Biology Reference
In-Depth Information
3
Comparative Genomics
of Insect Endosymbionts
Ivica Tamas and Siv G.E. Andersson
CONTENTS
Aphid Endosymbionts:
....................................................................................................40
Buchnera
Buchnera
and Their Free-Living Relatives .....................................................................................42
Divergence Dates for
Buchnera
.......................................................................................................43
Comparative Genomics of
Buchnera
...............................................................................................43
Deterioration of the
Genomes.........................................................................................44
Buchnera
INTRODUCTION
To date, approximately 60 microbial genomes have been sequenced, with sizes ranging from the
smallest known genome of
Mycoplasma genitalium
of only 0.58 megabases (Mb) to
Myxococcus
xanthus
of 9.5 Mb and 10 Mb, respectively (www.nlm.ncbi.nih.gov/
entrez/query.fcgi). The genomes of obligate host-associated bacteria tend to be smaller than the
genomes of their close free-living relatives. The smallest genomes are thought to be not ancestral
but the result of reductive genome evolution (Andersson and Kurland, 1998; Moran, 2002). The
sequence loss has come about through a massive loss of phages, mobile elements, and repeated
sequences that are present in abundance in most genomes of free-living bacteria as well as through
the loss of gene sequences that are essential in most other systems.
Unlike free-living bacteria, which are capable of growth in a variety of different environments,
intracellular bacteria grow optimally inside other eukaryotic cells, either freely in the cytoplasm
or inside phagosomes or within specialized cells whose sole function is to contain bacteria. Some
bacteria have lost their ability to grow in a free-living mode; these are obligate host-associated.
Others, referred to as facultative intracellular, are capable of switching between the intracellular
and the extracellular growth environments. In addition to differences in the extent of host-associ-
ation, intracellular bacteria also differ with respect to the type of host associations, as summarized
by the following terms:
and
Nostoc punctiforme
