Genetic And Molecular Analysis Of The Tomato Root Knot Nematode Resistance Locus Mi 1

Author : M. Dautova
Publisher :
ISBN 13 :
Total Pages : 126 pages
Book Rating : 4.:/5 (78 download)

DOWNLOAD NOW!

Download or read book Population and Molecular Genetics of Root-knot Nematodes written by M. Dautova and published by . This book was released on 2001 with total page 126 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nematodes constitute the largest and most ubiquitous physlum of the animal kingdom. They live predominantly in fresh water and soil. In soils they can be present in numbers ranging from 1.8 to 120 millions per square meter. Only a minority is parasitic on species from the plant kingdom. Plant parasitic nematodes are studiedmainly because of the economic damage they cause in crops as a result from morphological and physiological abnormalities induced in plants. The root-knot nematodes belong to the genus Meloidogyne (Goeldi, 1892) and constitute a major group of plant parasites of economic importance. Their wide distribution due to their ability to survive in diverse environmental conditions enamore than 2,000 plant species. The annual losses caused by root-knot nematodes are in the range of 10% worldwide. The damage inflicted in certain regions of developing countries may be as much as 25-50%. At present there are 80 nominal root-knot nematode species described. The four most common species, M. incognita, M. javanica, M. arenaria and M. hapla, are responsible for nearly 90% of the losses caused by root knot nematodes. One way of environmentally satisfactory control is growth of resistant varieties but virulent populations that overcome resistance occur and hamper the use of resistant cultivars. In spite of economic importnace of the tomato crop in Macedonia, knowledge on the distribution of Meloidogyne spp. is scarce and even completely absent if intraspecific variations are considered. These omissions seriously impede implementation of proper management system the distribution of Meloidogyne populations virulent and avirulent on tomato cultivars bearing the Mi-gene for resistance was assessed. Capter 2 - Seventy-three isolates from 9 locations (fields and glassHouses) were identified and their behaviour on two susceptible and two Mi resistant tomato cultivars was compared. M. incognita (47, 9%) and M. javanica (35.6%) are thepredominant species followed by M. arenaria (13,7%), and, sporadically, M. hapla (2,7%) was found in M. incognita (11%) and M. javanica (46%) as well as in M. arenaria(50%) infestations. M. hapla isolates were virulent on all tomato genotypes tested. The MI=gene does not confer resistance to M. hapla. The results presented in Chapter 2 show that growing Mi-resistant tomato cultivars in a cropping system is beneficial in case avirulent populations are present. A problem may be that after prolonged selection pressure virulent populations may arise from avirulent ones. Then combinations of control strategies are necessary, such as crop rotation, biological control and fallow, eventually extended with steam sterilisation or the use of traditional nematicides. Variability in the behaviour of a nematode population is a reflection of the genetic strutuvre of that population. Revealing the genetic information contained in the nematode's genome contributes to reliabre predictions of their behaviour in the field. The genome of the all animals consists of nuclear and mitochondrial DNA. Population genetic studies can be conducted by studying polymorphism at the DNA level of both subsets. In Chapter 3 - we have studied gentic variation in 16 M.incognita, M. javanica and M. arenaria, populations. In this chapter, genetic variation in the Meloidogyne spp. populations was assessed using amplified fragment length polymorphism (AFLP) markers in nuclear DNA and tandem repeats (TR) in mtDNA in order to reveal genetic divergence at different hierarchical levels. Even though the examined populations belong to obligatory mitotic parthenogenetic species genetic differences were detected within and among populations. The proportion of polymorphic fragments in the nuclear and mtDNA genomes respectively were lower in M. incognita (32% nad 20% respectively)as compared to portions in M. javanica (52% and 40% respectively) and M. arenaria (61% and 50% respectively). Genetic distances based on the AFLP patterns have assigned the populations into theree clusters comensurate with their respective species identities. The theree principle coordinate analysis situated M. incognita as the most genetically distinct of the three species. Polymorphisms generated by differences of mtDNA in the tandem array of 63-bp repeats, showed that M. incognita individuals were the most heteroplasmic, where as at the population level M. incognita was the most homogenous (13% overall diversity) followed by M.javanica (24%0 and M.arenaria (35%0. Comparing the intraspecific genetic distances based on nuclear and mtDNA markers has only revealid a positive correlation between both approaches with regard to the M. arenaria populations. Exploring the nematode's genome with molecular techniques may directly lead us to the genes involved in parasitism of plants. By definition these nematode genes are pivotal for host penetration, intercellular migration an feeding on plants. Their identification may help to design resistant strategies for pest control as they potentially make good targets for bioengineering anti-nematode strategies. In Chapter 4 - we have presented the analysis of 1,000 random sequences obtained from a cDNA library. These so-called expressed sequence tags (EST) have shown to be a powerful method to identify genes expressed at a certain time point of the nematode life cycle. The parasitic cycle involves various distinct stages, plant penetration and intercellular migration, and feeding site initiation and maintenance. There-fore, cDNA libraries convering these main stages may provide insight in the molecular fundaments of plant parasitism by root knot nematodes. In chapter 4 a cDNA library of preparasitic J2s of Meloidogyne incognita was used, which covered the initial phases of the parasitic cycle - plant penetration and intercellular migration. The ESTs were clustered into 9 functional groups. Candidate parasitism genes (3,1%) included all parasitism genes identified to date as well as novel ones. A more challenging group is the class 'Pioneering sequences' (33,9% of the ESTs). Several analytical steps that may help in assigning a function to these novel sequences are discussed. One of the expressed sequence tag was categorised into the Candidate parasitism genes group because it showed homology to a cell wall degrading enzyme - a xylanase (Chapter 5) The partial sequence of the EST was used to obtain a full-length transcript of 1220 nt encoding an open reading frame (Mi-Xy11) of 37.6 kDa. Hydrolase. Whole mount in situ hybridisation showed specific labelling of a Mi-xyll probe in the presence of two homologues in M. incognita whereas no hybridisation was found with genomic DNA fragments of Caenorhabditis elegans and cyst nematodes. Recombinant Mi-xyll protein, produced in Escherichia coli, exhibited hydrolytic activity on xylan and carboxymethylcellulose. The plant cell wall can be considered as an effective barrierthat protects the plant from invasion by pathogens and parasites. It is a highly organised network composed of different polysaccharides, proteins and phenolic compounds. Recently endoglucagenases were identified in plant-parasitic nematodes. In this thesis evidence is provided for the presence of hemicellulolytic enzymes in plant-parasitic nematodes. This finding suggests that plant parasitic nematodes make use of a suite of cell wall degrading enzymes with overlapping activities to faciliate plant invasion. Root knot nematodes are able to propagate on both monocots and dicots. It is hypothesised that the xylanases genes in these nematode species enable invasion of monocots, which have a significantly higher xyland content in the cell walls. Based on phylogenetic and hydrophobic cluster analysis the nematode xylanases seen to be closer related to bacterial xylanases than to homologyes in other animals, plants and fungi. This observation is commensurate with the findigns with pectinases and cellulases in nematodes, which suggests that this type of genes may have been acquired from bacteria by horizontal gene transfer. More lines of evidence are needed to support these hypotheses.