Practitioners of nematode taxonomy have rarely been explicit about what species represent or how data are being used to delimit species prior to their description. This lack of explicitness reflects the broader species problem common to all biology: there is no universally accepted idea of what species are and, as a consequence, scientists disagree on how to go about finding species in nature. However, like other biologists, nematologists seem to agree that species are real and discrete units in nature, and that they result from descent with modification. This evolutionary perspective provides a conceptual framework for nematologists to view species as independent evolutionary lineages, and provides approaches for their delimitation. Specifically, species may be delimited scientifically by methods that can test the hypothesis of lineage independence. For sequence data, such hypothesis testing should be based on sampling many individual organisms for multiple loci to avoid mistaking tokogeny and gene trees as evidence of species. Evolutionary approaches to analysing data and delimiting species avoid the inherent pitfalls in approaches that use all observed sequence differences to define species through calculation of a genetic distance. To illustrate evolutionary species delimitation, molecular data are used to test the hypothesis that hookworms parasitic in northern fur seals and in California sea lions represent separate species. The advantages and potential caveats of employing nucleotide sequence data for species delimitation are discussed, and the merits of evolutionary approaches are contrasted to inherent problems in similarity-based methods.
This zuta offers a presentation of the various printer’s marks used by Menasseh ben Israel, and especially the discovery of some sources in Dutch emblem literature for his most prominent and well-known mark and its motto, ‘Non odit tamen.’
Members of the family Cephalobidae (Nematoda) are among the most common and morphologically striking soil nematodes. Many members of Cephalobidae have extensive lip elaborations called probolae, but two taxonomically problematic genera, Acrobeloides and Cephalobus, have simple, low probolae. We sequenced a portion of the nuclear large subunit ribosomal DNA for 33 cultures of Acrobeloides and Cephalobus. A phylogenetic analysis of these data, plus sequences representing other members of Cephalobina, revealed a core clade of 22 closely related taxa, but did not represent Acrobeloides and Cephalobus as monophyletic. The dominant feature used in morphological taxonomy of Cephalobidae, the lip region, was homoplastic according to the molecular phylogenetic hypothesis. Contrary to previous suggestions, taxa with simple probolae have arisen multiple times from taxa with complex probolae. Cultures were also examined for mode of reproduction (presumed parthenogenetic vs sexual) and three morphological characters commonly used in generic diagnoses: the shape of the corpus in profile; the number of lateral incisures; and the terminal extent of the lateral field. Most cultures, including all 22 members of the core clade, lacked males and were presumed to be parthenogenetic, but several independent origins of sexually reproducing taxa were found. Of the morphological characters, only the corpus shape was consistent with the molecular phylogeny, however, the utility of this character is also questioned. Many genera with complex probolae were also paraphyletic, including Nothacrobeles, Zeldia and Cervidellus, indicating the need for more comprehensive phylogenies and a broad taxonomic revision of the family.
The genus Panagrellus currently comprises 12 known species. These nematodes have a worldwide distribution and have been found in a variety of habitats such as slime flux, thermal springs, insect frass and spoiled cider. Diagnosis of Panagrellus species is rather problematic since few morphological features can be used to discriminate between species and the original publications do not indicate the number of specimens measured and lack standard descriptions of variance. In this study we review the taxonomic status of several species from this genus combining classical morphological data and molecular sequences. Eleven live isolates and fixed material from currently available type specimens representing six Panagrellus species were included. Morphological analysis included the examination of qualitative and quantitative characters of males and females. The taxonomic utility of morphometric data was evaluated by means of multivariate statistics (principal component and canonical discriminant analyses). Phylogenetic inference was based on analysis of nucleotide sequences from the LSU rDNA gene and morphological characters. Parsimony tree topologies inferred from nucleotide datasets strongly supported monophyly of the P. dubius isolates, but not the P. redivivus isolates. Phylogenetic interpretation of these rDNA sequence data suggests that both the P. redivivus and P. dubius isolates each include more than a single species. Only two of the 15 morphological characters evaluated were variable within the ingroup taxa. A long spicule bifurcation length maps on the combined evidence trees as a putative synapmorphy for P. dubius, whereas male D% was homoplastic within isolates of that species. The diagnosis of the genus Panagrellus is emended.
A population of Placodira lobata from the USA is described using both light and scanning electron microscopy and compared with the descriptions of the type specimens of the same species. The phylogenetic relationships of the species are inferred from molecular data and places P. lobata in a clade that unites genera with mostly simple morphology of the labial region, like Cephalobus, Acrobeloides, Heterocephalobellus and Metacrobeles, but also includes species of Zeldia and Chiloplacus.
Acromoldavicus (Cephalobina, Cephaloboidea) with its highly distinctive lip region has only a single species, Acromoldavicus skrjabini Nesterov & Lisetskaya, originally described from Moldova and subsequently also detected at sites in the Middle East and near the Mediterranean. Herein, Acromoldavicus mojavicus n. sp. is described from sandy soil surrounding a Joshua tree (Yucca brevifolia) in a remote area of the Mojave Desert, California, USA. The lip region of A. mojavicus n. sp. is bilaterally symmetrical with three triangular probolae surrounded by three pairs of plate-like lips. The lip region is organised along similar lines as that of A. skrjabini, but differs in several respects, such as its larger size, presence of elongate posterior processes on each lip and division of the lateral lips into two lobes (excluding the dorso-sublateral guard processes). In addition, phylogenetic interpretation of sequence data from the large-subunit of ribosomal DNA provides further evidence for autapomorphies and separate species status for A. skrjabini and A. mojavicus n. sp. Characteristics shared with Cephaloboidea include the offset spermatheca and males with eight pairs of genital papillae. Both species of Acromoldavicus have a buccal capsule with a reduced gymnostom, a character that seems to be shared with the cephalobid Elaphonema and in part is a basis for placement of both genera in Elaphonematidae. The species A. mojavicus n. sp. exhibits additional similarities with Elaphonema spp. that further support this placement.
A detailed morphological description (light and scanning electron microscope observations) is given of two isolates currently used in molecular and developmental studies: PDL0024, Panagrobelus stammeri Rühm, 1956 and PDL0025, a new species herein described as Plectonchus hunti n. sp. We redescribe the lip region, interpret the orientation of the lips in P.stammeri and provide additional morphological and morphometric diagnostic features. Plectonchus hunti n. sp. is characterised by the bifurcating spicule tip and by the placement of the male genital papillae in a 3/3 + p + 2 arrangement. Additionally, females of this new species can be differentiated from other species of Plectonchus by the more anterior position of the vulva, the more anterior location of the excretory pore and the tail morphology. An emended diagnosis for this genus is provided. In this study we demonstrate that both studied taxa share morphological similarities, such as the presence of a 'cephaloboid-type' spermatheca. Our observations also indicate slight but consistent differences in cell composition of the female reproductive system between these two taxa.
Detailed descriptions are given of the amphimictic nematode strains PS1158, PS2052 and PS2160, which are unusual in that they only differ in predominant body handedness. Although these strains are morphologically identical in all other respects, published reproductive data and new DNA sequence data of the D2/D3 region of the large subunit rRNA gene show that they do represent two separate species. On the basis of comparison with type material, the left-handed strains PS1158 and PS2160 are identified as Acrobeloides bodenheimeri, and the right-handed strain PS2052 as A. camberenensis, which is re-instated as a valid species. A. bodenheimeri and its relatives exhibit various types of diagnostic and taxonomic problems at species level, and it is shown that D2/D3 sequence data provide an important new diagnostic tool for addressing these problems. Phylogenetic analysis shows that two right-handed parthenogenetic strains identified as A. maximus represent a third species which is more closely related to A. camberenensis than to A. bodenheimeri. Caracterisation morphologique et moleculaire de deux especes intersteriles de chiralite contraire (Nematoda: Cephalobidae) - Une description detaillee est donnee des souches amphimictiques de nematodes PS1158, PS2052 et PS2160, souches inhabituelles car differant par la chiralite du corps. Bien que ces souches soient morphologiquement identiques sous tout autre rapport, les resultats publies de tests de croisement et de nouvelles donnees concernant la sequence d'ADN de la region D2/D3 du gene de la grosse sous-unite d'ARN ribosomal montrent qu'elles representent en fait deux especes distinctes. Se fondant sur une comparaison avec le materiel type, les souches sinistres PS1158 et PS2160 sont identifiees comme Acrobeloides bodenheimeri et la souche dextre PS2052 comme A. camberenensis, ainsi retabli comme espece valide. A. bodenheimeri et les especes proches posent differents problemes diagnostiques et taxinomiques au niveau specifique, et nous montrons que les donnees de sequence D2/D3 fournissent un nouvel outil diagnostique important pour aborder ces problemes. L'analyse phylogenetique montre que deux souches parthenogenetiques dextres identifiees comme A. maximus representent en fait une troisieme espece, plus proche de A. camberenensis que de A. bodenheimeri.