POLLINATION AND FRUIT-SET IN ASCLEPIAS: A REAPPRAISAL

Experimental pollinations of Asclepias tuberosa L., the first for this species and second largescale effort for the genus, reveal trends toward local population differentiation. The species possesses a low level of self-compatibility, the first documented report for the genus. Analysis of flowers visited by natural pollinators in populations across the eastern half of the United States demonstrated a linear relationship between pollinia removal and pollinia insertion. Clarifications in older literature regarding the specificity of the pollination mechanism, effective levels of pollination in natural populations, and the performance of experimental manipulations are made, and it is concluded that fruit-set in Asclepias is regulated by the interaction of both mechanical aspects of pollination and physiological aspects of fertilization and fruit development. A model incorporating these restraints is seen to predict fairly well the observed fruit to flower ratio of about 1:100 in natural populations. WITH MORE RECENT attempts to quantify and explain pollination biology in evolutionary terms (e.g., Faegri and van der Pijl, 1966; Heinrich and Raven, 1972; Proctor and Yeo, 1972), there has been a resurgence of interest in the reproductive biology of asclepiads. The reproductive biology of the Asclepiadaceae, or milkweed family, is rivalled only by that of the Orchidaceae in its floral complexity (Fig. 1). The five antisepalous stamens have undergone extreme modification; the anther connectives extend upward to partially overlap the stigmatic head. In addition, coronal extensions (hoods) containing arching structures (horns) retain the nectar secreted by nectaries located inside the stigmatic chamber within the flower (Galil and Zeroni, 1965). The stigmatic surfaces have been shifted laterally and are enclosed by the closely adjoining anther wings. The pollen sacs (pollinia) of two adjacent anthers are joined by translator arms to a black body (corpusculum or gland) located just above the slit of the stigmatic chamber formed by the projecting anther wings. Pollinia removal is effected when a groove in the corpusculum catches on a hair of an insect leg. When the insect flies to another flower on a different plant the pollinium lodges in the chamber, thus effecting cross-pollination. Quantitative studies of the reproductive biology I Received for publication 26 June 1975. I wish to thank Janis Antonovics for supplying encouragement, ideas, and criticism. Gordon Whitaker provided able assistance in the field during the re-collection of the transect. Henry M. Wilbur and Mary F. Willson suggested improvements in an earlier draft of the manuscript. The line drawings were executed by Karen Teramura. Funds for use of the Duke University Phytotron were supplied by NSF Grant GB-28950 and for travel by an NSF Graduate Fellowship. of the Asclepiadaceae were first attempted in the 1940's when commercial use of species of Asclepias was proposed (Fischer, 1941; Whiting, 1943; Stevens, 1945; Moore, 1946, 1947; Sparrow and Pearson, 1948). These early studies raised many questions regarding reproductive biology, but no general agreement could be reached on even such basic matters as the precise mechanism of pollinia removal and insertion, the relative importance of mechanical versus physiological interspecific isolating mechanisms, the possibility of self-compatibility, and factors limiting fruit-set to levels of 1 %. Floral morphology (Safwat, 1962; Galil and Zeroni, 1965, 1969), specificity of pollinia transfer (Macior, 1965), and temporal and spatial aspects of the floral display (Willson and Rathcke, 1974) have all been re-examined. Life history characteristics (Wilbur, 1976) and interspecific variation in ovule and pollen production (Bell, 1974) have also been analyzed recently. This paper reports detailed studies on pollination, breeding system, and fruit-set in Asclepias tuberosa L., the common roadside "butterflyweed." This species was studied previously by Woodson (1947, 1953, 1962, 1964). MATERIALS AND METHODS-In July, 1973, twenty flowering individuals were taken from each of five populations located more or less equidistantly along Woodson's (1947, 1962) 1200-mile west-to-east transect. The populations were therefore separated by average distances of about 300 miles. The plants were cut back to the rootcrown, established in pots in a controlled environment greenhouse and grown under natural light in a temperature regime of 23 C day and 20 C night. The plants flowered after two months, and all