Jamieson, Fausset and Brown on 1 Chronicles 2:17

17. Jether the Ishmaelite--(compare 2Sa 17:25). In that passage he is called Ithra an Israelite; and there seems no reason why, in the early days of David, anyone should be specially distinguished as an Israelite. The presumption is in favor of the reading followed by the Septuagint, which calls him "Jetra the Jezreelite." The circumstance of his settling in another tribe, or of a woman marrying out of her own tribe, was sufficiently rare and singular to call for the statement that Abigail was married to a man of Jezreel.

1Ch 2:18-55. Posterity of Caleb.