How are pollen digested in the stomach although sporopollenin cannot be degraded by any known enzyme?

Question

How are pollen digested in the stomach although sporopollenin cannot be degraded by any known enzyme?

Answer 1

Pollen is a rich source of protein (Faegri and van der Pijl 1971, Stanley and Linskens 1974:154), but the extent to which it can be used as a nitrogen source by many animals is debatable because the exine, or outer coat of the pollen grain, is highly resistant to degradation by digestive enzymes (Heslop-Harrison 1971).

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Eight Major Job Trends For 2008 Pollen feeding has been reported for many species of arthropods (Gilbert 1972, Smith and Mommsen 1984, Peng et al. 1986), bats (Howell 1974, Law 1992), marsupials (Turner 1984, Richardson et al. 1986), and several species of birds (Churchill and Christensen 1970, Grant and Grant 1981, 1989, Paton 1981, Grant 1986, Wooller et al. 1988, Brice et al. 1989, Richardson and Wooller 1990). Digestion of pollen was examined in some of these studies and was found to vary considerably among bird species. This was due both to variation in digestive ability among bird species and to variation in digestibility of pollen among plant species (Brice et al. 1989). Digestion occurs when enzymes in the bird's gut gain access to the pollen cytoplasm, either by mechanically penetrating the exine or by passing through the germination pore or a pore in the exine weakened by changes in osmotic pressure (Simpson and Neff 1983). The empty exine is resistant to breakdown by digestive enzymes and is eliminated in the f***s of the bird. Thus, the proportion of empty grains in the f***s, when compared with the proportion in the ingested pollen, is a reliable indicator of the extent of digestion (Turner 1984, Brice et al. 1989).

Prior germination of pollen, stimulated by nectar, facilitates digestion in Heliconius butterflies (Gilbert 1972), and possibly in marsupials (Turner 1984); it might also occur in the crops of birds (Turner 1984). Germination of pollen is known to occur readily in in vitro solutions of sugar and water (Stanley and Linskens 1974:85). Germination of pollen grains stimulated by nectar in the crop would eliminate the need to degrade the exine; enzymes in the gut could act directly on the developing pollen tube or enter the pollen grain through the germination pore.

Law (1992) found that 53-57% of ingested pollen grains were digested by the Queensland blossom bat, Syconycteris australis. Similar proportions have been found for the marsupials Petaurus australis and P. breviceps (Goldingay et al. 1987, Goldingay 1990). As many as 90 to 100% of pollen grains were digested in the guts of the marsupials Cercartetus nanus and Tarsipes rostratus (Turner 1984, Richardson et al. 1986). Their f***s were packed with empty pollen grains, suggesting that much of their daily protein requirement was obtained from pollen. In contrast to this high digestive efficiency in marsupials, birds show low levels of pollen digestion. For example, Paton (1981) found that the percentage of empty pollen grains in the f***s of New Holland Honeyeaters, Phylidonyris novaehollandiae, did not differ significantly from the percent of empty grains ingested. Even the Purple-crowned Lorikeet, Glossopsitta porphyrocephala, which feeds extensively on pollen and nectar at certain times of the year (Churchill and Christensen 1970) and shows anatomical features of the tongue and gut to facilitate pollen and nectar digestion (Richardson and Woollet 1990), was only able to fully digest 48% of the pollen grains ingested, although many grains were partially digested (Wooller et al. 1988).

Indirect evidence suggests that some species of Darwin's finches on the Galapagos are more like marsupials than other birds in being able to extract sufficient nutrients from pollen and nectar to breed. Geospiza conirostris on Isla Genovesa and G. scandens on Isla Daphne breed before the rains, at a time when arthropods are scarce and birds are consuming pollen and nectar from Opuntia flowers (Millington and Grant 1983, 1984, Boag and Grant 1984, Grant and Grant 1989). However, the extent to which pollen is digested by adult and nestling finches was unknown. The opportunity to examine pollen digestion in adult and nestling G. scandens became available in January 1994, when five G. scandens pairs bred and successfully fledged young before the arrival of the rains. Opuntia echios were flowering abundantly at that time. In this paper, I report the results of the study on Isla Daphne Major from 1980 to 1991 and 1994 to address the importance of pollen digestion and its potential role in the early breeding of G. scandens.

Darwin's finches and Opuntia

Geospiza scandens, the Cactus Finch, has body mass of [approximately equal to]20 g and is a specialist on Opuntia cactus flowers and seeds (Grant and Grant 1980, 1981, 1993, Millington and Grant 1983, 1984, Boag and Grant 1984). Observations of foraging on Opuntia flowers from as short a distance as 2 m revealed that pollen and nectar, not insects, were taken. This was confirmed with closeup sequences on super-8 film of feeding behavior (Grant and Grant 1981).