Defying Muller's Ratchet?

Meloidogyne incognita in action

For most animal species sexual reproduction is favored over asexual reproduction. A proposed mechanism to explain this is Muller's ratchet which assumes that the genomes of an asexual population accumulate deleterious mutations in an irreversible manner. However, this negative effect may not be prevalent in organisms which, while they reproduce asexually, also undergo other forms of recombination. 

Root-knot nematodes (Meloidogyne spp.) exhibit a diversity of reproductive modes ranging from obligatory sexual to fully asexual reproduction. Intriguingly, the most widespread and devastating species to global agriculture are those that reproduce asexually, without meiosis.  Instead of hitting an evolutionary dead-end, these plant pests have a wider geographic range and can infect greater numbers of crops than sexual species. 

To investigate the reasons behind their success, researchers sequenced and assembled the genomes of the three most damaging root-knot nematodes and compared them to a sexual relative. The asexual genomes are large, with numerous duplicated regions resulting from past reproduction events where at least two individual genomes recently hybridized together. They detected signs of positive selection between these gene copies and confirmed functional divergence at the expression pattern level. The colleagues think that it is this peculiar hybrid genome structure that provide these nematodes with a potential for adaptation and plasticity and explains the paradoxical success in the absence of sex:

By analyzing and comparing their genomes, we provide large-scale evidence that these asexual nematodes underwent hybridization and are polyploid. Their duplicated hybrid genome architectures provide these nematodes with multi-copy genes showing diverged sequence and expression patterns where their sexual relatives have very closely related alleles. We suspect these multiple copies provide a reservoir to adapt to different environments and plant hosts, and constitute an evolutionary advantage over their sexual relatives (at least in the short term). Their intriguing parasitic success despite absence of sex could thus be due to their hybrid origin where they combined multiple genomes of adapted parasitic nematodes in one single species.

In addition, Transposable elements (TE) cover a ~1.7 times higher proportion of the genomes of the ameiotic asexual Meloidogyne compared to the sexual relative and might also participate in their plasticity. The intriguing parasitic success of asexually-reproducing Meloidogyne species could be partly explained by their TE-rich composite genomes, resulting from allopolyploidization events, and promoting plasticity and functional divergence between gene copies in the absence of sex and meiosis.

It becomes paramount to understand under what conditions these hybrids came to be. It is a scary thought that similar conditions could favor the rise of even more aggressive and devastating new hybrids.