Floral Conservatism in Neotropical Malpighiaceae

Over 950 species of Malpighiaceae grow in a variety of neotropical habitats and have evolved great diversity in habit, fruit, pollen, and chromosome number. Their flowers, in contrast, tend to be very similar in general architecture, especially in those aspects concerned with the attraction, orientation, and reward of pollinators. The flowers are visited only by Hymenoptera, principally female anthophorid bees and rrigonid bees. The anthophorids collect oil from the calyx glands, mix it with pollen, and use the mixture as food for their iarvae. The trigoilids collect pollen. The usual character-syndrome of the flower seems to be related to and maintained by pollination by oil-bees, and was probably ancestral in the family. Pollination by pollen-collecting bees is probably secondary in many genera and has shifted to primary importance in groups that have lost the calyx glards. Other families such as Polemoniaceae, which have evolved very diverse flowers, reward pollinators with a sugary nectar that attracts a variety of secondary pollinators. This faunal cliversity provides a bridge betm-een one character-syndrome in the flowers and another. The specialized rewards and resulting lack of diversity in pollinators in neotropical Malpighiaceae explain why the flowers have remained so conservative in spite of the evolution of great diversity in other aspects of the phenotype. Calyx glands seem to have been lost in most paleotropical lines in the absence of oil-bees, but field observations on the pollination of those plants are practically nonexistent. THEMALPIGHIACEAE are a family of mostly tropical fruits without obvious adaptations for dispersal (Anflowering plants, currently classified in approximately derson 1977) 60 genera comprising at least 1100 species. Of these, While the Malpighiaceae show great diversity in 950 species in 47 genera grow only in the Nen other respects, their flowers are at least superficially World (Niedenzu 1928, plus since-discovered taxa, very similar. Figure 1 is a comparison of three genera many of them still undescribed). The neotropical with very different habits and fruits but similar Malpighiaceae occupy a variety of habitats from flowers. It is the purpose of this paper to consider southwestern United States to temperate Argentina, why the flowers of this family have remained so with the great majority of the species between the similar while great changes were occurring in many tropics. They are most commonly found in relativeother structures. ly open habitats, from woodlands through dense and First, it is necessary to describe the structure and sparse savannas ( in the broad sense, including cervariation of the flowers, in order to decide in which rados), to open grasslands. Margins of rivers and respects they are actually conservative. The calyx forests are also common habitats, and some grow in comprises five free or basally connate sepals; four tall forests. A few species have become adapted to or all five sepals usually bear two large multicellular xeric habitats; none has succeeded in breaching the glands ("elaiophors") on the abaxial side, which barrier of cold weather that apparently bars the produce fatty oils (Vogel 1974). These glands are family from more "temperate" latitudes and from the reduced or absent in diverse populations or species high Andes in South America. As in most large of many genera and in most species of Galphimia, groups of plants with such wide ecological and and completely absent in all species of five small geographical amplitude, the Malpighiaceae have genera (Coleostachys, Echifzopterys, Lasiocavpas, Ptilevolved great diversity in many respects, most notably ochaeta, and Thryall isj . The five free petals are contheir habit and fruits (Niedenzu 1928), their pollen spicuously clawed, and often reflex between the (Lobreau 1967 ) ,and chromosome numbers (Andersepals, so that the calyx glands are readily accessible son 1977, Fouet 1966). Habit varies from large trees to an insect that has landed in the middle of the in lowland wet forests, to medium-sized trees to flower. The petals are most often carrot-yellow, shrubs to perennial herbs, and from large woody lemon-yellow, pink, or white. Blue petals are exceedlianas to small herbaceous (but perennial) vines. ingly rare, being found in a few species of iMascaglzia, Various groups produce bird-dispersed drupes, which and petals that are dark red at anchesis are also very I believe to have originated three times in the family, rare, although pink or white or even yellow petals a great variety of wind-dispersed schizocarpic samamay turn red with age, presumably an adaptation roid fruits, fruits adapted for dispersal by water, that prevents pollinated flowers from diverting the and diverse schizocarpic or indehiscent nutlet-like attention of insects from younger flowers. The limb FIGURE 1. Comparison of three genera of Malpighiaceae. a-c, Byrsonima: a tree, 12 m tall; b, flower, X 3.5; c, drupaceous fruit, X 3.5. d-f, Camarea: d, perennial herb, 40 cm tall; e, flower, X 3.5; f, dry fruit breaking apart into three aculeate nutlets, X 7. g-i, Mascagnia: g, vine in shrub, 2 m high; h, flower, X 3.5; i, dry fruit breaking apart into three samaras, X 2. Drawn bv Karin Douthit.