What tangled web: barriers to rampant horizontal gene transfer

In English Dawkins in his The Selfish Gene(1) quite aptly applies the term "selfish" to parasitic repetitive DNA sequences endemic to eukaryotic genomes, especially vertebrates. Doolittle and Sapienza (2) as well as Orgel and Crick (3) enlivened this notion of selfish DNA with the identification of such repetitive sequences as remnants of mobile elements such as transposons. In addition, Orgel and Crick (3) associated parasitic DNA with a potential to outgrow their host genomes by propagating both vertically via conventional genome replication as well as infectiously by horizontal gene transfer (HGT) to other genomes. Still later, Doolittle (4) speculated that unchecked HGT between unrelated genomes so complicates phylogeny that the conventional representation of a tree of life would have to be replaced by a thicket or a web of life. (4) In contrast, considerable data now show that reconstructions based on whole genome sequences are consistent with the conventional "tree of life".((5-10)) Here, we identify natural barriers that protect modern genome populations from the inroads of rampant HGT. (c) 2005 Wiley Periodicals, Inc.