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Two host clades, two bacterial arsenals: evolution through gene losses in
facultative endosymbionts
#MMPMID25714744
Rollat-Farnier PA
; Santos-Garcia D
; Rao Q
; Sagot MF
; Silva FJ
; Henri H
; Zchori-Fein E
; Latorre A
; Moya A
; Barbe V
; Liu SS
; Wang XW
; Vavre F
; Mouton L
Genome Biol Evol
2015[Feb]; 7
(3
): 839-55
PMID25714744
show ga
Bacterial endosymbiosis is an important evolutionary process in insects, which
can harbor both obligate and facultative symbionts. The evolution of these
symbionts is driven by evolutionary convergence, and they exhibit among the
tiniest genomes in prokaryotes. The large host spectrum of facultative symbionts
and the high diversity of strategies they use to infect new hosts probably impact
the evolution of their genome and explain why they undergo less severe genomic
erosion than obligate symbionts. Candidatus Hamiltonella defensa is suitable for
the investigation of the genomic evolution of facultative symbionts because the
bacteria are engaged in specific relationships in two clades of insects. In
aphids, H. defensa is found in several species with an intermediate prevalence
and confers protection against parasitoids. In whiteflies, H. defensa is almost
fixed in some species of Bemisia tabaci, which suggests an important role of and
a transition toward obligate symbiosis. In this study, comparisons of the genome
of H. defensa present in two B. tabaci species (Middle East Asia Minor 1 and
Mediterranean) and in the aphid Acyrthosiphon pisum revealed that they belong to
two distinct clades and underwent specific gene losses. In aphids, it contains
highly virulent factors that could allow protection and horizontal transfers. In
whiteflies, the genome lost these factors and seems to have a limited ability to
acquire genes. However it contains genes that could be involved in the production
of essential nutrients, which is consistent with a primordial role for this
symbiont. In conclusion, although both lineages of H. defensa have mutualistic
interactions with their hosts, their genomes follow distinct evolutionary
trajectories that reflect their phenotype and could have important consequences
on their evolvability.
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