606
The determination and analysis of complete genome sequences
has led to the suggestion that horiztonal gene transfer may be
much more extensive than previously appreciated. Many of
these studies, however, rely on evidence that could be
generated by forces other than gene transfer including
selection, variable evolutionary rates, and biased sampling.
Addresses
The Institute for Genomic Research, 9712 Medical Center Drive,
Rockville, Maryland 20850, USA; e-mail: jeisen@tigr.org;
WWW: http://www.tigr.org/~jeisen
Current Opinion in Genetics & Development 2000, 10:606–611
0959-437X/00/$ — see front matter
© 2000 Elsevier Science Ltd. All rights reserved.
Abbreviation
HGT
horzontal gene transfer
Introduction
‘The view commonly entertained by naturalists is
that species, when intercrossed, have been special-
ly endowed with sterility, in order to prevent their
confusion. This view certainly seems at first highly
probable, for species living together could hardly
have been kept distinct had they been capable of
freely crossing’
Charles Darwin, On the Origin of Species.
Analysis of complete genome sequences is providing
insights into many issues concerning microbe evolution.
One such area involves the transfer of genetic material
between distinct evolutionary lineages — a phenomena
known as horizontal, or lateral, gene transfer. (I prefer the
use of ‘transfer’ instead of ‘exchange’ to refer to the
process as, in most known cases, the process is unidirec-
tional and rarely involves reciprocal exchanges of DNA. In
addition, although the process can involve many genes or
parts of genes, for the sake of simplicity I refer to it simply
as ‘gene transfer’.) This contrasts with what is considered
the ‘normal’ process of inheritance — the transmission of
traits from parents to offspring, also known as vertical
inheritance. The occurrence of horizontal gene transfer
(HGT), which blurs the boundaries between species even
more than the hybridization referred to by Darwin quoted
above, has been generally accepted for many years