Sucrose octaacetate avoidance in nontaster mice is not enhanced by two type-A Prp transgenes from taster mice.

The Soa bitter-sensitivity and Prp salivary-protein loci map to distal mouse chromosome six. No recombination has been found between sucrose octaacetate (SOA)-avoidance phenotype and PRP haplotype in any mouse population. Soa and Prp, therefore, are either very near each other or identical. To assess the latter possibility, two type-A, proline-rich protein genes (MP2 and M14), situated approximately 30 kb apart at the Prp locus, were separately transferred from an SOA-taster inbred strain (SWR) to an SOA-nontaster inbred strain (FVB). Five MP2-transgenic mice and seven M14-transgenic mice were insensitive to 1 mM SOA in two-bottle tests, thus retaining the nontaster FVB phenotype. Each transgenic mouse was mated to control FVB mice. Their transgene-positive F1 and F2 offspring also were insensitive. Transgene expression varied among the founder lines, but SWR-like expression levels, higher than background FVB expression levels, were found in submandibular gland tissue of adult transgenic mice from two MP2 lines and one M14 line. F3 mice from one of these MP2 lines were mated to F2 mice from the M14 line. Nine offspring inherited both transgenes. All nine were insensitive to 1 mM SOA. These findings indicated that expression of mRNAs for both type-A Prp genes alone or together did not enhance SOA taste sensitivity in nontaster mice.

A common polygenic basis for quinine and PROP avoidance in mice.

Inbred strains of mice (Mus musculus) differ greatly in ability to taste various bitter compounds. For some compounds, the differences result from allelic variation at a single locus. However, segregation patterns incompatible with monogenic inheritance have been found for quinine avoidance. The Soa bitter sensitivity locus exerts some influence on this phenotype, but an unknown number of other loci also contribute. Relative avoidance patterns for quinine sulfate in panels of naive inbred strains resembled avoidance patterns for 6-n-propyl-2-thiouracil (PROP), suggesting a common genetic basis. In particular, C57BL/6J mice strongly avoided both 0.1 mM quinine sulfate and 1 mM PROP in two-bottle preference tests, whereas C3H/HeJ mice were indifferent to both. Therefore, 12 BXH/Ty recombinant inbred strains, derived from these strains, were tested with both solutions to begin identification of the unknown bitter loci. Naive mice were tested for four consecutive days with each compound (order counterbalanced). Some BXH/Ty strain means resembled those of the parent strains, but others were intermediate. This indicated recombination among loci affecting avoidance, and therefore polygenic inheritance. The strain means were highly correlated across compounds (r = 0.98), suggesting that the same polygenes controlled both phenotypes. The BXH/Ty means for both compounds were then compared with the strain genotypes at 212 chromosome position markers distributed throughout the genome. Eight markers on five chromosomes (3, 6, 7, 8 and 9) yielded significant correlations. Six of the markers were correlated with both phenotypes, again suggesting common polygenic inheritance. The marker with the highest correlation was Prp, tightly linked to Soa on chromosome 6. The correlated marker regions likely contain quantitative trait loci affecting bitter avoidance. The phenotypic similarity of PROP to quinine, rather than to phenylthiourea, apparently stemming from a common polygenic basis, indicates a difference between mice and humans in gustatory organization related to bitters.

SW.B6-Soa(b) nontaster congenic strains completed and a sucrose octaacetate congenic quartet tested with other bitters.

Ten SW.B6 SOA nontaster strains congenic with the SWR/J SOA taster inbred strain were bred via repeated backcross-intercross cycles, with selection for nontasting in each cycle. Preference ratio distributions and phenotypic proportions across cycles at 0.1 mM SOA were consistent with monogenic predictions. The SW.B6 mice completed a congenic quartet with the SWR/J, B6.SW SOA taster and C57BL/6J SOA nontaster strains. The Soa locus controlled avoidance differences within the quartet for SOA, raffinose undecaacetate, glucose pentaacetate and brucine. Background genes not linked to Soa controlled avoidance differences for L-phenylalanine and ethanol. Avoidance of bitter picric acid was influenced by the Soa locus, but avoidance of acetic acid was not. The quartet pattern for quinine HCl was unclear, with indications of both Soa and background effects. Two forms of ribose tetraacetate yielded different patterns. Avoidance differences controlled by the Soa locus were found for the pyranose form; however, all four strains avoided the furanose form. The pleiotropic effects of Soa allele substitution within the quartet were limited to a subset of bitter compounds.

Genetics of bitter perception in mice.

Inbred and congenic strains exhibited several patterns of relative sensitivity to bitter tastants in 48-h, two-bottle preference tests. With segregation analyses of descendents of crosses between contrasting strains, these patterns suggested at least three genetic loci influencing bitter perception. The extensively characterized Soa (sucrose octaacetate) locus underlies one pattern. Variation at this locus had pleiotropic effects on avoidance of other acetylated sugars, plus such structurally dissimilar bitter tastants as brucine, denatonium benzoate, and quinine sulfate. Unlike SOA, however, sensitivity to quinine sulfate was polygenically determined, and produced a second characteristic pattern. At least one, possibly several, additional unlinked loci contributed to quinine differences. Phenylthiocarbamide (PTC) aversion differences exemplified a third pattern. Segregation consistent with monogenic control of PTC aversion has been reported, and within segregating populations PTC aversion did not covary with SOA or quinine sulfate avoidance. Variants of the three major patterns may be useful for analysis of specific mechanisms. While both showed the SOA pattern, strychnine differences were markedly smaller than brucine (dimethoxystrychnine) differences. Likewise, a hop extract containing primarily iso-alpha acids (e.g., isohumulone) produced an SOA-like pattern, while an extract with nonisomerized alpha-acids (e.g., humulone) did not.

The B6.SW bilineal congenic sucrose octaacetate (SOA)-taster mice.

SWR/J inbred mice (Tasters) reliably avoid, whereas C57BL/6J inbred mice (Nontasters) are indifferent to, sucrose octaacetate (SOA) at certain concentrations. From these strains we have developed a set of bilineal congenic Taster mice. Approximately 4000 mice, from 2 isogenic and 12 segregating generations, were tested in a program designed to evaluate genetic models for SOA tasting during development of congenic strains. The criterion phenotype was avoidance or nonavoidance in preference tests of the bitter tastant SOA at concentrations of 10(-4) and 10(-5) M. Across the 12 segregating generations, the results were consistent with Mendelian expectations for a single autosomal locus with complete dominance of the Taster phenotype. The breeding program produced 12 replicate B6.SW lines containing the taster allele on the B6-Nontaster genomic background. The congenic Taster mice may facilitate a functional analysis of the sense of taste.