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Genetics, Vol. 172, 287-292, January 2006, Copyright © 2006
doi:10.1534/genetics.105.045831
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Institut für Tierzucht und Genetik, Veterinärmedizinische Universität Wien, 1210 Wien, Austria
1 Corresponding author: Institut für Tierzucht und Genetik, Veterinärmedizinische Universität Wien, Veterinarplatz 1, 1210 Wien, Austria.
E-mail: christian.schloetterer{at}vu-wien.ac.at
| ABSTRACT |
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Apart from some studies in humans and maize, Drosophila has been the primary target of systematic screens for the identification of recently selected alleles (HARR et al. 2002; GLINKA et al. 2003; KAUER et al. 2003b; ORENGO and AGUADE 2004; SCHÖFL and SCHLÖTTERER 2004). Drosophila melanogaster is particularly well suited for the identification of ecologically relevant alleles. Like humans, D. melanogaster originated in sub-Saharan Africa and colonized the rest of the world only recently (DAVID and CAPY 1988). This habitat expansion presumably required numerous adaptations, which have left their traces in the genome (HARR et al. 2002).
Despite the overwhelming evidence for a large number of adaptive mutations, the identification of the genomic region affected by a selective sweep is significantly complicated by demographic history. Non-sub-Saharan African D. melanogaster populations are most likely derived from a single founder event (BAUDRY et al. 2004), which was probably associated with a significant reduction in population size. Genetic drift during the early phase of the colonization could have led to a genetic signature in the genome that is almost indistinguishable from a selective sweep (BARTON 1998). Thus, one of the major challenges for the identification of genomic regions carrying a beneficial mutation is the distinction between demography and selection (NIELSEN 2001; WALL et al. 2002; DEPAULIS et al. 2003; JENSEN et al. 2005).
Recent theoretical work has suggested that the genomic signature of a selective sweep strongly depends on the population structure (SANTIAGO and CABALLERO 2005). In the case of low population differentiation, all populations show the same signature of a selective sweep: a valley of reduced variation and a surplus of rare, derived alleles (i.e., negative Tajima's D; TAJIMA 1989). For highly differentiated populations, however, only the population in which the beneficial mutation arose shows this characteristic pattern. The other populations, which acquire the beneficial mutation by migration, show a characteristically different pattern: only a very narrow genomic region around the target of selection has low variation and a negative Tajima's D. With an increasing distance from the target of selection, Tajima's D increases above neutral expectations (SANTIAGO and CABALLERO 2005). Hence, the analysis of multiple differentiated populations will provide a highly characteristic pattern of a selective sweep, with a very narrow genomic region carrying the beneficial mutation.
Until the discovery of the high differentiation between sub-Saharan African and cosmopolitan D. melanogaster populations (BEGUN and AQUADRO 1993), D. melanogaster was often considered a completely panmictic population. Even when a large number of highly informative markers were used, the differentiation among non-African D. melanogaster populations was found to be extremely low with FST values in the range of 0.0040.152 (CARACRISTI and SCHLÖTTERER 2003).
In this report we analyzed a large collection of Asian D. melanogaster populations. We find that Asian D. melanogaster are derived from the same colonization event as other non-African populations, but we detected extremely high levels of differentiation (up to 0.3) between them. This hitherto unappreciated high population structure in Asian D. melanogaster populations opens completely new possibilities for studying adaptation in natural D. melanogaster populations.
| MATERIALS AND METHODS |
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Measures of genetic variation, such as heterozygosity and number of alleles, were calculated using the MS-Analyzer (MSA) software, version 4.0 (DIERINGER and SCHLÖTTERER 2003). When more than a single population was typed for one continent, estimates of variability were calculated for each population separately and subsequently averaged. This treatment was chosen to avoid the Wahlund effect (WAHLUND 1928). The proportion of shared alleles was calculated by the MSA software (DIERINGER and SCHLÖTTERER 2003). The obtained distance matrix was converted into a dendrogram using the FITCH program, which is part of the PHYLIP package (FELSENSTEIN 1991), and graphically displayed with TREEVIEW (PAGE 1996). The statistical significance of the nodes of the dendrogram were evaluated by bootstrapping loci (EFRON and GONG 1983). To estimate population differentiation, pairwise
-values were determined as an unbiased estimate of FST (WEIR and COCKERHAM 1984) using the MSA software. The significance of pairwise FST values was tested by permuting genotypes among populations (10,000 times), as this method does not rely on HardyWeinberg assumptions (GOUDET et al. 1996). To account for multiple testing, we used the Bonferroni method (SOKAL and ROHLF 1995). Bayesian clustering and admixture analysis was performed with the BAPS3.1 software (CORANDER et al. 2004) using the default settings.
| RESULTS AND DISCUSSION |
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Using a Bayesian method (CORANDER et al. 2004), we identified 11 genetically distinct groups. Four of the identified clusters consisted of more than one population. The first cluster contained both African populations. The second cluster contained all populations from America, including the population from Belize, which was previously shown to harbor a relatively high proportion of African alleles (CARACRISTI and SCHLÖTTERER 2003). The third cluster combined all European populations. Finally, the populations from continental China and Taiwan were also clustered. All remaining Asian populations formed separate genetic entities. The conclusion that Asian populations are highly differentiated is further corroborated by a pairwise FST analysis (supplemental Table 1 at http://www.genetics.org/supplemental/). The population from Chiang Mai (Thailand) was the most differentiated one with an average pairwise FST value of 0.26. Interestingly, the two populations from mainland China and Taiwan were the Asian populations with the least differentiation from the European/American populations. With an average of 0.059, the differentiation between Taiwan and America was particularly low.
As this low level of differentiation between the Taiwanese population and America may be indicative of recent gene flow, we performed a Bayesian admixture analysis. Figure 1 depicts the 11 genetically distinct groups, with one color specific for each group. A thin vertical line consisting of only one color represents individuals without admixture. In admixed individuals, the line is partitioned into colored segments that represent the individual's estimated membership fraction in the corresponding clusters. The different colors occurring in one line (individual) represent the admixture proportions. While in most populations, the inferred admixture is low, the population from Taiwan has a significant proportion of alleles with coancestry in American populations. Apart from the Taiwanese population, Asian D. melanogaster populations showed the lowest degree of admixture (Figure 1).
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In D. melanogaster and D. simulans, X-linked and autosomal loci show significant differences between African and non-African populations (ANDOLFATTO 2001; KAUER et al. 2002; KAUER et al. 2003b; SCHÖFL and SCHLÖTTERER 2004) as well as among African populations (KAUER et al. 2003a). Hence, we also analyzed autosomal and X-linked loci separately. While we found an overall agreement between the two data sets (supplemental Table 2 and supplemental Figure 1 at http://www.genetics.org/supplemental/), we noted that the Chinese population grouped differently in both data sets. While for autosomal loci a grouping with the European and American populations was supported by a bootstrap support of 69%, for X-linked loci the Chinese population clustered with the Asian populations (supplemental Figure 1 at http://www.genetics.org/supplemental/). The reason for this discrepancy between X-linked and autosomal loci in the Chinese population is not clear and more data are required to distinguish between chance and directed (e.g., selection) effects.
Interestingly, previous studies using other genetic markers provided conflicting evidence for Asian flies. While a large survey of allozymes did not find higher differentiation among Asian populations (SINGH and RHOMBERG 1987), mtDNA data do indicate higher levels of differentiation among Asian populations (HALE and SINGH 1991; SOLIGNAC 2004).
Well-designed experiments based on the recapture of D. melanogaster mutants released at an orchard in Maryland showed a large dispersal potential and released flies were able to move to potential breeding sites (COYNE and MILSTEAD 1987). As the authors failed to find mutant flies in the subsequent year, they concluded that D. melanogaster does not overwinter and recolonizes the study site every year. Overall, these data strongly suggested that D. melanogaster is a highly migratory species. Consistent with these direct observations, indirect measurements also found low levels of population differentiation in American populations (SINGH and RHOMBERG 1987; CARACRISTI and SCHLÖTTERER 2003) (supplemental Table 1 at http://www.genetics.org/supplemental/). As European D. melanogaster also show low levels of differentiation, it could be assumed that they also have high dispersal capabilities. This picture contrasts with our observations in Asian D. melanogaster, where we observed high levels of differentiation, most likely caused by genetic drift. Thus, our data suggest that Asian D. melanogaster disperse to a much lower extent than European/American flies. Interestingly, the pronounced population structure in Asian D. melanogaster is paralleled by D. ananassae, which also shows strong population structure for populations collected in Southeast Asia (DAS et al. 2004). Thus, the high population structure of Southeast Asian Drosophila may be a more general phenomenon.
This difference in dispersal of Asian and other non-African D. melanogaster has important implications for the design of experiments screening for genes involved in adaptation to non-African habitat. The population bottleneck associated with the out-of-Africa habitat expansion could generate a signature that closely resembles the signature of a selective sweep. The high similarity of American and European populations strongly limits the joint analysis of multiple populations as an effective means for distinguishing between a population bottleneck and a beneficial mutation that spread after the colonization eventthe signature of the selective sweep has been found to be very similar among different populations (HARR et al. 2002).
Asian populations offer two conceptual advances for the identification of selective sweeps:
Most important, due to the fact that in the continent to which the allele is exported by migration a smaller window of reduced variation is expected, a higher mapping precision should be achieved when hitchhiking mapping is applied. Nevertheless, as the theoretical study by SANTIAGO and CABALLERO (2005) focused on extremely large FST valueshigher than the ones observed for the Asian populationsmore data are required to decide if the statistical power will be high enough to detect a selective sweep on the basis of the theoretical predictions of SANTIAGO and CABALLERO (2005).
| ACKNOWLEDGEMENTS |
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