Floral Biology of Myristica fragrans Houtt. (Myristicaceae), the Nutmeg of Commerce

Abstract

The flowering and pollination of Myristica fragrans were studied on plants cultivated in southern India. The staminate flowers are borne in indeterminate inflorescences and function only for a single night. Pistillate flowers are borne solitarily and appear receptive for 2-3 days. Staminate plants may produce over 50 times as many flowers as pistillate plants. Both types of flowers are strongly fragrant. A single species of beetle, Formicomus braminus (Anthicidae, ant-mimicking flower beetles), appears to be the best candidate for an effective pollinator of nutmeg in the area of the study, although other species of small beetles may be involved. The beetles are strong fliers, are active nocturnally around staminate flowers, and carry significant pollen loads. Pollen is the only evident pollinator reward. Pistillate flowers produce no reward, and spatial constraints of the perianth and stigma appear to prevent beetles from entering the flowers. Pollen carried on the beetle's head or body would be deposited on the stigma when the insect attempts to enter the flower. This is the first report of a food-deception automimicry in a cantharophilous pollination system. The pollen/ ovule ratio for M. fragrans is estimated to be 801,000/1, a very high P/0 ratio for an insect-pollinated plant. THERE ARE SEVERAL REASONS TO SUSPECT that the four Southeastern Asian genera of Myristicaceae may be beetle pollinated. These include (1) the predominance of white or cream to light-yellow flowers, (2) strong nocturnal fragrance, (3) lack of well-defined nectaries or other secretory structures (Wilson & Maculans 1967, Armstrong & Wilson 1978) (although reports of nectaries persist, e.g., Cobley 1976), and (4) the prevalence of beetle pollination among other members of the Annonales (Thien 1974, 1980; Faegri & van der Pijl 1979). However, flowers of Myristica, Horsfieldia, Knema, and Gymnacranthera differ morphologically from typical beetle-pollinated, annonalean flowers in several important respects. These genera are dioecious, as are all Myristicaceae, although there are reports of monoecious species in Iryanthera (Smith 1937). The flowers are small and do not form a broad cup or saucer. Pistillate flowers have one simple pistil containing a single ovule. Androecia consist of few to many, variously fused stamens rather than numerous, free, leaf-like stamens. In several genera, including Myristica, the perianth is fused into an urn-shaped enclosure. These differences suggest that Myristicaceae have either a nonbeetle pollinator or a different type of cantharophilous syndrome. Most accounts of nutmeg biology (e.g., Mcllroy 1967, Purseglove 1968, Cobley 1976) assume that pollination is effected by small insects, although wind pollination also is considered possible (Mcllroy 1967), and one researcher suggested that moths (Lepidoptera) are pollen vectors of M. fragrans (Deinum 1949). Pollen vectors of other annonalean families (Winteraceae, Degeneriaceae) include Diptera, Thysanoptera, primitive Lepidoptera, and Coleoptera (Thien 1980). In this study of M. fragrans we present preliminary information about the floral biology and pollination ecology of Myristicaceae. Specifically we made field observations and manipulated plants to determine (1) the phenology of staminate and pistillate flowers, (2) the relative abundance of staminate and pistillate flowers, (3) the nature of floral attractants and pollinator rewards, and (4) the identity and behavior of the effective pollinator.