[Methods for the identification of horizontal gene transfer (HGT) events and progress in related fields]

Assembly and comparative analysis of the first complete mitochondrial genome of Salix psammophila, a good windbreak and sand fixation shrub

Salix psammophila, commonly known as the sandlive willow, is a vital shrub species within the Salicaceae family, particularly significant for its ecological role in regions susceptible to desertification and sandy soils. In this study, we assembled the complete S. psammophila mitochondrial genome using Pacbio HiFi third-generation sequencing data. The genome was found to be a typical single circular structure, with a total length of 715,555 bp and a GC content of 44.89%. We annotated 33 unique protein-coding genes (PCGs), which included 24 core mitochondrial genes and 9 variable genes, as well as 18 tRNA genes (5 of which were multicopy genes) and 3 rRNA genes. Comparative analysis of the PCGs from the mitochondrial genomes of S. psammophila, Populus deltoides, Populus simonii, Salix wilsonii, and Salix suchowensis revealed that these genes are relatively conserved within the Salicaceae family, with variability primarily occurring in the ribosomal protein genes. The absence of the rps14, which encodes a ribosomal protein, may have played a role in the evolution of stress tolerance in Salicaceae plants. Additionally, we identified 232 SSRs, 19 tandem repeat sequences, and 236 dispersed repeat sequences in the S. psammophila mitochondrial genome, with palindromic and forward repeats being the most abundant. The longest palindromic repeat measured 260 bp, while the longest forward repeat was 86,068 bp. Furthermore, 324 potential RNA editing sites were discovered, all involving C-to-U edits, with the nad4 having the highest number of edits. These findings provide valuable insights into the phylogenetic and genetic research of Salicaceae plants.

Assembly and comparative analysis of the complete mitochondrial genome of Salix wilsonii using PacBio HiFi sequencing

Salix L. (willows) is one of the most taxonomically complex genera of flowering plants, including shrubs, tall trees, bushes, and prostrate plants. Despite the high species diversity, only five mitochondrial genomes (mitogenomes) have been released in this genus. Salix wilsonii is an important ornamental and economic willow tree in section Wilsonia of the genus Salix. In this study, the S. wilsonii mitogenome was assembled into a typical circular structure with a size of 711,456 bp using PacBio HiFi sequencing. A total of 58 genes were annotated in the S. wilsonii mitogenome, including 33 protein-coding genes (PCGs), 22 tRNAs, and 3 rRNAs. In the S. wilsonii mitogenome, four genes (mttB, nad3, nad4, and sdh4) were found to play important roles in its evolution through selection pressure analysis. Collinearity analysis of six Salix mitogenomes revealed high structural variability. To determine the evolutionary position of S. wilsonii, we conducted a phylogenetic analysis of the mitogenomes of S. wilsonii and 12 other species in the order Malpighiales. Results strongly supported the segregation of S. wilsonii and other five Salix species with 100% bootstrap support. The comparative analysis of the S. wilsonii mitogenome not only sheds light on the functional and structural features of S. wilsonii but also provides essential information for genetic studies of the genus Salix.

Genome of lethal Lepiota venenata and insights into the evolution of toxin-biosynthetic genes

BACKGROUND

Genomes of lethal Amanita and Galerina mushrooms have gradually become available in the past ten years; in contrast the other known amanitin-producing genus, Lepiota, is still vacant in this aspect. A fatal mushroom poisoning case in China has led to acquisition of fresh L. venenata fruiting bodies, based on which a draft genome was obtained through PacBio and Illumina sequencing platforms. Toxin-biosynthetic MSDIN family and Porlyl oligopeptidase B (POPB) genes were mined from the genome and used for phylogenetic and statistical studies to gain insights into the evolution of the biosynthetic pathway.

RESULTS

The analysis of the genome data illustrated that only one MSDIN, named LvAMA1, exits in the genome, along with a POPB gene. No POPA homolog was identified by direct homology searching, however, one additional POP gene, named LvPOPC, was cloned and the gene structure determined. Similar to ApAMA1 in A. phalloides and GmAMA1 in G. marginata, LvAMA1 directly encodes α-amanitin. The two toxin genes were mapped to the draft genome, and the structures analyzed. Furthermore, phylogenetic and statistical analyses were conducted to study the evolution history of the POPB genes. Compared to our previous report, the phylogenetic trees unambiguously showed that a monophyletic POPB lineage clearly conflicted with the species phylogeny. In contrast, phylogeny of POPA genes resembled the species phylogeny. Topology and divergence tests showed that the POPB lineage was robust and these genes exhibited significantly shorter genetic distances than those of the house-keeping rbp2, a characteristic feature of genes with horizontal gene transfer (HGT) background. Consistently, same scenario applied to the only MSDIN, LvAMA1, in the genome.

CONCLUSIONS

To the best of our knowledge, this is the first reported genome of Lepiota. The analyses of the toxin genes indicate that the cyclic peptides are synthesized through a ribosomal mechanism. The toxin genes, LvAMA1 and LvPOPB, are not in the vicinity of each other. Phylogenetic and evolutionary studies suggest that HGT is the underlining cause for the occurrence of POPB and MSDIN in Amanita, Galerina and Lepiota, which are allocated in three distantly-related families.