Figure A1.

Germ line transmission of ERVs is a form of Lamarckian inheritance of acquired characters, creating a reverse flow of genetic information. Before reaching germ cells, the virus must give a systemic infection of varying intensity, then infect reproductive tissue, and finally reach germ line cells, presumably spermatogonia or oogonia. There are several bottlenecks in this process; (1) A systemic infection must be established, then (2) reproductive tissue must be infected, and (3) finally the germ cell precursor must be infected, (4) the offspring must be fit enough to pass on the new acquired character, and (5) in a few cases the new ERV is “fixed” in the germ line, i.e., is present in all individuals in a species. The many restrictions and the ubiquity of fixed ERVs indicate that stages 1–4 are common, but surprisingly little is known about this process.