[rdancer]

Mitochondria are self-contained organelles. Gene transfer would be if their DNA became part of the nuclear DNA. What you suggest is that there was first a lateral transfer, and then that transferred DNA was ejected from the nucleus, and the cell somehow formed organelles that look surprisingly like bacteria. The theory I believe is that symbiosis evolved into mutual dependency, with the mitochondria starting to live inside the cells instead of outside them sometime along the way.