Balkan Journal of Medical Genetics

DETECTING SEX-BIASED GENE FLOW IN AFRICANAMERICANS THROUGH THE ANALYSIS OF INTRA- AND INTER-POPULATION VARIATION AT MITOCHONDRIAL DNA AND Y- CHROMOSOME MICROSATELLITES
Battaggia C1, Anagnostou P1,3, Bosch I4, Brisighelli F4, Destro-Bisol G1,3, Capocasa M2,3,*
*Corresponding Author: Dr. Marco Capocasa, Dipartimento di Biologia e Biotecnologie “Charles Darwin”, Università “La Sapienza”, Roma, Italy, P.le A. Moro 5, 00185 Roma, Italia; Tel.: +39-6-4991-2725; Fax: ++39-6-4991-2771; E-mail: marco.capocasa@uniroma1.it
page: 7

RESULTS AND DISCUSSION

Mitochondrial DNA and Y-Chromosomal Variation. DNA sequences of the HVR-1 were determined in 50 unrelated individuals (Table 1). We found nucleotide differences at 63 out of 360 positions with respect to the ‘consensus’ sequence [19], five of which were transversions [16114 (A>C); 16183 (A>C); 16188 (C>G); 16258 (T>A)]. In total, we were able to define 40 different lineages, with frequencies ranging from 0.002 to 0.008. We obtained a HD value of 0.987 ± 0.008 and a MNPD value of 7.673 ± 3.637. Y-Chromosomal haplotypes, built using seven microsatellite loci, were determined in 23 individuals (Table 2). Nineteen different haplotypes were observed, with frequencies ranging from 0.042 to 0.125. We obtained a HD value of 0.996 ± 0.014 and a MNPD value of 4.383 ±2.246. Intra- and Inter-population Diversity in African- American and African Populations. Since the seminal work by [20], the approach based on the simultaneous use of mtDNA and Y-chromosomal polymorphisms has been used in numerous studies of human genetic diversity [21]. This approach may also turn out to be useful in the case of African-American populations. In fact, the availability of uni linear DNA polymorphisms offers the opportunity to detect possible signatures of a sex-biased gene flow from Europeans to African-Americans. Intriguingly, whereas a male-biased gene flow was expected on the basis of historical knowledge, demographic data indicate that since 1960, most of the mixed marriages involved African-American males to females of European origin [22]. In a study carried out among African-American students at the Texas University at Austin, Austin, TX, USA, Hsieh and Sutton [23] compared admixture estimates based on mtDNA and protein loci, concluding that there is no evidence of a European male-biased gene flow. On the other hand, the latter case was supported by more recent studies carried out in a number of African-American populations from different areas and based on population specific mtDNA and Y-chromosome alleles [7-10]. Estimates of sex-specific admixture suffer from two different orders of limitations. Obtaining reliable African parental populations is difficult for three main reasons. First, present day African-Americans originate from populations scattered in a wide area from the central and western parts of the continent. Second, it is tricky to identify the populations in continuity with those from which slaves were taken to the US, as their demography was reshaped by slave capture and forced migrations [24]. Third, there is also substantial variation among sub-Saharan groups for mtDNA and Y-chromosome polymorphisms [25,26], so that results may vary substantially depending on the populations chosen as the parental ones. A further source of confusion may be created by geneticists and molecular anthropologists when they use inaccurate ethnolinguistic labels and/or assume particular populations as representative of wider groups [27]. The second considerable problem is in the large statistical uncertainty of admixture estimates based on mtDNA and Y-chromosome polymorphisms. Given their intrinsic nature of single loci in evolutionary terms, both the uni linearly transmitted genetic systems generally produce values with extended confidence intervals that substantially overlap (e.g.,see estimates for Maryland, Texas and Virginia of table 5 in [9]). A possible approach is represented by the search for possible effects of gene flow on intra- and interpopulation genetic variation. Since the two genetic systems cannot be directly compared due to substantial difference in types and rate of mutations and demographic dynamics, we have compiled a paired mtDNA and Y-chromosomal population database including both European and African populations [25,28]. This makes it possible to compare the extent of mtDNA and Y-chromosome variation within a given population, by contrasting it with data from other populations included in the database. An incoming European male-biased gene flow predicts a greater ratio of Y-chromosome to mtDNA diversity for African-Americans than for African populations. This expectation is met by our African- American sample. In fact, their mtDNA HD is equal or lower than in five African populations, whereas their Y-chromosomal value is higher (Table 3). The greater diversity of African-Americans is even more evident comparing MNPD, a measure which weighs molecular differences. In this case, the diversity between Europeans and Africans may have a greater impact than happens with HD, in which the extent of inter-haplotypic differentiation is not taken into account. In fact, there are eight African populations with a mtDNA MNPD higher than our African-Americans, and only one for the Y-chromosomal polymorphisms. It is noteworthy that the latter case involves the Fulbe, a population which has been shown to have undergone a male-biased gene flow due to African migrations [29]. Although no inference on asymmetric gene flow can be made for other African-American populations, it is remarkable that the ratio between molecular and haplotypic measures of intra-population diversity are the lowest for mtDNA and the highest for Y-chromosome olymorphisms (Figure 1). A male-biased gene flow in African-Americans from Chicago, IL, USA is also supported by the analysis of genetic distances (Table 4). They show the lowest genetic distance (both molecular and haplotypic from Europeans for Y-chromosomal polymorphisms but not for the mtDNA ones. Furthermore, the ratio of mtDNA to Y-chromosomal genetic distances from Europeans is markedly higher (18.3-38.3% for molecular and haplotypic distances, respectively) than all African groups. The introgression of Y-chromosomes into other African-American populations is consistent with their comparatively low genetic distance from Europeans. To sum up, our results coherently support the hypothesis that the African-American sample under study has undergone a European male-biased gene flow. On a more general note, our research showed that comparative analysis of intra- and inter-population variation for Y-chromosome and mtDNA polymorphisms in a broad dataset including African and Europeans groups, may help detect admixture signatures, providing a useful complement to methods for admixture estimates.

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