Multiple independent adaptations to different ranges of host plants indicate high adaptability despite clonal reproduction in the nematode pest Meloidogyne incognita

Abstract

The most devastating nematodes to worldwide agriculture are the root-knot nematodes with Meloidogyne incognita being the most widely distributed and damaging species. This parasitic and ecological success seem surprising given its supposed obligatory clonal reproduction. Clonal reproduction has been suspected based on cytological observations but, so far, never confirmed by population genomics data. At the species level, M. incognita is highly polyphagous with thousands of host plants. However, the host range varies among different M. incognita isolates that may present distinct and more restricted host compatibilities. Historically, four host races had been defined as a function of ranges of compatible and incompatible plants. We sequenced the genomes of 11 isolates across Brazil, covering these four distinct races to assess (i) how clonal reproduction is and (ii) how the level of genome variability associates with biological traits such as the host races, affected agronomic culture, and geographical distribution. By aligning the genomic reads of the isolates to the M. incognita reference genome assembly, we identified SNV and small-scale insertions/deletions. Analysis of linkage disequilibrium and 4-gametes test, showed no sign of recombination, confirming the clonal mode of reproduction of M. incognita. We showed that there are relatively few point variations between the different isolates, and these variations show no significant association with either the host races, the geographical origin of the samples or the host plant on which they have been collected. Due to the lack of phylogenetic signal underlying their existence, we recommend the discontinuation of the terminology race. Overall, these results suggest that multiple gains and losses of parasitic abilities and adaptations to different environmental conditions account for the broad host spectrum and wide geographic distribution of M. incognita. Hence, this nematode constitutes a model species to study adaptability without sexual recombination and overall low genomic variations in animals.