Overexpression of GmPAP4 Enhances Symbiotic Nitrogen Fixation and Seed Yield in Soybean under Phosphorus-Deficient Condition.

Legume crops establish symbiosis with nitrogen-fixing rhizobia for biological nitrogen fixation (BNF), a process that provides a prominent natural nitrogen source in agroecosystems; and efficient nodulation and nitrogen fixation processes require a large amount of phosphorus (P). Here, a role of GmPAP4, a nodule-localized purple acid phosphatase, in BNF and seed yield was functionally characterized in whole transgenic soybean (Glycine max) plants under a P-limited condition. GmPAP4 was specifically expressed in the infection zones of soybean nodules and its expression was greatly induced in low P stress. Altered expression of GmPAP4 significantly affected soybean nodulation, BNF, and yield under the P-deficient condition. Nodule number, nodule fresh weight, nodule nitrogenase, APase activities, and nodule total P content were significantly increased in GmPAP4 overexpression (OE) lines. Structural characteristics revealed by toluidine blue staining showed that overexpression of GmPAP4 resulted in a larger infection area than wild-type (WT) control. Moreover, the plant biomass and N and P content of shoot and root in GmPAP4 OE lines were also greatly improved, resulting in increased soybean yield in the P-deficient condition. Taken together, our results demonstrated that GmPAP4, a purple acid phosphatase, increased P utilization efficiency in nodules under a P-deficient condition and, subsequently, enhanced symbiotic BNF and seed yield of soybean.

Retention of an Endosymbiont for the Production of a Single Molecule.

Sap-feeding insects often maintain two or more nutritional endosymbionts that act in concert to produce compounds essential for insect survival. Many mealybugs have endosymbionts in a nested configuration: one or two bacterial species reside within the cytoplasm of another bacterium, and together, these bacteria have genomes that encode interdependent sets of genes needed to produce key nutritional molecules. Here, we show that the mealybug Pseudococcus viburni has three endosymbionts, one of which contributes only two unique genes that produce the host nutrition-related molecule chorismate. All three bacterial endosymbionts have tiny genomes, suggesting that they have been coevolving inside their insect host for millions of years.

Symbiotic revolutions at the interface of genomics and microbiology.

Symbiosis is an old idea with a contentious history. New genomic technologies and research paradigms are fueling a shift in some of its central tenets; we need to be humble and open-minded about what the data are telling us.

Fitness trade-offs and the origins of endosymbiosis.

Endosymbiosis drives evolutionary innovation and underpins the function of diverse ecosystems. The mechanistic origins of symbioses, however, remain unclear, in part because early evolutionary events are obscured by subsequent evolution and genetic drift. This Essay highlights how experimental studies of facultative, host-switched, and synthetic symbioses are revealing the important role of fitness trade-offs between within-host and free-living niches during the early-stage evolution of new symbiotic associations. The mutational targets underpinning such trade-offs are commonly regulatory genes, such that single mutations have major phenotypic effects on multiple traits, thus enabling and reinforcing the transition to a symbiotic lifestyle.

How do bacterial endosymbionts work with so few genes?

The move from a free-living environment to a long-term residence inside a host eukaryotic cell has profound effects on bacterial function. While endosymbioses are found in many eukaryotes, from protists to plants to animals, the bacteria that form these host-beneficial relationships are even more diverse. Endosymbiont genomes can become radically smaller than their free-living relatives, and their few remaining genes show extreme compositional biases. The details of how these reduced and divergent gene sets work, and how they interact with their host cell, remain mysterious. This Unsolved Mystery reviews how genome reduction alters endosymbiont biology and highlights a "tipping point" where the loss of the ability to build a cell envelope coincides with a marked erosion of translation-related genes.

Setting up Agrobacterium tumefaciens-mediated transformation of the tropical legume Aeschynomene evenia, a powerful tool for studying gene function in Nod Factor-independent symbiosis.

Most legumes are able to develop a root nodule symbiosis in association with proteobacteria collectively called rhizobia. Among them, the tropical species Aeschynomene evenia has the remarkable property of being nodulated by photosynthetic Rhizobia without the intervention of Nod Factors (NodF). Thereby, A. evenia has emerged as a working model for investigating the NodF-independent symbiosis. Despite the availability of numerous resources and tools to study the molecular basis of this atypical symbiosis, the lack of a transformation system based on Agrobacterium tumefaciens significantly limits the range of functional approaches. In this report, we present the development of a stable genetic transformation procedure for A. evenia. We first assessed its regeneration capability and found that a combination of two growth regulators, NAA (= Naphthalene Acetic Acid) and BAP (= 6-BenzylAminoPurine) allows the induction of budding calli from epicotyls, hypocotyls and cotyledons with a high efficiency in media containing 0,5 µM NAA (up to 100% of calli with continuous stem proliferation). To optimize the generation of transgenic lines, we employed A. tumefaciens strain EHA105 harboring a binary vector carrying the hygromycin resistance gene and the mCherry fluorescent marker. Epicotyls and hypocotyls were used as the starting material for this process. We have found that one growth medium containing a combination of NAA (0,5 µM) and BAP (2,2 µM) was sufficient to induce callogenesis and A. tumefaciens strain EHA105 was sufficiently virulent to yield a high number of transformed calli. This simple and efficient method constitutes a valuable tool that will greatly facilitate the functional studies in NodF-independent symbiosis.

A genome-centric view of the role of the Acropora kenti microbiome in coral health and resilience.

Microbial diversity has been extensively explored in reef-building corals. However, the functional roles of coral-associated microorganisms remain poorly elucidated. Here, we recover 191 bacterial and 10 archaeal metagenome-assembled genomes (MAGs) from the coral Acropora kenti (formerly A. tenuis) and adjacent seawater, to identify microbial functions and metabolic interactions within the holobiont. We show that 82 MAGs were specific to the A. kenti holobiont, including members of the Pseudomonadota, Bacteroidota, and Desulfobacterota. A. kenti-specific MAGs displayed significant differences in their genomic features and functional potential relative to seawater-specific MAGs, with a higher prevalence of genes involved in host immune system evasion, nitrogen and carbon fixation, and synthesis of five essential B-vitamins. We find a diversity of A. kenti-specific MAGs encode the biosynthesis of essential amino acids, such as tryptophan, histidine, and lysine, which cannot be de novo synthesised by the host or Symbiodiniaceae. Across a water quality gradient spanning 2° of latitude, A. kenti microbial community composition is correlated to increased temperature and dissolved inorganic nitrogen, with corresponding enrichment in molecular chaperones, nitrate reductases, and a heat-shock protein. We reveal mechanisms of A. kenti-microbiome-symbiosis on the Great Barrier Reef, highlighting the interactions underpinning the health of this keystone holobiont.

Anaerobic fungi in the tortoise alimentary tract illuminate early stages of host-fungal symbiosis and Neocallimastigomycota evolution.

Anaerobic gut fungi (AGF, Neocallimastigomycota) reside in the alimentary tract of herbivores. While their presence in mammals is well documented, evidence for their occurrence in non-mammalian hosts is currently sparse. Culture-independent surveys of AGF in tortoises identified a unique community, with three novel deep-branching genera representing >90% of sequences in most samples. Representatives of all genera were successfully isolated under strict anaerobic conditions. Transcriptomics-enabled phylogenomic and molecular dating analyses indicated an ancient, deep-branching position in the AGF tree for these genera, with an evolutionary divergence time estimate of 104-112 million years ago (Mya). Such estimates push the establishment of animal-Neocallimastigomycota symbiosis from the late to the early Cretaceous. Further, tortoise-associated isolates (T-AGF) exhibited limited capacity for plant polysaccharides metabolism and lacked genes encoding several carbohydrate-active enzyme (CAZyme) families. Finally, we demonstrate that the observed curtailed degradation capacities and reduced CAZyme repertoire is driven by the paucity of horizontal gene transfer (HGT) in T-AGF genomes, compared to their mammalian counterparts. This reduced capacity was reflected in an altered cellulosomal production capacity in T-AGF. Our findings provide insights into the phylogenetic diversity, ecological distribution, evolutionary history, evolution of fungal-host nutritional symbiosis, and dynamics of genes acquisition in Neocallimastigomycota.

New plastids, old proteins: repeated endosymbiotic acquisitions in kareniacean dinoflagellates.

Dinoflagellates are a diverse group of ecologically significant micro-eukaryotes that can serve as a model system for plastid symbiogenesis due to their susceptibility to plastid loss and replacement via serial endosymbiosis. Kareniaceae harbor fucoxanthin-pigmented plastids instead of the ancestral peridinin-pigmented ones and support them with a diverse range of nucleus-encoded plastid-targeted proteins originating from the haptophyte endosymbiont, dinoflagellate host, and/or lateral gene transfers (LGT). Here, we present predicted plastid proteomes from seven distantly related kareniaceans in three genera (Karenia, Karlodinium, and Takayama) and analyze their evolutionary patterns using automated tree building and sorting. We project a relatively limited ( ~ 10%) haptophyte signal pointing towards a shared origin in the family Chrysochromulinaceae. Our data establish significant variations in the functional distributions of these signals, emphasizing the importance of micro-evolutionary processes in shaping the chimeric proteomes. Analysis of plastid genome sequences recontextualizes these results by a striking finding the extant kareniacean plastids are in fact not all of the same origin, as two of the studied species (Karlodinium armiger, Takayama helix) possess plastids from different haptophyte orders than the rest.

The Wolbachia WalE1 effector alters Drosophila endocytosis.

The most common intracellular bacterial infection is Wolbachia pipientis, a microbe that manipulates host reproduction and is used in control of insect vectors. Phenotypes induced by Wolbachia have been studied for decades and range from sperm-egg incompatibility to male killing. How Wolbachia alters host biology is less well understood. Previously, we characterized the first Wolbachia effector-WalE1, which encodes an alpha-synuclein domain at the N terminus. Purified WalE1 sediments with and bundles actin and when heterologously expressed in flies, increases Wolbachia titer in the developing oocyte. In this work, we first identify the native expression of WalE1 by Wolbachia infecting both fly cells and whole animals. WalE1 appears as aggregates in the host cell cytosol. We next show that WalE1 co-immunoprecipitates with the host protein Past1, although might not directly interact with it, and that WalE1 manipulates host endocytosis. Yeast expressing WalE1 show deficiency in uptake of FM4-64 dye, and flies harboring mutations in Past1 or overexpressing WalE1 are sensitive to AgNO3, a hallmark of endocytosis defects. We also show that flies expressing WalE1 suffer from endocytosis defects in larval nephrocytes. Finally, we also show that Past1 null flies harbor more Wolbachia overall and in late egg chambers. Our results identify interactions between Wolbachia and a host protein involved in endocytosis and point to yet another important host cell process impinged upon by Wolbachia's WalE1 effector.

Symbiotic efficiency of Rhizobium leguminosarum sv. trifolii strains originating from the subpolar and temperate climate regions.

Red clover (Trifolium pratense L.) is a forage legume cultivated worldwide. This plant is capable of establishing a nitrogen-fixing symbiosis with Rhizobium leguminosarum symbiovar trifolii strains. To date, no comparative analysis of the symbiotic properties and heterogeneity of T. pratense microsymbionts derived from two distinct geographic regions has been performed. In this study, the symbiotic properties of strains originating from the subpolar and temperate climate zones in a wide range of temperatures (10-25 °C) have been characterized. Our results indicate that all the studied T. pratense microsymbionts from two geographic regions were highly efficient in host plant nodulation and nitrogen fixation in a wide range of temperatures. However, some differences between the populations and between the strains within the individual population examined were observed. Based on the nodC and nifH sequences, the symbiotic diversity of the strains was estimated. In general, 13 alleles for nodC and for nifH were identified. Moreover, 21 and 61 polymorphic sites in the nodC and nifH sequences were found, respectively, indicating that the latter gene shows higher heterogeneity than the former one. Among the nodC and nifH alleles, three genotypes (I-III) were the most frequent, whereas the other alleles (IV-XIII) proved to be unique for the individual strains. Based on the nodC and nifH allele types, 20 nodC-nifH genotypes were identified. Among them, the most frequent were three genotypes marked as A (6 strains), B (5 strains), and C (3 strains). Type A was exclusively found in the temperate strains, whereas types B and C were identified in the subpolar strains. The remaining 17 genotypes were found in single strains. In conclusion, our data indicate that R. leguminosarum sv. trifolii strains derived from two climatic zones show a high diversity with respect to the symbiotic efficiency and heterogeneity. However, some of the R. leguminosarum sv. trifolii strains exhibit very good symbiotic potential in the wide range of the temperatures tested; hence, they may be used in the future for improvement of legume crop production.

Identification and characterization of the TetR family transcriptional regulator NffT in Rhizobium johnstonii.

Symbiotic nitrogen fixation (SNF) by rhizobia is not only the main natural bionitrogen-source for organisms but also a green process leveraged to increase the fertility of soil for agricultural production. However, an insufficient understanding of the regulatory mechanism of SNF hinders its practical application. During SNF, nifA-fixA signaling is essential for the biosynthesis of nitrogenases and electron transfer chain proteins. In the present study, the TetR regulator NffT, whose mutation increased fixA expression, was discovered through a fixA-promoter-ß-glucuronidase fusion assay performed with Rhizobium johnstonii. Real-time quantitative PCR analysis showed that nffT deletion increased the expression of symbiotic genes including nifA and fixA in nifA-fixA signaling, and fixL, fixK, fnrN, and fixN9 in fixL-fixN signaling. nffT overexpression resulted in disordered nodules and reduced nitrogen-fixing efficiency. Electrophoretic mobility shift assays revealed that NffT directly regulated the transcription of RL0091-93, which encode an ATP-binding ABC transporter predicted to be involved in carbohydrate transport. Purified His-tagged NffT bound to a 68 bp DNA sequence located -32 to -99 bp upstream of RL0091-93 and NffT deletion significantly increased the expression of RL0091-93. nffT-promoter-ß-glucuronidase fusion assay indicated that nffT expression was regulated by the cobNTS genes and cobalamin. Mutations in cobNTS significantly decreased the expression of nffT, and cobalamin restored its expression. These results revealed that NffT affects nodule development and nitrogen-fixing reaction by participating in a complex regulatory network of symbiotic and carbohydrate metabolic genes and, thus, plays a pivotal regulatory role during symbiosis of R. johnstonii-Pisum sativum.IMPORTANCESymbiotic nitrogen fixation (SNF) by rhizobia is a green way to maintain soil fertility without causing environmental pollution or consuming chemical energy. A detailed understanding of the regulatory mechanism of this complex process is essential for promoting sustainable agriculture. In this study, we discovered the TetR-type regulator NffT, which suppressed the expression of fixA in Rhizobium johnstonii. Furthermore, NffT was confirmed to play pleiotropic roles in R. johnstonii-Pisum sativum symbiosis; specifically, it inhibited rhizobial growth, nodule differentiation, and nitrogen-fixing reactions. We revealed that NffT indirectly affected R. johnstonii-P. sativum symbiosis by participating in a complex regulatory network of symbiotic and carbohydrate metabolic genes. Furthermore, cobalamin, a chemical molecule, was reported for the first time to be involved in TetR-type protein transcription during symbiosis. Thus, NffT identification connects SNF regulation with genetic, metabolic, and chemical signals and provides new insights into the complex regulation of SNF, laying an experimental basis for the targeted construction of rhizobial strains with highly efficient nitrogen-fixing capacity.

How Did Thylakoids Emerge in Cyanobacteria, and How Were the Primary Chloroplast and Chromatophore Acquired?

The emergence of thylakoid membranes in cyanobacteria is a key event in the evolution of all oxygenic photosynthetic cells, from prokaryotes to eukaryotes. Recent analyses show that they could originate from a unique lipid phase transition rather than from a supposed vesicular budding mechanism. Emergence of thylakoids coincided with the great oxygenation event, more than two billion years ago. The acquisition of semi-autonomous organelles, such as the mitochondrion, the chloroplast, and, more recently, the chromatophore, is a critical step in the evolution of eukaryotes. They resulted from primary endosymbiotic events that seem to share general features, i.e., an acquisition of a bacterium/cyanobacteria likely via a phagocytic membrane, a genome reduction coinciding with an escape of genes from the organelle to the nucleus, and, finally, the appearance of an active system translocating nuclear-encoded proteins back to the organelles. An intense mobilization of foreign genes of bacterial origin, via horizontal gene transfers, plays a critical role. Some third partners, like Chlamydia, might have facilitated the transition from cyanobacteria to the early chloroplast. This chapter further details our current understanding of primary endosymbiosis, focusing on primary chloroplasts, thought to have appeared over a billion years ago, and the chromatophore, which appeared around a hundred years ago.

Fulvic Acid Promotes Legume-Rhizobium Symbiosis by Stimulating Endogenous Flavonoids Synthesis and Secretion.

Fulvic acid (FA) promotes symbiosis between legumes and rhizobia. To elucidate from the aspect of symbiosis, the effects of root irrigation of water-soluble humic materials (WSHM) or foliar spraying of its highly active component, FA, on soybean root exudates and on rhizosphere microorganisms were investigated. As a result, WSHM/FA treatments significantly altered root exudate metabolite composition, and isoflavonoids were identified as key contributors in both treatments compared to the control. Increased expression of genes related to the isoflavonoid biosynthesis were validated by RT-qPCR in both treatments, which notably elevated the synthesis of symbiotic signals genistein, daidzin, coumestrol, and biochanin A. Moreover, the WSHM/FA treatments induced a change in rhizosphere microbial community, coupled with an increase in the relative abundance of rhizobia. Our findings showed that WSHM/FA promotes symbiosis by stimulating the endogenous flavonoid synthesis and leads to rhizobia accumulation in the rhizosphere. This study provides new insights into mechanisms underlying the FA-mediated promotion of symbiosis.

Endosymbiont and gut bacterial communities of the brown-banded cockroach, Supella longipalpa.

The brown-banded cockroach (Supella longipalpa) is a widespread nuisance and public health pest. Like the German cockroach (Blattella germanica), this species is adapted to the indoor biome and completes the entirety of its life cycle in human-built structures. Recently, understanding the contributions of commensal and symbiotic microbes to the biology of cockroach pests, as well as the applications of targeting these microbes for pest control, have garnered significant scientific interest. However, relative to B. germanica, the biology of S. longipalpa, including its microbial associations, is understudied. Therefore, the goal of the present study was to quantitatively examine and characterize both the endosymbiont and gut bacterial communities of S. longipalpa for the first time. To do so, bacterial 16S rRNA gene amplicon sequencing was conducted on DNA extracts from whole adult females and males, early instar nymphs, and late instar nymphs. The results demonstrate that the gut microbiome is dominated by two genera of bacteria known to have beneficial probiotic effects in other organisms, namely Lactobacillus and Akkermansia. Furthermore, our data show a significant effect of nymphal development on diversity and variation in the gut microbiome. Lastly, we reveal significant negative correlations between the two intracellular endosymbionts, Blattabacterium and Wolbachia, as well as between Blattabacterium and the gut microbiome, suggesting that Blattabacterium endosymbionts could directly or indirectly influence the composition of other bacterial populations. These findings have implications for understanding the adaptation of S. longipalpa to the indoor biome, its divergence from other indoor cockroach pest species such as B. germanica, the development of novel control approaches that target the microbiome, and fundamental insect-microbe interactions more broadly.

Genome-wide identification and comparative analysis of the Amino Acid Transporter (AAT) gene family and their roles during Phaseolus vulgaris symbioses.

Amino acid transporters (AATs) are essential integral membrane proteins that serve multiple roles, such as facilitating the transport of amino acids across cell membranes. They play a crucial role in the growth and development of plants. Phaseolus vulgaris, a significant legume crop, serves as a valuable model for studying root symbiosis. In this study, we have conducted an exploration of the AAT gene family in P. vulgaris. In this research, we identified 84 AAT genes within the P. vulgaris genome sequence and categorized them into 12 subfamilies based on their similarity and phylogenetic relationships with AATs found in Arabidopsis and rice. Interestingly, these AAT genes were not evenly distributed across the chromosomes of P. vulgaris . Instead, there was an unusual concentration of these genes located toward the outer edges of chromosomal arms. Upon conducting motif analysis and gene structural analysis, we observed a consistent presence of similar motifs and an intron-exon distribution pattern among the subfamilies. When we analyzed the expression profiles of PvAAT genes, we noted tissue-specific expression patterns. Furthermore, our investigation into AAT gene expression under rhizobial and mycorrhizal symbiotic conditions revealed that certain genes exhibited high levels of expression. Specifically, ATLa5 and LHT2 was notably upregulated under both symbiotic conditions. These findings point towards a potential role of AATs in the context of rhizobial and mycorrhizal symbiosis in P. vulgaris, in addition to their well-established regulatory functions.

Dietary potential of the symbiotic fungus Penicillium herquei for the larvae of a nonsocial fungus-cultivating weevil Euops chinensis.

Many insect taxa cultivate fungi for food. Compared to well-known fungus cultivation in social insects, our knowledge on fungus cultivation in nonsocial insects is still limited. Here, we studied the nutritional potentials of the fungal cultivar, Penicillium herquei, for the larvae of its nonsocial insect farmer, Euops chinensis, a specialist on Japanese knotweed Reynoutria japonica. Overall, fungal hyphae and leaf rolls contained significantly higher carbon (C), stable isotopes of C (δ13C), and nitrogen (δ15N) but significantly lower C/N ratios compared to unrolled leaves, whereas insect bodies contained significantly higher N contents but lower C and C/N ratios compared to other types of samples. The MixSIAR model indicated that fungal hyphae contributed a larger proportion (0.626-0.797) to the diet of E. chinensis larvae than leaf materials. The levels of ergosterol, six essential amino acids, seven nonessential amino acids, and three B vitamins tested in fungal hyphae and/or leaf rolls were significantly higher than in unrolled leaves and/or larvae. The P. herquei genome contains the complete set of genes required for the biosynthesis of ergosterol, the essential amino acids valine and threonine, nine nonessential amino acids, and vitamins B2 and B3, whereas some genes associated with five essential and one nonessential amino acid were lost in the P. herquei genome. These suggest that P. herquei is capable of providing the E. chinensis larvae food with ergosterol, amino acids, and B vitamins. P. herquei appears to be able to synthesize or concentrate these nutrients considering that they were specifically concentrated in fungal hyphae. IMPORTANCE: The cultivation of fungi for food has occurred across divergent insect lineages such as social ants, termites, and ambrosia beetles, as well as some seldom-reported solitary insects. Although the fungal cultivars of these insects have been studied for decades, the dietary potential of fungal cultivars for their hosts (especially for those nonsocial insects) is largely unknown. Our research on the mutualistic system Euops chinensis-Penicillium herquei represents an example of the diverse nutritional potentials of the fungal cultivar P. herquei in the diet of the larvae of its solitary host, E. chinensis. These results demonstrate that P. herquei has the potential to synthesize or concentrate ergosterol, amino acids, and B vitamins and benefits the larvae of E. chinensis. Our findings would shed light on poorly understood fungal cultivation mutualisms in nonsocial insects and underscore the nutritional importance of fungal cultivars in fungal cultivation mutualisms.

Calcium Induces the Cleavage of NopA and Regulates the Expression of Nodulation Genes and Secretion of T3SS Effectors in Sinorhizobium fredii NGR234.

The type III secretion system (T3SS) is a key factor for the symbiosis between rhizobia and legumes. In this study, we investigated the effect of calcium on the expression and secretion of T3SS effectors (T3Es) in Sinorhizobium fredii NGR234, a broad host range rhizobial strain. We performed RNA-Seq analysis of NGR234 grown in the presence of apigenin, calcium, and apigenin plus calcium and compared it with NGR234 grown in the absence of calcium and apigenin. Calcium treatment resulted in a differential expression of 65 genes, most of which are involved in the transport or metabolism of amino acids and carbohydrates. Calcium had a pronounced effect on the transcription of a gene (NGR_b22780) that encodes a putative transmembrane protein, exhibiting a 17-fold change when compared to NGR234 cells grown in the absence of calcium. Calcium upregulated the expression of several sugar transporters, permeases, aminotransferases, and oxidoreductases. Interestingly, calcium downregulated the expression of nodABC, genes that are required for the synthesis of nod factors. A gene encoding a putative outer membrane protein (OmpW) implicated in antibiotic resistance and membrane integrity was also repressed by calcium. We also observed that calcium reduced the production of nodulation outer proteins (T3Es), especially NopA, the main subunit of the T3SS pilus. Additionally, calcium mediated the cleavage of NopA into two smaller isoforms, which might affect the secretion of other T3Es and the symbiotic establishment. Our findings suggest that calcium regulates the T3SS at a post-transcriptional level and provides new insights into the role of calcium in rhizobia-legume interactions.

Genomic Diversity in the Endosymbiotic Bacteria of Human Head Lice.

Insects have repeatedly forged symbioses with heritable microbes, gaining novel traits. For the microbe, the transition to symbioses can lead to the degeneration of the symbiont's genome through transmission bottlenecks, isolation, and the loss of DNA repair enzymes. However, some insect-microbial symbioses have persisted for millions of years, suggesting that natural selection slows genetic drift and maintains functional consistency between symbiont populations. By sampling in multiple countries, we examine genomic diversity within a symbiont species, a heritable symbiotic bacterium found only in human head lice. We find that human head louse symbionts contain genetic diversity that appears to have arisen contemporaneously with the appearance of anatomically modern humans within Africa and/or during the colonization of Eurasia by humans. We predict that the observed genetic diversity underlies functional differences in extant symbiont lineages, through the inactivation of genes involved in symbiont membrane construction. Furthermore, we find evidence of additional gene losses prior to the appearance of modern humans, also impacting the symbiont membrane. From this, we conclude that symbiont genome degeneration is proceeding, via gene inactivation and subsequent loss, in human head louse symbionts, while genomic diversity is maintained. Collectively, our results provide a look into the genomic diversity within a single symbiont species and highlight the shared evolutionary history of humans, lice, and bacteria.

Symbiosis modulates gene expression of symbionts, but not coral hosts, under thermal challenge.

Increasing ocean temperatures are causing dysbiosis between coral hosts and their symbionts. Previous work suggests that coral host gene expression responds more strongly to environmental stress compared to their intracellular symbionts; however, the causes and consequences of this phenomenon remain untested. We hypothesized that symbionts are less responsive because hosts modulate symbiont environments to buffer stress. To test this hypothesis, we leveraged the facultative symbiosis between the scleractinian coral Oculina arbuscula and its symbiont Breviolum psygmophilum to characterize gene expression responses of both symbiotic partners in and ex hospite under thermal challenges. To characterize host and in hospite symbiont responses, symbiotic and aposymbiotic O. arbuscula were exposed to three treatments: (1) control (18°C), (2) heat (32°C), and (3) cold (6°C). This experiment was replicated with B. psygmophilum cultured from O. arbuscula to characterize ex hospite symbiont responses. Both thermal challenges elicited classic environmental stress responses (ESRs) in O. arbuscula regardless of symbiotic state, with hosts responding more strongly to cold challenge. Hosts also exhibited stronger responses than in hospite symbionts. In and ex hospite B. psygmophilum both down-regulated gene ontology pathways associated with photosynthesis under thermal challenge; however, ex hospite symbionts exhibited greater gene expression plasticity and differential expression of genes associated with ESRs. Taken together, these findings suggest that O. arbuscula hosts may buffer environments of B. psygmophilum symbionts; however, we outline the future work needed to confirm this hypothesis.