Biology Reference
In-Depth Information
Symbiosis
Ð Association where one organism (the symbiont) lives within the body of another
organism (the host), regardless of the actual effect on the host; this term is often used to
describe physically associated mutualistic relationships
Mutualism
Ð Association in which both partners beneÝt from their interaction; develops
most often between organisms with widely different living requirements
Commensalism
Ð Association where one of the species obtains some beneÝt from the
interaction without either harming or beneÝting the other organism; this association is
most often seen between a larger host and a smaller commensal
Parasitism
Ð Relationship between two species in which one partner beneÝts at the expense
of the other
Most intracellular bacteria whose genomes have been sequenced are human parasites that often
kill their hosts after exploiting amino acids, nucleotides, and other small molecules present in the
nutrient-rich intracellular growth milieu.
is an excellent example of an obligate
intracellular human pathogen with a small genome (1.1 Mb) that is transmitted by insect vectors,
in this case lice (Andersson et al., 1998). Not surprisingly, the reduction of genome size has mostly
been achieved at the expense of genes coding for proteins involved in metabolic pathways that are
no longer required in the new, host-associated environment. Furthermore, most of the nonessential
genetic material, such as repetitive elements, insertion sequences, prophages, transposons, and other
kinds of foreign DNA, has been eliminated from these small genomes. Conversely, genes involved
in basic information processes are less affected by the reduction of genome size, and a basic set
of essential genes is present in most intracellular parasites irrespective of their different phylogenetic
afÝliation and the extent of damage caused to their host cells.
Other intracellular bacteria have established mutualistic interactions with their host cells. These
relationships are typically based on the bacterial supply of compounds such as amino acids and
other small metabolites to the host. The aphid endosymbiont
Rickettsia prowazekii
is perhaps the
best-known example of a bacterium that has established a long-term obligate relationship with its
host (Baumann et al., 1995). However, in contrast to obligate intracellular parasites, obligate
endosymbionts have often retained genes for basic biosynthetic processes and in some cases even
ampliÝed them on plasmids in response to the host demand for small molecules.
Thus, the process of reductive evolution is associated with host specialization as well as with
genome deterioration and loss of mutational variability in the population. This seems to be an
irreversible process in small, isolated, and asexual populations that frequently undergo bottlenecks
(Moran, 1996, 2002; Andersson and Kurland, 1998) irrespective of the effects on host phenotypes.
Thus, there is a theoretical danger that deleterious mutations will accumulate to such an extent that
the Ýtness of the bacterium (and eventually its host) will start to decrease. This is because few, if
any, genome variants are present in the population and because the loss of genes involved in DNA
repair and recombination processes further reduces the possibility of repairing a sequence loss by
recombination with a variant in which the lost fragment is still present.
Most published work on the interactions of endosymbionts with their hosts has focused on the
nutritional dependencies of the host and its symbiont and on the physiological consequences of the
interaction. However, recent research on the genomes of endosymbiotic bacteria has started to
reveal the genetic basis for this relationship from the bacterial perspective. In this review, we will
discuss the genome content and structure of bacteria that have developed mutualistic relationships
with insects, based primarily on the recent publication of genome-sequence information from aphid
endosymbionts, including our own genome-sequence data (Tamas et al., 2002).
Buchnera aphidicola
APHID ENDOSYMBIONTS:
BUCHNERA
Associations with intracellular prokaryotes are common among members of Arthropoda, which has
probably contributed to their evolutionary success (Goebel and Gross, 2001). This is the most
