Zinder and Lederberg (Nobel 1958) placed two auxotrophic strains of Salmonella on opposite sides of a Davis U-tube.

Prototrophs were recovered from the side containing LA-22 cells, but not from the side containing LA-2 cells.

Adding DNase, an enzyme that digests DNA, did not prevent recombination, so this is not transformation.

The filter prevented cell contact, so this is not conjugation.