Characterization of Admixture in an Urban Sample from Buenos

Characterization of Admixture in an Urban Sample from Buenos
Aires, Argentina, Using Uniparentally and Biparentally Inherited
Genetic Markers
VERÓNICA L. MARTÍNEZ MARIGNAC,1,3 BERNARDO BERTONI,2 ESTEBAN J. PARRA,3
AND NÉSTOR O. BIANCHI1
Abstract
In this study we analyzed a sample of the urban population of
La Plata, Argentina, using 17 mtDNA haplogroups, the DYS199 Y-chromosome polymorphism, and 5 autosomal population-associated alleles (PAAs).
The contribution of native American maternal lineages to the population of
La Plata was estimated as 45.6%, whereas the paternal contribution was
much lower (10.6%), clearly indicating directional mating. Regarding autosomal evidence of admixture, the relative European, native American, and
West African genetic contributions to the gene pool of La Plata were estimated to be 67.55% (Ⳳ2.7), 25.9% (Ⳳ4.3), and 6.5% (Ⳳ6.4), respectively.
When admixture was calculated at the individual level, we found a low correlation between the ancestral contribution estimated with uniparental lineages
and autosomal markers. Most of the individuals from La Plata with a native
American mtDNA haplogroup or the DYS199*T native American allele
show a genetic contribution at the autosomal level that can be traced primarily to Europe. The results of this study emphasize the need to use both uniparentally and biparentally inherited genetic markers to understand the
history of admixed populations.
The process of gene and cultural exchange in the Americas involved three main
populations: native Americans, Europeans, and West Africans. Gene introgression from other populations (e.g., South Asia, East Asia, and the Middle East)
has been much smaller and limited to speciÞc geographic regions. Throughout
the Americas, historical differences in the pattern of settlement and migration
have resulted in a wide range of parental genetic contributions to present rural
and urban populations (Sans 2000).
1
Multidisciplinary Institute of Cell Biology, Department of Population Molecular Genetics, Calle 526 e/10
y 11, C.C. 403, La Plata, C.P. 1900, Buenos Aires, Argentina.
2
Department of Genetics, College of Medicine, Universidad de la República, Avda. Gral. Flores 2125, C.P.
11800, Montevideo, Uruguay.
3
Department of Anthropology, University of Toronto at Mississauga, 3359 Mississauga Rd. North, Room
4019, South Building, Mississauga, ON L5L 1C6, Canada.
Human Biology, August 2004, v. 76, no. 4, pp. 543–557.
Copyright 䉷 2004 Wayne State University Press, Detroit, Michigan 48201-1309
KEY WORDS: Y CHROMOSOME, MITOCHONDRIAL DNA, mtDNA HAPLOGROUPS, AUTOSOMES, PV92, WI1319, DR2DI, OCA2, SB19.3, DYS199, ADMIXTURE, POPULATION-ASSOCIATED
ALLELES (PAAs), ARGENTINA, PARAGUAY, URUGUAY, SOUTH AMERICA.
544 / martínez marignac et al.
The presence of molecular markers of restricted geographic distribution in
the mitochondrial genome and the non-pseudo-autosomal region of the Y chromosome provides a powerful tool to explore the history of female and male
migrations. mtDNA and Y-chromosome-speciÞc sequences are haploid, do not
recombine, and are transmitted through the maternal and paternal lines, respectively. Given their mode of inheritance, mtDNA and Y-chromosome-speciÞc sequences provide a useful but limited view of the past history of an individual or
population. The full perspective can be gained only by combining these uniparental markers with autosomal markers, which provide the ‘‘average’’ history of
admixture through the generations. A particularly useful set of autosomal markers to study admixture are those markers showing large allele-frequency differences (⌬ values) between geographic populations (Shriver et al. 1997; Parra et
al. 1998, 2001). These markers have been termed population-speciÞc alleles
(PSAs) or population-associated alleles (PAAs) (Pfaff et al. 2001).
A good example of the application of uniparental and biparental markers
to understand population history was described by Bravi et al. (1997) and Sans
et al. (2002). These researchers used mtDNA, Y-chromosome, and autosomal
polymorphisms to study a relatively isolated Afro-Uruguayan population that,
according to historical and sociocultural data, was assumed to have primarily an
African genetic contribution. In this population 52% of the mtDNA lineages were
of African ancestry, 29% of native American ancestry, and 19% of European
ancestry, whereas the relative contributions observed for the Y-chromosome lineages were 30% African, 6% native American, and 64% European. The use of
blood groups and protein enzyme markers indicated a mixture composed of 47%,
15%, and 38% African, Amerindian, and European autosomal genetic components, respectively. This Afro-Uruguayan example was chosen to illustrate three
points: (1) Genetic mixture occurs in most human populations, even in seemingly
isolated ones; (2) admixture estimates provided by mtDNA, Y-chromosome, and
autosomal markers may differ because each system evaluates a distinct path of
gene introgression; and (3) genetic data provide information that is critical to
understanding the history of any given population, complementing and expanding the evidence that can be gathered from other sources.
Most urban populations in Argentina are assumed to have a predominantly
European genetic component (mainly because of immigration from Spain and
Italy and to a lesser extent from Germany and France) with low levels of Amerindian and African admixture (INDEC 1997; Sala et al. 1999; Goicoechea et al.
2000). However, estimation of admixture using genetic markers is almost nonexistent for Argentinean urban populations.
La Plata city, the capital of the province of Buenos Aires, is a city of
750,000 inhabitants. The city was founded in 1882 in a region that had no Amerindian population, and the early settlers were Europeans, mainly Italians, who
came to the city as workers in charge of building the ediÞces housing the political, cultural, and administrative branches of the provincial government (Bejarano
1967; Barba 1983). At the end of the 19th century the population of La Plata
Admixture in Buenos Aires / 545
Table 1. Samples and Number of Individuals Analyzed
Sample
Argentinean native Americans
Chorote
Toba
Mataco-Mataguayo
Tehuelche
Mapuche
Humahuaqueño and Quechua, province of Jujuy
Paraguayan native Americans
Lengua
Ayoreo
Europeans
CEPH
Urban sample, La Plata city
Women (n)
Men (n)
Total n
35
4
3
9
1
3
30
4
6
13
1
3
65
1
4
7
6
8
10
28
23
19
64
47
87
3
4
CEPH, Centre d’Etude du Poymorphisme Humain.
was approximately 40,000. The marked population expansion that resulted in the
approximately 640,000 inhabitants registered in the last La Plata census (2002)
occurred in the 20th century as a result of immigration, mainly from rural areas
in northern Argentina to Buenos Aires, La Plata, Rosario, and other industrialized
cities (Elizalde 1975; Fouscaldo 1985). For La Plata the two main immigration
periods were 1930–1935 and 1945–1955. In this study our main objective was to
estimate the Amerindian genetic proÞle of the present urban population of La
Plata. We have estimated admixture in a random sample using 17 mtDNA haplogroups, the DYS199 Y-chromosome polymorphism, and 5 autosomal populationassociated alleles (PAAs).
Materials and Methods
Populations Analyzed.
Relevant details regarding the samples analyzed in
this study are given in Table 1. Additional information is provided in the following paragraphs.
One of the main objectives of our investigation was to estimate the Amerindian genetic proÞle of the present urban population of La Plata. Because no
historical records of direct admixture of La Plata inhabitants with Amerindians
exist, we decided not to include in the study samples from donors with Amerindian surnames to avoid bias resulting from atypical and recent Amerindian gene
incorporations to the ‘‘main La Plata’’ genetic pool. The number of samples of
Amerindian origin not included in our analysis was low (⬍2%); therefore the
biological material used for this study can be considered a random sample representing the population of ‘‘main La Plata’’ city. The La Plata sample was represented by 87 DNA samples (64 males and 23 females) obtained from donors
whose surnames were all of European origin (Table 1). The sample included
546 / martínez marignac et al.
individuals attending a public hospital and also staff and faculty from the University of La Plata. At the time of collection, all the individuals were working and
living in the city of La Plata.
Our reference populations include the native American, European, and
West African populations used to obtain the PAA reference frequencies employed
to estimate admixture. Native American DNA samples were obtained from six
Argentinean and two Paraguayan communities inhabiting geographic areas historically occupied by their ancestors. Donors were selected only if they had welldocumented Amerindian genealogies and no family links with other donors in
the same population. The samples are composed of individuals from the following native American groups: (1) Humahuaqueño and Quechua from the province
of Jujuy in northwestern Argentina; (2) Tehuelche from Pampa de Chalṍa and
Loma Redonda, province of Neuquén; (3) Mapuche, from the province of Rṍo
Negro; (4) Mataco-Mataguayo from the village of Santa Victoria, northwest of
the province of Salta; (5) Chorote and (6) Toba from the village of Santa Victoria,
in the northwest province of Salta; and (7) Ayoreo and (8) Lengua from southern
Paraguay.
Europeans were represented by DNA samples corresponding to 19 males
and 28 females that were part of CEPH genealogies of known European ancestry.
In addition, we used previously published data on allele frequencies from Spain
and West Africa to estimate the parental frequencies that were used in the admixture analysis (Parra et al. 1998). These data are available in the dbSNP database
under the submitter handle PSU_ANTH (available at http://www.ncbi.nlm.nih
.gov/SNP/).
All La Plata donors gave informed consent for their participation in this
study. The native American and European samples analyzed in the laboratory
were submitted by various investigators who were in charge of obtaining informed consent from the donors. For the La Plata samples in which the surname
of the donors was made available to the research group, the correspondence between each sample and the surname of the donor was maintained in anonymity.
Molecular Markers. The samples were screened for the following mtDNA
haplogroups: A1, A2, B, C1, C2, D1, and D2 (native American–speciÞc haplogroups); H, I, J, K, T, U, V, W, and X (European-speciÞc haplogroups); and
L1/L2 (African-speciÞc haplogroups). Information on the experimental conditions
used to characterize these haplogroups, including primer sequences and PCR conditions, can be found in Bailliet et al. (1994) and Torroni et al. (1994, 1996).
The samples were screened for the presence of a C V T transition in the
DYS199 locus, a marker speciÞc to native American Y chromosomes, usually
found in 70–90% of South American native American populations (Underhill et
al. 1996; Bianchi et al. 1998).
The samples were characterized for Þve PAA biallelic markers (PV92,
WI1319, DR2DI, OCA2, and SB19.3). Further information on these polymorphisms is provided in Table 2. These Þve PAAs were selected because they show
Admixture in Buenos Aires / 547
Table 2. PAA Markers Used
Polymorphism
Type of
Marker
Chromosome
Locationa
PV92
Alu insertion
16q24.1, ss4387048
WI14319
C to T transition,
RsaI RSP
15q14, ss4387037
DR2DI
C to T transition,
BclI RSP
Intronic region of
DR2D gene, 11q23.2,
ss4387021
OCA2
G to A synonymous
transition, HaeIII
RSP
15q11.2–q12,
ss4387028
SB19.3
Alu insertion
19p13.11, ss4387049
Primers
F: 5⬘ TGA GTT CTC AAC TCC
TGT GTG TTA G 3⬘
R: 5⬘ AAC TGG GAA AAT TTG
AAG AGA AAG T 3⬘
F: 5⬘ TGT TTT TGA TTG AAG
AAA CAT CTC T 3⬘
R: 5⬘ AAT TCT GTA CTG TCC
CAC CCC 3⬘
F: 5⬘ CCT CCT CTC CGT TCT
CCA 3⬘
R: 5⬘ AGG GCC TTA AAA GTC
A 3⬘
F: 5⬘ CTT TCG TGT GTG CTA
ACT CC 3⬘
R: 5⬘ ACC TCT AGC ATG GTT
CTT GGG C 3⬘
F: 5⬘ TCT AGC CCC AGA TTT
ATG GTA ACT G 3⬘
R: 5⬘ AAG CAC AAT TGG TTA
TTT TCT GAC 3⬘
a. ss ⳱ dbSNP NCBI submitted number.
high frequency differences between the parental populations (native Americans,
Europeans, and West Africans) and therefore are extremely informative to estimate the relative genetic contribution of each of these parental populations to the
admixed population. The frequencies of these PAAs in several native American
samples, European samples, West African samples, and the sample from La Plata
are depicted in Table 3.
Statistical Analyses. PAA allele frequencies were directly estimated by allele
counting. The Þt of genotype frequencies to Hardy–Weinberg proportions was
tested using the Fisher exact test implemented in the GDA, version 1.1, program
(Lewis and Zaykin 2002). Population admixture was calculated using the gene
identity method (Chakraborty 1985), implemented in the Admix95 program (provided by B. Bertoni and available at www.genetica.fmed.edu.uy/software.html),
using a model with three parental populations (native Americans, Europeans, and
West Africans).
We used the Structure 2.0 program (Pritchard et al. 2000) to evaluate
whether there was evidence of population structure in the samples and to estimate
individual admixture proportions. To determine whether there was population
stratiÞcation (population structure) in any of the samples (e.g., native American
samples from different communities, European samples from CEPH and Spain,
or the sample from La Plata), we ran the program with K ⳱ 1, K ⳱ 2, and K ⳱ 3
548 / martínez marignac et al.
Table 3. Allele-Frequency Results for Each Sample and Average Frequency Used for
Population Admixture Estimation
Sample
N
Native Americans
Humahuaqueño65
Quechua
Mataco-Mataguayo
13
Toba
6
Chorote
4
Mapuche
3
Tehuelche
1
Lengua
8
Ayoreo
10
Native American
110
average
Europeans
CEPH
47
86
Spaina
European average
133
Africans (Nigeria,
251
Central African Republic,
and Sierra Leone)a
La Plata
87
PV92 Alu WI14319*C DRD2*C OCA2*A
Alu
Frequency Frequency Frequency Frequency Frequency
0.808
0.731
0.531
0.208
0.508
0.692
0.750
1.000
0.667
1.000
0.688
0.000
0.714
0.500
0.750
0.875
0.833
0.500
0.875
0.900
0.736
0.615
0.750
0.705
0.667
1.000
0.563
1.000
0.614
0.731
0.500
0.625
0.833
0.500
0.563
0.700
0.391
0.462
0.583
0.625
0.333
0.000
0.500
0.600
0.509
0.117
0.140
0.132
0.203
0.287
0.174
0.214
0.378
0.096
0.140
0.124
0.058
0.840
0.709
0.756
0.118
0.702
0.895
0.827
0.438
0.362
0.345
0.178
0.724
0.621
a. Available at dbSNP, under the submitter handle PSU_ANTH.
as the predetermined number of populations, using 50,000 iterations for the burnin period and 200,000 additional iterations to obtain parameter information. For
all calculations we used a model with uncorrelated frequencies and independent
alphas. The presence of population stratiÞcation in a sample will result in models
with K ⳱ 2 (or higher) having a higher posterior probability than the model with
K ⳱ 1. To estimate autosomal admixture in each of the individuals from La Plata,
we prepared an input Þle that included genotype data from each of the parental
populations (native Americans, Europeans, and West Africans) as well as from
the sample from La Plata. Structure 2.0 was then run using a model with K ⳱ 3
populations. The number of iterations was 50,000 for the burn-in period and
200,000 to obtain parameter information. The resulting output Þle indicates the
relative genetic contribution of each of the parental populations to the individuals
from La Plata.
To estimate the relative maternal and paternal contribution to the urban
sample from La Plata using the mtDNA and Y-chromosome markers, we used
Bernstein’s equation:
m⳱
pH ⳮ p1
,
p2 ⳮ p1
(1)
where pH is the allele frequency in La Plata and p1 and p2 are the allele frequencies in the parental populations.
Admixture in Buenos Aires / 549
Results
Parental Samples. Ninety-nine percent of the native American samples (109/
110) had a native American mtDNA lineage, and 88% of the males (53/60)
showed the DYS199*T native American allele. A combination of Y-chromosome
and mtDNA native American lineages was observed in 52 males (87%). All
CEPH samples showed European-speciÞc mtDNA lineages and showed the
DYS199*C allele.
Table 3 shows the frequencies of the autosomal PAAs analyzed in this
study. We used the Fisher exact test to evaluate the goodness of Þt to Hardy–
Weinberg proportions for each locus and over all loci (3,200 shufßing runs).
Single-locus testing showed signiÞcant departures from Hardy–Weinberg proportions for PV92, DR2D, OCA2, and SB19.3 in the native American sample, for
PV92 and DR2D in the CEPH samples, and for PV92, DR2DI, and SB19.3 loci
in the sample from La Plata.
The genetic structure analyses using Structure 2.0 showed that there was
no evidence of the presence of a signiÞcant genetic structure in the European
parental sample composed of individuals from Spain and the CEPH. The model
with the highest posterior probability was K ⳱ 1 in this sample. However, Structure 2.0 indicated the presence of genetic structure in the native American group.
The model with K ⳱ 2 had the highest posterior probability (ln P(D) ⳱ ⳮ668.4
for K ⳱ 2; ln P(D) ⳱ ⳮ709.9 for K ⳱ 1). Of the two groups deÞned by Structure
2.0, one group consisted of Ayoreos (10 subjects) and 18 additional individuals
from several other aboriginal communities (11 Humahuaqueños and Quechuas,
1 Mapuche, 3 Mataco-Mataguayos, 2 Lenguas, and 1 Toba). The second group
included the remaining native American samples.
Urban Sample.
As mentioned, before the characterization of admixture proportions in the sample from La Plata, we genotyped a set of mtDNA, Y-chromosome, and autosomal PAA markers in several samples from European (CEPH)
and native American groups (Omaguacas or Humahuaqueños, Quechuas, Tehuelches, Mapuches, Mataco-Mataguayos, Chorotes, Tobas, Ayoreos, and Lenguas).
We used these frequencies, in addition to published data for other European and
African samples, to estimate admixture contributions in the sample from La
Plata.
Table 4 displays the mitochondrial results for La Plata: 44.8 % (39/87) of
the samples showed a native American maternal lineage, 41.4% (36/87) were
non–native American mtDNA haplogroups, and for the remaining 13.8% we
could not determine the lineage using the 17 mitochondrial markers analyzed in
this study. Regarding the Y chromosome, 9.4% of the samples (6 of the 64 males)
had the DYS199*T allele. According to Bernstein’s formula, the contribution of
native American maternal lineages to the gene pool of La Plata was estimated as
45.2%, whereas the paternal contribution was much lower (10.6%).
The assessment of ethnic admixture using autosomal markers showed that
550 / martínez marignac et al.
Table 4.
La Plata mtDNA Haplogroups and Their Geographic Ancestral Origin
Haplogroupa
AluI 10397/
DdeI 10394
A1
A2
B
C1
C2
D1
D2
H
I
J
K
T
U
V
W
X
Nb
ⳮ/ⳮ
ⳮ/ⳮ
ⳮ/ⳮ
Ⳮ/Ⳮ
Ⳮ/Ⳮ
Ⳮ/Ⳮ
Ⳮ/Ⳮ
HaeIII Ⳮ663, HaeIII Ⳮ16517
HaeIII Ⳮ663, HaeIII ⳮ16517
Del 9-bp region V, HaeIII Ⳮ16517
HincII ⳮ13259, HaeIII Ⳮ16517
HincII ⳮ13259, HaeIII ⳮ16517
AluI ⳮ5176, HaeIII Ⳮ16517
AluI ⳮ5176, HaeIII ⳮ16517
3
4
10
6
6
1
9
Native American
(44.8%)
ⳮ/ⳮ
Ⳮ/ⳮ
Ⳮ/ⳮ
Ⳮ/ⳮ
ⳮ/ⳮ
ⳮ/ⳮ
ⳮ/ⳮ
ⳮ/ⳮ
ⳮ/ⳮ
AluI ⳮ7025
DdeI ⳮ1715, HaeIII ⳮ8250
HinfI Ⳮ16065
A V G 12308, HhaI ⳮ9053
BamHI Ⳮ13366
NlaIII ⳮ4577
HaeII ⳮ8250
A V G 12308 bp
DdeI ⳮ1715, HaeIII Ⳮ8250,
EcoRV Ⳮ16274
11
3
5
0
3
6
0
0
3
European (35.6%)
L1/L2
Ⳮ/ⳮ
HpaI Ⳮ3592
1
Other
ⳮ/ⳮ
No A, B, C, D, H, T, U, V, W or X
haplogroup
No A, B, C, or D haplogroup
No I, J, K, L, X, A, D, or C haplogroup
2
ⳮ/ⳮ
Ⳮ/ⳮ
No data
Total
Lineage
Ancestral Origin
Restriction Sites Characterized
1
1
12
87
African (1.2%)
Non–Native
American (4.6%)
(13.8%)
a. Haplogroups are according to Bailliet et al.’s (1994) and Torroni et al.’s (1996) nomenclature for
native American and European lineages, respectively.
b. Number of individuals.
the relative European, native American, and West African contributions to the
gene pool of La Plata were 67.6% (Ⳳ2.7), 25.9% (Ⳳ4.3), and 6.5% (Ⳳ6.4),
respectively. The R2 for the trihybrid model was 99.9%.
The Structure 2.0 program was applied to test for the presence of genetic
structure in the sample from La Plata and to estimate admixture proportions at
the individual level. There was no evidence of genetic structure in La Plata,
which is most likely represented by a model with K ⳱ 1 (ln P(D) ⳱ ⳮ529.4)
rather than K ⳱ 2 (ln P(D) ⳱ ⳮ578.1). The results of individual admixture
showed a high European contribution to all individuals, even to those with both
uniparental lineages of native American origin (Figure 1). The average European
contribution at the individual level was 67.7% (Ⳳ4.5%), the Native American
contribution was 25.6% (Ⳳ4.35%), and the African average contribution was
6.7% (Ⳳ0.9%). These values are similar to the population admixture levels estimated with Chakraborty’s gene identity method.
Individual admixture proportions in the sample of La Plata. (A) Results of uniparental ancestor
lineages for each individual (striped bars, DYS199*T or native American mtDNA; solid bars,
DYS199*C or non–native American mtDNA; open bars, no data available). (B) Results of admixture proportions for each individual obtained with the Structure 2.0 program (open bars, percentage
of native American contribution; gray bars, percentage of African contribution; solid bars, percentage of European contribution).
Admixture in Buenos Aires / 551
Figure 1.
552 / martínez marignac et al.
Comparison of Admixture Estimates Using Y-Chromosome-SpeciÞc, mtDNA,
and PAA Systems in La Plata. Our results showed a lack of correlation between mtDNA and Y-chromosome systems versus the PAA systems used to determine the ethnic genetic proÞle in the La Plata sample (Figure 1): Only 2 of
the 42 individuals showing at least one Amerindian parental lineage also had a
predominant Amerindian autosomal background, whereas in all other cases PAA
markers indicated a prevalent European endowment. On the contrary, in the native American sample, uniparental and autosomal systems coincided in showing
a prevalent Amerindian genetic proÞle (Figure 2).
Discussion
The aim of this study was to characterize the parental genetic contributions
to an urban sample from Argentina through the analyses of uniparentally and
biparentally inherited genetic markers, both at the population and the individual
level.
We used two sets of DNA samples representing well-deÞned European and
native American populations in order to test the efÞciency of the uniparental and
biparental markers employed in this study to estimate the genetic admixture in
the La Plata population. No evidence of genetic structure was observed in the
samples of European origin. Conversely, a marked structuring (K ⳱ 2) and a
signiÞcant Hardy–Weinberg disequilibrium were detected in Amerindian populations, very likely resulting from well-documented population reductions associated with genetic drift and endogamy (Ubelaker 1992; Guerreiro et al. 1994;
Trachtenberg et al. 1996; Zago et al. 1996; Tourret et al. 2000; Goicoechea et al.
2001). The Structure 2.0 program showed a signiÞcant non-Amerindian genetic
proÞle in only 3 of the 110 Amerindians included in this study (Figure 2). This
Þnding further supports the assumption that isolation and genetic drift and not
gene introgression are the major factors inducing the genetic structuring detected
in Amerindians (Cavalli-Sforza, et al. 1996; Salzano and Bortolini 2002). The
wide dispersion of allele frequencies detected in native American populations
(Table 4) is in good agreement with this hypothesis.
Both mtDNA and Y-chromosome data show evidence of a native American
genetic contribution to the urban sample from La Plata. However, the native
American paternal contribution was substantially lower (10.6%) than the native
American maternal contribution (45.2%). Clearly, there has been sex-biased gene
ßow in La Plata, in agreement with historical reports indicating that during colonial times, Spanish men embarking on the conquest of South America commonly
practiced polygamous unions with native American women (Herren 1992). Previous studies have also shown directional mating in other South American populations (Dipierri et al. 1998; Batista dos Santos et al. 1999; Rodriguez-DelÞn et al.
2001). The autosomal markers showed an intermediate picture, with a high European genetic contribution (68%) and native American and African estimates of
Individual admixture proportions in the native American parental sample. (A) Results of uniparental ancestor lineages for each individual (striped bars, DYS199*T or native American mtDNA; solid
bars, DYS199*C or non–native American mtDNA; open bars, no data available). (B) Results of
admixture proportions for each individual obtained with the Structure 2.0 program (open bars,
percentage of native American contribution; gray bars, percentage of African contribution; solid
bars, percentage of European contribution).
Admixture in Buenos Aires / 553
Figure 2.
554 / martínez marignac et al.
26% and 6%, respectively. The PAA data in La Plata show concordance with
previous studies in urban regions from Argentina, where a high European ancestral contribution was detected but relatively small native American and African
contributions were also present (Martṍnez Marignac et al. 1999; Avena et al.
2001).
In La Plata the autosomal native American contribution is relatively low at
the individual level, even in those individuals showing native American maternal
or paternal lineages. This Þnding indicates that the native American contribution
to La Plata did not take place in recent times. The uniparental and biparental
admixture data from the urban sample of La Plata can be interpreted to be a
result of admixture during colonial times or the Argentinean state conformation.
It is important to note that the present population of La Plata can be traced back
not only to relatively recent immigrants from Europe but also to immigration of
previously admixed individuals from rural areas and neighboring countries. In
addition, it is known that Africans also migrated to the region (Picotti 2001), and
indeed a minor African genetic contribution was detected in the sample from La
Plata.
The data indicate that admixture studies with the goal of understanding our
past should integrate the use of uniparental and biparental markers. Y-chromosome-speciÞc markers and mtDNA can provide information about male and female gene ßow patterns and are useful for detecting directional mating. In
addition to the Y-chromosome and mtDNA evidence, the autosomal evidence
provides a picture of the global history of admixture through the generations.
The precision of the autosomal admixture estimates is dependent on the number
and ⌬’s of the markers studied. Although the analysis relied on only Þve autosomal markers, these markers showed high frequency differences between native
American, European, and West African populations and were informative for
estimating admixture proportions. In our judgment, admixture studies based only
on uniparental markers must be interpreted with caution because of the potential
effect of directional mating. In places such as La Plata, the results of mtDNA,
Y-chromosome, and autosomal DNA can be substantially different. Native
American contribution is highest at the level of mtDNA, intermediate at the autosomal level, and lowest at the level of the Y chromosome. In addition, most of
the individuals from La Plata who showed a native American mtDNA haplogroup
or the DYS199*T native American allele also showed a genetic contribution at
the autosomal level that can be traced primarily to Europe.
Finally, we would like to stress that the characterization of population and
individual ancestry is important, and not just from the historical and anthropological perspectives. This subject far exceeds its scientiÞc limits; several constitutions in South American countries have been recently reformed (e.g., in
Argentina in 1994, in Colombia in 1991, and in Ecuador in 1998). The new
constitutions recognize the existence and rights of indigenous people, but currently there is no reliable deÞnition of what ‘‘indigenous’’ means for legal matters. The inclusion and recognition of rights to the ‘‘indigenous people’’ in the
Admixture in Buenos Aires / 555
Argentinean National Constitution has raised controversial interpretations, and
some populations have become interested in the characterization of their genetic
proÞles because of the potential need to defend their own rights (Bianchi and
Martṍnez Marignac 2001; Bianchi et al. 2003). In this sense, it is important to
mention that the history of populations and individuals is complex, and understanding this history requires gathering evidence from a variety of sources, not
just genetics.
Received 22 July 2003; revision received 26 November 2003.
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