NEW YORK (AP) ¿ Shares of Investment Technology Group tumbled Wednesday, undermined by a March drop in its average commission rate that pressured margins, and a subsequent analyst downgrade. Shares of ITG, which provides electronic trading and brokerage services, tumbled $5.76, or 21.6 percent, to $20.92 in afternoon trading. Shares have traded between $13 and $53.35 during the past year. Howard Naphtali, ITG's chief financial officer, said in a statement earlier Wednesday that full-service institutional clients cut back on business, while direct market access clients increased trading. The shift in business mix led to a 17 percent decline in commissions per share, pressuring margins, Naphtali said. The company also said its non-U.S. revenue was down 17 percent to 19 percent during the first quarter. BMO Capital Markets analyst Michael Vinciquerra cut his rating on ITG to "market perform" from "outperform" based on the weakening commission trends and international revenues. Vinciquerra said the decline in commissions was well below his expectations, falling to about 67 cents per 100 shares versus his model of 79 cents per 100 shares.