Tardigrade community microbiomes in North American orchards include putative endosymbionts and plant pathogens

The microbiome of tardigrades, a phylum of microscopic animals best known for their ability to survive extreme conditions, is poorly studied worldwide and completely unknown in North America. An improved understanding of tardigrade-associated bacteria is particularly important because tardigrades have been shown to act as vectors of the plant pathogen Xanthomonas campestris in the laboratory. However, the potential role of tardigrades as reservoirs and vectors of phytopathogens has not been investigated further. This study analyzed the microbiota of tardigrades from six apple orchards in central Iowa, USA, and is the first analysis of the microbiota of North American tardigrades. It is also the first ever study of the tardigrade microbiome in an agricultural setting. We utilized 16S rRNA gene amplicon sequencing to characterize the tardigrade community microbiome across four contrasts: location, substrate type (moss or lichen), collection year, and tardigrades versus their substrate. Alpha diversity of the tardigrade community microbiome differed significantly by location and year of collection but not by substrate type. Our work also corroborated earlier findings, demonstrating that tardigrades harbor a distinct microbiota from their environment. We also identified tardigrade-associated taxa that belong to genera known to contain phytopathogens (Pseudomonas, Ralstonia, and the Pantoea/Erwinia complex). Finally, we observed members of the genera Rickettsia and Wolbachia in the tardigrade microbiome; because these are obligate intracellular genera, we consider these taxa to be putative endosymbionts of tardigrades. These results suggest the presence of putative endosymbionts and phytopathogens in the microbiota of wild tardigrades in North America.This is a preprint made available through bioRxiv at doi:10.1101/2022.01.28.478239. It is made available under a CC-BY-NC-ND 4.0 International license

FISHing for Rickettsia in tardigrades: additional evidence for tardigrade endosymbionts

Many ecdysozoans harbour endosymbiotic bacteria within their microbiota, and these endosymbionts can have a range of positive and negative effects on their hosts. Recent 16S rRNA gene amplicon sequencing studies have provided evidence for endosymbionts within the tardigrade microbiota. In a previous amplicon study, we determined that sequences corresponding to the endosymbiotic genus Rickettsia were significantly more associated with tardigrades than with the substrate from which they were isolated. Here, we performed fluorescence in situ hybridization (FISH) using a Rickettsia-specific probe, RickB1, to determine if Rickettsia could be found in tardigrades. RickB1 and a probe targeting most bacteria, EUB338, colocalized within tardigrade tissues, indicating the presence of Rickettsia. We also performed FISH using RickB1 and a nonsense probe, which allowed us to distinguish between false-positives and true positives. This method revealed RickB1 signals in tardigrades that were not due to erroneous probe binding, providing further evidence that Rickettsia is present in tardigrades. Future research will be necessary to determine the effects, if any, of these endosymbionts on their tardigrade hosts.This article is published as Bienvenido W Tibbs-Cortes and others, FISHing for Rickettsia in tardigrades: additional evidence for tardigrade endosymbionts, Zoological Journal of the Linnean Society, 2023;, zlad081, https://doi.org/10.1093/zoolinnean/zlad081. Posted with permission.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited

Combining GWAS and TWAS to identify candidate causal genes for tocochromanol levels in maize grain

Tocochromanols (tocopherols and tocotrienols, collectively vitamin E) are lipid-soluble antioxidants important for both plant fitness and human health. The main dietary sources of vitamin E are seed oils that often accumulate high levels of tocopherol isoforms with lower vitamin E activity. The tocochromanol biosynthetic pathway is conserved across plant species but an integrated view of the genes and mechanisms underlying natural variation of tocochromanol levels in seed of most cereal crops remains limited. To address this issue, we utilized the high mapping resolution of the maize Ames panel of ∼1,500 inbred lines scored with 12.2 million single-nucleotide polymorphisms to generate metabolomic (mature grain tocochromanols) and transcriptomic (developing grain) data sets for genetic mapping. By combining results from genome- and transcriptome-wide association studies, we identified a total of 13 candidate causal gene loci, including 5 that had not been previously associated with maize grain tocochromanols: 4 biosynthetic genes (arodeH2 paralog, dxs1, vte5, and vte7) and a plastid S-adenosyl methionine transporter (samt1). Expression quantitative trait locus (eQTL) mapping of these 13 gene loci revealed that they are predominantly regulated by cis-eQTL. Through a joint statistical analysis, we implicated cis-acting variants as responsible for colocalized eQTL and GWAS association signals. Our multiomics approach provided increased statistical power and mapping resolution to enable a detailed characterization of the genetic and regulatory architecture underlying tocochromanol accumulation in maize grain and provided insights for ongoing biofortification efforts to breed and/or engineer vitamin E and antioxidant levels in maize and other cereals