Nematodes are considered among the most difficult animals to identify. DNA-based diagnostic methods have already gained acceptance in applications ranging from quarantine determinations to assessments of biodiversity. Researchers are currently in an information-gathering mode, with intensive efforts applied to accumulating nucleotide sequence of 18S and 28S ribosomal genes, internally transcribed spacer regions, and mitochondrial genes. Important linkages with collateral data such as digitized images, video clips and specimen voucher web pages are being established on GenBank and NemATOL, the nematode-specific Tree of Life database. The growing DNA taxonomy of nematodes has lead to their use in testing specific short sequences of DNA as a "barcode" for the identification of all nematode species.
There are many nematode species that, following formal description, are seldom mentioned again in the scientific literature. Lobocriconema thornei and L. incrassatum are two such species, described from North American forests, respectively 37 and 49 years ago. In the course of a 3-year nematode biodiversity survey of North American ecoregions, specimens resembling Lobocriconema species appeared in soil samples from both grassland and forested sites. Using a combination of molecular and morphological analyses, together with a set of species delimitation approaches, we have expanded the known range of these species, added to the species descriptions, and discovered a related group of species that form a monophyletic group with the two described species. In this study, 148 specimens potentially belonging to the genus Lobocriconema were isolated from soil, individually measured, digitally imaged, and DNA barcoded using a 721 bp region of cytochrome oxidase subunit 1 (COI). One-third of the specimens were also analyzed using amplified DNA from the 3' region of the small subunit ribosomal RNA gene (18SrDNA) and the adjacent first internal transcribed spacer (ITS1). Eighteen mitochondrial haplotype groups, falling into four major clades, were identified by well-supported nodes in Bayesian and maximum likelihood trees and recognized as distinct lineages by species delimitation metrics. Discriminant function analysis of a set of morphological characters indicated that the major clades in the dataset possessed a strong morphological signal that decreased in comparisons of haplotype groups within clades. Evidence of biogeographic and phylogeographic patterns was apparent in the dataset. COI haplotype diversity was high in the southern Appalachian Mountains and Gulf Coast states and lessened in northern temperate forests. Lobocriconema distribution suggests the existence of phylogeographic patterns associated with recolonization of formerly glaciated regions by eastern deciduous forest, but definitive glacial refugia for this group of plant parasitic nematodes have yet to be identified. Unlike agricultural pest species of plant-parasitic nematodes, there is little evidence of long-distance dispersal in Lobocriconema as revealed by haplotype distribution. Most haplotype groups were characterized by low levels of intragroup genetic variation and large genetic distances between haplotype groups. The localization of nematode haplotypes together with their characteristic plant communities could provide insight into the historical formation of these belowground biotic communities.
Fruit production in the Okanagan Valley of British Columbia is dominated by apple, sweet cherry, and wine grape. The relative importance of sweet cherry and grape has increased in recent decades, but little was known of the plant-parasitic nematodes associated with those crops. Soil samples analyzed for plant-parasitic nematodes were collected from a total of 39 apple orchards, 61 cherry orchards, and 57 vineyards; most were collected in 2018, but 36 cherry orchards were sampled in 2012. Soil properties were also assessed and related to nematode population densities. Nematode genera of potential significance were, in order of prevalence, Pratylenchus, Mesocriconema, Xiphinema, Paratylenchus, Paratrichodorus, Hemicycliophora, and Meloidogyne. Pratylenchus were found in 79, 98, and 81% of the apple, cherry, and grape plantings, respectively; Mesocriconema were found in 51, 79, and 82%; and Xiphinema were found in 59, 51, and 77%. Population densities of the three dominant genera were influenced more by soil texture than any other soil characteristics, with Pratylenchus being negatively correlated with percentage clay, Mesocriconema positively correlated with percentage sand, and Xiphinema positively correlated with percentage silt. The high prevalence of Mesocriconema in cherry orchards and vineyards in this region is significant because Mesocriconema is known to be an important pest of other Prunus crop species and grapevines in other regions. This study therefore provides a rationale for increasing grower awareness and research efforts on the impacts and management of Mesocriconema and other plant-parasitic nematodes in orchards and vineyards in the region.
Specimens of Heterodera have been collected from alfalfa fields in Kearny County, Kansas and Carbon County, Montana. DNA barcoding with the COI mitochondrial gene indicate that the species is not Heterodera glycines, soybean cyst nematode, H. schachtii, sugar beet cyst nematode, or H. trifolii, clover cyst nematode. Maximum likelihood phylogenetic trees show that the alfalfa specimens form a sister clade most closely related to H. glycines, with a 4.7% mean pairwise sequence divergence across the 862 nucleotides of the COI marker. Morphological analyses of juveniles and cysts conform to the measurements of H. medicaginis, the alfalfa cyst nematode originally described from the USSR in 1971. Initial host testing demonstrated that the nematode reproduced on alfalfa, but not on soybeans, tomato, or corn. Collectively, the evidence suggests that this finding represents the first record of H. medicaginis in North America. Definitive confirmation of this diagnosis would require COI sequence of eastern European isolates of this species.
This case reinforces the concept of coevolution as a reciprocal change in genetic structure between or among two or more populations, by having students analyze and interpret data, build a descriptive model of the system, and use data to make scientific arguments. The case study is designed for a single 50-minute class period after students have completed a brief pre-class reading assignment introducing coevolution. Students analyze evidence for interactions among red squirrels (Tamiasciurus hudsonicus), red crossbills (Loxia curvirostra), and lodgepole pines (Pinus contorta v. latifolia). The case describing the interactions among these species invites students to answer three questions: 1) What evidence is required for demonstrating coevolution? 2) What specific evidence supports the conclusion that that red squirrels, red crossbills, and lodgepole pines are coevolving (or not) in this system? 3) Why does the evidence support coevolution (or not)? In this discussion-and jigsaw-based case study, students advance both their core conceptual knowledge and their proficiency with scientific practices.
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