The anhydrobiotic potential and molecular phylogenetics of species and strains of Panagrolaimus (Nematoda, Panagrolaimidae).

Members of the genus Panagrolaimus are bacterial-feeding nematodes that occupy a diversity of niches ranging from Antarctic and temperate soils to terrestrial mosses. Some members of this genus are able to survive extreme desiccation by entering into a state of suspended animation known as anhydrobiosis. We have assembled a collection of Panagrolaimus species and strains and have investigated their anhydrobiotic phenotypes. Our data show that within the genus Panagrolaimus there is a continuum of strains ranging from those unable to survive exposure to low relative humidity (RH) without prior preconditioning at high RH (slow desiccation strategists), through strains that have limited ability to survive rapid desiccation but whose anhydrobiotic ability improves upon preconditioning, to strains such as P. superbus that can readily survive immediate exposure to severe desiccation (fast desiccation strategists). Using this panel of nematodes we investigated the effect of preincubation at high RH on the accumulation of trehalose and on the nematodes' anhydrobiotic potential. We found that there is a strong correlation between trehalose induction and anhydrobiotic survival in Panagrolaimus. Furthermore, the high trehalose levels observed in fully hydrated P. superbus (10% dry mass) suggest that constitutive expression of trehalose pre-adapts this fast dehydration strategist to combat desiccation. All the strains observed, regardless of survival rates, undertook both coiling and clumping, which has the effect of reducing surface area and slowing the rate of water loss during desiccation. Phylogenetic analyses were carried out to investigate whether the observed anhydrobiotic phenotypes were the result of convergent evolution or represented a single phylogenetic lineage. These analyses, derived from alignments of the rDNA ITS and D3 sequences, indicate that the strongly anhydrobiotic strains of Panagrolaimus form a single phylogenetic lineage, which is separate from the weakly anhydrobiotic strains. The weakly anhydrobiotic strains are also phylogenetically divergent from each other. Our data indicate that Panagrolaimus has the potential to be an excellent model system for the investigation of molecular aspects of nematode anhydrobiosis.