Homogeneity in the structure of the medaka fish transposable element Tol2

The hAT family is a group of transposable elements of the terminal inverted repeat class, which includes Ac of maize, hobo of Drosophila and Tam3 of A ntirrhinum (snapdragon). AU the members of this family so far examined are known to comprise complete and defective copies, with a good correspondence to autonomous and non-autonomous elements, respectively. Internal deletion is the most common cause of defective copies. Tol2, a transposable element of the medaka fish Oryzias latipes, is a member of the hAT family. We exam ined, mainly by the genomic Southern blot analysis, variation in the struct ure of copies of this element, and revealed that there are few or no intern ally deleted copies. This situation is unusual in a member of the hAT famil y. Possible causes of this anomaly are discussed.