Abstract
Three closely related parthenogenetic species of root-knot nematodes, collectively termed the Meloidogyne incognita-group, are economically significant pathogens of diverse crop species. Remarkably, these asexual root-knot nematodes are capable of acquiring heritable changes in virulence even though they lack sexual reproduction and meiotic recombination. Characterization of a near isogenic pair of M. javanica strains differing in response to tomato with the nematode resistance gene Mi-1 showed that the virulent strain carried a deletion spanning a gene called Cg-1. Herein, we present evidence that the Cg-1 gene lies within a member of a novel transposable element family (Tm1; Transposon in Meloidogyne-1). This element family is defined by composite terminal inverted repeats of variable lengths similar to those of Foldback (FB) transposable elements and by 9 bp target site duplications. In M. incognita, Tm1 elements can be classified into three general groups: 1) histone-hairpin motif elements; 2) MITE-like elements; 3) elements encoding a putative transposase. The predicted transposase shows highest similarity to gene products encoded by aphids and mosquitoes and resembles those of the Phantom subclass of the Mutator transposon superfamily. Interestingly, the meiotic, sexually-reproducing root-knot nematode species M. hapla has Tm1 elements with similar inverted repeat termini, but lacks elements with histone hairpin motifs and contains no elements encoding an intact transposase. These Tm1 elements may have impacts on root-knot nematode genomes and contribute to genetic diversity of the asexual species.
Highlights
Root-knot nematodes (RKNs), comprising the genus Meloidogyne, are obligate parasites causing damage to plant roots [1] and weakening plants by acting as carbon sinks [2,3]
Cg-1 locus (GenBank EU214531.1) [24] to query the published genome sequence of M. incognita, a RKN species closely related to M. javanica
We noted that sequences 59 of the M. javanica Cg-1 transcript are present on contig 1763 in an inverted-repeat orientation flanked by 9 bp direct repeats, suggestive of a transposable element (Figure 1A)
Summary
Root-knot nematodes (RKNs), comprising the genus Meloidogyne, are obligate parasites causing damage to plant roots [1] and weakening plants by acting as carbon sinks [2,3]. All three are parthenogenetic species in which the eggs are products of mitosis and develop without fertilization [5,6,7] These asexual species are closely related and are thought to have originated from interspecific hybridization followed by the loss of meiosis [8,9,10]. Despite their lack of meiosis, this group of RKNs is evolutionarily successful, capable of parasitizing thousands of plant species and acquiring heritable virulent phenotypes [7]. The mechanisms by which these asexual RKN species acquire the ability to parasitize additional host taxa are unknown, but genomic change mediated by transposable elements and repetitive sequences has been proposed [11,12]
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