The correlation of PTC perception with the perception of other toxic bitter substances and the association of the inability to perceive PTC with disease susceptibility combine to suggest that natural selection may have been an important factor in the evolution of this trait. R. A. Fisher hypothesized that the pervasive phenotypic variation in PTC perception is due to balancing natural selection, which may have favored heterozygotes (Fisher et al. 1939). However, such hypotheses have been difficult to test without knowledge of the genetic underpinnings of the trait. The recent discovery of a gene that accounts for up to 85% of the phenotypic variance in PTC perception allowed us to address this problem (Drayna et al. 2003; Kim et al. 2003).
Ptc Evolution 4 5 Nulled Scripts
Download File: https://chriswisketawd.blogspot.com/?file=2vEdoc
The minimum spanning tree revealed that the human sample was dominated by two major haplotypes, hsA and hsG, differing by three amino acid substitutions, as shown in figure 3A. These two haplotypes, which account for >90% of sampled chromosomes (fig. 3), are strongly associated with taster (hsA) and nontaster (hsG) status, respectively (Drayna et al. 2003; Kim et al. 2003). In addition, the hsA and hsG haplotypes were both found at intermediate frequencies: 0.55 and 0.38. A variety of factors, including population subdivision and balancing natural selection, can lead to the presence of two or more intermediate-frequency haplotypes in gene genealogies (Marjoram and Donnelly 1994; Bamshad and Wooding 2003). The evolution of two or more intermediate-frequency clusters is also surprisingly common under selectively neutral conditions (Slatkin and Hudson 1991).
To test whether patterns of DNA sequence variation in PTC fit expectations under the hypothesis of evolutionary neutrality, we analyzed the sequences by use of the DT, DF, and F statistics. These statistics are functions of the number of variable nucleotide positions in a sample of sequences, the mean pairwise difference between sequences, and the number of derived variants that are only observed once in the sample, all of which are affected by natural selection (Tajima 1989; Fu and Li 1993). For example, positive natural selection leading to the rapid fixation of a single, advantageous haplotype will result in a decrease in the expected number of polymorphic sites, a decrease in the mean pairwise difference between sequences, and an increase in the number of variants observed only once in the sample (Fu and Li 1993). In contrast, balancing natural selection can lead to an increase in all three of these values (Fu and Li 1993).
Evidence for balancing selection at the PTC locus does not imply that other selective pressures have been absent. For instance, it is possible that positive natural selection led to the rapid evolution of the nontaster allele, which was then maintained by balancing selection. This possibility might explain the unusually large number of nonsynonymous nucleotide substitutions found in this gene. It is also possible that specific PTC alleles have been favored by positive natural selection in particular environments, resulting in local adaptation. Such effects might account for the high frequency of PTC taster alleles in New World populations and the significant low DT, DF, and F in our North American sample.
2ff7e9595c
Comments