Lactose digestion capacity in Tokelauans: A case for the role of gene flow and genetic drift in establishing the lactose absorption allele in a Polynesian population

Abstract

Throughout the Pacific, lactose absorption occurs at rates of 0-46%. Most authors explain the current rate of lactose absorption in the Pacific in terms of gene flow with Europeans. However, researchers have not been able to determine the exact historical circumstances of the introduction of the lactose absorption gene. The availability of genealogical material, historical information, and mal/absorption frequencies for Tokelau (a Polynesian population) presents a unique opportunity to examine the role of gene flow in establishing lactose absorption in Pacific isolates. In an earlier study, Cheer and Allen ([1997] Am. J. Hum. Biol. 93:1-34) determined lactose digestion capacities for 58 Tokelauans. In the current study, we constructed pedigrees for the same 58 individuals, using the Tokelau Island Migrant Study genealogies (Wessen et al. [1992]). Results indicated that 8 of 13 lactose-absorbing individuals have European ancestors, compared with only 4 of 35 lactose malabsorbers (Chi(2) = 15.75, P < 0.01). Six lactose-absorbing individuals have either American or Portuguese ancestors dating back to the mid-1800s. Historical data were used to establish the contribution of Portuguese and American genetic material to the Tokelau population following massive depopulation after the Peruvian slave raids of 1863. This study provides clear evidence for the role of a population bottleneck followed by gene flow and genetic drift in establishing lactose absorption in the Tokelau population. Furthermore, it is the first study of a Pacific population to directly link lactose absorption with gene flow.