Phylogenetic analyses do not support horizontal gene transfers from bacteria to vertebrates

Horizontal gene transfer (HGT) has long been recognized as a principal forc e in the evolution of genomes(1). Genome sequences of Archaea and Bacteria have revealed the existence of genes whose similarity to loci in distantly related organisms is explained most parsimoniously by HGT events(2-4). In m ost multicellular organisms, such genetic fixation can occur only in the ge rm line. Therefore, it is notable that the publication of the human genome reports 113 incidents of direct HGT between bacteria and vertebrates(5), wi thout any apparent occurrence in evolutionary intermediates, that is, non-v ertebrate eukaryotes. Phylogenetic analysis arguably provides the most obje ctive approach for determining the occurrence and directionality of HGT(6,7 ). Here we report a phylogenetic analysis of 28 proposed HGT genes, whose p resence in the human genome had been confirmed by polymerase chain reaction (PCR)(5). The results indicate that most putative HGT genes are present in more anciently derived eukaryotes (many such sequences available in non-ve rtebrate EST databases) and can be explained in terms of descent through co mmon ancestry. They are, therefore, unlikely to be examples of direct HGT f rom bacteria to vertebrates.