Morphological and molecular taxonomy of Longidorus and Xiphinema (Nematoda: Longidoridae) occurring in Arkansas, United States

Some species in the genera Longidorus and Xiphinema are important plant parasites as well as vectors for plant nepoviruses. Species discrimination based primarily on morphology is often questionable and controversial because of the morphometric overlap and the lack of a phylogenetic framework for this group. In a survey from natural sites for longidorids, 828 soil samples were collected from 37 Arkansas counties in 1999–2001 in addition to some specimens collected earlier. Longidorid nematodes were recovered from 542 of the 828 soil samples (65.5%), usually with several mixed species. The species delimitation in this study was supported by combined data on morphology, statistical analysis and rDNA sequencing.; Sixteen species of Longidorus from 142 populations and 4 species of Xiphinema from 127 populations were found in Arkansas. The Longidorus species found are L. breviannulatus, L. crassus, L. diadecturus, L. fragilis, L. biformis n. sp., L. glycines n. sp., L. grandis n. sp., L. paralongicaudatus n. sp., L. paravineacola n. sp. and seven unidentified Longidorus species. The Xiphinema species found are X. americanum, X. bakeri, X. chambersi and X. krugi. In Arkansas, X. americanum, L. diadecturus and L. crassus are the most frequently occurring longidorids. L. breviannulatus, L. crassus and L. fragilis are new records for this state. A nematode database was designed to manage the information on the sample, identification results, slides and DNA sequences. A checklist of longidorids associated with their host plants in Arkansas is provided.; Means comparison, stepwise discriminant analysis and hierarchical cluster analysis on morphometrics were successfully applied in studying the interspecies and intraspecies variation, grouping and discriminating the species or populations of Longidorus. A dendrogram was generated by hierarchical cluster analysis for 131 published Longidorus species. The dendrogram was further tested using 83 populations belonging to 19 Longidorus species from Arkansas and various other locations. The cluster analysis best illustrated the grouping and morphometric relationships of the species and it may lead to species identification for unidentified Longidorus species and the description of new species.; Phylogenetic analyses using the DNA sequences of nuclear ribosomal DNA ITS1 and the 18S gene were conducted to construct evolutionary trees and evaluate taxonomic relationship among Longidorus and Xiphinema species. The aligned sequences of the ITS1 region including the 3′ end of the 18S rDNA gene and 5′ end of the 5.8S rDNA gene from each species ranged from 870 by to 1354 by for all longidorids. This fragment is highly variable, only 15.9% sequence were constant within longidorids examined. The length of the 18S rDNA gene ranged from 1698 by to 1772 by among longidorids, 87.1% of the nucleotides were constant. ITS1 has evolved much faster than 18S within the genus Longidorus and Xiphinema. Both intraspecies and interspecies sequence variations were observed in rDNA sequence. Phylogeny analysis based on rDNA sequences suggested 2 clades: species of Xiphinema and species of Longidorus, except for L. diadecturus in the cluster of Xiphinema which is an intermediate form between Longidorus and Xiphinema and could represent a separate genus. The phylogenetic analyses largely corresponded to the genera and species def...