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The Success of Nematodes in the Absence of Sex Partly Explained

Root-knot nematodes are devastating species to global agriculture; those that reproduce asexually are especially thriving. Researchers from CEA-Genoscope and their partners proposed a hypothesis to explain this extraordinary success.
Published on 12 June 2017
Responsible for more than $100 billion of crop loss each year, plant-parasitic nematodes can devastate entire crops with root-knot nematodes (genus Meloidogyne) being the most damaging species. They exhibit a diversity of reproductive modes ranging from sexual to asexual. Against all odds, the most widespread and devastating species are those that strictly reproduce asexually, without taking advantage of the mix of genes produced through sexual reproduction.

Researchers from INRA, Université Côte d'Azur, Genoscope (CEA) and CNRS have set out to understand what causes this unexpected success. They have highlighted notable differences between the genomes of nematodes multiplying strictly asexually, and those of others. The genomes of the former were found to be three to five times larger than that of the nematodes that reproduce sexually—between 185 and 300 megabases (Mb) instead of 50 to 60 Mb.

The same cell contains several copies of these genomes (three to four copies) with very strong divergence. The analysis of the evolutionary history of these genome copies shows that they are a result of hybridization events. Unlike the high divergence in the nuclear genome within a single species, there is low divergence in the mitochondrial genome between different nematode species. This finding suggests that these hybrids share a recent common maternal ancestor.

By exploring the functional consequences of the hybrid origin of nematodes reproducing asexually, the team of scientists has shown that the structure of their genome may have a significant functional impact, potentially contributing to their success: in these nematodes, more than 60% of the copies of genes present in the duplicated regions exhibit different expression profiles for proteins whose biochemical functions are different.

Where the sexuated species have two almost identical alleles of the same gene, the asexual species generally have three to four copies that are very divergent in terms of sequence, and potentially, in terms of function. Moreover, half of their genomes is composed of transposable elements—repeated mobile DNA sequences capable of generating mutations by shifting and, consequently, strongly contributing to the modifications undergone by the genome, compared to only one third for that of the nematodes that can reproduce sexually.

This result was the subject of a press release.

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