. Typical Abbeville Red Iris flower. Iris flowers have three pollination units, each of which is composed of a sepal (A) and a stylar branch (B) subtended by a single anther and the nectary. 2012 Southeastern Naturalist Notes 143 was removed and inspected under an ultraviolet light in a dark room. If no dye was found upon further inspection, the visit was considered an unsuccesful transfer. A total of 67 visiting bouts were recorded during the 4 days of observation, most of which included visits to multiple flowers. Forty-four bouts were classified as legitimate, 18 were classified as illegitimate, 1 bout contained both legitimate and illegitimate visits, and the legitimacy of 3 of the bouts could not be determined. A majority of the legitimate visits (39/44, 88.6%) were made by Archilochus colubris L. (Ruby-throated Hummingbirds). Of the 39 visits made by hummingbirds, all were classified as legitimate. Two butterfly species, Danaus plexippus L. (Nymphalidae; Monarch Butterfly) and Phoebis sennae L. (Pieridae; Cloudless Sulphur), and a black capenter bee, Xylocopa sp., also made legitimate visits to I. nelsonii flowers, though the effectiveness of these insect species as pollen transfer agents was not documented as none of the individuals visited dyed focal flowers. The Monarch Butterfly made 2 observed legitimate visits and 0 illegitimate visits. Cloudless Sulphur butterflies made 3 visiting bouts for a total of 8 flowers and visited 6 of the 8 flowers legitimately. A Papilio polyxenes Fabricius (Nymphalidae; Black Swallowtail) individual appeared to visit legitimately, but the observer’s sight line was impaired, so the legitimacy of the Black Sallowtail visit was classified as unknown. Illegitimate floral visitors included a variety of wasps, Thorybes sp. (Hesperiid Butterfl y) and Apis mellifera L. (Honey Bee). Fifteen wasp visiting bouts were recorded. In a majority of the recorded visits, the wasps visited the flower illegitimately and robbed nectar. However, for 2 of the 15 visits, sight lines of the observers were impaired so the legitimacy of the visits was classified as unknown. Archilochus colubris (L.) (Ruby-throated Hummingbird) were the only floral visitors that we assayed for pollen transfer success (April 17). Hummingbirds were largely successful at transferring the pollen analogue from the dyed flower to the undyed flower visited immediately after the dyed flower. Of the 10 primary transfer data points that were collected, 8 of the visited flowers received dye, while the remaining 2 flowers did not receive dye. Hummingbirds were also successful at transferring the pollen analogue to subsequent flowers, although not all subsequently visited flowers were collected to view under ultraviolet light. Of those collected, dye was present on the second and third flowers visited in the same bout, but pollen was not detected on a second flower visited in a different bout. The floral visitors of the Abbeville Red Iris are largely different from two of its progenitors, the Zigzag Iris and the Dixie Iris (its geographically closest progenitor). However, the Abbeville Red Iris shares primary pollinators with the Copper Iris, Figure 2. Example of legitimate visitation (A) and illegitimate visitation (B) to an Abbeville Red Iris flower. 144 Southeastern Naturalist Vol. 11, No. 1 suggesting that other barriers (e.g., habitat isolation; Randolph 1966) are responsible for isolation between the Abbeville Red Iris and the Copper Iris. Hummingbirds are largely successful at intraspecific Abbeville Red Iris pollen transfer (this study) and intraspecific Copper Iris pollen transfer (N.H. Martin, unpubl. data). However, the two species differ in floral morphology (Randolph 1966), and the ability of hummingbirds to transfer pollen between flowers of these two species is still unknown. Studies of reproductive isolation between the Abbeville Red Iris and its progenitors are continuing to investigate the mechanisms that prevent gene flow between this geographically restricted species and its progenitors. Acknowledgments. The authors thank Warren Perrin and the Broussard family for access to the sampling localities; Amity Bass, Christopher Reid, and Mark Schexnayder from the Louisiana Department of Wildlife and Fisheries for their work on the conservation of I. nelsonii; and Chris Nice and Tony Bosworth for assistance in visitor identification. This study would not have been possible without the invaluable assistance of John Joseph. We also thank Kevin from Pop-a-lock. This study was funded by NSF DEB-0816905 to N.H. Martin and the American Iris Society Foundation to S.J. Taylor. S.J. Taylor was also supported by National Science Foundation DGE-0742306. Literature Cited Arnold, M.L. 1993. Iris nelsonii: Origin and genetic composition of a homoploid hybrid species. 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