peopling of Sao Tome (Gulf of Guinea): Origins of slave settlers and admixture with the Portuguese, The

Abstract The geographic origins of African slave settlers and the Portuguese genetic contribution to the population of Sao Tome (Gulf of Guinea) were assessed through the analysis of beta-globin haplotypes in 44 chromosomes bearing the betas^sup s^ allele and through the study of the genetic variation in eight autosomal markers (APOA 1, AT3, FY, LPL, OCA2, RB1, Sb 19.3, and GC) informative for admixture in a sample of 224 individuals. The observed betas^sup s^ haplotype distribution (36.4% Bantu, 52.3% Benin, 4.5% Cameroon, 4.5% Senegal, and 2.3% atypical) is in accordance with the historical information on the major geographic sources of slave settlers of Sdo Tome, although it captures a more important contribution of Central-West Africa regions than previously anticipated. European admixture, estimated to be 10.7 +/-0.9%, has created a considerable level of genetic structure, as indicated by the finding of significant linkage disequilibrium between 33% of unlinked marker loci pairs. Recent admixture was found to have an important contribution to these values, since removal of individuals with Portuguese or Cape Verdian parents or grandparents from the sample dropped the miscegenation level to 6.5 +/- 0.8% and reduced significant linkage disequilibrium to 11% of unlinked marker pairs. Taken together, these results indicate that the peopling of Sao Tome might have provided one of the first examples of the combination of diverse African contributions and European admixture that emerged from the overseas population relocations promoted by the Atlantic slave trade.

The archipelago of Sao Tome Principe, located on the Equator in the Gulf of Guinea, lies approximately 300 km off the coast of Gabon and consists of the two major islands of Sao Tome (836 km^sup s^) and Principe (128 km^sup s^) (Figure 1). It is generally accepted that the islands were uninhabited until their discovery by Portuguese navigators in the early 1470s, but their suitability for sugar cane plantation and the proximity to the African mainland favored a settlement process based on the importation of large numbers of slaves from the adjacent coasts. At present, the population size of the archipelago is over 150,000.

Besides its importance as a sugar colony, Sdo Tome Principe soon became a central entrepot for the assemblage and redistribution of slaves, first within the limits of the Gulf of Guinea, and later in the intercontinental trade with the Americas, especially with Brazil (Curtin 1969; Curtin et al. 1995; Thomas 1997; Klein 1999). In more recent times, an economic boom following the introduction of cacao at the end of the 19th century has led to the importation of indentured contract laborers from the Portuguese colonies of Angola, Cape Verde, and Mozambique, since the local inhabitants refused to work in plantations once slavery had been formally abolished (Tenreiro 1961; Klein 1999; Henriques 2000).

European settlers, mainly Portuguese, have always remained a small fraction of the total population of the archipelago, in spite of measures implemented by the Portuguese crown to stimulate their settling, which included encouragement of miscegenation with the Africans and protection of the social rights of admixed families (Tenreiro 1961; Caldeira 1999; Henriques 2000).

The diversity of contributions to the settlement of Sdo Tome Principe is reflected in the three Creole languages created in the archipelago: Saotomense and Angolan both spoken in Sao Tome, and Principense, spoken in Principe. These Creole languages are characterized by an African substratum provided by Kwa and Western Bantu languages, and they share significant proportions of their lexicon, which is predominantly derived from archaic and modern Portuguese (Ferraz 1979; Hagemeijer 1999). Angolan, the most differentiated of the three languages, has the strongest Bantu component and is spoken by the so-called Angolares, a group living in the southern part of Sdo Tome.

Genetic studies in the archipelago have been centered in the island of Sdo Tome and have so far comprised the characterization of mitochondrial DNA sequence diversity (Mateu et al. 1997) and a number of autosomal markers (Prata et al. 1996; Albarran et al. 1998; Seixas et al. 1999; Pereira et al. 2000). All these studies have shown that the population of Sdo Tome has retained the high global levels of mitochondrial and nuclear DNA diversity that are generally observed in the continental mainland, as expected from a settlement pattern based on massive importation of slaves from different regions. More recently, a study on the Y-chromosome microsatellite variation has detected signs of a slight genetic microdifferentiation between Angolares and the rest of the population (Trovoada et al. 2001). However, some important aspects of the peopling of Sao Tome Principe still remain to be clarified, such as the proportional contribution of different areas of origin of the African settlers or the determination of the impact of admixture with the Portuguese. In the present work, we have addressed the first question by studying the distribution of beta-globin gene cluster haplotypes within chromosomes bearing the hemoglobin betas^sup s^ mutation from the island of Sdo Tome. In addition, we have used a recently established set of informative autosomal markers (Parra et al. 1998) to estimate the extent of Portuguese admixture and to evaluate the patterns of linkage disequilibrium generated in the miscegenation process.