Release of Alfalfa, Medicago sativa, Pollen Cytoplasm in the Gut of the Honey Bee, Apis mellifera (Hymenoptera: Apidae)

Abstract

The mechanism and process of pollen digestion by honey bee, Apis mellifera L., were studied by histological observations on the digestive tracts of adult workers that were hand-fed with suspensions of alfalfa, Medicago sativa L., pollen. Pollen grains showed no visible morphological or chemical changes as they entered the anterior region of the midgut from the crop ca. 30 min after pollen feeding. As the pollen bolus progressed into the middle region of the midgut, ca. 1 h after feeding, pollen grains began to show initial swelling and cytoplasmic protrusion at the thin and weakened germination pores. During the next 2 h, as the pollen progressed through distal regions of the midgut, pollen cytoplasm was observed to extrude gradually from the broken germination pores, and nutrients became available for enzymic digestion and absorption. Empty pollen and fragments of pollen wall were observed in the rectum 15 h after feeding. Our present observations provide histological and histochemical evidence demonstrating that alfalfa pollen must undergo initial chemical changes in the pollen wall. Then it can respond to the changes of osmotic pressure in the gut, swell subsequently at the germination pores, and finally, cytoplasm can be extruded through the germination pores and released into the gut lumen.