Signatures of the Evolution of Parthenogenesis and Cryptobiosis in the Genomes of Panagrolaimid Nematodes

Most animal species reproduce sexually and fully parthenogenetic lineages are usually short lived in evolution. Still, parthenogenesis may be advantageous as it avoids the cost of sex and permits colonization by single individuals. Panagrolaimid nematodes have colonized environments ranging from arid deserts to Arctic and Antarctic biomes. Many are obligatory meiotic parthenogens, and most have cryptobiotic abilities, being able to survive repeated cycles of complete desiccation and freezing. To identify systems that may contribute to these striking abilities, we sequenced and compared the genomes and transcriptomes of parthenogenetic and outcrossing panagrolaimid species, including cryptobionts and non-cryptobionts. The parthenogens are triploids, most likely originating through hybridization. Adaptation to cryptobiosis shaped the genomes of panagrolaimid nematodes and is associated with the expansion of gene families and signatures of selection on genes involved in cryptobiosis. All panagrolaimids have acquired genes through horizontal gene transfer, some of which are likely to contribute to cryptobiosis.

A role for the Parkinson's disease protein DJ-1 as a chaperone and antioxidant in the anhydrobiotic nematode Panagrolaimus superbus

Mutations in the human DJ-1/PARK7 gene are associated with familial Parkinson's disease. DJ-1 belongs to a large, functionally diverse family with homologues in all biological kingdoms. Several activities have been demonstrated for DJ-1: an antioxidant protein, a redox-regulated molecular chaperone and a modulator of multiple cellular signalling pathways. The majority of functional studies have focussed on human DJ-1 (hDJ-1), but studies on DJ-1 homologues in Drosophila melanogaster, Caenorhabditis elegans, Dugesia japonica and Escherichia coli also provide evidence of a role for DJ-1 as an antioxidant. Here, we show that dehydration is a potent inducer of a dj-1 gene in the anhydrobiotic nematode Panagrolaimus superbus. Our secondary structure and homology modelling analyses shows that recombinant DJ-1 protein from P. superbus (PsuDJ-1.1) is a well-folded protein, which is similar in structure to the hDJ-1. PsuDJ-1.1 is a heat stable protein; with T1/2 unfolding transition values of 76 and 70 °C obtained from both circular dichroism (CD) and Fourier transform infrared spectroscopy (FTIR) measurements respectively. We found that PsuDJ-1.1 is an efficient antioxidant that also functions as a 'holdase' molecular chaperone that can maintain its chaperone function in a reducing environment. In addition to its chaperone activity, PsuDJ-1.1 may also be an important non-enzymatic antioxidant, capable of providing protection to P. superbus from oxidative damage when the nematodes are in a desiccated, anhydrobiotic state.

Expression profiling and cross-species RNA interference (RNAi) of desiccation-induced transcripts in the anhydrobiotic nematode Aphelenchus avenae

BACKGROUND

Some organisms can survive extreme desiccation by entering a state of suspended animation known as anhydrobiosis. The free-living mycophagous nematode Aphelenchus avenae can be induced to enter anhydrobiosis by pre-exposure to moderate reductions in relative humidity (RH) prior to extreme desiccation. This preconditioning phase is thought to allow modification of the transcriptome by activation of genes required for desiccation tolerance.

RESULTS

To identify such genes, a panel of expressed sequence tags (ESTs) enriched for sequences upregulated in A. avenae during preconditioning was created. A subset of 30 genes with significant matches in databases, together with a number of apparently novel sequences, were chosen for further study. Several of the recognisable genes are associated with water stress, encoding, for example, two new hydrophilic proteins related to the late embryogenesis abundant (LEA) protein family. Expression studies confirmed EST panel members to be upregulated by evaporative water loss, and the majority of genes was also induced by osmotic stress and cold, but rather fewer by heat. We attempted to use RNA interference (RNAi) to demonstrate the importance of this gene set for anhydrobiosis, but found A. avenae to be recalcitrant with the techniques used. Instead, therefore, we developed a cross-species RNAi procedure using A. avenae sequences in another anhydrobiotic nematode, Panagrolaimus superbus, which is amenable to gene silencing. Of 20 A. avenae ESTs screened, a significant reduction in survival of desiccation in treated P. superbus populations was observed with two sequences, one of which was novel, while the other encoded a glutathione peroxidase. To confirm a role for glutathione peroxidases in anhydrobiosis, RNAi with cognate sequences from P. superbus was performed and was also shown to reduce desiccation tolerance in this species.

CONCLUSIONS

This study has identified and characterised the expression profiles of members of the anhydrobiotic gene set in A. avenae. It also demonstrates the potential of RNAi for the analysis of anhydrobiosis and provides the first genetic data to underline the importance of effective antioxidant systems in metazoan desiccation tolerance.