Abstract
(Englisch)
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During the last year the comparison in various animal species of their gustatory responses to compounds eliciting a sweet taste in humans, extended our knowledge of the great biodiversity of the taste responses and evidenced some specialization and/or phyletic trends within species. Our interest was focused on responses to natural sugars, polyols and naturally-occurring sweeteners, but also on various artificial sweetening compounds, including the very powerful guanidine sweeteners. Once more I made experiments with dogs, mice (129P2/OlaHsd), birds, reptiles and compared these data with new results obtained in kangaroos. The observed biodiversity for the sweet taste between species may be considered as the result of two main evolutive forces: specialization and phyletic trends.
An example of specialization may be seen in some insect species such as the flies, honey bees and ants. For the tested polyols and carbohydrates, all known to be sweet in humans (except methyl-ß-D-glucopyranoside), the fly Calliphora erythrocephala (Diptera) has a preference for fourteen compounds, the ant Myrmica rubra (Hymenoptera) for twelve, the ant Lasius niger (Hymenoptera) for eight, the ant Myrmica rubida and the honey bee Apis mellifica (Hymenoptera) for only seven compounds.
Interestingly Calliphora and Myrmica rubra which display preferences for a larger number of the compounds tested are classified regarding their main feeding habits, in the omnivorous group. The other three insect genera are more specialized for their feeding sources. For example Myrmica rubida is carnivorous while the honey bee is rather phytophagous and is specialized in the consumption of some specific plant nectars. In a discussion at an ECRO-Symposium in 1979, Schoonhoven already mentioned, 'that in insects there are no two species with a completely identical sensory system'.
For instance the carnivores: the cats (n=18; in three groups) and dogs (n=33; in five groups) - belonging not to the type of pet cats and dogs - don't respond to the sugars tested, to aspartame, neotame, thaumatin and other miscellaneous compounds and don't prefer the guanidine derivatives. The results with the sugars in dogs are not in agreement with other authors, but Kare mentioned already that 'there was substantial individual variation in dogs'. Maybe there exist a relationship to the daily diet of the individual dog, or we have - like in mice - genetic variation among inbred dog subspecies/or strains in taste responses to several sweeteners.
The new data from a group of ten swamp wallabies (Wallabia bicolor bicolor), which originated from the South-western region of Australia, show that they respond to six natural sugars (D-glucose, D-fructose, L-sorbose, D-maltose, D-sucrose, D-raffinose), but not to D-galactose and D-lactose. When I tested the naturally occurring polyols in the marsupials, I found that they respond positive to all tetrols, pentols and hexols tested but in very high concentrations (between 0.125-1.0 mol/L).
The fact that kangaroos show no responses to dipeptide based derivatives (neotame, superaspartame, ASPE, aspartame, ASME, ampame), to phenylurea derivatives (CAMPA, suosan), to guanidine sweeteners (lugduname, carrelame, sucrononate, sucrooctate) and to other miscellaneous artificial sweeteners tested (magapame, sucralose, NHDHC, saccharine, acesulfame-K, Na-cyclamate) as well as to some naturally occurring complicated sweeteners (thaumatin, stevioside, glycyrrhizic acid), strengthen the idea that the ability of tasting artificial sweeteners and more generally non-carbohydrate sweet compounds is to link to a phyletic trend signaling a major change in the sweet taste receptors of more recent animal species.
The first time such a major difference was found was with thaumatin and this naturally occurring protein elicits an intensely sweet taste in humans and all other catarrhine primates, but no response at all in prosimians and South American platyrrhine primates. Later we have ascertained that the 'thaumatin feature' should be a basic one since we found two dipeptide derivatives i.e. aspartame and neotame displaying the same effect of separating the order of primates at the same intersection line than thaumatin.
Now again, with kangaroos (originated about 130 MYA) which show no positive responses to these three same compounds, it is observed once more, that animals, which evolved before the Oligocene (about 37 MYA), are not able to taste thaumatin, aspartame and neotame.
The receptor modification which allows higher animal species to taste the complicated artificial sweet substances must be then appeared in the Eutheria, with the higher evolved mammals from the Lower Cretaceous i.e. at about 100 million years ago. Since animal sweetness receptors was not designed to taste artificial sweeteners, the question of the advantage of this major modification of their sweetness receptors is still to elucidate. Additional efforts in studying comparative gustatory physiology could be thus helpful in clarifying the evolution of the mammalians in the animal kingdom.
These results, especially from the farm animals, are in relationship of nutrient uptake, health and wellbeing and therefore some international working companies for feed additives have great interest on these data: SEGHERS, Belgium (chicken), NIZO, The Netherlands (cats and dogs), Phodè, France (pigs), BIOLEX Int., Denemark (pigs); MMRL Ltd., India (pigs), BRAES Feed Ingredients, USA (pigs).
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