Introduction
Comparative genomic studies of microorganisms have disrupted the paradigm of vertical
inheritance with modification. First in bacteria, and more recently in microscopic
and even multicellular eukaryotes, horizontal gene transfer (HGT) has been implicated
in genomic and ecological evolution. HGT is the exchange of genetic material between
organisms that occurs independently of meiotic and mitotic recombination between mating
or hybridizing individuals. HGT occurs as viral and plasmid-mediated transfer, and
transformation by environmental DNA via known or yet-unknown mechanisms [1]. The existence
of environmental gene pools and pan-genomes is supported by decades of functional
and phylogenetic studies in bacteria that have highlighted the exchange and proliferation
of virulence factors and antibiotic resistance mechanisms [2–4]. Presently, accumulating
reports of HGT in eukaryotes raise similar questions of how exposure to such gene
pools has impacted the evolution of eukaryotic microbes, and whether or not human
activities influence HGT dynamics. Here, we describe the evidence supporting HGT in
eukaryotic microbial pathogens from divergent lineages that impact human, animal,
and plant health (S1 Table). We consider three interacting dimensions affecting the
prevalence of HGT (genetic network structure, selectable functions, and opportunity
for contact) in order to better understand how HGT manifests in this important group
of organisms.
Does HGT Really Contribute to Eukaryotic Microbial Pathogen Genomes?
Reports of HGT among eukaryotic microbial pathogens have accumulated in recent years,
largely driven by comparative genomic analyses showing an unexpected distribution
and phylogenetic placement of gene sequences. However, these analyses require appropriate
sampling and methodology, and should be interpreted with caution. Objections to the
veracity and extent of HGT include the absence of a reproducible transfer mechanism
in some lineages, and the plausibility of alternative explanations for the distributions
and phylogenies of HGT candidate gene sequences [5]. The alternative interpretations
of unexpected gene distributions and phylogenies hinge on different assumptions about
the parsimony of a small number of gene transfer events versus a large number of the
more widely accepted processes of gene duplication and loss [5]. Proponents of HGT
hypotheses emphasize the importance of robust phylogenetic analyses coupled with multiple
additional lines of evidence to support claims [6,7]. The most convincing cases have
thus combined model-based phylogenetic approaches, which compare the topologies of
gene trees to species phylogenies, with additional support from genome structure,
sequence identity, codon usage, GC nucleotide content, and evidence of benefits to
the recipient (S1 Table). Analyses relying on but a few of these methods, especially
supporting methods in isolation, are rarely sufficient to strongly support HGT, and
can result in false positive identification of HGT [7]. Unsampled genetic diversity
at the population and species levels, which can impact reconstructions of gene distribution
and inheritance, may also lead to false positive identification of HGT, underscoring
the importance of robust taxon sampling [8]. For example, in a 2011 study, genes encoding
the greatly expanded Crinkler protein family in the amphibian pathogen Batrachochytrium
dendrobatidis had a distribution and phylogeny consistent with HGT from plant pathogenic
oomycetes; however, the recent detection of Crinkler homologs in two additional fungal
lineages opens the possibility that the current distribution is compatible with vertical
inheritance and widespread gene loss [9–11]. Similar sampling biases can also lead
to the underestimation of HGT in lineages of eukaryotic microbial pathogens, due to
recent HGTs that are not fixed in populations and transferred genes that are prone
to subsequent loss under changing selective pressures. This is illustrated by the
rapid, differential degeneration of a horizontally acquired gene cluster among members
of the necrotrophic fungal genus Botrytis [12]. More thorough sampling efforts currently
underway may reduce erroneous inferences about HGT, help resolve the timing and direction
of HGT events, and provide better estimates of their ecological contexts [13].
Does Genetic Network Complexity Influence HGT?
The complexity hypothesis posits that genes that are more modular in nature (i.e.,
residing at the periphery of gene connectivity networks) are more likely to be successfully
transferred, because they are less disruptive to host networks and require establishment
of fewer connections (Fig 1) [2,14]. This hypothesis is supported in bacteria, in
which low network connectivity is found to enhance a gene’s “transferability,” which
is largely independent of its specific biological function [15,16]. In eukaryotic
microbial pathogens, genes encoding virulence factors may provide a fitness advantage
to the recipient without extensive integration into genetic networks. Examples include
genes encoding secreted effector proteins and specialized metabolic genes, especially
those located in complete multigene clusters, which encode mechanisms for regulation,
compartmentalization, secretion of products, and stoichiometric control of toxic intermediates
[17–24]. Biased rates of gene transfer and loss of function found in systematic investigations
of pathogen genomes also support the complexity hypothesis. Notably, in the human
pathogen Trichomonas vaginalis, only 2% of 152 horizontally transferred genes were
“informational” (more often involved in highly conserved, connected cellular processes
like transcription), compared to 65% that encoded metabolic enzymes, which are “operational”
genes associated with less conserved processes [25,26]. However, a survey of metabolic
enzymes in select pathogens found that the degree of network connectivity of horizontally
transferred genes was not different from the connectivity of vertically inherited
genes [27]. The authors note that this could be because the horizontally transferred
genes integrated into existing networks during the extensive time period since they
were acquired [28].
10.1371/journal.ppat.1005156.g001
Fig 1
The interacting dimensions of horizontal gene transfer in eukaryotic microbial pathogens.
The probability of horizontal gene transfer (HGT) and retention of a gene in recipient
organisms is proposed to be under three main interacting influences or dimensions:
(1) the genetic network structure, defined as the sum of functional connections between
the gene and all other genes within a genome; (2) the selectability of the phenotype
conferred by its function in a host environment; and (3) opportunity for contact,
i.e., the rate and intimacy of meetings between donor DNA and recipient organisms
throughout their life cycles. In this depiction of a general model, the probability
of HGT increases with increasing dot intensity and size. Some pathogen-specific parameters
influencing each dimension are listed above each circle’s perimeter. While HGT and
the subsequent maintenance of genes in recipient genomes might be possible under the
influence of only one or two dimensions, it is predicted to have the highest probability
when all dimensions interact.
A notable consequence of limited gene connectivity associated with horizontally transferred
genes is the relaxation of network-imposed selection pressures, giving rise to sequence
divergence, thus increasing the capacity for adaptive evolution. This effect has been
reported in plant pathogenic Pyrenophora spp., in which ten horizontally transferred
genes present significantly more diversifying sequence change compared to corresponding
homologs in donor species [29].
What Biological Functions Favor Successful HGT in Eukaryotic Microbial Pathogens?
Environmental selection in pathogen niches may favor the acquisition of certain gene
functions. To date, there have been few formal investigations of general trends in
functions of genes transferred to eukaryotic microbial pathogens (but see [6,30,31]
for discussion of trends in fungi and oomycetes), and few functional confirmations
of the roles horizontally transferred genes play in virulence [27,28]. Individual
reports suggest three functional categories that are often horizontally transferred
to divergent pathogen lineages: secreted molecules, membrane modifications, and metabolism
specialized to host interactions and environments (Fig 1). Secreted molecule genes
that have been transferred include those for degradative enzymes, such as the 22 different
plant polysaccharide depolymerization enzymes that were transferred from phytopathogenic
fungi to oomycetes [18]. Genes for production of toxic metabolites that disrupt normal
cellular function are also reported transferred, including the fumonisin mycotoxin
gene cluster between phytopathogenic Fusarium and Aspergillus fungi [22]. Membrane
modifications identified in HGT reports may directly mediate cellular contact between
hosts and pathogens, or mask pathogen membranes from host defense responses [32].
For example, a septin trans-membrane protein acquired by the microsporidian Ordospora
colligata may facilitate the endocytosis-mediated infection of its Daphnia hosts [33].
Finally, the environment, i.e., the host, selects for the ability in the pathogen
to metabolize and/or utilize host defenses and resources, or other sources of fitness
in or on the host. For example, osmoregulatory genes acquired by phytopathogenic fungi
may facilitate cellular osmotic balance in vascular fluids, and an anaerobic sulfur
mobilization gene may increase survival of Blastocystis (a stramenopile suspected
to be a pathogen) in anaerobic gut environments [19,20]. Some genes or gene clusters
may be considered “repeat offenders,” having been transferred multiple times, possibly
due to advantages conferred in specific pathogenic ecologies. For example, the complex
distribution of epipolythiodioxopiperazine toxin gene clusters in ascomycete fungi
suggests at least three independent transfers between divergent lineages [23].
The extent to which de novo gene gain promotes rapid pathogen emergence is largely
unknown. One apparently contemporary transfer of a secreted toxin-encoding gene required
for complete virulence on wheat, from Stagonospora nodorum to Pyrenophora tritici-repentis,
has been reported [17]. The role of HGT in pathogen emergence is additionally supported
by functional studies of transferred genes. For example, deletions of two horizontally
acquired genes from the grass pathogen Fusarium pseudograminearum, and of a horizontally
acquired osmoregulatory gene from vascular wilt fungi, resulted in reduced virulence
[19,34]. Conversely, gain of virulence was documented in a fungal endophyte transformed
with a membrane modification gene that the related entomopathogenic fungus, Metarhizium
robertsii, may have ancestrally acquired from insects [32].
The fitness benefits conferred by horizontally acquired genes may range from none
to highly beneficial, and may not directly relate to pathogenesis. Some pathogens
exhibit complex life cycles that alternate between pathogenic and non-pathogenic states,
and alternatively the gain of adaptive genes with no pathogenic function could facilitate
attenuation of pathogenicity or transition to free-living status under selection from
host density. For example, it was speculated that the transfer of a nitrate assimilation
gene cluster to the mycoparasitic Trichoderma fungi may promote a transition to the
nitrogen-limited wood-decay niche, and the transfer of a sugar utilization gene cluster
to Schizosaccharomyces yeast could be part of an ecological transition from pathogen
to fermenter [35,36].
Do Eukaryotic Microbial Pathogens Become “Who They Meet?”
The frequency of physical contact between donors and recipients should be considered
a driving force behind the likelihood of HGT events, and is a function of an organism’s
ecology. Bacteria isolated from the same human body site, for example, exchange genes
more frequently, and the genes they exchange are more frequently associated with niche-specific
functions [3]. Three groups of ecologically adjacent organisms are often shown to
be involved in horizontal gene exchange with eukaryotic microbial pathogens: co-infecting
pathogens, non-pathogens symbiotically associated with the host, and the hosts themselves.
Examples of transfers between potentially co-infecting pathogens include a host-specific
toxin gene between two fungal wheat pathogens and a plant defense compound degradation
cluster between fungal grass pathogens [17,21]. Non-pathogenic gut commensal bacteria
are thought to have contributed diverse metabolic genes to Trichomonas vaginalis and
Blastocystis genomes, including those involved in carbohydrate and amino acid metabolism
[20,25,26]. Remarkably, there are cases of host-gene acquisition by insect- and plant-pathogenic
fungi. These include the acquisition of a sterol binding protein by the entomopathogenic
fungus Metarhizium robertsii that enables it to incorporate host-derived cholesterol
into its cell membrane during infections, and the acquisition of a purine salvage
pathway gene by obligate intracellular microsporidian pathogens, among others [32,33,37,38].
Considering the range of genetic exchange between ecological associates outside of
predator–prey relationships, we suggest that the “you are what you eat” hypothesis
proposed as a mechanism of HGT in phagotrophic eukaryotes may be rebranded “you are
who you meet” for eukaryotic microbial pathogens [39]. We propose that the frequency
of meetings between pathogens and specific classes of organisms may be influenced
by virulence, localization in host, and host range (Fig 1). Less virulent pathogens
may have sustained encounters with genomes of co-occurring pathogens, non-pathogenic
symbionts, and the host because of their low impact on host mortality. In contrast,
increasingly virulent pathogens may disproportionately encounter genomes from the
greater environment as increased virulence correlates with increased survival time
outside of hosts [40]. Obligate intracellular pathogens might be more frequently exposed
to host genomes, while extracellular pathogens may often encounter genomes of other
host-associated organisms. Similarly, generalists encounter a greater diversity of
host genomes compared to specialists, and facultative pathogens may encounter more
genes from the greater environment. Factors that contribute to a net increase in exposure
to foreign DNA may favor acquisition of novel adaptive functions or drive an HGT ratchet
by replacing pathogen genes with foreign genes (as proposed by Doolittle [39]). Furthermore,
the gradually converging ecologies resulting from successive “meetings” may promote
further transfers of ecology-specific genes such that decreasing ecological proximity
results in acceleration of gene acquisition and vice versa. This could explain in
part the repeated transfers of phytopathogenic genes from fungi to oomycetes, as well
as the repeated acquisition of genes by the plant pathogenic Fusarium lineage from
other plant pathogenic fungi, including the Verticillium, Aspergillus, and Collectotrichum
genera [18,19,22,31,34]. It remains to be investigated whether specific pathogen lineages,
virulence levels, host localizations, or specializations are more prone to horizontal
gene exchange, but we note that lineage-specific biases in the rates of HGT were recently
shown in a large comparative analysis of fungi [28].
Do Human Activities Impact HGT in Eukaryotic Microbial Pathogens?
Human activities that accelerate environmental changes may impose selection pressures
and precipitate dispersal events, which can influence the likelihood of HGT among
eukaryotic microbial pathogens. Strong selection pressures exerted by decreased host
diversity and intensive management practices in agro-ecosystems, including antimicrobials
and other chemical control agents, may be expected to increase the prevalence of horizontally
transferred genes, similar to the horizontal proliferation of antibiotic-resistant
genes in bacteria due to modern overuse [4]. Homogeneous host environments increase
the density of host-specific pathogens, non-pathogens, and the opportunities for them
to interact, while at the same time relaxing density-dependent selection against virulent
pathogens. Other human activities, such as global trade and travel, influence both
the frequency and diversity of the close physical encounters required for horizontal
gene flow, which can lead to emergence and evolution of pathogens in the near-term
[17]. Furthermore, the migration of host ranges associated with climate and land-use
changes provides new opportunities for encounters between pathogens established in
previously isolated environments [41]. The extent of human impact on HGT in eukaryotic
microbial pathogens is not known, but recent HGT discoveries in these organisms argue
for careful consideration of pathogen emergence by HGT as a consequence of ecosystem
management.
Supporting Information
S1 Table
Well-supported reports of HGT in eukaryotic microbial pathogens.
This table details 21 references with at least one report of HGT among eukaryotic
microbial pathogens. Recipient lineage, donor lineage, detection methods, putative
contact opportunity, and information on gene functions are listed.
(PDF)
Click here for additional data file.