Figure 2. Heat map showing normalized average PID values between Hsp60 sequences belonging to sub-clusters Arthropoda #2, Arthropoda #3, and Nematoda #1 and Hsp60 sequences belonging to each of 19 phyla. The average PID values were normalized using the min-max normalization method. The 19 phyla were sorted by NCBI Taxonomy.
According to Figure 2, the highest average PIDs were observed between Arthropoda sub-clusters #2 and #3 and phyla Mollusca and Arthropoda, respectively. Since these phyla belong to Metazoa, the presence of these sub-clusters in clusters II and IV can be explained as a feature of dendrogram construction and the selected percent of cophenetic distance (see Hierarchical clustering of sequences).
On the other hand, the average PID value between the Nematoda #1 sub-cluster and Proteobacteria phylum was 37.8±0.3% (Supplementary, Metazoan artifacts), which was higher than for all metazoan phyla including Nematoda (28.7±0.9%). This feature may be the result of a database error.
In turn, this may be explained by horizontal gene transfer39. Accordingly, possible cases of horizontal Hsp60 gene transfer were identified using the obtained data (Supplementary, Horizontal Hsp60 gene transfer). It was found that horizontal Hsp60 gene transfer is most common in Bacteria, which is obvious. In turn, the Hsp60 sequences from the Nematoda #1 sub-cluster have the maximum PID in the following pairs:
Onchocerca volvulus (CAA70570.1, Nematoda)/Wolbachia sp.(WP_014869024.1, Proteobacteria), PID 78.3%;
Trichuris trichiura (CDW56974.1, Nematoda)/Enterobacter sp. (WP_015572446.1, Proteobacteria), PID 88.6%.
Hence, the transmission paths can be tracked. Endosymbiotic bacteriaWolbachia sp. are present in filarial nematodes40, to which Onchocerca volvulusbelongs. At the same time, Trichuris trichiura roundworms parasitize in human intestine, where Enterobacter sp. bacteria are also present. These data are not conclusive evidence of horizontal Hsp60 gene transfer between these organisms and require in-depth review.