Amino acid polymorphism and rare electrophoretic variants of G6PD from natural populations of Drosophila melanogaster

Contrasting histories of G6PD molecular evolution and malarial resistance in humans and chimpanzees

Although mutations in the glucose-6-phosphate dehydrogenase (G6PD) gene result in several blood-related diseases in humans, they also confer resistance to malarial infection. This association between G6PD and malaria was supported by population genetic analyses of the G6PD locus, which indicated that these mutations may have recently risen in frequency in certain geographic regions as a result of positive selection. Here we characterize nucleotide sequence variation in a 5.2-kb region of the G6PD locus in a population sample of 56 chimpanzees, as well as among 7 other nonhuman primates, to compare with that in humans in determining whether other primates that are impacted by malaria also exhibit patterns of G6PD polymorphism or divergence consistent with positive selection. We find that chimpanzees have several amino acid variants but that the overall pattern at G6PD in chimpanzees, as well as in Old and New World primates in general, can be explained by recent purifying selection as well as strong functional constraint dating back to at least 30-40 MYA. These comparative analyses suggest that the recent signature of positive selection at G6PD in humans is unique.

DNA diversity in Hawaiian endemic plant Schiedea globosa

This is the first report of a study devoted to the population genetics of speciation in the endemic Hawaiian plant genus Schiedea (Caryophyllaceae). Here, we report the estimates of DNA sequence diversity and divergence in a newly isolated nuclear gene from Maui and Oahu Schiedea globosa populations. Overall, the species-wide average heterozygosity per silent site is pi = 0.3%. The silent DNA diversity on the older island of Oahu (pi = 0.24%) is almost twice as high as on the younger Maui (pi = 0.14%). Consistent with this, the haplotype phylogeny suggests a more recent origin of the Maui populations. There is no significant isolation between the two Maui populations (F(st)=0.027), while isolation between the two islands is high (F(st)=0.57, P<0.0001). Pairwise mismatch distributions suggest population growth approximately 660 and 310 thousand generations ago for the Oahu and the Maui populations, respectively, which may be the minimal age for these populations. This is consistent with a fairly neutral frequency spectrum (Tajima's D is 0.34 and -0.94 for the Oahu and the Maui populations, respectively), suggesting that both populations are sufficiently old to have recovered from any initial founder effects. Relatively high nuclear DNA diversity in the S. globosa populations illustrates the usefulness of a DNA sequence-based approach to the population genetics of island plant populations.

Historical selection, amino acid polymorphism and lineage-specific divergence at the G6pd locus in Drosophila melanogaster and D. simulans

The nucleotide diversity across 1705 bp of the G6pd gene is studied in 50 Drosophila melanogaster and 12 D. simulans lines. Our earlier report contrasted intraspecific polymorphism and interspecific differences at silent and replacement sites in these species. This report expands the number of European and African lines and examines the pattern of polymorphism with respect to the common A/B allozymes. In D. melanogaster the silent nucleotide diversity varies 2.8-fold across localities. The B allele sequences are two-to fourfold more variable than the derived A allele, and differences between allozymes are twice as among B alleles. There is strong linkage disequilibrium across the G6pd region. In both species the level of silent polymorphism increases from the 5' to 3' ends, while there is no comparable pattern in level of silent site divergence or fixation. The neutral model is not rejected in either species. Using D. yakuba as an outgroup, the D. melanogaster lineage shows a twofold greater rate of silent fixation, but less than half the rate of amino acid replacement. Lineage-specific differences in mutation fixation are inconsistent with neutral expectations and suggest the interaction of species-specific population size differences with both weakly advantageous and deleterious selection.