worm~lab/BioC2
University of Cologne
Title: Virgins frozen in time: evolution of animal biodiversity in extreme environments
Abstract: Animals have evolved different strategies to cope with harsh environmental conditions. These include cryptobiosis, which can, for example, be found in some tardigrades, rotifers, and nematodes. Under desiccation or freezing cryptobiotic soil organisms arrest their metabolism completely, lose almost all cellular water, and reside in that state until favourable conditions return. As an extreme example, we revived one such nematode from Siberian permafrost after 46,000 years. The species is also parthenogenetic, and this combination of traits is quite common: cryptobiosis and asexuality have arisen together repeatedly across the invertebrate tree of life.
Nematodes globally make up much of the diversity of soils. Around 28,000 nematode species are formally described against estimates running into the millions, and fewer than 500 genomes are public. My group works on this gap, studying nematode biodiversity across the globe, with a particular focus on extreme environments, like the Atacama and the Namib deserts or the Australian Outback.
In the Atacama, the driest non-polar desert on earth, we sampled nematode communities along gradients of aridity and elevation. We find that climate and reproductive mode together shape these communities, with parthenogenetic lineages dominating the harshest sites. We are now polarising these findings by looking at the Namib, a younger and slightly less dry west-coast desert.
Some of the asexual lineages are also triploid. Polyploidy is widespread across the tree of life, but not very common in Metazoa. Consequences for genome architecture, adaptation and organismal resilience remain poorly understood in polyploid animals. Triploid parthenogenetic species present a particularly fascinating system, as there are three divergent homeologous copies of every gene, no meiotic recombination to homogenise them, and instant fixation of genomic states. We study this from an evolutionary perspective using Panagrolaimus nematodes, a genus in which triploidy arose through hybridisation independently twice and is invariably associated with parthenogenesis, while the sexual species in the genus are diploid. We generated phased chromosome-level assemblies resolving diploid genomes of sexuals and all three homeologs in asexuals. These reveal extensive intra- and interchromosomal rearrangements between haplotypes, structural divergence absent in diploid sexuals, accumulating freely in the absence of recombination. Whether the cryptobiotic lifestyle amplifies this plasticity is an open question we are currently addressing: repeated cycles of desiccation and freezing might induce DNA breakage, repaired by a permissive machinery that joins previously non-contiguous fragments.
Together, parthenogenesis and cryptobiosis create a genomic environment of exceptional plasticity, with implications for how these nematodes persist and adapt in some of the most extreme environments on earth.
Host: Oliver Hobert
