Adaptive radiation in 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 four major floral types in the genus, indicate that this southern African genus of 52 species is cross pollinated by a wide variety of insects. The pollination ecology of Ixia species can be divided into several distinct systems exploiting insects of four insect orders (Coleoptera, Diptera, Hymenoptera, and Lepidoptera). Ixia atrandra, I. curta, I. lutea, I. maculata, I. metelerkampiae, and I. versicolor have salver-shaped, nectarless flowers, in bright colors contrasting with dark "beetle marks" and are pollinated exclusively by hopliine scarab beetles. Four Ixia species with narrowly tubular flowers, spreading tepals, and ample nectar are pollinated by long-proboscid flies (Moegistorhynchus longirostris and Philoliche species). Three additional species with tubular flowers, 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 largely pollinated by anthophorine bees or Apis mellifera, but bee pollination comprises three discrete systems. Species pollinated by Anthophora and Pachymelus species (I. capillaris, I. latifolia, I. odorata, I. rapunculoides, I. thomasiae) have cup-shaped flowers that secrete nectar. Salver-shaped flowers of I. flexuosa secrete no nectar, but are pollinated by pollen-collecting 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 anthers dehiscing incompletely from the base. Outgroup comparison suggests that the ancestral pollination system in Ixia is the one in which flowers are cup-shaped, produce nectar, and are pollinated by large anthophorine bees. Exaggeration of the perianth into an elongate tube containing ample nectar, or the closure of the perianth tube and absence of nectar, or the development of basal anther dehiscense must be regarded as specialized adaptations related to their derived pollination strategies.