Let's add to Eugene's questions the following one:


How does a bee digest pollen?


Pollen capsules are so tough they can survive for millions of years in sediments. Pollen grain capsules are made out of sporopollenin<https://en.wikipedia.org/wiki/Sporopollenin> which passes undigested through the gut of animals such as ourselves (making pollen supplementation rather useless, but people buy it anyway).


Quoting from Roulston and Cane:


Six basic methods have been suggested for animals to extract pollen contents:

I Crack open the pollen wall mechanically
II Pierce the pollen wall with sharp mouthparts

III Dissolve the pollen wall with enzymes
IV Induce germination or pseudo-germination

V Burst the pollen wall through osmotic shock

VI Penetrate the pollen wall with digestive enzymes


Pollen digestion has been studied most extensively in bees, but a complex and somewhat confusing set of interpretations has emerged. Adult bees possess a crop, in which nectar and pollen may mix, thus providing a pre-treatment that could lead to germination or pseudo-germination. The crop leads through the proventricular valve to the gut, a region that differs substantially in osmotic pressure from the crop. Thus, pollen consumed by adult bees is subjected to immersion in a sugar solution followed by an abrupt osmotic gradient. In contrast, larval bees have no crop. Pollen enters the gut without internal exposure to nectar sugars or sudden changes in osmotic pressure. The pollen consumed by larval bees, however, is part of a food provision including a large quantity of nectar. Thus, larval pollen has been exposed to a liquid sugar environment prior to ingestion. For most bee species, larvae consume the pollen within a few days of pollen collection. For honey bees, however, pollen may be stored in the hive for an extended period of time. Adult honey bees add secretions that inhibit pollen germination (Klungness and Peng 1983). Additionally, the stored pollen undergoes considerable biochemical change due to the activities of microbial organisms. Thus, there may be differences in pollen digestion between larvae and adult bees, and among bees that differ substantially in sociality.

Kroon et al. (1974) noted that pollen grains will burst when transferred suddenly between chambers that differed in osmotic pressure. These researchers determined that the osmotic differential between the honey bee crop and ventriculus was sufficient to burst pollen grains, and suggested that the burst grains then traveled through the gut where their contents were accessible to digestive enzymes. They hypothesized that this "osmotic shock" was a necessary prerequisite to digestion. Peng et al., however, (1985, 1986) did not find burst pollen grains in the anterior midgut of the honey bee. Instead, these researchers found that Medicago sativa (alfalfa) and Taraxacum officinale (dandelion) pollen slowly degraded and lost their contents during their passage through the midgut. Taraxacum pollen swelled at the germination pores in the bee's anterior midgut, but showed no further signs of germination (Peng et al. 1985). The pollen walls of Taraxacum gradually became misshapen as the cytoplasm drained through the germ pores. Medicago pollen showed germination-like cytoplasmic protrusions, still contained by the bulging intine, in the middle midgut (Peng et al. 1986). As the Medicago pollen reached the posterior midgut, the pollen wall partially broke down and the protruding intine ruptured, releasing the cytoplasm. Peng et al. (1986) hypothesized that the digestion process comprised enzymatic degradation of the pollen wall, followed by extrusion of the cytoplasm due to osmotic pressure, weakening of the intine by proteases, and eventual rupture of the intine by continued osmotic pressure. In this interpretation, cytoplasmic protrusions resembling germination are caused by cell wall breakdown and osmotic pressure rather than induction of germination. Although pollen wall components such as sporopollenin and cellulose are unlikely to be digested by honey bee digestive enzymes, Klungness and Peng (1984) found that honey bees could partially digest the hemicellulose and pectic acid components.


_____


Further studies have shown that numerous bacteria are part of the bee's gut microflora, and it is more than likely that these bacteria are an essential part of their ability to glean the maximum and best benefit from digestion of pollen.  The bacteria are NOT the same as the ones marketed as "probiotic" to humans.  These are unique to bees.  Conversations on Bee-L have previously discussed gut microflora, and the implications of changes in that microflora.


So...how do bees digest pollen?


Christina

             ***********************************************
The BEE-L mailing list is powered by L-Soft's renowned
LISTSERV(R) list management software.  For more information, go to:
http://www.lsoft.com/LISTSERV-powered.html