After 90 years of taxonomic uncertainties, using phenotypic, phylogenetic, and population genetics analyses, the two uncultivated fungi causing skin disease in humans and dolphins, long known as Lacazia loboi8, are now placed as separate species within the genus Paracoccidioides. Early studies using phenotypic or phylogenetic data alone erroneously placed these two fungal pathogens in different genera and species3,4,5,6,7,8,12,13,15,16,17,24,25. This trend persisted for years2,13,16,17,25. For instance, recent studies using several partial DNA sequences recovered from Brazilian humans with skin disease in phylogenetic analyses concluded that the genus Lacazia, the accepted name at that time, was an independent taxon from Paracoccidioides species16,24,25. Their phylogenetic data was correct, but their analyses missed the inclusion of DNA from the uncultivated pathogen causing skin disease in dolphins. This was an understandable mistake, since the collection and processing specimens from infected dolphins is highly regulated and the fact that the etiology of dolphins’ disease was long believed to be the same as that in humans, as shown in Fig. 1 and Table 1. Although P. cetii has numerous phenotypic differences with Paracoccidioides species (Table 1, Fig. 1), in the pass used to group them in separated clusters2,3,7,8, our data showed they share several phylogenetic features in common (Figs. 4, 5 and 6). With the addition of P. cetii DNA sequences, the phylogenetic support of closely related Paracoccidioides species dramatically changed. For example, P. loboi clustered in a monophyletic group sister to P. lutzii, even with the inclusion of homologous dimorphic Onygenales DNA sequences as outgroup (Figs. 4b, 5), whereas the support of monophyletic species within the genus weakened (Figs. 4, 5 and 6). More dolphin DNA sequences from different geographical locations must be sequenced to understand P. cetii´s true evolutionary traits.
Several studies reported geographical cryptic speciation among Paracoccidioides species14,24,26,27,28. In those analyses the presence of at least five species within the genus, including P. lutzii, was found14,15,24,27,29,30. Recent genome sequencing in phylogenetic analysis tend to validate these findings26,28,29. Although the DNA sequences of P. loboi were used in some of the analyses, the human skin pathogen was always placed as an independent genus from that in Paracoccidioides species16,24,25. The placement of P. cetii sister to P. americana DNA sequences in this study, indicates the use of phenotypic or phylogenetic characteristics without the inclusion of anomalous species, can lead to inaccuracies in the taxonomic and phylogenetic classification of these type of microbes. For instance, our data, using several statistical tools, consistently showed the presence of different clusters within Paracoccidioides species. In our analyses, P. americana, P. cetii, P. lutzii, and P. loboi were placed in monophyletic groups sister to the remaining Paracoccidioides species (Figs. 2, 3, 4, 5 and 6). Therefore, the addition of P. cetii to the genus Paracoccidioides not only confirmed that the genus has indeed a high level of speciation but, indicates that the concept of species delimitation in this genus must be revisited12,31.
Recently, Vilela et al.16, using phylogenetic analysis of five different genes, showed P. loboi shared the same ancestor with Paracoccidioides species. The results in our study support their proposal. The main obstacle of this hypothesis at that time was the phenotypic features of P. loboi (Fig. 1). However, if P. loboi and P. cetii (both uncultivated and subcutaneous pathogens) share the same ancestor with other Paracoccidioides species (cultivated and causing systemic infections), the likelihood that the ancestor of Paracoccidioides species could growth in culture, as previously suggested, is a strong possibility16. If this concept is correct, when in the evolutionary history of P. cetii and P. loboi they lost the capacity to grow in culture? What evolutionary pressure triggered such a change? Sadly, as is common in neglected pathogens such as P. cetii and P. loboi key questions such as these, remain without an answer. Interestingly, the uncultivated feature found in these two neglected fungi was also reported in a strain of Histoplasma capsulatum infecting monkeys, suggesting that an uncultivated ancestral trait in the Onygenales dimorphic fungi may be at work32. However, the evolutionary pressures that triggered such ancestral feature remains an enigma.
The report of new human cases of paracoccidioidomycosis loboi acquired by traveling to endemic areas2,3,4,5,33,34,35,36, suggests P. loboi may has a similar phenotype (hyphae with conidia) to the one displayed by Paracoccidioides species in nature and in culture. Thus, it may be present in specific ecological niches in the endemic areas (around the Amazon basin and other Latin American big rivers)2,14,15,25. Therefore, it is possible P. cetii and P. loboi may have a phenotype in nature similar to that of Paracoccidioides species (hyphae with conidia). Under this scenario, both uncultivated pathogens display a mycelia form with conidia and the classic life cycle style of dimorphic fungi in nature25. As is the case in other dimorphic fungi, these propagules could then contact susceptible hosts (human, dolphins) switching from hyphae → yeast thus, causing subcutaneous infections. Perhaps due to abnormalities on the molecular mechanisms of yeast → hyphae conversion (mutations?), once the hyphae → yeast conversion occurs, it cannot longer switch back from yeast to hyphal phase. However, the yeast phase of both pathogens can infect other hosts, as had been demonstrated in accidental and experimental infection with yeast-like cells from infected humans and dolphins2,37,38,39,40,41,42. Despite attempts made by the Broad Institute (www.broadinstitute.org/fungal-genome-initiative/lacazia-loboi-sequencing), only fragmented genomic information is available for P. loboi, and the genome of P. cetii is yet to be sequence. We hypothesize that the genomes of both uncultivated pathogens may hide important genomic clues that could answer this and other evolutionary questions.
Several P. cetii DNA sequences recovered from dolphins captured in Brazil, Cuba, Japan, and the USA are currently available in the database (Table S1)19,20,21,22,23. The complete ITS DNA sequences from Brazilian and Cuban dolphins with paracoccidioidomycosis ceti, showed high percentage of identify with the DNA sequences in this study (ITS = 100%) whereas the partial Gp43 DNA sequences from a Japanese dolphin (471 bp) had 98.62% identity with P. cetii DNA sequences from dolphins captured in the Americas. During Gp43 DNA alignment of Japanese and USA dolphins, a five nucleotides gap was consistently present in the DNA sequences of USA dolphins. Moreover, two additional 266 bp GP43 DNA sequences extracted from a Japanese dolphin (Lagenorhynhus obliquidens) with paracoccidioidomycosis ceti showing, 99.62% identity with P. brasiliensis (sensu lato). In our analyses, these two sequences (only 110 bp could be used) clustered also with P. brasiliensis (Fig. 4, red rectangle). However, the same DNA sequences clustered close to P. cetii in haplotype analysis indicating a fragile relationship (Fig. 3). If P. cetii DNA sequences from Japanese dolphins are accurate, the differences in the genetic makeup of these two populations of uncultivated pathogens is intriguing and deserve further analysis. Our data suggest P. cetii strains causing paracoccidioidomycosis ceti in Japanese and USA dolphins, likely are evolving into two different populations.
According to Teixeira et al.24, the estimated time for genetic divergence in Paracoccidioides species was calculated around 33 million years. Although, others have questioned this result31, Carruthers et al.43, cautioned that the use of linage-specific data usually demonstrate approximate divergence time regardless of the number of loci interrogated. Nonetheless, according to these reports, Paracoccidioides species probably diverged from their ancestor from a fraction of a million of years (P. restrepiensis and P. venezuelensis) to 10–30 million of years (P. lutzii and P. brasiliensis, sensu lato)24,31. Conversely, dolphins evolved into aquatic mammals ~ 50 to 30 million years ago, around late Paleocene period (Eocene, Oligocene epochs)44. According to fossil records, South America at this time had a large body of water crossing from the north Atlantic Ocean to what is today Bolivia, Brazil, Ecuador, Colombia, Peru and Venezuela45, all endemic areas of these species3,4,5,24,26,29, that lasted for millions of years. A similar situation occurred in what is today the estuary of the Amazon River. The current location of Paracoccidioides species (including P. loboi), coincide with the locations of such geological periods, and then it is quite possible that during the time following these geological events, an ancestor of P. cetii first encountered dolphins entering these areas. Since humans came to South Americas only ~ 15,000-year ago46, likely the ancestor of Paracoccidioides species infected dolphin first and later humans. Whether this event had a role on the pathogenic capabilities of the genus to infect mammals is difficult to determine, nonetheless it is an intriguing possibility.
Working with uncultivated pathogens infecting the skin of mammals is challenging. Not only because collecting specimens from these species (dolphins are protected species and human cases are located in poor remote rural areas) is extremely difficult, but because open lesions usually harbor numerous environmental contaminants, which in the past had led to erroneous conclusions on the classifications of these two anomalous pathogens2,8,15,16,25,47. Furthermore, these unusual fungi are not in the list of neglected pathogens, thus discouraging investigators to submit proposals to funding organizations. Previous studies using P. loboi in phenotypic or phylogenetic analyses placed this anomalous pathogen away from the genus Paracoccidioides2,4,15,16,25. This study found that the use of phenotypic or phylogenetic approaches without the inclusion of DNA from infected dolphins, likely led previous studies to flawed data15,16,25. Thus, the failure of including organisms sharing a common ancestor, based in phenotypic or phylogenetic traits alone, could result in incomplete or incorrect assessment of the investigated populations. This study showed that the interpretation of taxonomic and/or phylogenetic data could be affected by missing neighboring anomalous taxa.
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