Limits to local adaptation in six populations of the annual plant Diodia teres

* Local adaptation is common, but tests for adaptive differentiation frequently compare populations from strongly divergent environments, making it unlikely that any influence of stochastic processes such as drift or mutation on local adaptation will be detected. Here, the hypothesis that local adaptation is more likely to develop when the native environments of populations are more distinct than when they are similar was tested. * A reciprocal transplant experiment including two populations from each of three habitats was conducted to determine the pattern of local adaptation. In addition to testing for local adaptation at the population level, the hypothesis was tested that local adaptation is more common between populations from different habitats than between populations from the same habitat. * Local adaptation was not common, but more evidence was found of local adaptation between populations from different habitats than between populations from the same habitat. Two instances of foreign genotype fitness advantage confirm that stochastic processes such as drift can limit local adaptation. * These results are consistent with the hypothesis that stochastic processes can inhibit local adaptation but are more likely to be overwhelmed by natural selection when populations occur in divergent environments.     

THE GENETIC STRUCTURE OF HOST PLANT ADAPTATION IN A SPATIAL PATCHWORK: DEMOGRAPHIC VARIABILITY AMONG RECIPROCALLY TRANSPLANTED PEA APHID CLONES

Populations of insect herbivores that feed on several host plant species may experience different selective forces on each host. When the hosts cooccur in a local area, herbivore populations can provide useful models for the study of evolutionary mechanisms in patchy environments. A first step in such a study involves determination of the genetic structure of host adaptation in the region: how is genetic variation for host use structured within and between subpopulations of herbivores on each host? The structure of genetic variation for host use reveals patterns of local adaptation, probable selective consequences of migration between hosts, and the potential for further evolution. To estimate the population structure of host adaptation in a patchwork, 7–11 pea aphid clones were collected at the beginning of the summer from each of two alfalfa and two red clover fields within a very localized area (about 15–20 km²). Using a reciprocal transplant in the field, replicates of these 35 clones were allowed to develop individually on each of the two crops. A complete life table was made for each replicate. Individual fitness was calculated from the life tables as the expected rate of population increase; longevity, age at first reproduction, and total fecundity were also measured for each clonal replicate. Currently, experimental estimates of genetic variation in complete life tables are virtually nonexistent for natural populations, even for single environments (Charlesworth, 1987); field studies are even less common. Because clones from each of two source crops were tested reciprocally on both hosts, variation in relative genotypic fitness on alfalfa and clover could be partitioned among clones within source crops, between fields of the same crop, and between source crops (alfalfa or red clover), providing a view of population structure. Significant clonal variation in relative performance on alfalfa and red clover was found: clones tended to have higher fitness on the crop from which they had been collected (the “home” crop) than they did on the “away” crop, suggesting local adaptation in response to patchy patterns of selection. Clonal variability within collections from the two crops suggests the potential for changes in the genetic constitution of these aphid populations within established fields as a result of clonal selection during the summer season. Significantly negative genetic correlations across crops were found for fitness and its major components. The possibility that these negative cross-environment correlations could act as evolutionary constraints on adaptation to the patchwork is considered.     

Performance of native grape phylloxera on host plants within and among terrestrial islands in Arizona, USA

In Arizona, USA, the canyon grape, Vitis arizonica Englemann, and grape phylloxera (Daktulosphaira vitifoliae Fitch, Homoptera, Phylloxeridae) are distributed among mountain ranges that are surrounded by expanses of desert lacking Vitis habitat, thus forming a system of terrestrial islands. Both herbivore and host populations may have diverged genetically among mountain ranges under the influence of restricted gene flow and variable selection among sites. Herbivore adaptation to local hosts would be expected to ensue, with the potential to promote divergence, both in traits under selection and by further reducing the probability of interisland colonization. To test the hypothesis that phylloxera are adapted to local hosts, demographic components of fitness of field-collected native grape phylloxera were measured in the greenhouse on vines of V. arizonica that were categorized as either natal, neighboring, and or isolated hosts. There was no evidence for greater adaptation to natal or neighboring hosts but there were significant interactions between herbivore and host treatments in one experiment. There was genetic variation for gall formation among six clones tested. Though a failure to detect local adaptation could have resulted from low statistical power, benign experimental conditions, or inadequate genetic variation, the divergence of isolated grape populations is suggested to have been insufficient to promote local adaptation in grape phylloxera at the level of isolated mountain ranges. It is further suggested that, within populations, adaptation to individual host plants could be forestalled by selection for ’general purpose genotypes’ through wind-borne displacement of colonizers into the unpredictable environment of a heterogeneous array of hosts. In addition, short-term extinction/colonization dynamics could promote gene flow such that time is insufficient for adaptive mutations or gene combination to arise. Received: 26 December 1998 / Accepted: 24 May 1999     

Geographical divergence in host use ability of a marine herbivore in alga–grazer interaction

When the selective environment differs geographically, local herbivore populations may diverge in their host use ability and adapt locally to exploit the sympatric host population. We tested whether populations of the marine generalist herbivore Idotea baltica have diverged in host us ability and whether they locally adapted to exploit the sympatric population of their main host species, the bladderwrack Fucus vesiculosus. We fed isopods from three local populations reciprocally with the sympatric and two allopatric populations of the host. The bladderwrack populations varied in their quality as food for isopods suggesting variation in the selective environment. The ability to exploit the main host showed considerable divergence among the isopod populations. There was no significant interaction between host and isopod origin, indicating that the patterns observed in the reciprocal feeding experiment could be explained by differences in overall suitability of the hosts and differences in overall performance of the isopod populations. Isopod population that was sympatric to a bladderwrack population with low phlorotannin content showed high performance on the algae from the sympatric but low performance on the algae from the two allopatric populations. Performance of isopods, especially in this population, decreased quickly with the increasing phlorotannin content of food algae. We therefore hypothesize that the isopods adapted to a low phlorotannin content were unable to utilize high-phlorotannin algae efficiently. Isopod populations sympatric to the high-phlorotannin bladderwrack populations may be generally better adapted to deal with phlorotannins, being thereby able to utilize a range of bladderwrack populations.     

Host-plant adaptation in an herbivorous marine amphipod: genetic potential not realized in field populations

Evolutionary responses of herbivores to their host plants depend not only on selection from plants, but also on the genetic basis of traits relating to host use. The genetic basis of such traits has been investigated extensively among terrestrial insect herbivores, but has received almost no attention among marine herbivores. We tested whether performance traits in the herbivorous marine amphipod Peramphithoe parmerong display heritable variation and, for the first time for a marine herbivore, whether selection has resulted in local adaptation to host plants on two spatial scales. Peramphithoe parmerong displayed heritable genetic variation for survival on two host macroalgae, the high-quality Sargassum linearifolium and the poor-quality Padina crassa, and for growth on S. linearifolium. Differences in performance on different hosts thus have the potential to select for differential use of hosts by this amphipod. Despite this potential, there was no evidence among field populations of local adaptation to host algae on either scale tested: between hosts within a site or among sites differing in algal species composition. Within a site, amphipods were not more likely to prefer or perform better on the host on which they were collected. Similarly, amphipods collected from sites in which P. crassa was present were not more likely to perform well on this host than amphipods collected from sites where this alga was not found. Ecological factors that may explain the persistence of P. parmerong on P. crassa and the possibility of phylogenetic constraints on host use by P. parmerong are discussed.     

Sex-specific effects of a parasite evolving in a female-biased host population

Background

Males and females differ in many ways and might present different opportunities and challenges to their parasites. In the same way that parasites adapt to the most common host type, they may adapt to the characteristics of the host sex they encounter most often. To explore this hypothesis, we characterized host sex-specific effects of the parasite Pasteuria ramosa, a bacterium evolving in naturally, strongly, female-biased populations of its host Daphnia magna.

Results

We show that the parasite proliferates more successfully in female hosts than in male hosts, even though males and females are genetically identical. In addition, when exposure occurred when hosts expressed a sexual dimorphism, females were more infected. In both host sexes, the parasite causes a similar reduction in longevity and leads to some level of castration. However, only in females does parasite-induced castration result in the gigantism that increases the carrying capacity for the proliferating parasite.

Conclusions

We show that mature male and female Daphnia represent different environments and reveal one parasite-induced symptom (host castration), which leads to increased carrying capacity for parasite proliferation in female but not male hosts. We propose that parasite induced host castration is a property of parasites that evolved as an adaptation to specifically exploit female hosts.

     

Local adaptation in host use among marine invertebrates

The study of interactions between small invertebrates and their larger plant and animal hosts has a long tradition. One persistent theme within this literature is that spatially‐segregated populations of terrestrial and freshwater invertebrates commonly adapt to local hosts across their geographic ranges. Marine examples are rare, which leaves the impression that marine populations are less likely to adapt to locally abundant hosts and more likely to evolve generalized or phenotypically‐plastic strategies. Here, I review a short but growing list of marine invertebrates that appear to display local adaptation in host use. As expected, most of the marine examples are brooded animals with weak dispersal potential. However, some species with pelagically dispersed larvae have apparently adapted to local hosts. This surprising result is consistent with recent evidence that pelagically‐dispersed larvae are not always broadly dispersed, that strong selective pressures maintain local differences in host use, or both. The presence of host‐mediated adaptation in the sea alters predictions on how marine communities respond to disturbance, supports the notion that marine consumer‐prey interactions can coevolve, and indicates that hosts play fundamental roles in the differentiation and perhaps speciation of small marine invertebrates.     

Counteracting selective regimes and host preference evolution in ecotypes of two species of walking-sticks

The evolution of ecological specialization has been a central topic in ecology because specialized adaptations to divergent environments can result in reproductive isolation and facilitate speciation. However, the order in which different aspects of habitat adaptation and habitat preference evolve is unclear. Timema walking-stick insects feed and mate on the host plants on which they rest. Previous studies of T. cristinae ecotypes have documented divergent, host-specific selection from visual predators and the evolution of divergent host and mate preferences between populations using different host-plant species (Ceanothus or Adenostoma). Here we present new data that show that T. podura, a nonsister species of T. cristinae, has also formed ecotypes on these host genera and that in both species these ecotypes exhibit adaptive divergence in color-pattern and host preference. Color-pattern morphs exhibit survival trade-offs on different hosts due to differential predation. In contrast, fecundity trade-offs on different hosts do not occur in either species. Thus, host preference in both species has evolved before divergent physiological adaptation but in concert with morphological adaptations. Our results shed light onto which traits are involved in the initial stages of ecological specialization and ecologically based reproductive isolation.     

The evolution of specificity in evolving and coevolving antagonistic interactions between a bacteria and its phage

The evolution of exploitative specificity can be influenced by environmental variability in space and time and the intensity of trade-offs. Coevolution, the process of reciprocal adaptation in two or more species, can produce variability in host exploitation and as such potentially drive patterns in host and parasite specificity. We employed the bacterium Pseudomonas fluorescens SBW25 and its DNA phage Phi2 to investigate the role of coevolution in the evolution of phage infectivity range and its relation with phage growth rate. At the phage population level, coevolution led to the evolution of broader infectivity range, but without an associated decrease in phage growth rate relative to the ancestor, whereas phage evolution in the absence of bacterial evolution led to an increased growth rate but no increase in infectivity range. In contrast, both selection regimes led to phage adaptation (in terms of growth rates) to their respective bacterial hosts. At the level of individual phage genotypes, coevolution resulted in within-population diversification in generalist and specialist infectivity range types. This pattern was consistent with a multilocus gene-for-gene interaction, further confirmed by an observed cost of broad infectivity range for individual phage. Moreover, coevolution led to the emergence of bacterial genotype by phage genotype interactions in the reduction of bacterial growth rate by phage. Our study demonstrates that the strong reciprocal selective pressures underlying the process of coevolution lead to the emergence and coexistence of different strategies within populations and to specialization between selective environments.     

Geographic variation in performance of a widespread generalist insect herbivore

1. Generalist herbivores are often widespread and occur in a variety of environments. Due to their broad distribution, it is likely that some populations of generalists will encounter host plants with geographic variation in traits that could affect the herbivore's growth and survival (i.e. performance). However, the geographic pattern of performance has rarely been studied for generalists, especially across large geographic ranges. 2. This study used one of the most generalist herbivore species known, the fall webworm (Hyphantria cunea Drury 1773, Erebidae, Lepidoptera), to experimentally test how the performance of a local population of fall webworms varies with increasing geographic distance of the host plant population from the local herbivore population. Specifically, a transplant experiment was used to compare the performance of one fall webworm population feeding on its local host plants with its performance on host populations from two other locations, 1300 and 2600 km away. 3. It was found that fall webworms performed better on their local host plant populations than on populations from other regions, with performance at its lowest when reared on hosts of the same species from the farthest region. It was also found that local fall webworms do not perform well on hosts commonly used by fall webworms at the other two, more distant sites. 4. This study helps to elucidate how the performance of generalist herbivores varies along their geographic range and suggests possible local adaptation to different sets of hosts across sites.     

Local adaptation and ecological genetics of host-plant specialization in a leaf beetle

The tendency of insect species to evolve specialization to one or a few plant species is probably a major reason for the remarkable diversity of herbivorous insects. The suggested explanations for this general trend toward specialization include a range of evolutionary mechanisms, whose relative importance is debated. Here we address two potentially important mechanisms: (i) how variation in the geographic distribution of host use may lead to the evolution of local adaptation and specialization; (ii) how selection for specialization may lead to the evolution of trade‐offs in performance between different hosts. We performed a quantitative genetic experiment of larval performance in three different populations of the alpine leaf beetle Oreina elongata reared on two of its main host plants. Due to differences in host availability, each population represents a distinctly different selective regime in terms of host use including selection for specialization on one or the other host as well as selection for utilizing both hosts during the larval stage. The results suggest that selection for specialization has lead to some degree of local adaptations in host use: both single‐host population had higher larval growth rate on their respective native host plant genus, while there was no difference between plant treatments in the two‐host population. However, differences between host plant treatments within populations were generally small and the degree of local adaptation in performance traits seems to be relatively limited. Genetic correlations in performance traits between the hosts ranged from zero in the two‐host population to significantly positive in the single‐host populations. This suggests that selection for specialization in single host populations typically also increased performance on the alternative host that is not naturally encountered. Moreover, the lack of a positive genetic correlation in the two host‐population give support for the hypothesis that performance trade‐offs between two host plants may typically evolve when a population have adapted to both these plants. We conclude that although there is selection for specialization in larval performance traits it seems as if the genetic architecture of these traits have limited the divergence between populations in relative performance on the two hosts.     

Host-parasite local adaptation after experimental coevolution of Caenorhabditis elegans and its microparasite Bacillus thuringiensis

Coevolving hosts and parasites can adapt to their local antagonist. In studies on natural populations, the observation of local adaptation patterns is thus often taken as indirect evidence for coevolution. Based on this approach, coevolution was previously inferred from an overall pattern of either parasite or host local adaptation. Many studies, however, failed to detect such a pattern. One explanation is that the studied system was not subject to coevolution. Alternatively, coevolution occurred, but remained undetected because it took different routes in different populations. In some populations, it is the host that is locally adapted, whereas in others it is the parasite, leading to the absence of an overall local adaptation pattern. Here, we test for overall as well as population-specific patterns of local adaptation using experimentally coevolved populations of the nematode Caenorhabditis elegans and its bacterial microparasite Bacillus thuringiensis. Furthermore, we assessed the importance of random interaction effects using control populations that evolved in the absence of the respective antagonist. Our results demonstrate that experimental coevolution produces distinct local adaptation patterns in different replicate populations, including host, parasite or absence of local adaptation. Our study thus provides experimental evidence of the predictions of the geographical mosaic theory of coevolution, i.e. that the interaction between parasite and host varies across populations.     

Local adaptation and enhanced virulence of Nosema granulosis artificially introduced into novel populations of its crustacean host, Gammarus duebeni

Local adaptation theory predicts that, on average, most parasite species should be locally adapted to their hosts (more suited to hosts from local than distant populations). Local adaptation has been studied for many horizontally transmitted parasites, however, vertically transmitted parasites have received little attention. Here we present the first study of local adaptation in an animal/parasite system where the parasite is vertically transmitted. We investigate local adaptation and patterns of virulence in a crustacean host infected with the vertically transmitted microsporidian Nosema granulosis. Nosema granulosis is vertically transmitted to successive generations of its crustacean host, Gammarus duebeni and infects up to 46% of adult females in natural populations. We investigate local adaptation using artificial horizontal infection of different host populations in the UK. Parasites were artificially inoculated from a donor population into recipient hosts from the sympatric population and into hosts from three allopatric populations in the UK. The parasite was successfully established in hosts from all populations regardless of location, infecting 45% of the recipients. Nosema granulosis was vertically (transovarially) transmitted to 39% of the offspring of artificially infected females. Parasite burden (intensity of infection) in developing embryos differed significantly between host populations and was an order of magnitude higher in the sympatric population, suggesting some degree of host population specificity with the parasite adapted to its local host population. In contrast with natural infections, artificial infection with the parasite resulted in substantial virulence, with reduced host fecundity (24%) and survival (44%) of infected hosts from all the populations regardless of location. We discuss our findings in relation to theories of local adaptation and parasite-host coevolution.     

Local adaptation in adult feeding preference and juvenile performance in the generalist herbivore Idotea balthica

Populations can respond to environmental heterogeneity by genetic adaptation to local conditions. Evidence for local adaptation in herbivores with relatively broad host breadth is scarce, either because generalists rarely locally adapt or because fewer studies have tested for local adaptation. The marine isopod Idotea balthica, a small (<3?cm) generalist herbivore common to estuaries of the northwestern Atlantic, is found on multiple macroalgae and sea grasses north of 42°N, while more southerly populations utilize sea grass-dominated and macroalgal-poor habitats. Feeding preference assays revealed a latitudinal shift in preference hierarchy that mirrors this geographic variation in host availability. Northern populations have higher feeding preference for fresh and freeze-dried tissue of the brown macroalga Fucus vesiculosus and consumed more of its water-soluble and lipophilic extracts relative to southern populations. In contrast, southern populations have a relatively higher preference for the green macroalga Ulva linza and sea grass Zostera marina. The rank of hosts in feeding assays exhibited by northern adults (Fucus?=?Ulva?>?Zostera) and southern adults (Ulva?>?Fucus?>?Zostera) closely mirrored ranking of juvenile growth rates, suggesting that preference and performance are strongly correlated across these macrophytes. Several of our assays included isopods that had parents reared under uniform laboratory conditions, indicating that geographic differences are genetically mediated and unlikely to reflect phenotypic plasticity or maternal effects. Local adaptation in host use traits may be common in broadly distributed, generalist herbivores in marine and terrestrial systems, and will manifest itself as local shifts in the preference ranking of hosts.     

Cuckoo females preferentially use specific habitats when searching for host nests

Egg mimicry is an important adaptation of common cuckoos, Cuculus canorus, against rejection of eggs by their respective hosts. A precondition for the maintenance of egg mimicry is that female cuckoos find hosts with a matching egg type. Experimental evidence indicated that habitat imprinting may be important for host selection. We tested whether the spacing and laying patterns of female cuckoos in the field are compatible with the supposed habitat-imprinting mechanism. We observed 16 females, with the help of radiotelemetry; of seven females, we observed directly 26 egg layings and 27 nest visits without laying. As expected if females were imprinted on different vegetation types, (1) the distribution of vegetation types differed between female home ranges, (2) female habitat use differed from average habitat availability within the egg-laying area (habitat preference), (3) females visited nests and deposited their eggs in the habitat they preferred, and (4) females laid their eggs consistently in a particular habitat type, irrespective of the host species. These results indicate that cuckoo females show habitat preference when searching for suitable host nests. Hence our data are compatible with the habitat-imprinting hypothesis, but owing to the habitat specificity of hosts the data cannot disprove a potential role of host specificity in cuckoo females.Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Host-range evolution in Aphidius parasitoids: fidelity, virulence and fitness trade-offs on an ancestral host

The diversity of parasitic insects remains one of the most conspicuous patterns on the planet. The principal factor thought to contribute to differentiation of populations and ultimately speciation is the intimate relationship parasites share with hosts and the potential for disruptive selection associated with using different host species. Traits that generate this diversity have been an intensely debated topic of central importance to the evolution of specialization and maintenance of ecological diversity. A fundamental hypothesis surrounding the evolution of specialization is that no single genotype is uniformly superior in all environments. This "trade-off" hypothesis suggests that negative fitness correlations can lead to specialization on different hosts as alternative stable strategies. In this study we demonstrate a trade-off in the ability of the parasitoid, Aphidius ervi, to maintain a high level of fitness on an ancestral and novel host, which suggests a genetic basis for host utilization that may limit host-range expansion in parasitoids. Furthermore, behavioral evidence suggests mechanisms that could promote specialization through induced host fidelity. Results are discussed in the context of host-affiliated ecological selection as a potential source driving diversification in parasitoid communities and the influence of host species heterogeneity on population differentiation and local adaptation.     

Herbivore host-associated genetic differentiation depends on the scale of plant genetic variation examined

Herbivore adaptation to plant genetic variation can lead to reproductive isolation and the formation of host-associated lineages (host-associated differentiation, or HAD). Plant genetic variation exists along a scale, ranging from variation among individual plant genotypes to variation among plant species. Along this scale, herbivores may adapt and diverge at any level, yet few studies have examined whether herbivore differentiation exhibits scaling with respect to host variation (e.g., from genotypes to species). Determining at which level(s) herbivore differentiation occurs can provide insight into the importance of plant genetic variation on herbivore evolution. Previous studies have found strong genetic differentiation in the eriophyid mite, Aceria parapopuli, between hybrid Populus hosts and parental Populus species, but minimal neutral-locus differentiation among individual trees of the same species. We tested whether genetic differentiation in A. parapopuli scales with genetic variation in its Populus hosts. Using mite ITS1 sequence data collected among host species and among host populations, two key patterns emerged. (1) We found strong differentiation of A. parapopuli among Populus species, supporting the hypothesis that plant species differences drive reproductive isolation and HAD. (2) We did not find evidence of host-driven genetic differentiation in mites at the level of plant populations, suggesting that this level of plant variation is insufficiently strong to drive differentiation at a neutral locus. In combination with previous studies, we found that HAD occurs at the higher levels of plant genetic variation, but not at lower levels, and conclude that HAD depends on the scale of plant genetic variation examined.     

Adaptation to enemy shifts: rapid resistance evolution to local Vibrio spp. in invasive Pacific oysters

One hypothesis for the success of invasive species is reduced pathogen burden, resulting from a release from infections or high immunological fitness of invaders. Despite strong selection exerted on the host, the evolutionary response of invaders to newly acquired pathogens has rarely been considered. The two independent and genetically distinct invasions of the Pacific oyster Crassostrea gigas into the North Sea represent an ideal model system to study fast evolutionary responses of invasive populations. By exposing both invasion sources to ubiquitous and phylogenetically diverse pathogens (Vibrio spp.), we demonstrate that within a few generations hosts adapted to newly encountered pathogen communities. However, local adaptation only became apparent in selective environments, i.e. at elevated temperatures reflecting patterns of disease outbreaks in natural populations. Resistance against sympatric and allopatric Vibrio spp. strains was dominantly inherited in crosses between both invasion sources, resulting in an overall higher resistance of admixed individuals than pure lines. Therefore, we suggest that a simple genetic resistance mechanism of the host is matched to a common virulence mechanism shared by local Vibrio strains. This combination might have facilitated a fast evolutionary response that can explain another dimension of why invasive species can be so successful in newly invaded ranges.     

Local adaptation in a plant herbivore interaction depends on the spatial scale

Local adaptation has central importance in the understanding of co-evolution, maintenance of sexual reproduction, and speciation. We investigated local adaptation in the alkaloid-bearing legume Crotalaria pallida and its seed predator, the arctiid moth Utetheisa ornatrix , at different spatial scales. When we studied three populations from south-east Brazil (150 km apart), we did not find evidence of local adaptation, although we did find interpopulational differences in herbivore performance, and a significant interaction between herbivore sex and plant population. These results indicate that both moth and plant populations are differentiated at the regional scale. In a comparison of populations from Brazil and Florida, the herbivore showed local adaptation to its host plant; for both moth populations, the pupae were heavier when the larvae ate the sympatric than the allopatric host population. We discuss the scale dependence of our results and the possible causes for the lack of local adaptation at the regional scale, even in the presence of plant and moth differentiation. The results obtained demonstrate the importance of studying co-evolution and local adaptation at different geographical scales.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 494–502.     

The Gut Bacterial Community of Mammals from Marine and Terrestrial Habitats

After birth, mammals acquire a community of bacteria in their gastro-intestinal tract, which harvests energy and provides nutrients for the host. Comparative studies of numerous terrestrial mammal hosts have identified host phylogeny, diet and gut morphology as primary drivers of the gut bacterial community composition. To date, marine mammals have been excluded from these comparative studies, yet they represent distinct examples of evolutionary history, diet and lifestyle traits. To provide an updated understanding of the gut bacterial community of mammals, we compared bacterial 16S rRNA gene sequence data generated from faecal material of 151 marine and terrestrial mammal hosts. This included 42 hosts from a marine habitat. When compared to terrestrial mammals, marine mammals clustered separately and displayed a significantly greater average relative abundance of the phylum Fusobacteria. The marine carnivores (Antarctic and Arctic seals) and the marine herbivore (dugong) possessed significantly richer gut bacterial community than terrestrial carnivores and terrestrial herbivores, respectively. This suggests that evolutionary history and dietary items specific to the marine environment may have resulted in a gut bacterial community distinct to that identified in terrestrial mammals. Finally we hypothesize that reduced marine trophic webs, whereby marine carnivores (and herbivores) feed directly on lower trophic levels, may expose this group to high levels of secondary metabolites and influence gut microbial community richness.