* Buyers included Renaissance Technologies, Adage, Scion

* Sellers included Bridgewater, Marshall Wace

NEW YORK, May 15 (Reuters) - Fund managers zeroed in on the U.S. financial sector during the first quarter, a period during which lenders’ shares were rocked by volatility following the worst banking crisis since 2008, regulatory filings showed on Monday.

The S&P Regional Banking Index fell approximately 25% during the quarter as a run on deposits sank Silicon Valley Bank and Signature Bank in March, both of which were at the time the largest banking failures since the Great Financial Crisis. The S&P Regional Banking index is now down 36% for the year to date.

Funds' positions were revealed in quarterly securities fillings known as 13Fs. While backward looking, these snapshots show what funds owned on the last day of the quarter and are one of the few ways that hedge funds and other institutional investors have to declare their positions. The filings do not indicate exact timing of purchases or sales and may not reflect current holdings.

Among the buyers was billionaire investor Jim Simons' Renaissance Technologies LLC, which has more than $100 billion in assets under management. The fund bought approximately 7.1 million shares of embattled First Republic Bank during the quarter and still held them as of March 31, according to securities filings released Monday. The lender collapsed on May 1 in what was the largest bank failure since 2008.

The securities filings did not show whether the firm sold any of its position before First Republic's May 1 seizure by regulators. The stock closed at $13.99 a share on March 31 after starting the year at around $120.

Famed "Big Short" investor Michael Burry's Scion Asset Management, meanwhile, added a number of new positions in regional banks, including stakes in First Republic, PacWest and Western Alliance Bancorp.

Boston-based Adage Capital Partners added a new position of approximately 185,000 shares of First Republic during the quarter, while New York-based Alpine Global Management LP added a new position of approximately 1.7 million shares, filings showed.

Renaissance Technologies, Adage Capital, Alpine Global, and Scion did not respond to requests to comment.

Shares of regional banks have remained volatile in recent weeks, with some investors wary of more tumult to come in the sector.

Treasury Secretary Janet Yellen said on Saturday that the current banking environment and pressures on earnings of some U.S. regional banks may lead to some concentration in the sector, and regulators will likely be open to such mergers.

Among prominent sellers of First Republic's stock in the first quarter was Ray Dalio's Bridgewater Associates, one of the world's largest hedge funds, which closed its position amid a broad portfolio shift away from financial firms.

The fund liquidated its positions in a range of big U.S. banks including Bank of America Corp, Goldman Sachs Group Inc, and Morgan Stanley, and slashed its positions in smaller banks such as Bank of Hawaii Corp, PacWest, PNC Financial Services Group, Citizens Financial Group and Capital One Financial Corp.

London-based Marshall Wace sold 51,300 shares of First Republic in the first quarter, closing its position in the bank.

Berkshire Hathaway, the conglomerate run by billionaire Warren Buffett, added a new position in Capital One Financial while dissolving its positions in US Bancorp and Bank of New York Mellon Corp, filings showed. (Reporting by David Randall; Editing by Ira Iosebashvili, Marguerita Choy, Lisa Shumaker and Sonali Paul)