Peter Mullin

Tropical nematode diversity: vertical stratification of nematode communities in a Costa Rican humid lowland rainforest

Comparisons of nematode communities among ecosystems have indicated that, unlike many organisms, nematode communities have less diversity in the tropics than in temperate ecosystems. There are, however, few studies of tropical nematode diversity on which to base conclusions of global patterns of diversity. This study reports an attempt to estimate nematode diversity in the lowland tropical rainforest of La Selva Biological Research Station in Costa Rica. We suggest one reason that previous estimates of tropical nematode diversity were low is because habitats above the mineral soil are seldom sampled. As much as 62% of the overall genetic diversity, measured by an 18S ribosomal barcode, existed in litter and understorey habitats and not in soil. A maximum‐likelihood tree of barcodes from 360 individual nematodes indicated most major terrestrial nematode lineages were represented in the samples. Estimated ‘species’ richness ranged from 464 to 502 within the four 40 × 40 m plots. Directed sampling of insects and their associated nematodes produced a second set of barcodes that were not recovered by habitat sampling, yet may constitute a major class of tropical nematode diversity. While the generation of novel nematode barcodes proved relatively easy, their identity remains obscure due to deficiencies in existing taxonomic databases. Specimens of Criconematina, a monophyletic group of soil‐dwelling plant‐parasitic nematodes were examined in detail to assess the steps necessary for associating barcodes with nominal species. Our results highlight the difficulties associated with studying poorly understood organisms in an understudied ecosystem using a destructive (i.e. barcode) sampling method.

Phylogenetic relationships of Nygolaimina and Dorylaimina (Nematoda: Dorylaimida) inferred from small subunit ribosomal DNA sequences

Phylogenetic reconstructions based on 18S rDNA sequence data indicate that Dorylaimida, comprising the suborders Nygolaimina and Dorylaimina, is a monophyletic lineage, but that there is a deep division within Nygolaimina, giving rise to the possibility that Nygolaimina is paraphyletic. A well-supported clade comprising members of the traditional orders Mermithida and Mononchida (including Bathyodontina) forms the sister taxon to the Dorylaimida. Inferred relationships within this clade indicate that Mermithida shares more recent common ancestry with Mononchina than does Bathyodontina. Vertebrate parasites within Dorylaimia (Dioctophymida and Trichinellida) are reconstructed in a sister-taxon relationship with the Mononchida/Dorylaimida lineage. The enigmatic order Isolaimida (represented by Isolaimium) appears to be ancestral to all other Dorylaimia sampled. Expanded taxon sampling for phylogenetic analyses of the subclass raises new possibilities for the reconstruction of hypothetical character states in the common ancestor of Dorylaimia.

Diversity and phylogenetic relationships within the spiral nematodes of Helicotylenchus Steiner, 1945 (Tylenchida: Hoplolaimidae) as inferred from analysis of the D2-D3 expansion segments of 28S rRNA gene sequences

The spiral nematodes of the genus Helicotylenchus are globally distributed and associated with the root system of diverse groups of plants in cultivated and uncultivated areas. Several species are considered serious parasites of crops. The identification of many Helicotylenchus species is not always reliable, in part because many species share very similar diagnostic characters and high intraspecific variation. To verify species identification of geographically distant populations of Helicotylenchus , we tested monophyly of some classical morphospecies and studied their phylogenetic relationships; specifically, we conducted sequence and phylogenetic analysis of 89 sequences of the D2-D3 expansion segments of 28S rRNA gene sequences from 54 Helicotylenchus isolates, including species identified as H. brevis , H. digonicus , H. dihystera , H. labiodiscinus , H. leiocephalus , H. martini , H. multicinctus , H. platyurus , H. pseudorobustus and H. vulgaris , together with three outgroup taxa. Phylogenetic analysis distinguished nine highly or moderately supported major clades within Helicotylenchus. Using the molecular approach we were able to confirm congruence with morphologicalbased identification of samples of H. dihystera and H. multicinctus . However, sequence and phylogenetic analysis using Bayesian inference and maximum parsimony analysis showed that isolates collected in different countries and morphologically identified as H. pseudorobustus , H. digonicus or H. vulgaris were each representative of several different and, sometimes, unrelated lineages. Further detailed comparative morphometrics and morphological studies will help to elucidate if there is some misidentification or if putative species actually comprise a complex of cryptic species. Molecular analysis also revealed that 14 samples were classified as representatives of 11 unidentified species. Molecular characterisation of known Helicotylenchus species especially, using samples collected from type localities, is needed for future reliable identification of species of this genus.

First Report of Columbia Root Knot Nematode (Meloidogyne chitwoodi) in Potato in Texas

Columbia root-knot nematode, Meloidogyne chitwoodi Golden et al. (1) was identified from potatoes, Solanum tuberosum L., collected from Dallam County, Texas in October 2000. Seed potatoes are the most likely source for this introduction. This nematode is currently found infecting potatoes grown in California, Colorado, Idaho, New Mexico, Nevada, Oregon, Utah, and Washington. Some countries prohibit import of both seed and table stock potatoes originating in states known to harbor M. chitwoodi. Lesions on the potatoes had discrete brown coloration with white central spots in the outer 1 cm of the tuber flesh. Female nematode densities averaged 3 per square centimeter of a potato section beneath the lesions. Nematodes were morphologically identified as M. chitwoodi based on the perineal pattern of mature females and the tail shape of juveniles per Golden et al. (1). Using polymerase chain reaction-RFLP of the rDNA ITS1 region and the mtDNA COII-16S rRNA region (2), individual juveniles were identified as M. chitwoodi based on their restriction fragment patterns. This is the first report of Columbia root-knot nematode infecting potatoes in Texas. The distribution of this nematode in potato fields throughout central United States should be determined. References: (1) A. N. Golden et al. J. Nematol. 12:319, 1980. (2) T. O. Powers and T. S. Harris. J. Nematol. 25:1, 1993.

Description of Mesocriconema ericaceum n. sp. (Nematoda: Criconematidae) and notes on other nematode species discovered in an ericaceous heath bald community in Great Smoky Mountains National Park, USA

A new species of Mesocriconema and a unique assemblage of plant-parasitic nematodes was discovered in a heath bald atop Brushy Mountain in Great Smoky Mountains National Park. Mesocriconema ericaceum n. sp., a species with males, superficially resembles M . xenoplax . DNA barcoding with the mitochondrial COI gene provided evidence of the new species as a distinct lineage. SEM revealed significant variability in arrangement of labial submedian lobes, plates, and anterior and posterior annuli. Three other nematodes in the family Criconematidae were characterised from the heath bald. Ogma seymouri , when analysed by statistical parsimony, established connections with isolates from north-eastern Atlantic coastal and north-western Pacific coastal wet forests. Criconema loofi has a southern Gulf Coast distribution associated with boggy soils. Criconema cf. acriculum is known from northern coastal forests of California. Understanding linkages between these species and their distribution may lead to the broader development of a terrestrial soil nematode biogeography.