Seasonal niche segregation of two sympatric xenocyprinid fishes

Xenocypris microlepis (Bleeker, 1871) and Xenocypris davidi (Bleeker, 1871) are two closely related xenocyprinid species potentially subject to exploitative competition. They often coexist in many freshwater ecosystems, which offers an opportunity to analyze their relationships. The present study describes the seasonal feeding habits of age‐3 X. microlepis and X. davidi in Xin'anjiang Reservoir, located in Zhejiang, China, as determined by stable isotope analysis. Particulate organic matter (POM; 50%) and periphyton (50%) were the major items observed in the X. microlepis diet, whereas periphyton (>60%) was prevalent in the X. davidi diet. Seasonal variations were observed in the diet compositions of both xenocyprinid species. Whereas X. microlepis tended to ingest more sediment organic matter (SOM) in autumn than in spring and summer, SOM was the secondary food item for X. davidi over the three seasons. The seasonal diet shift observed in this work indicates niche segregation of the sympatric pair. The results of this study contribute to the current knowledge of seasonal trophic interactions in predator–prey systems where sympatric xenocyprinids are present.     

Insights into historical drainage evolution based on the phylogeography of the chevron snakehead fish (Channa striata) in the Mekong Basin

1. The phylogeography of freshwater taxa is often integrally linked with landscape changes such as drainage re‐alignments that may present the only avenue for historical dispersal for these taxa. Classical models of gene flow do not account for landscape changes and so are of little use in predicting phylogeography in geologically young freshwater landscapes. When the history of drainage formation is unknown, phylogeographical predictions can be based on current freshwater landscape structure, proposed historical drainage geomorphology, or from phylogeographical patterns of co‐distributed taxa. 2. This study describes the population structure of a sedentary freshwater fish, the chevron snakehead (Channa striata), across two river drainages on the Indochinese Peninsula. The phylogeographical pattern recovered for C. striata was tested against seven hypotheses based on contemporary landscape structure, proposed history and phylogeographical patterns of co‐distributed taxa. 3. Consistent with the species ecology, analysis of mitochondrial and microsatellite loci revealed very high differentiation among all sampled sites. A strong signature of historical population subdivision was also revealed within the contemporary Mekong River Basin (MRB). Of the seven phylogeographical hypotheses tested, patterns of co‐distributed taxa proved to be the most adequate for describing the phylogeography of C. striata. 4. Results shed new light on SE Asian drainage evolution, indicating that the Middle MRB probably evolved via amalgamation of at least three historically independent drainage sections and in particular that the Mekong River section centred around the northern Khorat Plateau in NE Thailand was probably isolated from the greater Mekong for an extensive period of evolutionary time. In contrast, C. striata populations in the Lower MRB do not show a phylogeographical signature of evolution in historically isolated drainage lines, suggesting drainage amalgamation has been less important for river landscape formation in this region.     

Pinpointing the level of isolation between two cryptic species sharing the same microhabitat: a case study with a scarabaeid species complex

Understanding the processes of speciation is an important challenge in improving knowledge of the origin of biodiversity. One crucial point is to assess the causes of reproductive isolation, especially in the case of co‐occurring species. Differences in microscale spatial distribution in small organisms may blur the issue. We explored biological processes underlying speciation within dung beetles belonging to the vacca species complex (Scarabaeidae: Onthophagus). The two taxa of this complex, O. vacca and O. medius, not only are known to have a large overlapping Palearctic distribution range but also share the same cowpat with no physical barriers and no observed specific aggregated patterns in the local distribution. The present study aimed at determining the level of isolation between the two taxa and discusses the most likely scenario of the speciation (sympatry vs. allopatry) based on the Coyne & Orr's (2004) four criteria. We conducted a full study on populations sampled within the Mediterranean region integrating morphological analysis (digital image analysis of the elytral melanism pattern), two‐gene phylogenies, population genetic analyses on populations sampled from an area where both species occur and another one with O. vacca only, as well as intra‐ and interspecific mating and crossing bioassays. The variation in the elytral melanism pattern clearly followed a bimodal distribution, with O. medius being more melanic than O. vacca, with a very limited overlapping area. The two taxa are reproductively isolated, with a strong postzygotic incompatibility despite the absence of sexual isolation. Sequence analysis of both nuclear and mitochondrial markers revealed a deep divergence between the two taxa dating back to 8.7 Mya. All findings concurred with some phenological observations and the conclusion that the most likely scenario for speciation in the vacca complex was an allopatric speciation followed by secondary contact.     

Ancient vicariance and climate‐driven extinction explain continental‐wide disjunctions in Africa: the case of the Rand Flora genus Canarina (Campanulaceae)

Transoceanic distributions have attracted the interest of scientists for centuries. Less attention has been paid to the evolutionary origins of ‘continent‐wide’ disjunctions, in which related taxa are distributed across isolated regions within the same continent. A prime example is the ‘Rand Flora’ pattern, which shows sister taxa disjunctly distributed in the continental margins of Africa. Here, we explore the evolutionary origins of this pattern using the genus Canarina, with three species: C. canariensis, associated with the Canarian laurisilva, and C. eminii and C. abyssinica, endemic to the Afromontane region in East Africa, as case study. We infer phylogenetic relationships, divergence times and the history of migration events within Canarina using Bayesian inference on a large sample of chloroplast and nuclear sequences. Ecological niche modelling was employed to infer the climatic niche of Canarina through time. Dating was performed with a novel nested approach to solve the problem of using deep time calibration points within a molecular dataset comprising both above‐species and population‐level sampling. Results show C. abyssinica as sister to a clade formed by disjunct C. eminii and C. canariensis. Miocene divergences were inferred among species, whereas infraspecific divergences fell within the Pleistocene–Holocene periods. Although C. eminii and C. canariensis showed a strong genetic geographic structure, among‐population divergences were older in the former than in the latter. Our results suggest that Canarina originated in East Africa and later migrated across North Africa, with vicariance and aridification‐driven extinction explaining the 7000 km/7 million year divergence between the Canarian and East African endemics.     

Discordance between morphological and molecular species boundaries among Caribbean species of the reef sponge Callyspongia

Sponges are among the most species‐rich and ecologically important taxa on coral reefs, yet documenting their diversity is difficult due to the simplicity and plasticity of their morphological characters. Genetic attempts to identify species are hampered by the slow rate of mitochondrial sequence evolution characteristic of sponges and some other basal metazoans. Here we determine species boundaries of the Caribbean coral reef sponge genus Callyspongia using a multilocus, model‐based approach. Based on sequence data from one mitochondrial (COI), one ribosomal (28S), and two single‐copy nuclear protein‐coding genes, we found evolutionarily distinct lineages were not concordant with current species designations in Callyspongia. While C. fallax, C. tenerrima, and C. plicifera were reciprocally monophyletic, four taxa with different morphologies (C. armigera, C. longissima, C. eschrichtii, and C. vaginalis) formed a monophyletic group and genetic distances among these taxa overlapped distances within them. A model‐based method of species delimitation supported collapsing these four into a single evolutionary lineage. Variation in spicule size among these four taxa was partitioned geographically, not by current species designations, indicating that in Callyspongia, these key taxonomic characters are poor indicators of genetic differentiation. Taken together, our results suggest a complex relationship between morphology and species boundaries in sponges.     

Interindividual patterns of resource use in three subtropical Atlantic Forest frogs

Studies with disparate taxa suggest that generalist populations are composed of relatively specialist individuals that use a narrow part of a population's resource pool. Models based on optimum diet theory (ODT) can be used to predict different patterns of variation in resource use among individuals. In this work, we investigated the diet and measured the degree of individual specialization of three anuran species, Hypsiboas leptolineatus, Pseudis cardosoi and Scinax granulatus, from the Atlantic Forest of southern Brazil. The study is based on gut content obtained using a stomach‐flushing technique. Additionally, we tested for a correlation between the individuals’ diet and morphological variation. Furthermore, we applied methods based on network theory to investigate patterns of resource use among individuals of each species. All three study species showed generalized diets and significant values of individual specialization. However, we did not find any correlation between diet and morphology, indicating that diet variation is not a consequence of morphological trade‐offs. The individual‐resource networks of H. leptolineatus and S. granulatus showed a nested pattern. This result indicates the presence of individuals with more diverse diets than others, and the diets of the more specialist individuals are a predictable subset of the diets of the more generalist ones. The individual‐resource network of P. cardosoi did not show a distinct pattern, diverging from what was predicted by optimal diet theory‐based models. Although nested or modular patterns are commonly found in individual‐resource networks, our results indicate that they are not ubiquitous and that random patterns can also be found.     

Inter‐individual variation in colour patterns in noctuid moths characterizes long‐distance dispersers and agricultural pests

A high capacity for long‐distance dispersal is a key to success for species confronted with environmental heterogeneity, habitat modification, fragmentation and loss. However, dispersal capacity is difficult to quantify and therefore poorly known in most taxa. Here, we report on a test for an association of variation in dispersal capacity with variable colouration of noctuid moths. First, using data from 12 experienced lepidopterologists, we showed that despite variation among experts in average assessments, different species are consistently classified as having non‐variable, variable or highly variable colour patterns when assessed by different experts. We then compared the incidence of non‐resident species with high inter‐individual variation in colour patterns recorded on the isolated island Utklippan (n = 47), with that in a species pool of potential long‐distance dispersers from the nearest mainland (n = 295). Species with high inter‐individual colour pattern variation were over‐represented on the island compared with species having non‐variable colouration. This finding constitutes rare evidence from the wild of long‐distance dispersal, measured on a spatial scale relevant for moths when tracking habitats in fragmented and changing landscapes or when keeping pace with environmental challenges associated with climate change. Finally, we showed that Swedish noctuid moths classified as agricultural pests (n = 28) had more variable colour patterns compared with non‐pests (n = 368). The majority of agricultural pests were also recorded on the isolated island, an outcome that is indicative of pest species having high dispersal capacity. Data on colour pattern variation may thus offer a simple and cost‐effective proxy to estimate dispersal capacity and can also help identify potential pest species. Our findings are potentially useful when modelling and predicting population and range dynamics of species in spatiotemporally heterogeneous environments, with direct implications for conservation biology and pest management.     

New insights into the systematics of Malagasy mongoose‐like carnivorans (Carnivora,Eupleridae, Galidiinae) based on mitochondrial and nuclear DNA sequences

The Malagasy carnivorans (Eupleridae) comprise seven genera and up to ten species, depending on the authority, and, within the past decades, two new taxa have been described. The family is divided into two subfamilies, the Galidiinae, mongoose‐like animals, and the Euplerinae, with diverse body forms. To verify the taxonomic status of Galidiinae species, including recently described taxa, as well as some recognized subspecies, we studied intrageneric genetic variation and structure, using both mitochondrial and nuclear markers. Our results suggest the recognition of four species in the Galidiinae, rendering each genus monospecific. We propose to recognize three subspecies in Galidia elegans (G. e. dambrensis, G. e. elegans, and G. e. occidentalis), two subspecies in Mungotictis decemlineata (M. d. decemlineata and M. d. lineata), and two subspecies in Galidictis fasciata (G. f. fasciata and G. f. grandidieri, the latter was recently described as a distinct species). Our results indicate also that Salanoia durrelli should be treated as a junior synonym of Salanoia concolor. Low levels of intraspecific divergence revealed some geographical structure for the Galidiinae taxa, suggesting that environmental barriers have isolated certain populations in recent geological time. All taxa, whether at the species or subspecies level, need urgent conservation attention, particularly those with limited geographical distributions, as all are threatened by forest habitat degradation.     

Dorsal pattern polymorphism in female Iberian wall lizards: differences in morphology,dorsal coloration,immune response,and reproductive investment

Sex‐specific colour polymorphisms have been extensively documented in many different taxa. When polymorphism in colour pattern is restricted to females, the condition is known as female‐limited pattern polymorphism (FPP), which has been less commonly addressed in vertebrates. FPP is present in several lizard species, although most research on lizards has focused on carotenoid‐ and pteridine‐based coloration and not on melanin‐based polymorphisms. In the present study, we focus on Iberian wall lizards, Podarcis hispanicus, where two female melanin‐based dorsal patterns can be clearly distinguished: striped and reticulated‐blotched. We indirectly tested the hypothesis that selection acts differentially among P. hispanicus female morphs to create alternative morph‐specific phenotypic optima at different levels by investigating whether morphs differ in fitness proxies. We specifically examined whether the two female dorsal pattern morphs differed in adult morphology, dorsal coloration, immune response, reproductive investment, and growth. We did not find a relationship between melanin‐based coloration and hatchling growth and immune response, despite a correlation between these traits possibly being expected as a result of pleiotropy in the melanocortin system. However, our results show that female dorsal morphs in P. hispanicus differ in terms of adult morphology, dorsal coloration, and reproductive investment. Reticulated‐blotched P. hispanicus females had deeper heads and longer femora, less melanin, and more brownish coloration, and also had larger and heavier hatchlings than striped females.     

Survival in northern microrefugia in an endemic Carpathian gammarid (Crustacea: Amphipoda)

Gammarus leopoliensis (Crustacea: Amphipoda) is considered a north‐eastern Carpathian endemic species and therefore can be regarded as an appropriate model for testing the hypothesis of Quaternary glacial survival in northern microrefugia. However, 250 km south, the south‐western Carpathians harbour populations that resemble phenotypically both G. leopoliensis and Gammarus kischineffensis, a similar species distributed east of the Carpathians. We used maximum‐likelihood and Bayesian methods to evaluate the phylogenetic relationships of these three taxa based on mitochondrial and nuclear markers, and quantitatively compared diversity patterns, phylogeography and divergence times among north‐eastern and south‐western Carpathian taxa. Results indicate that G. leopoliensis and the south‐western populations form together a strongly supported group (G. leopoliensis s.l.) which, along with G. kischineffensis, belongs to the Gammarus balcanicus clade. This group contains 12 lineages mainly of Pliocene age. G. leopoliensis consists of two widely distributed and recently expanded allopatric sister lineages that diverged from the southern ones ca. 4 Ma, indicating long‐term survival in northern microrefugia. The southern lineages are micro‐endemic and display a scattered distribution, suggesting a more ancient, relict pattern. We conclude that the contrasting diversity patterns between the disjunct distributional areas of G. leopoliensis s.l. reflect differential survival of lineages across the latitudinal gradient, offering a promising system for comparing the evolutionary ecology of lineages persisting in latitudinally disconnected microrefugia. These results fill an important gap in the knowledge of European gammarid biogeography and reveal that all Carpathian Gammarus taxa are ancient and diverse species complexes.     

The mycobiome of Australian tree hollows in relation to the Cryptococcus gattii and C. neoformans species complexes

Cryptococcosis is a fungal infection caused by members of the Cryptococcus gattii and C. neoformans species complexes. The C. gattii species complex has a strong environmental association with eucalypt hollows (particularly Eucalyptus camaldulensis), which may present a source of infection. It remains unclear whether a specific mycobiome is required to support its environmental survival and growth. Conventional detection of environmental Cryptococcus spp. involves culture on differential media, such as Guizotia abyssinica seed agar. Next‐generation sequencing (NGS)‐based culture‐independent identification aids in contextualising these species in the environmental mycobiome. Samples from 23 Australian tree hollows were subjected to both culture‐ and amplicon‐based metagenomic analysis to characterize the mycobiome and assess relationships between Cryptococcus spp. and other fungal taxa. The most abundant genera detected were Coniochaeta, Aspergillus, and Penicillium, all being commonly isolated from decaying wood. There was no correlation between the presence of Cryptococcus spp. in a tree hollow and the presence of any other fungal genus. Some differences in the abundance of numerous taxa were noted in a differential heat tree comparing samples with or without Cryptococcus‐NGS reads. The study expanded the known environmental niche of the C. gattii and C. neoformans species complexes in Australia with detections from a further five tree species. Discrepancies between the detection of Cryptococcus spp. using culture or NGS suggest that neither is superior per se and that, rather, these methodologies are complementary. The inherent biases of amplicon‐based metagenomics require cautious interpretation of data through consideration of its biological relevance.     

Free‐living calamyzin chrysopetalids (Annelida) from methane seeps,anoxic basins,and whale falls

Members of Calamyzinae, a clade of free‐living and ectoparasitic chrysopetalids, are mainly associated with deep‐sea chemosynthetic environments. The three currently known free‐living calamyzin species are placed in Vigtorniella. A new free‐living calamyzin species similar to these is described here. Phylogenetic analyses of Calamyzinae using mitochondrial (cytochrome c oxidase subunit I and 16S rDNA) and nuclear (Histone H3 and 18S rDNA) loci showed that Vigtorniella and the new species form a grade with respect to an ectoparasitic clade, requiring two new genera to be erected. All free‐living calamyzins show a similar anterior end and chaetal morphology. Micospina auribohnorum gen. et sp. nov. is described for the small‐bodied new species from deep‐sea whale falls off California and methane seeps off Costa Rica. The maximum‐likelihood and Bayesian analyses show Micospina gen. nov. as sister to the ectoparasitic clade. Boudemos gen. nov. is named for the clade of two larger‐bodied species: Boudemos flokati gen. et comb. nov. and Boudemos ardabilia gen. et comb. nov., which is sister group to all other Calamyzinae. Vigtorniella is retained for the type species, Vigtorniella zaikai (Kiseleva, 1992), with the adults found amongst bacterial mats at the boundary of the hydrogen sulphide zone in the Black Sea. Micospina gen. nov., Boudemos gen. nov., and Vigtorniella form a grade of free‐living taxa that is associated with feeding on organic‐enriched sediments, and the latter two taxa display ontogenetic jaw change. Jaws are absent in Micospina auribohnorum gen. et sp. nov. and most of the calamyzin clade of parasitic forms.     

Insights into the evolutionary history of Cervus (Cervidae,tribe Cervini) based on Bayesian analysis of mitochondrial marker sequences,with first indications for a new species

Molecular phylogeny and evolutionary history of Cervus, the most successful and widespread cervid genus, have been extensively addressed in Europe, fairly in eastern Asia, but scarcely in central Asia, where some populations have never been phylogenetically investigated with DNA‐based methods. Here, we applied a coalescent Bayesian approach to most Cervus taxa using complete mitochondrial cytochrome b gene and control region to provide a temporal framework for species differentiation and dispersal, with special emphasis on the central Asian populations from the Tarim Basin (C. elaphus bactrianus, C. elaphus yarkandensis) and Indian Kashmir (C. elaphus hanglu) aiming at assessing their phylogenetic and phylogeographic patterns. Red deer (C. elaphus), wapiti (C. canadensis) and sika deer (C. nippon) are confirmed as highly differentiated taxa, with genetic distances, divergence times and phylogenetic positions compatible with the rank of species. Similarly, the red deer of the Tarim group, hitherto considered as subspecies of C. elaphus, showed a comparable pattern of genetic distinction in the phylogeny and, according to our results, are thus worthy of being raised to the species level. The systematic position of the endangered red deer from Indian Kashmir is assessed here for the first time, and implications for its conservation are also outlined. Based on phylogeny and divergence time estimates, we propose a novel evolutionary pattern for the genus Cervus during the Mio/Pliocene, in the light of palaeo‐climatological information.     

Odorella benthonica gen. & sp. nov. (Pleurocapsales,Cyanobacteria): an odor and prolific toxin producer isolated from a California aqueduct

Pleurocapsales are one of the least understood groups of cyanobacteria in terms of molecular systematics and biochemistry. Considering the high number of cryptic taxa within the Synechococcales and Oscillatoriales, it is likely that such taxa also occur in the Pleurocapsales. The new genus described in our research is the first known pleurocapsalean cryptic taxon. It produces off‐flavor and a large number of bioactive metabolites (n = 38) some of which can be toxic including four known microcystins. Using a polyphasic approach, we propose the establishment of the genus Odorella with the new species O. benthonica from material originally isolated from the California Aqueduct near Los Angeles.     

The genus Pseudo-nitzschia (Bacillariophyceae) in a temperate estuary with description of two new species: Pseudo-nitzschia plurisecta sp. nov. and Pseudo-nitzschia abrensis sp. nov.

The genus Pseudo‐nitzschia contains potentially toxic species of problematic taxonomy, making it one of the most intensively studied diatom genera. The study of 35 clonal strains isolated from the Bilbao estuary, an area that experiences recurrent blooms of Pseudo‐nitzschia, revealed the presence of two new species, P. abrensis and P. plurisecta, differing from their congeners in both morphology and gene sequence. The morphological features were analyzed by LM and EM, whereas molecular analyses were based on the internal transcribed spacer (ITS) and large subunit (LSU) regions of the rDNA. P. plurisecta appears closely related to P. cuspidata/P. pseudodelicatissima in the phylogenetic tree, whereas P. abrensis forms a moderately supported clade with P. heimii/P. subpacifica and P. caciantha/P. circumpora. Comparison of the secondary structure of ITS2 regions reveals marked differences in the most highly conserved regions among related taxa. Morphologically, the new species differ from their closest congeners in the arrangement of the poroid sectors and the density of valve striae and fibulae. The two species share similar pigment composition, and belong to the group of Pseudo‐nitzschia species containing only chlorophyll c₂ and c₃.     

Polyphasic approach using multilocus analyses supports the establishment of the new aerophytic cyanobacterial genus Pycnacronema (Coleofasciculaceae,Oscillatoriales)

A new Phormidium‐like genus was found during an investigation of Oscillatoriales diversity in Brazil. Eight aerophytic populations from south and southeastern regions were isolated in monospecific cultures and submitted to polyphasic evaluation. The populations presented homogeneous morphology with straight trichomes, not attenuated, and apical cell with thickened cell wall. Phylogenetic analyses based on 16S rRNA gene sequences showed that these populations, plus the Brazilian strain Phomidium sp. B‐Tom from GenBank, formed a highly supported and distinctive clade, which corresponds to the new genus Pycnacronema, comprising six new species: P. brasiliensis (type species), P. arboriculum, P. conicum, P. marmoreum, P. rubrum, and P. savannensis. These results were confirmed and supported by rpoC1 and rbcL genes evaluated independently and by the concatenated analysis of 16S rRNA, rpoC1 and rbcL genes (for all species but P. savannensis). Secondary structures of the D1‐D1′, box‐B, and V3 regions of the internal transcribed spacer were informative at specific level, being conserved in P. brasiliensis and variable among the other strains, also confirming the phylogenetic analyses. The generic name and specific epithets of the new taxa are proposed under the provisions of the International Code of Nomenclature of algae, fungi, and plants.     

Divergence in genital morphology may contribute to mechanical reproductive isolation in a millipede

Genitalia appear to evolve rapidly and divergently in taxa with internal fertilization. The current consensus is that intense directional sexual selection drives the rapid evolution of genitalia. Recent research on the millipede Antichiropus variabilis suggests that the male genitalia are currently experiencing stabilizing selection – a pattern of selection expected for lock‐and‐key structures that enforce mate recognition and reproductive isolation. Here, we investigate how divergence in genital morphology affects reproductive compatibility among isolated populations of A. variabilis. Females from a focal population were mated first to a male from their own population and, second, to a male from one of two populations with divergent genital morphology. We observed variation in mating behavior that might indicate the emergence of precopulatory reproductive barriers: males from one divergent population took significantly longer to recognize females and exhibited mechanical difficulty in genital insertion. Moreover, we observed very low paternity success for extra‐population males who were successful in copulating. Our data suggest that divergence in genital shape may be contributing to reproductive isolation, and incipient speciation among isolated populations of A. variabilis.     

New remains of the dryolestoid mammal <Emphasis Type="Italic">Leonardus cuspidatus</Emphasis> from the Los Alamitos Formation (Late Cretaceous,Argentina)

The great diversity of mammals from the Los Alamitos Formation (Campanian–Maastrichtian) in Río Negro Province, Argentina has provided significant information about the evolution of South American dryolestoids. Among these mammals the alleged dryolestid Leonardus cuspidatus Bonaparte was described based on a fragment of maxilla with four molariforms. A right mandibular fragment with two molariforms from the same site as the maxilla is here assigned to L. cuspidatus. The lower molariforms are compatible in the expected morphology with those from the holotype. Even though referred to Dryolestidae, Leonardus shows unique features: (1) the presence of a huge and dome-like stylocone, disconnected and more centrally placed from the other cusps than in other dryolestoids such as Groebertherium, but contacting the preparacrista in the first preserved molar; (2) the absence of cingulae in both upper and lower molars, the latter being similar in shape to the former; (3) the presence of three roots in at least one of the lower molars, which has only been documented in the mesungulatid Coloniatherium for the m1. These characters confirm the diversity and uniqueness of the South American Mesozoic mammals.     

Molecular evolution in Panagrolaimus nematodes: origins of parthenogenesis, hermaphroditism and the Antarctic species P. davidi

Background  

As exemplified by the famously successful model organism Caenorhabditis elegans, nematodes offer outstanding animal systems for investigating diverse biological phenomena due to their small genome sizes, short generation times and ease of laboratory maintenance. Nematodes in the genus Panagrolaimus have served in comparative development and anhydrobiosis studies, and the Antarctic species P. davidi offers a powerful paradigm for understanding the biological mechanisms of extreme cold tolerance. Panagrolaimus nematodes are also unique in that examples of gonochoristic, hermaphroditic and parthenogenetic reproductive modes have been reported for members of this genus. The evolutionary origins of these varying reproductive modes and the Antarctic species P. davidi, however, remain enigmatic.