Leverett R. SMITH
Chemistry Department, Contra Costa College, 2600 Mission Bell Drive, San Pablo,
CA 94806, U.S.A.
Russell J. MOLYNEUX*
Western Regional Research Center, Agricultural Research Service, USDA, 800 Buchanan Street,
Albany, CA 94710, U.S.A.
Polyhydroxyindolizidines comprise a variety of toxic alkaloids found primarily in the family Leguminosae (1), for example in the genera Astragalus and Oxytropus (2), Swainsona (3), and Castanopermum (4). The toxicity of these alkaloids derives typically from their activity as glycosidase inhibitors (5); some potential chemotherapeutic properties have been suggested (6-9). In the plants where such compounds occur, they presumably exert a defensive function by deterring predation (10,11 ); varying distribution of toxins within a plant may, however, offer opportunities for selective feeding. ln the present study, we have looked at the distribution of swainsonine, the principal toxic alkaloid of the Diablo locoweed (Stanislaus milk-vetch, Astragalus oxyphysus), in the seeds of the plant. This work was prompted by the observation that although larvae of the beetle Acanthoscelides aureolus (Horn), develop within the seeds of the Diablo locoweed, they avoid eating the outer seed coat. When the adult beetles emerge. they also appear to avoid eating the seed coat, cutting a circular opening rather than simply chewing their way out. This mode of behavior raised the suggestion that swainsonine might occur in higher concentrations in the outer coat. In that the predated seeds are completely hollowed out, this also provided a relatively convenient approach to comparing seed coat concentrations of swainsonine to concentrations in intact (unpredated) seeds.
Dry seed pods were collected from A. oxyphysus plants in Arroyo del Puerto (Stanislaus County, CA, USA) in late spring, and stored at room temperature in ventilated containers through the summer. Beetles were collected as they emerged, killed by freezing or by ether vapor, and allowed to air dry; a variety of small wasps, perhaps predators on the beetle larvae, were also collected but not examined further. After several months, when beetle emergence had ceased. intact seeds and predated empty seed coats were separated after removal from the pods. Average weight of an intact seed was about 8 mg; of a predated seed, about 4 mg; of an air-dried beetle, about 0.6 mg. Samples of the outer husks of the pods, intact seeds, predated seeds, and beetles were finely ground, and thereafter stored in the freezer pending extraction and analysis. After Soxhlet extraction with methanol, then evaporation, the alkaloid traction was isolated by an ion-exchange column cleanup, and aliquots were analyzed by GC or GCIMS following trimethylsilylation (1). Initial results indicate that swainsonine concentrations are higher in the seeds than in the outer husks, and higher in the seed coats than in the interior of the seeds. We found no evidence of alkaloid accumulation in the beetles.
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