Biomedical and genetic characteristics of the Ryukyuans: demographic history, diseases and physical and physiological traits

Context: The Ryukyu Islands stretch across a southwestern area of the Japanese Archipelago. Because of their unique geographical and historical backgrounds, Ryukyuans have their own genetic and phenotypic characteristics, which have been disclosed in previous anthropological and biomedical studies. Objective: The history, peopling and biomedical and genetic characteristics of Ryukyuans are reviewed and future research directions are discussed. Conclusion: Morphological and genetic studies have suggested the complex demographic history of Ryukyuans and their relationships with other Asian populations. Knowledge of population formation processes is important to understand the distribution of pathogens. In viral infectious diseases, some strains that may be associated with disease symptoms are specific to Ryukyuans. Dramatic changes in diet have played an important role among Ryukyuans in terms of increases in lifestyle-related diseases and mortality risks. To achieve a better understanding of pathogenic disease factors, further integration of findings regarding the genetic and biomedical characteristics of the Ryukyuans is needed.

A genetic analysis of the Sakishima islanders reveals no relationship with Taiwan aborigines but shared ancestry with Ainu and main-island Japanese

The Sakishima islands are members of the Ryukyu island chain, stretching from the southwestern tip of the Japanese archipelago to Taiwan in the East China Sea. Archaeological data indicate cultural similarities between inhabitants of prehistoric Sakishima and Neolithic Taiwan. Recent studies based on tooth crown traits show remarkably high inter-island diversity among Ryukyu islanders, suggesting that the Sakishima islanders might have genetic backgrounds distinct from main-island Okinawa people. To investigate the genetic diversity of the Ryukyu islanders, we analyzed mtDNA, Y chromosome, and autosomal short tandem repeat loci in a sample of main-island Okinawa people and Sakishima (Miyako and Ishigaki) islanders whose participated in a previous study of tooth crown morphology. Our phylogenetic analysis of maternal (mtDNA) and paternal (Y chromosome) lineages shows that the Sakishima islanders are more closely related to people from the Japanese archipelago than to Taiwan aborigines. Miyako islanders and the Hokkaido Ainu have the first and second highest frequencies (respectively) of the Y-chromosomal Alu-insertion polymorphism, which is a presumable Jomon marker. Genetic diversity statistics show no evidence of demographic reduction or of extreme isolation in each island's population. Thus, we conclude that 1) Neolithic expansion from Taiwan did not contribute to the gene pool of modern Sakishima islanders, 2) male-lineage of the Ryukyu islanders likely shares a common ancestor with the Hokkaido Ainu who are presumably direct descendants of the Jomon people, and 3) frequent reciprocal gene flow among islands has masked the trace of common ancestry in the Ryukyu island chain.

A common variation in EDAR is a genetic determinant of shovel-shaped incisors

Shovel shape of upper incisors is a common characteristic in Asian and Native American populations but is rare or absent in African and European populations. Like other common dental traits, genetic polymorphisms involved in the tooth shoveling have not yet been clarified. In ectodysplasin A receptor (EDAR), where dysfunctional mutations cause hypohidrotic ectodermal dysplasia, there is a nonsynonymous-derived variant, 1540C (rs3827760), that has a geographic distribution similar to that of the tooth shoveling. This allele has been recently reported to be associated with Asian-specific hair thickness. We aimed to clarify whether EDAR 1540C is also associated with dental morphology. For this purpose, we measured crown diameters and tooth-shoveling grades and analyzed the correlations between the dental traits and EDAR genotypes in two Japanese populations, inhabitants around Tokyo and in Sakishima Islands. The number of EDAR 1540C alleles in an individual was strongly correlated with the tooth-shoveling grade (p = 7.7 x 10(-10)). The effect of the allele was additive and explained 18.9% of the total variance in the shoveling grade, which corresponds to about one-fourth of the heritability of the trait reported previously. For data reduction of individual-level metric data, we applied a principal-component analysis, which yielded PC1-4, corresponding to four patterns of tooth size; this result implies that multiple factors are involved in dental morphology. The 1540C allele also significantly affected PC1 (p = 4.9 x 10(-3)), which denotes overall tooth size, and PC2 (p = 2.6 x 10(-3)), which denotes the ratio of mesiodistal diameter to buccolingual diameter.

Revisiting the peopling of Japan: an admixture perspective

The first inhabitants of Japan, the Jomon hunter-gatherers, had their culture significantly modified by that of the Yayoi farmers, who arrived at a later stage from mainland Asia. How this change took place is still debated, but it has been suggested that modern Japanese are the product of an admixture between these two populations. Here, we applied for the first time an admixture approach to study the Jomon-Yayoi transition, using Y-chromosomal data published earlier. Our results suggest that the Neolithic transition, in this part of the world, probably took place by a process of demic diffusion. We also show that for two populations that could not have contributed to this process, our approach is able to detect inconsistencies when they are used as parental populations. However, despite these promising results, we could not locate precisely the geographical origin of the Yayoi in mainland Asia, as different potential sources gave similarly good results. This suggests that more loci would be required for a better understanding of the peopling of Japan.