Genome Evolution in an Insect Cell: Distinct Features of an Ant-Bacterial Partnership

Abstract

Bacteria that live exclusively within eukaryotic host cells include not only well-known pathogens, but also obligate mutualists, many of which occur in diverse insect groups such as aphids, psyllids, tsetse flies, and the ant genus Camponotus (Buchner, 1965; Douglas, 1998; Moran and Telang, 1998; Baumann et al., 2000; Moran and Baumann, 2000). In contrast to intracellular pathogens, these primary (P) endosymbionts of insects are required for the survival and reproduction of the host, exist within specialized host cells called bacteriocytes, and undergo stable maternal transmission through host lineages (Buchner, 1965; McLean and Houk, 1973). Due to their long-term host associations and close phylogenetic relationship with well-characterized enterobacteria (Fig. 1), P-endosymbionts of insects are ideal model systems to examine changes in genome content and architecture that occur in the context of beneficial, intracellular associations. Since these bacteria have not been cultured outside of the host cell, they are difficult to study with traditional genetic or physiological approaches. However, in recent years, molecular and computational approaches have provided important insights into their genetic diversity and ecological significance. This review describes some recent insights into the evolutionary genetics of obligate insect-bacteria symbioses, with a particular focus on an intriguing association between the bacterial endosymbiont Blochmannia and its ant hosts.

Variation in the strength of selected codon usage bias among bacteria

“…Using an approach to estimate the correlations after removing the effects of shared ancestry (39), the correlation coefficient for rRNA and tRNA gene copy numbers is 0.82, while the correlations between S and rRNA and tRNA gene copy numbers are 0.49 and 0.44, respectively (all values are highly statistically significant). While the phylogenetic relationships shown in Figure 1 are broadly consistent with those derived from analyses of other sequence data sets (45,46), there are some differences, such as Escherichia and Haemophilus being more closely related to each other than to Vibrio and Wigglesworthia lying within the radiation of Buchnera strains (47). However, we found that using alternative trees with such minor differences in topology had very little impact on the magnitude of the correlation coefficients.…”

An overview of chemosynthetic symbioses in bivalves from the North Atlantic and Mediterranean Sea

“…Genome sequencing indicates the loss of several genes and strong genome reduction compared to free-living relatives. This trait, shared with many symbionts in insect, is characteristic of maternally inherited bacterial symbionts lacking a free-living stage, and is thus congruent with previous findings (Gil et al, 2004;Wernegreen et al, 2003). In Cand.…”

Host–Symbiont Stability and Fast Evolutionary Rates in an Ant–Bacterium Association: Cospeciation of Camponotus Species and Their Endosymbionts, Candidatus Blochmannia

“…comm. ;Wernegreen et al, 2003), and that the bacterium might also be involved in the production of pheromones used for food recruitment (Sauer, 2000). If Blochmannia provides a more subtle or sporadic selective advantage than is typical of nutritional P endosymbionts, it may exhibit a more labile host association.…”