GENETIC DIVERSITY AND COMPARATIVE MITOCHONDRIAL GENOMICS OF ROOT-KNOT NEMATODES (MELOIDOGYNE SPP.)
Humphreys Pereira, Danny Antonio
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The nematode genus Meloidogyne comprises approximately 100 species with a high level of reproductive plasticity, varied environmental preferences and host specificity. Knowledge of the genetic diversity in this genus is essential to avoid misidentifications of quarantine species, required for effective breeding programs and will help guide nematode control strategies. In Chapters 2 and 3, the four currently described types of the Columbia root-knot nematode M. chitwoodi were characterized using light and scanning electron microscopy, isozyme analysis and molecular markers. Significant morphological variation was recognized and molecular markers were demonstrated to be stable and reliable diagnostic tools. Population analyses using a mtDNA marker revealed limited genetic differentiation possibly indicative of gene flow. In Chapter 4, a new Meloidogyne species associated with coffee in southern Costa Rica was described and compared to other Meloidogyne species infecting coffee. Morphological features of M. lopezi differed from other coffee-associated species mainly in female lips, male body length, stylets and second-stage juvenile body and tail morphology. A novel esterase phenotype was observed in M. lopezi that can be used to discriminate among species. In addition, a PCR-RFLP system was developed to differentiate Meloidogyne species parasitizing coffee. Phylogenetic analyses positioned M. lopezi close to other tropical Meloidogyne spp. that infect coffee in Central and South America such as M. arabicida, M. izalcoensis and M. paranaensis. The mt genomes of five Meloidogyne species were sequenced to study the evolutionary history of Meloidogyne within the phylum Nematoda in Chapters 5 and 6. This genome data was used for a comparative mitochondrial analysis between species with different reproductive modes and sampled from different environments. Architecture of the mt genomes and phylogenies based on 12 protein coding genes supported the hypothesis that the migratory endoparasitic nematode genus Pratylenchus shares a recent common ancestor with Meloidogyne. The results also suggested that the sedentary endoparasitic nematodes, Meloidogyne and Heterodera evolved through convergent evolution. In addition, the tropical and mitotic parthenogenetic species, M. arenaria, M. enterolobii, M. incognita and M. javanica were placed in the same monophyletic group, and the meiotic parthenogenetic species, M. chitwoodi and M. graminicola in a different group.