Obligate Association Research Articles

Phylogenomics, biogeography and diversification of obligate mealybug-tending ants in the genus Acropyga

Acropyga ants are a widespread clade of small subterranean formicines that live in obligate symbiotic associations with root mealybugs. We generated a data set of 944 loci of ultraconserved elements (UCEs) to reconstruct the phylogeny of 41 representatives of 23 Acropyga species using both concatenation and species-tree approaches. We investigated the biogeographic history of the genus through divergence dating analyses and ancestral range reconstructions. We also explored the evolution of the Acropyga-mealybug mutualism using ancestral state reconstruction methods. We recovered a highly supported species phylogeny for Acropyga with both concatenation and species-tree analyses. The age for crown-group Acropyga is estimated to be around 30Ma. The geographic origin of the genus remains uncertain, although phylogenetic affinities within the subfamily Formicinae point to a Paleotropical ancestor. Two main Acropyga lineages are recovered with mutually exclusive distributions in the Old World and New World. Within the Old World clade, a Palearctic and African lineage is suggested as sister to the remaining species. Ancestral state reconstructions indicate that Old World species have diversified mainly in close association with xenococcines from the genus Eumyrmococcus, although present-day associations also involve other mealybug genera. In contrast, New World Acropyga predominantly evolved with Neochavesia until a recent (10–15Ma) switch to rhizoecid mealybug partners (genus Rhizoecus). The striking mandibular variation in Acropyga evolved most likely from a 5-toothed ancestor. Our results provide an initial evolutionary framework for extended investigations of potential co-evolutionary interactions between these ants and their mealybug partners.

Taxonomic recovery of the ant cricket Myrmecophilus albicinctus from M. americanus (Orthoptera, Myrmecophilidae).

Myrmecophilus americanus and Myrmecophilus albicinctus are typical myrmecophilous insects living inside ant nests. These species are ecologically important due to the obligate association with tramp ant species, including harmful invasive ant species. However, the taxonomy of these “white-banded ant crickets” is quite confused owing to a scarcity of useful external morphological characteristics. Recently, Myrmecophilus albicinctus was synonymized with Myrmecophilus americanus regardless of the apparent host use difference. To clarify taxonomical relationship between Myrmecophilus albicinctus and Myrmecophilus albicinctus, we reexamined morphological characteristics of both species mainly in the viewpoint of anatomy. Observation of genitalia parts, together with a few external body parts, revealed that Myrmecophilus albicinctus showed different tendency from them of Myrmecophilus americanus. Therefore, we recover Myrmecophilus albicinctus as a distinct species on the basis of the morphology.

Opportunistic Occupation of Nests of Microcerotermes spp. Silvestri (Termitidae, Termitinae) by Partamona seridoensis Camargo & Pedro (Apidae, Meliponini) in the Brazilian Tropical Dry Forest

Social bees make use of natural or animal-built structures to protect their colonies from environmental stressors. Here, particularly attractive shelters are active termite nests because they provide a stable climatic environment for inquilines. Several social bee species form obligatory associations with termites, among these the stingless bee Partamona seridoensis (Apidae, Meliponini), whose distribution is limited to the Tropical Dry Forest in the Brazilian Northeast. So far, colonies of this meliponine species have been found mainly in arboreal nests of the termite Constrictotermes cyphergaster, which suggests a tight relationship between these two social insect species. The present study was conducted in an area of the Tropical Dry Forest in the Brazilian State of Rio Grande do Norte, where P. seridoensis naturally occurs albeit the absence of C. cyphergaster. We registered 14 colonies of P. seridoensis, all occupying active arboreal nests of termites of the genus Microcerotermes. The only other termites with arboreal nests present in the study area, Nasutitermes corniger, never housed P. seridoensis. This selective preference of the bees for Microcerotermes nests might be due to differences between termites concerning defense mechanisms or concerning thermal stability within the nests.

Unraveling the Functions of the Macroalgal Microbiome.

Macroalgae are a diverse group of photosynthetic eukaryotic lower organisms and offer indispensable ecosystem services toward sustainable productivity of rocky coastal areas. The earlier studies have mainly focused on elucidation of the roles of the epiphytic bacterial communities in the ecophysiology of the host macroalga. However, mutualistic interactions have become topic of current interest. It is evident from recent studies that a fraction of epiphytic bacterial communities can be categorized as “core microbial species”, suggesting an obligate association. Epiphytic bacterial communities have also been reported to protect macroalgal surfaces from biofouling microorganisms through production of biologically active metabolites. Because of their intrinsic roles in the host life cycle, the host in turn may provide necessary organic nutrients in order to woo pelagic microbial communities to settle on the host surfaces. However, the precise composition of microbiomes and their functional partnership with hosts are hardly understood. In contrast, the microbial studies associated with human skin and gut and plants have significantly advanced our knowledge on microbiome and their functional interactions with the host. This has led to manipulation of the microbial flora of the human gut and of agricultural plants for improving health and performance. Therefore, it is highly imperative to investigate the functional microbiome that is closely involved in the life cycles of the host macroalgae using high-throughput techniques (metagenomics and metatranscriptomics). The findings from such investigations would help in promoting health and productivity in macroalgal species through regulation of functionally active microbiome.

Phylogenetic ecology of gall crabs (Cryptochiridae) as associates of mushroom corals (Fungiidae).

Coral-associated fauna is a relatively understudied topic. Hence, the nature of the relationship between an associated organism and its host is usually unknown. In the present study, the obligate associations between gall crabs (Decapoda: Cryptochiridae) and mushroom corals (Scleractinia: Fungiidae) are reviewed from a phylogenetic perspective. Based on field surveys, examination of museum material and a literature review, a total of 35 fungiid species have been found that act as hosts for four gall crab species. Fungiid-associated gall crabs appear to be more geographically widespread than previously known, with new records showing their occurrences from the Red Sea and western Indian Ocean all the way to the central Pacific Ocean. The obligate nature of the association between cryptochirids and their hosts makes them an ideal model taxon to test for possible cospeciation events. The congruence between their phylogenies was tested by using the program Jane 4.0, resulting in cospeciation and duplication events between the crabs and their host corals. The sharing of several closely related host coral species by the same gall crab species or genus may provide support to models indicating phylogenetic relationships within the Scleractinia.

Diversity of Hemerodromia Meigen, 1822 (Diptera: Empididae) in Thailand, the tip of a tropical iceberg?

The genus Hemerodromia in Thailand is revised and full descriptions and keys are provided for all 25 species. Twenty new species are recognised: H. alphalutea sp. nov., H. anisoserrata sp. nov., H. anomala sp. nov., H. attenuata sp. nov., H betalutea sp. nov., H. conspecta sp. nov., H. deltalutea sp. nov., H. deminuta sp. nov., H. demissa sp. nov., H. epsilutea sp. nov., H. etalutea sp. nov., H. gammalutea sp. nov., H. isochita sp. nov., H. namtokhinpoon sp. nov., H. ocellata sp. nov., H. oriens sp. nov., H. phahompokensis sp. nov., H. songsee sp. nov., H. systoechon sp. nov. and H. zetalutea sp. nov. Five species known previously from China are recognised: H. acutata Grootaert, Yang & Saigusa, H. flaviventris Yang & Yang, H. furcata Grootaert, Yang & Saigusa, H. fusca Yang & Yang and H. yunnanensis Yang & Yang. Hemerodromia songsee sp. nov. and H. fusca Yang & Yang are also recorded from Vietnam. Distribution maps of all species are presented. Four categories of distribution patterns of apparently endemic species were identified in (1) the northern mountains (2) the northern lowlands (3) the south, and (4) east of Thailand. Some lowland species with wide distributions in eastern Asia were interpreted as 'old Oriental elements'. Other montane species have wide distributions extending between the Himalayas and southeast China. Three lowland species have an apparently obligate association with alkaline, mineralised water courses where tufa deposition was evident. Tufa-linked assemblages of Hemerodromia may indicate a previously unrecognised and potentially diverse habitat for aquatic Empididae in Southeast Asia. Major historical factors determining contemporary distribution patterns were analysed in reference to a Climate History Model (Plant et al. 2012) and included (i) latitudinal migrations in response to climatically induced changes in the distribution of habitat (ii) radiation of high-elevation endemics from more widespread lowland forms (iii) historical connectivity and fragmentation of hydrological networks with possible marooning of taxa in stable tufa spring systems (iv) persistence of lowland forms in climatically 'buffered' stream environments during progressive aridification. Analysis of sampling methodology concluded that hand collecting was 2,000X more efficient at collecting numbers of Hemerodromia with a species discovery rate 775X greater than that with passive trapping methods (Malaise, flight interception and pan traps etc.) although both approaches are needed for full assessment of species richness. Consideration of the climatic, ecological and biogeographic complexity of tropical Southeast Asia suggests that an extremely rich Hemerodromia fauna awaits discovery in the region.

Визначення складу мікрофлори репродуктивного тракту жінок із використанням тест-системи «Фемофлор-17»

The data about composition of the vaginal microflora of women with use of the test system "Femoflor-17" was analyzed. Established that 92.7% of samples contained lactobacilli. For quantitaty of lactobacilli vaginal flora characterized as normobiosis. The dysbiosis in the background detection of mycoplasma, ureaplasma and yeast of genus Candida identified for 38.2% of the samples. Mycoplasma, ureaplasma and candida as monoinfection detected in 71% of cases and as an association of two pathogens in 29% of cases. In establishing compliance obtained by the test system "Femoflor-17" results to clinical and bacteriological markers of vaginal dysbiosis characterized as moderate anaerobic dysbiosis--6% of the samples, moderate aerobic dysbiosis--10%, moderate anaerobic-aerobic dysbiosis--35%, pronounced anaerobic dysbiosis--12% of the samples, expressed aerobic dysbiosis--10%, anaerobic-aerobic expressed dysbiosis--27% of the samples. Showing dominance of obligate anaerobic microorganisms associations among other microorganisms.

A note on the obligate symbiotic association between crab Zebrida adamsii White, 1847 (Decapoda: Pilumnidae) and Flower Urchin Toxopneustes pileolus (Lamarck, 1816) (Camarodonta: Toxopneustidae) from the Gulf of Mannar, India

The association of Parthenopid crab Zebrida adamsii and flower urchin Toxopneustes pileolus is reported for the first time from Gulf of Mannar. The Parthenopid crab Zebrida adamsii has been found to be associated exclusively with these sea urchins and this obligatory association has been studied by introducing another host sea urchin Salmacis virgulata to understand host preferences.

Germination and seedling establishment in orchids: a complex of requirements.

Seedling recruitment is essential to the sustainability of any plant population. Due to the minute nature of seeds and early-stage seedlings, orchid germination in situ was for a long time practically impossible to observe, creating an obstacle towards understanding seedling site requirements and fluctuations in orchid populations. The introduction of seed packet techniques for sowing and retrieval in natural sites has brought with it important insights, but many aspects of orchid seed and germination biology remain largely unexplored. The germination niche for orchids is extremely complex, because it is defined by requirements not only for seed lodging and germination, but also for presence of a fungal host and its substrate. A mycobiont that the seedling can parasitize is considered an essential element, and a great diversity of Basidiomycota and Ascomycota have now been identified for their role in orchid seed germination, with fungi identifiable as imperfect Rhizoctonia species predominating. Specificity patterns vary from orchid species employing a single fungal lineage to species associating individually with a limited selection of distantly related fungi. A suitable organic carbon source for the mycobiont constitutes another key requirement. Orchid germination also relies on factors that generally influence the success of plant seeds, both abiotic, such as light/shade, moisture, substrate chemistry and texture, and biotic, such as competitors and antagonists. Complexity is furthermore increased when these factors influence seeds/seedling, fungi and fungal substrate differentially. A better understanding of germination and seedling establishment is needed for conservation of orchid populations. Due to the obligate association with a mycobiont, the germination niches in orchid species are extremely complex and varied. Microsites suitable for germination can be small and transient, and direct observation is difficult. An experimental approach using several levels of environmental manipulation/control is recommended.

Sulcia symbiont of the leafhopper Macrosteles laevis (Ribaut, 1927) (Insecta, Hemiptera, Cicadellidae: Deltocephalinae) harbors Arsenophonus bacteria.

The leafhopper Macrosteles laevis, like other plant sap-feeding hemipterans, lives in obligate symbiotic association with microorganisms. The symbionts are harbored in the cytoplasm of large cells termed bacteriocytes, which are integrated into huge organs termed bacteriomes. Morphological and molecular investigations have revealed that in the bacteriomes of M. laevis, two types of bacteriocytes are present which are as follows: bacteriocytes with bacterium Sulcia and bacteriocytes with Nasuia symbiont. We observed that in bacteriocytes with Sulcia, some cells of this bacterium contain numerous cells of the bacterium Arsenophonus. All types of symbionts are transmitted transovarially between generations. In the mature female, the bacteria Nasuia, bacteria Sulcia, and Sulcia with Arsenophonus inside are released from the bacteriocytes and start to assemble around the terminal oocytes. Next, the bacteria enter the cytoplasm of follicular cells surrounding the posterior pole of the oocyte. After passing through the follicular cells, the symbionts enter the space between the oocyte and follicular epithelium, forming a characteristic “symbiont ball.”

Photosynthetic performance in Antarctic lichens with different growth forms reflect the diversity of lichenized algal adaptation to microhabitats

AbstractLichens, as typical obligate associations between lichenized fungi and their photosynthetic partners, are dominant in Antarctica. Three Antarctic lichens, Ochrolechia frigida, Umbilicaria antarctica, and Usnea aurantiacoatra with different growth forms, were sampled nearby the Great Wall Station, King George Island. Molecular data revealed that the photosynthetic algae in these three lichens were Trebouxia jamesii. The net photo− synthesis (Pn) of three individuals from these species, together with environmental factors such as light and temperature, were recorded by CO2gas exchange measurements using a CI−340 portable photosynthetic system in situ. Differences between T(leaf) (the temperature of the thalli) and T(air) (the air temperature) for these lichens were not consistent, which reflected that environment and the growth form of thalli could affect T(leaf) significantly. Strong irradiation was expected to have adverse effects on Pn of Ochrolechia frigida and Umbilicaria antarctica whose thalli spread flat; but this photoinhibition had little effect on Usnea aurantiacoatra with exuberant tufted thallus. These results indicated that photosynthetic activity in lichens was affected by the growth forms of thalli besides microhabitat factors. One species of lichenized alga could exhibit diversified types of photosynthetic behavior when it was associated with various lichenized fungi in different microhabitats. It will be helpful for understanding how lichens are able to adapt to and colonize in extreme environments.

High prevalence of obligate coral-dwelling decapods on dead corals in the Chagos Archipelago, central Indian Ocean

Small and cryptic organisms that live within the interstices of reef habitats contribute greatly to coral reef biodiversity, but are poorly studied. Many species of cryptofauna have seemingly obligate associations with live coral and are therefore considered to be very vulnerable to coral mortality. Here we report the unanticipated prevalence of obligate coral-dwelling decapod crustaceans on dead colonies of branching corals in the Chagos Archipelago (British Indian Ocean Territory) in the central Indian Ocean. A total of 205 obligate coral-dwelling decapods, including Trapezia crabs, were recorded from 43 (out of 54) dead coral colonies of Acropora and Pocillopora collected across five different atolls. Trapezia individuals found on dead corals were mainly juveniles, and the few adults were almost exclusively male. Among the shrimps (Pontoniinae), however, it was predominantly adult females found on dead corals. Obligate coral-dwelling species that typically occur only on live Pocillopora hosts (e.g., Trapezia spp.) were recorded on dead Acropora. These findings suggests that these obligate coral-dwelling decapods are not simply persisting on coral hosts that have died, but may be explicitly recruiting to or moving to dead coral hosts at certain stages in their life cycle. Variation in the abundance of live coral among sites had no affect on the presence or abundance of obligate coral-dwelling decapods on dead corals. This study shows that habitat associations of obligate coral-dwelling organisms, and their reliance on different habitat types, are complex and further work is required to establish their vulnerability to widespread habitat degradation on coral reefs.

High levels of heterospecific cohabitation among anemonefishes in Hoga Island, Indonesia

Anemonefishes have an obligate association with host sea anemones and normally occur in conspecific groups. Occasionally, heterospecific social groups are observed (Fautin and Allen 1997). Here, we report the highest documented frequency of heterospecific cohabitation in the world. Observations on coral reefs around Hoga Island (Southwest Sulawesi, Indonesia) were conducted on the reef crest and slope habitats (3–15 m depth) during July and August 2014, and cohabitation was recorded in the sea anemones Entacmaea quadricolor, Heteractis crispa and Stichodactyla mertensii. Surveys revealed that 55 out of 106 surveyed sea anemones (52 %) were occupied by more than one species of anenomefish; all other observed sea anemones were occupied by only one.

Elevated Atmospheric CO2 Affects Ectomycorrhizal Species Abundance and Increases Sporocarp Production under Field Conditions

Anthropogenic activities during the last century have increased levels of atmospheric CO2. Forest net primary productivity increases in response to elevated CO2, altering the quantity and quality of carbon supplied to the rhizosphere. Ectomycorrhizal fungi form obligate symbiotic associations with the fine roots of trees that mediate improved scavenging for nutrients in exchange for a carbohydrate supply. Understanding how the community structure of ectomycorrhizal fungi is altered by climate change is important to further our understanding of ecosystem function. Betula pendula and Fagus sylvatica were grown in an elevated CO2 atmosphere delivered using free air carbon dioxide enrichment (FACE) under field conditions in the U.K., and Picea abies was grown under elevated CO2 in glass domes in the Czech Republic. We used morphotyping and sequencing of the internal transcribed spacer region of the fungal ribosomal operon to study ectomycorrhizal community structure. Under FACE, un-colonised roots tips increased in abundance for Fagus sylvatica, and during 2006, sporocarp biomass of Peziza badia significantly increased. In domes, ectomycorrhizal community composition shifted from short-distance and smooth medium-distance to contact exploration types. Supply and competition for carbon belowground can influence ectomycorrhizal community structure with the potential to alter ecosystem function.

Towards the resolution of a long-standing evolutionary question: muscle identity and attachments are mainly related to topological position and not to primordium or homeotic identity of digits.

Signaling for limb bone development usually precedes that for muscle development, such that cartilage is generally present before muscle formation. It remains obscure, however, if: (i) tetrapods share a general, predictable spatial correlation between bones and muscles; and, if that is the case, if (ii) such a correlation would reflect an obligatory association between the signaling involved in skeletal and muscle morphogenesis. We address these issues here by using the results of a multidisciplinary analysis of the appendicular muscles of all major tetrapod groups integrating dissections, muscle antibody stainings, regenerative and ontogenetic analyses of fluorescently-labeled (GFP) animals, and studies of non-pentadactyl human limbs related to birth defects. Our synthesis suggests that there is a consistent, surprising anatomical pattern in both normal and abnormal phenotypes, in which the identity and attachments of distal limb muscles are mainly related to the topological position, and not to the developmental primordium (anlage) or even the homeotic identity, of the digits to which they are attached. This synthesis is therefore a starting point towards the resolution of a centuries-old question raised by authors such as Owen about the specific associations between limb bones and muscles. This question has crucial implications for evolutionary and developmental biology, and for human medicine because non-pentadactyly is the most common birth defect in human limbs. In particular, this synthesis paves the way for future developmental experimental and mechanistic studies, which are needed to clarify the processes that may be involved in the elaboration of the anatomical patterns described here, and to specifically test the hypothesis that distal limb muscle identity/attachment is mainly related to digit topology.

The most relictual fungus-farming ant species cultivates the most recently evolved and highly domesticated fungal symbiont species.

Fungus-farming (attine) ant agriculture is made up of five known agricultural systems characterized by remarkable symbiont fidelity in which five phylogenetic groups of ants faithfully cultivate five phylogenetic groups of fungi. Here we describe the first case of a lower-attine ant cultivating a higher-attine fungus based on our discovery of a Brazilian population of the relictual fungus-farming ant Apterostigma megacephala, known previously from four stray specimens from Peru and Colombia. We find that A. megacephala is the sole surviving representative of an ancient lineage that diverged ∼39 million years ago, very early in the ∼55-million-year evolution of fungus-farming ants. Contrary to all previously known patterns of ant-fungus symbiont fidelity, A. megacephala cultivates Leucoagaricus gongylophorus, a highly domesticated fungal cultivar that originated only 2-8 million years ago in the gardens of the highly derived and recently evolved (∼12 million years ago) leaf-cutting ants. Because no other lower fungus-farming ant is known to cultivate any of the higher-attine fungi, let alone the leaf-cutter fungus, A. megacephala may provide important clues about the biological mechanisms constraining the otherwise seemingly obligate ant-fungus associations that characterize attine ant agriculture.

Redescription of Hemieuryale pustulata von Martens, 1867 (Echinodermata, Ophiuroidea) based on Brazilian specimens, with notes on systematics and habitat association.

We redescribe Hemieuryale pustulata on the basis of 325 specimens obtained from the continental shelf off northeastern and southeastern Brazil. This is the first record of the species for Brazil. We illustrate for the first time details of the dental plate, oral plate, and vertebrae. Few morphological variations were observed in our extensive material. All specimens were associated with the gorgonian Nicella guadalupensis, suggesting an obligatory association in the Brazilian littoral. H. pustulata did not reveal a particular pattern of distribution on its host gorgonian, but larger gorgonians tend to support a larger number of ophiuroids. In addition to expanding the knowledge on morphology and ecology, we extend the known bathymetric distribution of the species, which is now known from 18 to 330 m.

Heritable symbiosis: The advantages and perils of an evolutionary rabbit hole

Many eukaryotes have obligate associations with microorganisms that are transmitted directly between generations. A model for heritable symbiosis is the association of aphids, a clade of sap-feeding insects, and Buchnera aphidicola, a gammaproteobacterium that colonized an aphid ancestor 150 million years ago and persists in almost all 5,000 aphid species. Symbiont acquisition enables evolutionary and ecological expansion; aphids are one of many insect groups that would not exist without heritable symbiosis. Receiving less attention are potential negative ramifications of symbiotic alliances. In the short run, symbionts impose metabolic costs. Over evolutionary time, hosts evolve dependence beyond the original benefits of the symbiosis. Symbiotic partners enter into an evolutionary spiral that leads to irreversible codependence and associated risks. Host adaptations to symbiosis (e.g., immune-system modification) may impose vulnerabilities. Symbiont genomes also continuously accumulate deleterious mutations, limiting their beneficial contributions and environmental tolerance. Finally, the fitness interests of obligate heritable symbionts are distinct from those of their hosts, leading to selfish tendencies. Thus, genes underlying the host–symbiont interface are predicted to follow a coevolutionary arms race, as observed for genes governing host–pathogen interactions. On the macroevolutionary scale, the rapid evolution of interacting symbiont and host genes is predicted to accelerate host speciation rates by generating genetic incompatibilities. However, degeneration of symbiont genomes may ultimately limit the ecological range of host species, potentially increasing extinction risk. Recent results for the aphid–Buchnera symbiosis and related systems illustrate that, whereas heritable symbiosis can expand ecological range and spur diversification, it also presents potential perils.

An alien ectosymbiotic branchiobdellidan (Annelida: Clitellata) adopting exotic crayfish: a biological co-invasion with unpredictable consequences

Invasive alien species present a global threat to biodiversity, particularly where pathogens and symbionts are involved. Branchiobdellidans are clitellate annelids with an obligate ectosymbiotic association primarily on astacoidean crayfish. There are several examples of branchiobdellidan species adopting a geographically exotic host where endemic and exotic crayfishes cohabit the same water body. The first records of a western North American branchiobdellidan, Xironogiton victoriensis, adopting the eastern North American crayfish, Procambarus clarkii, in 2 river basins in Spain provide further evidence of the ectosymbionts’ tolerance to adopt an exotic host. Given worldwide translocations of these and other commercial crayfish species, limnologists and agency managers need to be alert for further introductions of X. victoriensis and other branchiobdellidans. Impacts of these exotic ectosymbionts on habitat and biota at a new location are unknown, as are their consequences on native biodiversity.

Genomic classification of serous ovarian cancer with adjacent borderline differentiates RAS pathway and TP53-mutant tumors and identifies NRAS as an oncogenic driver.

Low-grade serous ovarian carcinomas (LGSC) are Ras pathway-mutated, TP53 wild-type, and frequently associated with borderline tumors. Patients with LGSCs respond poorly to platinum-based chemotherapy and may benefit from pathway-targeted agents. High-grade serous carcinomas (HGSC) are TP53-mutated and are thought to be rarely associated with borderline tumors. We sought to determine whether borderline histology associated with grade 2 or 3 carcinoma was an indicator of Ras mutation, and we explored the molecular relationship between coexisting invasive and borderline histologies. We reviewed >1,200 patients and identified 102 serous carcinomas with adjacent borderline regions for analyses, including candidate mutation screening, copy number, and gene expression profiling. We found a similar frequency of low, moderate, and high-grade carcinomas with coexisting borderline histology. BRAF/KRAS alterations were common in LGSC; however, we also found recurrent NRAS mutations. Whereas borderline tumors harbored BRAF/KRAS mutations, NRAS mutations were restricted to carcinomas, representing the first example of a Ras oncogene with an obligatory association with invasive serous cancer. Coexisting borderline and invasive components showed nearly identical genomic profiles. Grade 2 cases with coexisting borderline included tumors with molecular features of LGSC, whereas others were typical of HGSC. However, all grade 3 carcinomas with coexisting borderline histology were molecularly indistinguishable from typical HGSC. Our findings suggest that NRAS is an oncogenic driver in serous ovarian tumors. We demonstrate that borderline histology is an unreliable predictor of Ras pathway aberration and underscore an important role for molecular classification in identifying patients that may benefit from targeted agents.