Nematodes are important: parasitic nematodes threaten the health of plants, animals and humans on a global scale; interstitial nematodes pervade sediment and soil ecosystems in overwhelming numbers; and Caenorhabditis elegans is a favourite experimental model system. A lack of clearly homologous characters and the absence of an informative fossil record have prevented us from deriving a consistent evolutionary framework for the phylum. Here we present a phylogenetic analysis, using 53 small subunit ribosomal DNA sequences from a wide range of nematodes. With this analysis, we can compare animal-parasitic, plant-parasitic and free-living taxa using a common measurement. Our results indicate that convergent morphological evolution may be extensive and that present higher-level classification of the Nematoda will need revision. We identify five major clades within the phylum, all of which include parasitic species. We suggest that animal parasitism arose independently at least four times, and plant parasitism three times. We clarify the relationship of C. elegans to major parasitic groups; this will allow more effective exploitation of our genetic and biological knowledge of this model species.

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An inverse correlation between dispersal ability and genetic differentiation among populations of a species is frequently observed in the marine environment. We investigated the population genetic structure of the free-living marine nematode Pellioditis marina. A total of 426 bp of the mitochondrial cytochrome oxidase subunit 1 (COI) gene were surveyed on a geographical scale of approximately 100 km during spring 2003. Nematodes were collected from 2 coastal locations in Belgium, and from 2 estuaries and a saltwater lake (Lake Grevelingen) in The Netherlands. Molecular variation was assessed with the single-strand conformation polymorphism (SSCP) method. In total, 32 different haplotypes were observed, and sequence divergence among 452 individuals ranged from 0.2 to 10.6%. We discovered 4 distinct mitochondrial lineages, with low divergences within the lineages (0.2 to 1.6%) and high divergences between the lineages (5.1 to 10.6%). The nuclear ribosomal ITS (internal transcribed spacer) region showed concordant phylogenetic patterns, suggesting that nematode species diversity may be considerably underestimated. Analysis of molecular variance (AMOVA) indicated a strong genetic differentiation among populations. The Lake Grevelingen population was clearly differentiated from all other populations, but genetic structuring was also significant within the Westerschelde and was correlated with gradients in salinity and pollution. The observed population genetic structure is in accordance with the limited active dispersal capacity of P. marina, but is at variance with its significant potential for passive dispersal. We therefore suggest that autecological characteristics, including short generation time, high colonization potential and local adaptation, may be at the basis of this nematode's population genetic structure.KEY WORDS: Pellioditis marina · Single-strand conformation polymorphism · Cryptic species · Population genetic structure Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 300: [91][92][93][94][95][96][97][98][99][100][101][102][103] 2005 influencing gene flow in marine species are roughly divided into physical (e.g. ocean currents, habitat characteristics) and biological (e.g. spawning season, predation, larval and adult behavior) categories (Hohenlohe 2004). These characteristics limit the dispersal abilities of planktonic larvae, and render marine environments less open than previously thought. Furthermore, the issue of spatial scale may further complicate the discussion about open versus closed marine populations (Cowen et al. 2000, Camus & Lima 2002.In addition, population genetic surveys have also revealed that many marine 'species' are in fact species complexes involving morphologically cryptic taxa (Knowlton 1993, Todaro et al. 1996, Bond & Sierwald 2002, Mathews et al. 2002, McGovern & Hellberg 2003. Such complexes are especially prominent in small invertebrates with few taxonomically diagnostic characters (Rocha-Olivares et al. 2001). T...

We estimated the phylogeny of the order Odonata, based on sequences of the nuclear ribosomal genes 5.8 S, 18S, and ITS1 and 2. An 18S-only analysis resolved deep relationships well: the order Odonata, as well as suborders Zygoptera and Epiprocta (Anisoptera+Epiophlebia), emerged as monophyletic. Some other deep clades resolved well, but support for more recently diverged clades was generally weak. A second, simultaneous, analysis of the 5.8S and 18S genes with the intergenic spacers ITS1 and 2 resolved some recent branches better, but appeared less reliable for deep clades with, for example, suborder Anisoptera emerging as paraphyletic and Epiophlebia superstes recovered as an Anisopteran, embedded within aeshnoid-like anisopterans and sister to the cordulegastrids. Most existing family levels in the Anisoptera were confirmed as monophyletic clades in both analyses. However, within the corduliids that form a major monophyletic clade with the Libellulidae, several subclades were recovered, of which at least Macromiidae and Oxygastridae are accepted at the family level. In the Zygoptera, the situation is complex. The lestid-like family groups (here called Lestomorpha) emerged as sister taxon to all other zygopterans, with Hemiphlebia sister to all other lestomorphs. Platystictidae formed a second monophylum, subordinated to lestomorphs. At the next level, some traditional clades were confirmed, but the tropical families Megapodagrionidae and Amphipterygidae were recovered as strongly polyphyletic, and tended to nest within the clade Caloptera, rendering it polyphyletic. Platycnemididae were also non-monophyletic, with several representatives of uncertain placement. Coenagrionids were diphyletic. True Platycnemididae and non-American Protoneurids are closely related, but their relationship to the other zygopterans remains obscure and needs more study. New World protoneurids appeared relatively unrelated to old world + Australian protoneurids. Several recent taxonomic changes at the genus level, based on morphology, were confirmed, but other morphology-based taxonomies have misclassified taxa considered currently as Megapodagrionidae, Platycnemididae and Amphipterygidae and have underestimated the number of family-level clades

Pinpointing processes that structure the geographical distribution of genetic diversity of marine species and lead to speciation is challenging because of the lack of obvious dispersal barriers and the likelihood of substantial (passive) dispersal in oceans. In addition, cryptic radiations with sympatric distributions abound in marine species, challenging the allopatric speciation mechanism. Here, we present a phylogeographical study of the marine nematode species complex Rhabditis (Pellioditis) marina to investigate processes shaping genetic structure and speciation. Rhabditis (P.) marina lives on decaying macroalgae in the intertidal, and may therefore disperse over considerable distances. Rhabditis (P.) marina consists of several cryptic species sympatrically distributed at a local scale. Genetic variation in the COI gene was screened in 1362 specimens from 45 locations around the world. Two nuclear DNA genes (ITS and D2D3) were sequenced to infer phylogenetic species. We found evidence for ten sympatrically distributed cryptic species, seven of which show a strong genetic structuring. A historical signature showed evidence for restricted gene flow with occasional long-distance dispersal and range expansions pre-dating the last glacial maximum. Our data also point to a genetic break around the British Isles and a contact zone in the Southern Bight of the North Sea. We provide evidence for the transoceanic distribution of at least one cryptic species (PmIII) and discuss the dispersal capacity of marine nematodes. The allopatric distribution of some intraspecific phylogroups and of closely related cryptic species points to the potential for allopatric speciation in R. (P.) marina.

Species identification in the phylum Nematoda is complicated due to the paucity of easily obtainable diagnostic morphological features. Furthermore, the cosmopolitan distribution of several species despite low dispersal abilities makes cryptic diversity potentially substantial within this phylum. We conducted a population genetic survey in the marine nematode Geomonhystera disjuncta in Belgium and The Netherlands in two seasons. The mitochondrial cytochrome oxidase c subunit 1 (COI) gene was screened with the single-strand conformation polymorphism method in 759 individuals. The 43 haplotypes were grouped into five lineages, with low divergences within (< 3%) and high divergences between lineages (> 14%). Analysis of the nuclear ITS region yielded concordant tree topologies, indicating the presence of five cryptic taxa within G. disjuncta. Analysis of Molecular Variance (AMOVA) illustrated a significant structuring in all lineages and temporal fluctuations in haplotype frequencies within and between locations. Metapopulation dynamics and/or priority effects best explained this structuring. Finally, our data indicate that the COI gene may be useful for DNA barcoding purposes.

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