Agronomy and Horticulture Department


Date of this Version

Summer 6-27-2011


A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Agronomy, (Plant Pathology), Under the Supervision of Professor Thomas O. Powers. Lincoln, Nebraska: June, 2011

Copyright 2011 Angel Ramirez-Suarez


The False Root-Knot nematode Nacobbus aberrans is a plant parasitic nematode that causes yield losses in several crops and plant protection agencies have established regulations to prevent infestations from spreading to new agricultural areas. The taxonomic status of the nematode has been the subject of controversy due to wide degree of variation exhibited by the species. This variability has led to the suggestion that N. aberrans is actually a species complex rather than a single species. In order to test this hypothesis, we compared twelve N. aberrans populations, which include isolates from western Nebraska, two distinct regions of Mexico and the lowlands of Argentina. The specimens were obtained from different hosts and analyzed by physiological, morphological, phylogenetic approaches. The physiological response of the nematode cultured on a common host together with the variation detected in the multivariate morphometric analysis revealed a slight tendency to differentiate groups. This intraspecific variation was confirmed and clarified using phylogenetic inference of nuclear and mitochondrial markers. Analysis of these molecular markers resolved four groups that displayed a well-defined geographic pattern of distribution. The following discrete groupings were recognized: I. Nebraska, II. Mexican North region, III. Mexican Central region, IV. The Argentinean lowlands. The median-joining network analysis of mitochondrial marker was congruent with the four geographic and genetic lineages and separated from each other by a genetic distance of 63-92 mutational steps. AMOVA analysis of the four groups revealed significant differences among the hierarchical levels, with the vast majority of the genetic variation concentrated among the four geographic groups. A character-based barcode analysis allowed identification of “pure” diagnostic characters for each group that contributed further evidence that N. aberrans is a species complex composed of at least four species. The population from Nebraska meets the criteria for consideration as a separate species. We recommend that this isolate be reinstated to N. batatiformis Thorne and Schuster, 1956 as it was originally described.

Advisor: Thomas O. Powers