Symbiosis, an interdependent relationship between two species, is an important driver of
evolutionary novelty and ecological diversity. Microbial symbionts in particular have been
major evolutionary catalysts throughout the 4 billion years of life on earth and have
largely shaped the evolution of complex organisms. Endosymbiosis is a specific type of
symbiosis in which one—typically microbial—partner lives within its host and represents the
most intimate contact between interacting organisms. Mitochondria and chloroplasts, for
example, result from endosymbiotic events of lasting significance that extended the range
of acceptable habitats for life. The wide distribution of intracellular bacteria across
diverse hosts and marine and terrestrial habitats testifies to the continued importance of
endosymbiosis in evolution.
Among multicellular organisms, insects as a group form exceptionally diverse
associations with microbial associates, including bacteria that live exclusively within
host cells and undergo maternal transmission to offspring. These microbes have piqued the
interest of evolutionary biologists because they represent a wide spectrum of evolutionary
strategies, ranging from obligate mutualism to reproductive parasitism (Buchner 1965;
Ishikawa 2003) (Box 1; Table 1). In this issue of
PLoS Biology , the publication of the full genome sequence of the
reproductive parasite
Wolbachia allows the first genomic comparisons across this range
(Wu et al. 2004).
Lifestyle Extremes in Insect Endosymbionts
At one end of the spectrum, beneficial endosymbionts provide essential nutrients to
about 10%–15% of insects and provide models for highly specialized, long-term mutualistic
associations (Figure 1). These ‘primary’ endosymbionts are required for the survival and
reproduction of the host, most of which feed on unbalanced diets such as plant sap, blood,
or grain, and occur within specialized host cells called bacteriocytes (or mycetocytes)
(Baumann et al. 2000; Moran and Baumann 2000). Molecular phylogenetic analyses demonstrate
stability of these obligate mutualists over long evolutionary periods, ranging from tens to
hundreds of millions of years. By allowing their hosts to exploit otherwise inadequate food
sources and habitats, the acquisition of these mutualists can be viewed as a key innovation
in the evolution of the host (Moran and Telang 1998). Owing to their long-term, stable
transmission from generation to generation (vertical transmission), these cytoplasmic
genomes have been viewed as analogs to organelles.
By contrast, reproductive parasites of insects, including
Wolbachia (O'Neill et al. 1998) and the more recently discovered
endosymbiont in the Bacteroidetes group (also called CFB or CLO) (Hunter et al. 2003),
propagate in insect lineages by manipulating host reproduction. These maternally inherited
bacteria inflict an impressive arsenal of reproductive alterations to increase the
frequency of infected female offspring, often at the expense of their hosts. Such
mechanisms include cytoplasmic incompatibility, parthenogenesis, and male killing or
feminization. As parasites, these bacteria rely on occasional horizontal transmission to
infect new populations (Noda et al. 2001) and, by directly altering reproductive patterns,
may be a causative agent of host speciation (Bordenstein et al. 2001).
Comparative molecular analysis of insect endosymbionts over the past decade has provided
new insights into their distribution across hosts, their varying degrees of stability
within host lineages (ranging from cospeciation to frequent host-switching), and their
impressive genetic diversity that spans several major bacterial groups. More recently,
studies in genomics of obligate mutualists—and now
Wolbachia —illuminate mechanisms of host–symbiont integration
and the distinct consequences of this integration in various symbiotic systems. In
addition, since hosts and symbionts often have different evolutionary interests, the
diverse features of insect–bacterial associations can be understood as different outcomes
in the negotiation of genetic conflicts. Some recent highlights and tantalizing research
areas are described below.
Endosymbiont Genomes: Spanning the Gamut from Static to Plastic
The distinct lifestyle of endosymbionts has clear effects on rates and patterns of
molecular evolution. Compared to free-living relatives, endosymbionts are thought to have
reduced effective population sizes due to population bottlenecks upon transmission to host
offspring and, in the case of obligate mutualists that coevolve with their hosts, limited
opportunities for gene exchange. The nearly neutral theory of evolution (Ohta 1973)
predicts accelerated fixation of deleterious mutations through random genetic drift in
small populations, a phenomenon that has been observed in endosymbionts (Moran 1996;
Lambert and Moran 1998). Over time, this lifestyle-associated accumulation of deleterious
mutations may negatively affect the fitness of both the host and symbiont.
It is increasingly clear the distinct lifestyle of endosymbionts also shapes the
architecture and content of their genomes, which include the smallest, most AT-rich
bacterial genomes yet characterized (Andersson and Kurland 1998; Moran 2002). A common
theme is substantial gene loss, or genome streamlining, which is thought to reflect an
underlying deletion bias in bacterial genomes combined with reduced strength or efficacy of
selection to maintain genes in the host cellular environment. As a result of gene loss,
these bacteria completely rely on the host cell for survival. Because they cannot be easily
cultured apart outside of the host for traditional genetic or physiological techniques,
analysis of genome sequence offers a valuable tool to infer metabolic functions that
endosymbionts have retained and lost and to elucidate the steps in the evolutionary
processes of genome reduction.
Since 2000, full genome sequences have been published for
Buchnera of three aphid host species,
Wigglesworthia of tsetse flies, and
Blochmannia of ants (Shigenobu et al. 2000; Akman et al. 2002;
Tamas et al. 2002; Gil et al. 2003; van Ham et al. 2003). Parallels among these mutualist
genomes include their small size (each smaller than 810 kb), yet retention of specific
biosynthetic pathways for nutrients required by the host (for example, amino acids or
vitamins). However, genomes also show signs of deleterious deletions. Early gene loss in
Buchnera involved a few deletions of large contiguous regions of
the ancestral genome and often included genes of unrelated functions (Moran and Mira 2001).
These ‘large steps’ imply that genome reduction involved some random chance (due to the
location of genes in the ancestral chromosome) and selection acting on the combined fitness
of large sets of genes, rather than the fitness of individual loci. Such deletions are
apparently irreversible in obligate mutualists, which lack recombination functions and
genetic elements, such as prophages, transposons, and repetitive DNA that typically mediate
gene acquisition. The scarcity of these functions, combined with limited opportunities to
recombine with genetically distinct bacteria, may explain the unprecedented genome
stability found in
Buchnera compared to all other fully sequenced bacteria (Tamas
et al. 2002) and a lack of evidence for gene transfer in other mutualist genomes. Stability
also extends to the level of gene expression, as obligate mutualists have lost most
regulatory functions and have reduced abilities to respond to environmental stimuli (Wilcox
et al. 2003).
The
Wolbachia genome presented in this issue allows the first genome
comparisons among bacteria that have adopted divergent evolutionary strategies in their
associations with insects (Wu et al. 2004). Like other parasites, but unlike long-term
mutualists,
Wolbachia may experience strong selection for phenotypic
variation, for example, to counter improved host defenses, to compete with distinct
Wolbachia strains that coinfect the same host, or to increase
its transmission to new host backgrounds. High levels of recombination in
Wolbachia (for example, Jiggins et al. 2001) may allow rapid
genetic changes in this parasite and may be catalyzed by the exceptionally high levels of
repetitive DNA and mobile elements in its genome (Wu et al. 2004). Other bacteria that
colonize specialized niches for long periods and lack co-colonizing strains also possess
high levels of repetitive chromosomal sequences. For example, among ulcer-causing
Helicobacter pylori in primate guts, repetitive DNA mediates
intragenomic recombination and may provide an important source of genetic variation for
adaptation to dynamic environmental stresses (Aras et al. 2003). The potential
contributions of repetitive DNA and phage to intragenomic and intergenomic recombination in
Wolbachia are exciting areas of research (Masui et al. 2000).
The
Wolbachia genome also provides a valuable tool for future
research to test whether plasticity extends to gene content variation among
Wolbachia strains and labile gene expression patterns.
Between these two extremes of obligate mutualism and reproductive parasitism lies a
spectrum of secondary symbionts of insects, most of which have not yet been studied in
detail. Such ‘guest’ microbes transfer among diverse host species (Sandström et al. 2001),
may provide more subtle or occasional benefits (for example, relating to host defense
against parasitoids [Oliver et al. 2003]), and could represent an intermediate stage
between a free-living lifestyle and obligate endosymbiosis. Genome-level data from these
secondary symbionts promise to shed light on the range of lifestyles between obligate
mutualism and reproductive parasitism and on the early stages in the transition to each.
Microarray-based comparisons of gene content among
Escherichia coli , a facultative mutualist of tsetse flies (
Sodalis glossinidius ), and a relatively young mutualist of
weevils (
Sitophilus oryzae primary endosymbiont [SOPE]) show that genome
streamlining in the endosymbionts may preclude extracellular existence, and highlight
modifications in metabolic pathways to complement specific host physiology and ecology (Rio
et al. 2003). In addition, these endosymbionts may employ similar mechanisms as
intracellular parasites in overcoming the shared challenges of entering host cells,
avoiding or counteracting host defense mechanisms, and multiplying within a host cellular
environment (Hentschel et al. 2000). The rapidly growing molecular datasets for secondary
(or young primary) insect endosymbionts have identified pathways that are considered to be
required for pathogenicity, such as Type III secretion (Dale et al. 2001, 2002). Such
pathways may therefore have general utility for bacteria associated with host cells and may
have evolved in the context of beneficial interactions.
Genetic Conflicts and Host–Symbiont Dynamics
Given their diverse evolutionary strategies, insect endosymbionts also provide a rich
playing field to explore genetic conflicts (Frank 1996a, 1996b), which might involve the
mode of symbiont transmission, the number of symbionts transmitted, and the sex of host
offspring. Genetic conflicts described between organelle and nuclear genomes of the same
organism (Hurst 1995) can provide a context to understand the evolutionary dynamics of
insect–bacterial associations and the diverse outcomes of these relationships. For example,
the uniparental (maternal) mode of inheritance of both mitochondria and insect
endosymbionts may reflect host defense against invasion by foreign microbes with strong
deleterious effects, which spread more easily under biparental inheritance (Law and Hutson
1992).
Host–symbiont conflicts over offspring sex ratio are quite apparent in reproductive
parasites (Vala et al. 2003). While the bacteria favor more female offspring and employ a
variety of mechanisms to achieve this, the host typically favors a more balanced sex ratio.
This conflict may lead to changes in the host that counter the symbiont's effect on sex
ratio. For example, the spread of
Wolbachia in a spider mite population caused selection on host
nuclear genes that decrease the symbiont-induced sex ratio bias (Noda et al. 2001).
Obligate mutualists also experience genetic conflicts with the host regarding
transmission mode and number. In general, symbionts generally favor dispersal out of the
host to avoid competition with their close relatives, while hosts are expected to restrict
symbiont migration and thus reduce the virulent tendencies (Frank 1996b). In obligate
mutualisms, there may be little room for negotiation. For example, the highly conserved,
host-controlled determination of aphid bacteriocytes (Braendle et al. 2003) and the
phylogenetic congruence observed in numerous studies suggest that aphids have won this
conflict over symbiont transfer. However, the number of bacteria transmitted may be more
flexible and is known to vary among aphid taxa (Mira and Moran 2002). Models indicate that
the fixation rate for symbiont-beneficial (selfish) mutations increase with the number of
symbionts transmitted, reflecting greater efficacy of selection among bacteria within a
given host (Rispe and Moran 2000).
Prospects
In sum, the past few years have witnessed a surge of new empirical and theoretical
approaches to understand the dynamics of bacterial–insect relationships. These tools have
shed light on the roles of recombination, selection, and mutation on endosymbiont genome
evolution and have highlighted parameters that shape the outcome of genetic conflicts
between hosts and symbionts. These data provide a foundation for studying the evolution of
mutualism and parasitism and modes of transitions between them. In the near future, we can
look forward to full genome sequences that span a broader ecological and phylogenetic
diversity of endosymbionts and provide a richer comparative framework to test existing
models and develop new ones.
Developments in endosymbiosis are important not only to questions in basic research, but
may have important practical applications. Blood-feeding insects such as mosquitoes and
tsetse flies are vectors for parasites that cause significant global infectious diseases
such as malaria, dengue virus, and trypanosomiasis, many of which have frustrated attempts
at vaccine development. The same insects that transmit these devastating human parasites
often possess a diversity of mutualistic and parasitic bacterial endosymbionts. A very
promising and urgent area of endosymbiont research is the manipulation of these bacteria to
control vector populations in the field. Current studies already provide evidence that
endosymbiont manipulation is a promising strategy to reduce the lifespan of the insect
vector or limit its transmission of disease-causing parasites (Aksoy et al. 2001;
Brownstein et al. 2003). Each advance in our understanding of endosymbiont genomics and
evolutionary dynamics represents one step closer to that goal.
