Mainland Asia includes the lower basin of Chinas Yangtze River as well as Korea  But being specific is fine too  I just found a citation for a more comprehensive DNA study by Hammer below rather than our generarizations and speculation so far    Citation for Yayoi culture was brought to Japan by migrants from Korea who in turn trace their roots to southeast Asiasouth China     DNA study by Hammer  Describes the Yayoi migration from Korea based on the OSRY genes and other genes with close lineage haplogroups OM and OM Reiterates that the entire O haplogroup has been proposed to have a Southeast Asian origin  Their definition of Southeast Asia includes southern China  Then hypothesizes that the dispersals of Neolithic farmers from Southeast Asia also brought haplogroup O lineages to Korea and eventually to Japan  In the concluding paragraph it states we propose that the Yayoi Y chromosomes descend from prehistoric farmers that had their origins in southeastern Asia perhaps going back to the origin of agriculture in this region  Hammers DNA study is based on a global sample consisted of   males from  Asian populations including six populations sampled from across the Japanese archipelago