Lance nematodes (Hoplolaimus spp.) are migratory ecto-endo plant-parasitic. They have been found from a wide range of the world that feed on the roots of a diversity of monocotyledonous and dicotyledonous plants, and have caused a great agricultural damage. Since more taxonomic knowledge and molecular references are demanded for the lance nematode phylogeny and population study, four chapters of lance nematode researches on three species were presented here: (1) A new species, Hoplolaimus smokyensis n. sp., was discovered from a mixed forest sample of maple (Acer sp.), hemlock (Tsuga sp.) and silverbell (Halesia carolina) from the Great Smoky Mountains National Park. It is characterized by possession of a lateral field with four incisures, an excretory pore posterior to the hemizonid, esophageal glands with three nuclei, phasmids anterior and posterior to the vulva, and the epiptygma absent. Phylogenetic analyses based on ribosomal and mitochondrial gene sequences also suggest H. smokyensis n. sp. to be an independent lineage distinct from all other reported Hoplolaimus species. (2) Additional morphological characteristics of Hoplolaimus columbus were described. Photos of its esophageal gland cell nuclei, a H. columbus male and abnormal female tails were presented. (3) The first complete de novo assembly of mitochondrial genome of Hoplolaimus columbus using Whole Genome Amplification and Illumina MiSeq technique was reported as a circularized DNA of 25228bp. The annotation results using two genetic codes were diagnosed and compared. Including H. columbus, phylogenetic relationships, gene content and gene order arrangement of 92 taxa nematodes were analyzed. (4) The phylogenetic informativeness of mitochondrial genes in Nematoda phylum is analyzed with two quantitative methods using mitochondrial genomes of 93 nematode species, including H. columbus and H. galeatus. Results from both methods agree with each other, indicate that the nad5 and nad4 contain higher informativeness than other candidates. Traditional markers like the cox1 and cytb genes contain medium informativeness. The nad4l and nad3 contain the lowest informativeness comparing with other protein-coding genes. Results also indicate that the phylogenetic informativeness is independent of the molecular sequence length of a phylogenetic marker. Concatenated-genes marker could present better phylogenetic informativeness if selected genes are higher informative
