Crossroads of assembling a moss genome: navigating contaminants and horizontal gene transfer in the moss Physcomitrellopsis africana

The first chromosome-scale reference genome of the rare narrow-endemic African moss Physcomitrellopsis africana is presented here. Assembled from 73x nanopore long reads and 163x BGI-seq short reads, the 414 Mb reference comprises 26 chromosomes and 22,925 protein-coding genes (BUSCO: C:94.8%[D:13.9%]). This genome holds two genes that withstood rigorous filtration of microbial contaminants, have no homolog in other land plants and are thus interpreted as resulting from two unique horizontal gene transfers from microbes. Further, Physcomitrellopsis africana shares 176 of the 273 published HGT candidates identified in Physcomitrium patens, but lacks 98 of these, highlighting that perhaps as many as 91 genes were acquired in P. patens in the last 40 million years following its divergence from its common ancestor with P. africana. These observations suggest rather continuous gene gains via HGT followed by potential losses, during the diversification of the Funariaceae. Our findings showcase both dynamic flux in plant HGTs over evolutionarily "short" timescales, alongside enduring impacts of successful integrations, like those still functionally maintained in extant Physcomitrellopsis africana. Furthermore, this study describes the informatic processes employed to distinguish contaminants from candidate HGT events. Article SummaryThe first draft genome of the rare South African endemic moss Physcomitrellopsis Africana is presented. The 414 Mb assembly contains 22,925 genes, including two uniquely horizontally transferred genes, but lacks 97 of the microbial genes previously identified in the closely related model, Physcomitrium patens - highlighting the dynamic role of HGT in the evolution of these moss genomes and loss. This study presents best practices for contamination detection and new insights into HGT identification.