Tokyo stocks opened nearly flat Tuesday, as selling to lock in gains after the recent 34-year highs of key indexes was offset by buying of export-related issues on a weaker yen against the U.S. dollar. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average rose 19.29 points, or 0.05 percent, from Monday to 38,489.67. The broader Topix index was down 0.24 point, or 0.01 percent, at 2,639.45. On the top-tier Prime Market, gainers were led by marine transportation and pharmaceutical issues, while insurance, and oil and coal product issues led decliners. At 9 a.m., the U.S. dollar …