Adaptive radiation of pollination mechanisms in Ixia (Iridaceae : Crocoideae)

Field observations, floral dissections, and pollen load analyses of insects captured on 20 species of Ixia (Iridaceae), representing examples of the f our major floral types in the genus, indicate that this southern African ge nus of 52 species is cross pollinated by a wide variety of insects. The pol lination ecology of Ixia species can be divided into several distinct syste ms exploiting insects of four insect orders (Coleoptera, Diptera, Hymenopte ra, and Lepidoptera). Ixia atrandra, I. curta, I. lutea, I. maculata, I. me telerkampiae, and I. versicolor have salver-shaped, nectarless flowers, in blight colors contrasting with dark "beetle marks" and are pollinated exclu sively by hopliine scarab beetles. Four Ixia species with narrowly tubular flowers, spreading tepals, and ample nectar are pollinated by long-probosci d flies (Moegistorhynchus longirostris and Philoliche species). Three addit ional species with tubular Rowers, and modest nectar volumes, appear to be pollinated by the pieriid butterfly, Colias electo (Ixia orientalis), or by a combination of hopliine beetles and tabanid flies with short probosces ( I. aurea, I. esterhuyseniae, I. tenuifolia). The remaining species are larg ely pollinated by anthophorine bees or Apis mellifera, hut bee pollination comprises three discrete systems. Species pollinated by Anthophora and Pach ymelus species (I. capillaris, I. latifolia, I. odorata, I. rapunculoides, I. thomasiae) have cup-shaped flowers that secrete nectar. Salver-shaped fl owers of I. flexuosa secrete no nectar, but are pollinated by pollen-collec ting Apis mellifera. Buzz pollination by Amegilla fallax in I. scillaris is associated with vertical floral presentation, nectarless flowers, unusual in having a short, closed perianth tube, short, stubby filaments, and anthe rs dehiscing incompletely from the base. Outgroup comparison suggests that the ancestral pollination system in Ixia is the one in which flowers are cu p-shaped, produce nectar. and are pollinated by large anthophorine bees. Ex aggeration of the perianth into an elongate tube containing ample nectar, o r the closure of the perianth tube and absence of nectar, or the developmen t of basal anther dehiscense must he regarded as specialized adaptations re lated to their derived pollination strategies.