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Pollination Biology of Lapeirousia subgenus Lapeirousia (Iridaceae) in southern Africa; floral divergence and adaptation for long-tongued fly-pollination 1

Peter Goldblatt 2, John C. Manning 3 & Peter Bernhardt 4

1. Support for this study by grant 4816-92 from the National Geographic Society is gratefully acknowledged. We thank C. D. Michener and R. W. Brooks, Snow Entomological Museum, University of Kansas, for the anthophorid and megachilid bee identifications, and W. Whitehead, South African Museum, Cape Town, for the loan of Parafidelia bees seen foraging on Lapeirousia barklyi; B.­E. van Wyk, Rand Afrikaans University, Johannesburg, for the nectar analyses; the South African Department of Environment Affairs and the Cape Department of Nature and Environmental Conservation for providing collecting permits; and Donna Stevens for the insect illustrations.
2. B. A. Krukoff Curator of African Botany, Missouri Botanical Garden, P. O. Box 299, St. Louis, Missouri 63166, U.S.A.
3. Compton Herbarium, National Botanical Institute, P. Bag. X7, Claremont 7735, South Africa.
4. Department of Biology, St. Louis University, St. Louis, Missouri 63103, U.S.A.

Originally published in the Annals of the Missouri Botanical Garden 83: 67-86. 1996. Copyright Missouri Botanical Garden 1996.


Abstract | Materials and Methods | Results | Discussion | Literature

ABSTRACT

Lapeirousia subgenus Lapeirousia (Iridaceae) consists of 21 species endemic to sub-Saharan Africa, with the mediterranean southwestern part of southern Africa the center of diversity. Analysis of the floral ecology of 20 species shows that the majority secrete large amounts of sucrose-rich or sucrose-dominant nectar and the 16 species for which pollinator observations were obtained are cross-pollinated exclusively by insects with elongated mouth parts, representing three orders (Diptera, Hymenoptera, and Lepidoptera). In the absence of pollinators, at least four species (L. arenicola, L. jacquinii, L. oreogena, L. simulans) were found to set seed without cross pollination and are assumed to be mechanically autogamous. The 20 species segregate into three pollination types based on divergent patterns of pigmentation, scent production, perianth tube length, and pollinator taxa. Species with flowers of the L. silenoides-type are pollinated exclusively by Prosoeca species (Diptera: Nemestrinidae), and species with flowers of the L. fabricii-type are pollinated exclusively by Moegistorhynchus longirostris (Diptera: Nemestrinidae) and Philoliche gulosa (Diptera: Tabanidae). The latter two fly species, together with the Prosoeca species, have the longest mouth parts of all pollinators examined on species of subgenus Lapeirousia. The L. divaricata-type flower is pollinated by a combination of bees, mostly female Anthophoridae (e.g., Anthophora, Amegilla, Tetraloniella) and native Apis mellifera (Apidae), and to a lesser extent by bombyliid flies and some Lepidoptera. Observations of insect pollen load analysis suggest that the evolution of these three flower types has encouraged ethological isolation between species, but is a small component in broader pollination guilds encompassing co-blooming species in other genera and families, including Asteraceae, Geraniaceae, Scrophulariaceae, and Sterculiaceae. Mapping of pollination syndromes on a phylogenetic tree of subgenus Lapeirousia demonstrates the extreme adaptive radiation in the subgenus and the convergent development of the same pollination strategy repeatedly across the subgenus.

Africa is the center of diversity for the family Iridaceae, with the majority of species concentrated in the temperate and mediterranean regions in the southern part of the continent (Goldblatt, 1978; 1994). One striking feature of the iridaceous flora of southern Africa is the variation in floral form and color (see Marloth, 1917-1932; Jeppe, 1989). By far the greater proportion of species of Iridaceae in southern Africa have flowers in which the perianth forms an elongated tube and the stamens and style are often prominently displayed. Although the tepals of such flowers do not have the characteristic shape found in Iris or Moraea, these tubular flowers exhibit a wide variety of colors, color patterns, and tepal orientation. The floral ecology of Iridaceae with exaggerated floral tubes has received less attention than those genera of the family with gullet blossoms (sensu Faegri & van der Pijl, 1979).

Within the southern African Iridaceae there are genera with more typical Iris-type flowers in which each flower is a meranthium. That is, each flower functions as three separate reproductive units, each consisting of a stamen and a stigma hidden between ornate style crests and a spreading tepal (e.g., Moraea, Dietes, Gynandriris). Initial work on the floral ecology of Moraea (Vogel, 1954; Goldblatt et al., 1989) suggested that this genus has a pollination mechanism similar to those Iris species in the Northern Hemisphere (Faegri & van der Pijl, 1979) in which the pollination biology has been studied. Here heavy insects (usually bees and wasps) depress the drooping tepal to probe the floral interior for nectar and contact the sexual organs of the flower.

Most of the literature on the pollination ecology of tubular-flowered Iridaceae in southern Africa consists of little more than brief descriptions (Scott Elliot, 1890, 1891; Marloth, 1908; 1917-1932; Vogel, 1954; Johnson, 1992). This literature suggests that Iridaceae with tubular flowers may be pollinated by a wider variety of animals with much elongated mouth parts, including sunbirds, bees, flies in the families Nemestrinidae, Tabanidae, and Bombyliidae, sphinx moths, and other Lepidoptera, notably Meneris tulbaghia (Satyridae). Both the taxonomic literature and the brief reports of pollination observations indicate that tubular flowers specialized for pollination by these vectors have evolved in many families distributed through southern Africa, including the Amaryllidaceae, Crassulaceae, Ericaceae, Geraniaceae, Orchidaceae, Proteaceae, Scrophulariaceae, etc. (Vogel, 1954, Johnson, 1992; Johnson & Bond, 1994; Johnson et al., 1993).

Long-term fieldwork and quantitative analysis of the floral ecology of tubular-flowered Iridaceae has been confined so far to work on the genus Nivenia (Goldblatt & Bernhardt, 1990). Flowers in this genus are homostylous or heterostylous and are pollinated primarily by long-tongued flies in the genus Prosoeca (Nemestrinidae) and secondarily by female anthophorid bees. Is this pollination syndrome characteristic of other tubular-flowered Iridaceae in southern Africa?

Lapeirousia represents a far more promising genus for fieldwork on floral ecology of the African Iridaceae. This genus consists of 41 species in two subgenera, of which subgenus Lapeirousia has the more visibly diverse and specialized floral forms. Subgenus Lapeirousia is also easier to study, since many of its species have a narrowly circumscribed distribution, compared with the almost pan-sub-Saharan African subgenus Paniculata (Goldblatt, 1990). Nineteen of the 21 species in subgenus Lapeirousia occur along the west coast and near interior of southern Africa and flowering sites for most species are easily accessible (Goldblatt, 1972; Goldblatt & Manning, 1994). Two more species, L. littoralis Baker and L. odoratissima Baker, are widespread across south tropical Africa, and are thus outside our study area. To elucidate the dynamics of pollination in Lapeirousia, observations will be compared with results of the earlier study of Nivenia and with what is known about the pollination ecology of tubular flowers of southern Africa in general.

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