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piqtree is an easy to use, open-source Python package that directly exposes IQ-TREE’s phylogenetic inference engine. It offers Python functions for performing many of IQ-TREE’s capabilities including phylogenetic reconstruction, ultrafast bootstrapping, branch length optimisation, ModelFinder, rapid neighbour-joining, and more. By exposing IQ-TREE’s algorithms within Python, piqtree greatly simplifies the development of new phylogenetic workflows through seamless interoperability with other Python libraries and tools mediated by the cogent3 package. It also enables users to perform interactive analyses with IQ-TREE through, for instance, Jupyter notebooks. We present the key features available in the piqtree library and a small case study that showcases its interoperability. The piqtree library can be installed withpip install piqtree, with the documentation available at https://piqtree.readthedocs.io and source at https://github.com/iqtree/piqtree. 

Bacteroides, the prominent bacteria in the human gut, play a crucial role in degrading complex polysaccharides. Their abundance is influenced by phages belonging to theCrassviralesorder. Despite identifying over 600Crassviralesgenomes computationally, only few have been successfully isolated. Continued efforts in isolation of moreCrassviralesgenomes can provide insights into phage-host-evolution and infection mechanisms. We focused on wastewater samples, as potential sources of phages infecting variousBacteroideshosts. Sequencing, assembly, and characterization of isolated phages revealed 14 complete genomes belonging to three novelCrassviralesspecies infectingBacteroides cellulosilyticusWH2. These species,Kehishuvirussp. ‘tikkala’ strain Bc01,Kolpuevirussp. ‘frurule’ strain Bc03, and ‘Rudgehvirus jaberico’ strain Bc11, spanned two families, and three genera, displaying a broad range of virion productions. Upon testing all successfully culturedCrassviralesspecies and their respective bacterial hosts, we discovered that they do not exhibit co-evolutionary patterns with their bacterial hosts. Furthermore, we observed variations in gene similarity, with greater shared similarity observed within genera. However, despite belonging to different genera, the three novel species shared a unique structural gene that encodes the tail spike protein. When investigating the relationship between this gene and host interaction, we discovered evidence of purifying selection, indicating its functional importance. Moreover, our analysis demonstrated that this tail spike protein binds to the TonB-dependent receptors present on the bacterial host surface. Combining these observations, our findings provide insights into phage-host interactions and present threeCrassviralesspecies as an ideal system for controlled infectivity experiments on one of the most dominant members of the human enteric virome.