Adaptive radiation of bee-pollinated Gladiolus species (Iridaceae) in southern Africa

Observations on the flowers of 45 of 166 species of southern African Gladiolus (in sects. Blandus, Densiflorus, Hebea, Heterocolon, Homoglossum, Linearifolius, Ophiolyza) show that 42 species are pollinated largely by polylectic bees in the family Apidae, 2 species by bees of the families Andrenidae or Halictidae, and 1 by a combination of hopliine beetles (Scarabaeidae) and Andrenidae. The floral phenology, attractants, diversity of floral foragers, and sometimes the rewards, vary according to geography and are not correlated with taxonomy. Flowering in most Gladiolus species in the southern African winter-rainfall zone coincides with the end of the wet season, August to October, but a few flower from February to April at the end of the dry season. They have sweetly fragrant flowers with a wide range of colors and markings. These species receive a diversity of floral foragers including bees in the genera Allodape, Amegilla, Andrena, Anthophora, Apis, and Pachymelus. Most Gladiolus species of the summer-rainfall zone flower from December to April, from the middle to the end of the wet season, but a few species bloom from August to November, at the end of the dry season. Their flowers have no discernible fragrance and are colored mostly in shades of pink to mauve or white. Most floral foragers collected on these species were bees in the genus Amegilla, but other bee genera, as well as flies in the genera Prosoeca and Stenobasipteron, were captured. Among the southern African species of Gladiolus pollinated by bees, there are two distinct pollination strategies. The majority have bilabiate, "gullet" flowers or ’flag" flowers that secrete sucrose-rich nectar at the base of an obliquely funnel-shaped floral tube 9-20 mm long with the lower, narrow part 5-15 mm long. Apis mellifera and large-bodied anthophorine bees (especially Amegilla and Anthophora) contact dehiscent anthers and/or receptive stigmas while probing the tube for nectar with elongated probosces. In contrast, G. brevitubus, G. quadrangularis, and G. stellatus have rotate, actinomorphic (or subactinomorphic) perianths offering little or no nectar at the base of tubes less than 7 mm long. Andrena species or Apis mellifera contact both dehiscent anthers and receptive stigmas of G. stellatus or G. quadrangularis, respectively, while foraging for pollen. An additional 53 Gladiolus species also have bilabiate, gullet or flag flowers with obliquely funnel-shaped tubes 920 mm long (the most common flower type in the genus), and are presumed also to be adapted for pollination by long-tongued anthophorine and honey bees. The actinomorphic, rotate floral form is present in 2 more species. Thus, 60% of the Gladiolus species in southern Africa may be regarded as being pollinated by bees, and the overwhelming majority of these species (95%) have gullet or flag flowers and are visited primarily by long-tongued anthophorine bees that are foraging for nectar. The remaining species of Gladiolus in southern Africa have flowers with elongate perianth tubes and are adapted for pollination by sunbirds or insects other than bees, most importantly long-tongued flies (Nemestrinidae, Tabanidae), moths, and the large satyrid butterfly, Aeropetes.