Brief communication mtDNA variation in North Cameroon Lack of asian lineages and implications for back migration from Asia to sub-Saharan Africa.код для вставкиСкачать
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 128:678 – 681 (2005) Brief Communication: mtDNA Variation in North Cameroon: Lack of Asian Lineages and Implications for Back Migration From Asia to Sub-Saharan Africa Valentina Coia,1 Giovanni Destro-Bisol,1* Fabio Verginelli,2 Cinzia Battaggia,1 Ilaria Boschi,3 Fulvio Cruciani,4 Gabriella Spedini,1 David Comas,5 and Francesc Calafell5 1 Department of Animal and Human Biology, University “La Sapienza,” 00185 Rome, Italy Department of Oncology and Neurosciences, Section of Molecular Pathology, University “G. d’Annunzio,” and Center for Research and Training on Cancer in Sub-Saharan Africa, University G. d’Annunzio Foundation, 66013 Chieti, Italy 3 Institute of Legal Medicine, Catholic University, 00168 Rome, Italy 4 Department of Genetics and Molecular Biology, University “La Sapienza,” 00185 Rome, Italy 5 Department de Ciències Experimentals i de la Salut, University “Pompeu Fabra,” 08003 Barcelona, Spain 2 KEY WORDS mtDNA U6 haplogroup; Y-chromosome R1*-M173 haplogroup; Fulbe ABSTRACT The hypervariable region-1 and four nucleotide positions (10400, 10873, 12308, and 12705) of the coding region of mitochondrial DNA (mtDNA) were analyzed in 441 individuals belonging to eight populations (Daba, Fali, Fulbe, Mandara, Uldeme, Podokwo, Tali, and Tupuri) from North Cameroon and four populations (Bakaka, Bassa, Bamileke, and Ewondo) from South Cameroon. All mtDNAs were assigned to ﬁve haplogroups: three sub-Saharan (L1, L2, and L3), one northern African (U6), and one European (U5). Our results contrast with the observed high frequencies of a Y-chromosome haplogroup of probable Asian origin (R1*-M173) in North Cameroon. As a ﬁrst step toward a better understanding of the evident discrepancy between mtDNA and Y-chromosome data, we propose two contrasting scenarios. The ﬁrst one, here termed “migration and asymmetric admixture,” implies a back migration from Asia to North Cameroon of a population group carrying the haplotype R1*-M173 at high frequency, and an admixture process restricted to migrant males. The second scenario, on the other hand, temed “divergent drift,” implies that modern populations of North Cameroon originated from a small population group which migrated from Asia to Africa and in which, through genetic drift, Y-chromosome haplotype R1*-M173 became predominant, whereas the Asian mtDNA haplogroups were lost. Am J Phys Anthropol 128:678 – 681, 2005. © 2005 Wiley-Liss, Inc. In a recent study of 77 biallelic polymorphisms located in the nonrecombining portion of the Y-chromosome, Cruciani et al. (2002) detected the haplotype R1*-M173 (YChromosome Consortium, 2002; Jobling and Tyler-Smith, 2003). It was found among six well-deﬁned populations from Cameroon (Fali, Tali, Uldeme, Fulbe, Daba, and Ewondo), with the highest frequency in North Cameroon (from 6.7% among the Tali to 95.2% among the Uldeme). This ﬁnding has important implications for the peopling of Africa, since it could signal an ancient back migration from Asia (Cruciani et al., 2002). In fact, the haplotype R1*-M173 harbors the derived allele at the M9 site, which characterizes haplogroups K to R (Y-Chromosome Consortium, 2002; Jobling and Tyler-Smith, 2003) , all of nonAfrican origin (Underhill et al., 2001; Semino et al., 2000; Wells et al., 2001; Cinnioglu et al., 2004; Luis et al., 2004). Other lineages derived from the M9 superclade have not been detected in sub-Saharan Africa, the only exception being the haplotype K2 found among some sub-Saharan populations (Cruciani et al., 2002; Luis et al., 2004). These ﬁndings are difﬁcult to reconcile with an African origin of the R1*-M173 subclade, unless a considerable extinction of lineages is assumed (Cruciani et al., 2002). Furthermore, according to present studies, the frequency of haplogroup R1*-M173 in Europe, the Middle East, and North Africa seems to be substantially lower than in North Cam- eroon (North Africa) (Bosch et al., 2001; Cruciani et al., 2002; Cinnioglu et al., 2004; Luis et al., 2004; Arredi et al., 2004). This makes it unlikely that the presence of R1*M173 in North Cameroon is due to recent gene ﬂow from neighboring regions. Salas et al. (2002) suggested that the presence of the haplotype R1*-M173 in northern Cameroon populations could be due to recent gene ﬂow from North Africa. These authors maintained that the processes leading to the present-day distribution of haplogroup R1*-M173 could © 2005 WILEY-LISS, INC. Grant sponsor: M.I.U.R.; Grant number: 2003054059; Grant sponsor: University of Rome “La Sapienza;” Grant number: G101819; Grant sponsor: European Science Foundation; Grant number: BFF2002-10206-E; Grant sponsor: Spanish Ministry of Science and Technology; Grant number: BOS2001-0794. *Correspondence to: Giovanni Destro-Bisol, Department of Animal and Human Biology, University “La Sapienza,” P. le A. Moro 5, 00185 Rome, Italy. E-mail: firstname.lastname@example.org Received 25 October 2003; accepted 29 June 2004. DOI 10.1002/ajpa.20138 Published online 13 May 2005 in Wiley InterScience (www.interscience.wiley.com). mtDNA VARIATION IN NORTH CAMEROON have followed the same route of the mtDNA haplogroup U6, widespread among Berbers (Rando et al., 1998; Macaulay et al., 1999), which is thought to have arrived in North Africa from the Near East between 20,000 –50,000 years ago. According to Salas et al. (2002), the Fulbe or other pastoralists might have conveyed both mtDNA sequences belonging to the U6 and H haplogroups and the haplotype R1*-M173 from Berbers to North Cameroon populations. This hypothesis is based on the presence of the Fulbe in North Cameroon and on the evidence that the Fulbe who settled in Nigeria show signs of gene ﬂow of maternally transmitted characters from Berbers, namely sequences belonging to the haplogroups U6 and H (Watson et al., 1997). As a logical development of the study by Cruciani et al. (2002), we analyzed mtDNA variation in North Cameroon. The present work is aimed at understanding whether the presence of Y-chromosome haplotypes of Asian origin has a counterpart in terms of maternally transmitted characters. It is also useful to test the role of the Fulbe from Cameroon as possible carriers of maternal lineages from Berbers to North Cameroon, since mtDNA data from this area were not available to Salas et al. (2002). MATERIALS AND METHODS We analyzed mitochondrial variation in a total of 12 populations and 441 individuals from Cameroon. Informed consent was obtained from all donors. The sampling covers the eight populations from Cameroon studied by Cruciani et al. (2002). In addition, we examined four other populations: Mandara, Tupuri, and Podokwo from North Cameroon, and Bassa from South Cameroon. For information about the populations studied, readers are referred to Table 2 and Figure 1 in Spedini et al. (1999). Specimens collected in K3EDTA were maintained at 4°C (for ⬍7 days) until their arrival at the Laboratory of Anthropology of the University of Rome “La Sapienza.” Genomic DNA was extracted from blood by a standard phenol-chloroform protocol (Gill et al., 1985). It was then quantitated by direct comparison in agarose minigels. We sequenced the hypervariable region-1 (HVR-1) from np 16024 –16390 according to Vigilant et al. (1989), with minor modiﬁcations. Each sequence was assigned to a haplogroup according to the networks published by Salas et al. (2002). A total of 102 haplotypes which could not be unambiguously classiﬁed into L1, L2, or L3 haplogroups on the basis of their HVR-1 sequence alone was further analyzed for their variation at nucleotide positions 10400, 10873, 12308, and 12705 of the mtDNA coding region which deﬁnes the almost exclusively non-African haplogroups M, N, R, and U. The SnapShot method (SNaPshot™ ddNTP Primer Extension Kit, Applied Biosystems) was used to type those sites (Comas et al., 2004). The samples assigned to haplogroup U6 were further investigated by a survey of the MnlI restriction site at np 11203, whose presence is typical of the Ethiopian U6 mtDNAs, and which distinguishes them from those of North Africa (A. Torroni, unpublished data). Sequence data are available at the address: http://www.scienzemfn. uniroma1.it/labantro/database.html. RESULTS AND DISCUSSION Five different haplogroups were found (Table 1): three of sub-Saharan origin (L1, L2, and L3), one of European origin (U5) (Richards et al., 2000), and one of North African origin (U6) (Rando et al., 1998; Macaulay et al., 1999). 679 Considering the overall sampling, the most frequent haplogroups were L3 (46.7%), L2 (26.5%), and L1 (25.2%), which were observed in all 12 populations examined. All sequences observed in South Cameroon belong to macrohaplogroup L. The non-African haplogroups U5 (0.9%) and U6 (0.7%) were conﬁned to North Cameroon, occurring among the Fulbe and among the Podokwo and Uldeme, respectively. All of the U6 mtDNAs lacked the MnlI site at site 11203, which indicates that they do not belong to the Ethiopian subclade of this haplogroup (A. Torroni, unpublished data). We did not detect sequences belonging to haplogroups currently found in Central and East Asia (superhaplogroups M (haplogroups M7–M10, C, D, E, and G), N (A, N9, and Y), and R (R9, B, and F); Kivisild et al., 2002). Therefore, according to our data, the processes leading to the spread of Y chromosomes of Asian origin in North Cameroon did not leave any detectable sign in terms of maternally transmitted characters, at least if one considers the current mtDNA phylogeography of Asian populations. Certainly the lack of Asian sequences in our samples does not necessarily mean that such sequences are absent from the populations. The probability of not ﬁnding by chance a particular sequence that is present in a population at a frequency f in a sample of size N is given by ␣ ⫽ (1 ⫺ f)N. Then for ␣ ⫽ 0.05 and an overall sample size of 441 chromosomes, we obtain a frequency value of 0.7%. Considering only populations from North Cameroon, where haplotype R1*-M173 is most frequent, the sample size is 244 chromosomes, which leads to a maximum nondetectable frequency of 1.22%. Thus, with a 95% probability, it can be ruled out that Asian mtDNA lineages are present in North Cameroon at frequencies over 1.22%, whereas the Y-chromosome haplotype R1*-M173 is found at frequencies of 7–95%, with an average of 39.5%. Our study also points toward the existence of gene ﬂow from North Africa to North Cameroon. In fact, one of the two U6 sequences found in northern Cameroon (in one Podokwo and one Uldeme individual; HVR-1 transitions at nucleotide positions 16172, 16219, and 16278 relative to the Cambridge reference sequence; Anderson et al., 1991) matches with sequences found among Berbers from Morocco, Mauritanians, West Saharans, and Canarians (Rando et al., 1998, 1999; Brakez et al., 2001). The remaining U6 sequence (HVR-1 transitions at np 16172, 16219, and 16311), found in one Uldeme individual, is a one-step neighbor of sequences found among Mauritanians, non-Berber Moroccans, and Wolof from Senegal (Rando et al., 1998, 1999). These results suggest that the U6 sequences found in the Podokwo and Uldeme could be a signature of gene ﬂow from North Africa. At the same time, the presence of this mitochondrial haplogroup among the population where R1*-M173 reaches its highest frequency (Uldeme) makes it worth testing the hypothesis that the Y-chromosomal haplogroup mentioned above could have reached northern Cameroon starting from the Near East, as suggested for U6 (Salas et al., 2002; Rando et al., 1998; Macaulay et al., 1999). Even taking the abovementioned hypothesis for granted, a statistically signiﬁcant difference between the frequency of R1*-M173 and U6 in North Cameroon persists. In fact, U6 ranges from 0 –7% in North Cameroon, with an average frequency of 1.23%. These ﬁgures are much lower than those obtained for the R1*-M173 in the same dataset (see above). Furthermore, the 95% conﬁdence interval range of U6 for North Cameroon is of 0 –2.6%, compared to 27.8 –52.4% for R1*-M173 (recalculated from Cruciani et al., 2002). Fi- 2 4 2 5 2 2 2 10 29 15 15 4 7 6 3 10 34 9 27 13 25 16 21 2 15 14 6 24 21 6 2 7 6 2 4 7 20 15 4 15 5 5 12 11 5 5 8 11 4 5 10 2 6 11 5 5 12 For Bamileke and Ewondo, original data are reported in Destro Bisol et al. (2004). For other populations, sequence data are available at http://www.scienzemfn.uniromal.it/labantro/ database.html. 2 8 2 4 10 11 6 26 10 13 11 6 2 4 2 6 3 3 4 4 8 5 3 2 12 6 13 4 5 5 2 20 12 12 11 13 10 4 11 10 5 18 16 15 15 28 18 L3d L3b L3* L2d L2c L2b L2a L2* L1* L1a L1b L1c L1e North Cameroon Daba (20) Fali (41) Fulbe (34) Mandara (37) Podokwo (39) Tali (20) Tupuri (25) Uldeme (28) South Cameroon Bakaka (50) Bamileke (48) Bassa (46) Ewondo (53) 1 7 3 12 3 5 3 5 15 12 4 5 7 5 30 10 12 8 26 20 24 11 L3e L3f L3g U5a U L3 Population (no. of individuals) Frequency of haplogroups (%) L2 L1 nally, it is important to note that the high frequency of R1*-M173 and the low frequency of U6 found in North Cameroon contrast sharply with the low frequency of R1*M173 and the high frequency of U6 in North Africa (Bosch et al., 2001; Cruciani et al., 2002; Cinnioglu et al., 2004; Luis et al., 2004; Arredi et al., 2004; Salas et al., 2002). Therefore, our study indicates a discrepancy between Y-chromosomal and mitochondrial variation in North Cameroon populations, either considering the current phylogeography in Asia or assuming U6 to be the maternal counterpart of R1*-M173. This is not unexpected, since the two genetic systems are transmitted independently. Previous studies showed that the phylogeography of mtDNA and the Y chromosome may differ substantially even in the same population. For instance, North Africans and Near Eastern Arabs show a much larger maternal than paternal sub-Saharan contribution (Plaza et al., 2003; Richards et al., 2003). We can imagine two extreme scenarios which could explain difference which we have in the frequency of “Asian” Y-chromosome and mtDNA lineages in North Cameroon. In the ﬁrst one, called “migration and asymmetric admixture,” a back migration from Asia to Africa was conducted by groups carrying the Y-chromosome haplotype R1* -M173 at a high frequency. Such groups admixed with resident populations, but the admixture process involved only males of the migrant population. In the second one, namely “divergent drift,” the present-day North Cameroon populations originated from a small group that migrated from Asia to Africa in which, through genetic drift, Y-chromosome haplotype R1*-M173 became the predominant Y-chromosome haplotype, whereas most, if not all, Asian mtDNA lineages were lost. The lack of robust information on movements of prehistoric populations to North Cameroon means that these scenarios must be considered initial working hypotheses which need to be supported by further data on Y-chromosomal and mitochondrial variation in Africa and Asia. Finally, our results fail to support the hypothesis that the Fulbe carried the U6 haplogroup into North Cameroon. In fact, the non-L sequences found in the Fulbe from Cameroon differ substantially from those found among the Berbers, since they belong to the U5 haplogroup (which has a very broad distribution in Eurasia). The fact that the U5 Fulbe sequence (transitions at np 16189, 16192, 16270, and 16320) is a one-step derivative of mtDNA types found in Moroccans (Brakez et al., 2001), Saharawi, and Tunisians (Plaza et al., 2003) suggests that this population had some genetic contact with North African populations. However, lacking a highly diagnostic haplogroup such as U6, thought to be a Northwestern African marker (Rando et al., 1998), our results do not provide support to the role of the Fulbe from Cameroon as U6 carriers to North Cameroon. Finally, the lack of the two mtDNA haplogroups U6 and H in the Fulbe from Cameroon already detected among the Fulbe from Nigeria (Watson et al., 1997) provides evidence of a substantial genetic heterogeneity among Fulbe populations from different areas. This feature is already noticeable in Y-chromosomal data (see Table 2 in Cruciani et al., 2002), where the Fulbe from Burkina Faso lack completely haplogroup R1*-M173. Such heterogeneity makes it difﬁcult to apply the results obtained for a Fulbe population of a certain area to those settled in other areas. U6 V. COIA ET AL. TABLE 1. mtDNA haplogroup frequencies (%) in 12 populations from Cameroon1 680 mtDNA VARIATION IN NORTH CAMEROON ACKNOWLEDGMENTS We thank the blood donors whose availability made this study possible. We are indebted to Rosaria Scozzari and Cristian Capelli for their advice, and to Antonio Torroni for sharing his unpublished data. This research was supported by grants from M.I.U.R. 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