Clues to an Evolutionary Mystery: The Genes for T-Toxin, Enabler of the Devastating 1970 Southern Corn Leaf Blight Epidemic, Are Present in Ancestral Species, Suggesting an Ancient Origin

The Southern corn leaf blight (SCLB) epidemic of 1970 devastated fields of T-cytoplasm corn planted in monoculture throughout the eastern United States. The epidemic was driven by race T, a previously unseen race of Cochliobolus heterostrophus. A second fungus, Phyllosticta zeae-maydis, with the same biological specificity, appeared coincidentally. Race T produces T-toxin, while Phyllosticta zeae-maydis produces PM-toxin, both host-selective polyketide toxins necessary for supervirulence. The present abundance of genome sequences offers an opportunity to tackle the evolutionary origins of T- and PM- toxin biosynthetic genes, previously thought unique to these species. Using the C. heterostrophus genes as probes, we identified orthologs in six additional Dothideomycete and three Eurotiomycete species. In stark contrast to the genetically fragmented race T Tox1 locus that encodes these genes, all newly found Tox1-like genes in other species reside at a single collinear locus. This compact arrangement, phylogenetic analyses, comparisons of Tox1 protein tree topology to a species tree, and Tox1 gene characteristics suggest that the locus is ancient and that some species, including C. heterostrophus, gained Tox1 by horizontal gene transfer. C. heterostrophus and Phyllosticta zeae-maydis did not exchange Tox1 DNA at the time of the SCLB epidemic, but how they acquired Tox1 remains uncertain. The presence of additional genes in Tox1-like clusters of other species, although not in C. heterostrophus and Phyllosticta zeae-maydis, suggests that the metabolites produced differ from T- and PM-toxin.

Reconstructing protein and gene phylogenies using reconciliation and soft-clustering

The architecture of eukaryotic coding genes allows the production of several different protein isoforms by genes. Current gene phylogeny reconstruction methods make use of a single protein product per gene, ignoring information on alternative protein isoforms. These methods often lead to inaccurate gene tree reconstructions that require to be corrected before phylogenetic analyses. Here, we propose a new approach for the reconstruction of gene trees and protein trees accounting for alternative protein isoforms. We extend the concept of reconciliation to protein trees, and we define a new reconciliation problem called MinDRGT that consists in finding a gene tree that minimizes a double reconciliation cost with a given protein tree and a given species tree. We define a second problem called MinDRPGT that consists in finding a protein supertree and a gene tree minimizing a double reconciliation cost, given a species tree and a set of protein subtrees. We propose a shift from the traditional view of protein ortholog groups as hard-clusters to soft-clusters and we study the MinDRPGT problem under this assumption. We provide algorithmic exact and heuristic solutions for versions of the problems, and we present the results of applications on protein and gene trees from the Ensembl database. The implementations of the methods are available at https://github.com/UdeS-CoBIUS/Protein2GeneTree and https://github.com/UdeS-CoBIUS/SuperProteinTree .

Further refinement of the phylogeny of the Halobacteriaceae based on the full-length RNA polymerase subunit B′ (rpoB′) gene

A considerable number of species of the Halobacteriaceae possess multiple copies of the 16S rRNA gene that exhibit more than 5 % divergence, complicating phylogenetic interpretations. Two additional problems have been pointed out: (i) the genera Haloterrigena and Natrinema show a very close relationship, with some species being shown to overlap in phylogenetic trees reconstructed by the neighbour-joining method, and (ii) alkaliphilic and neutrophilic species of the genus Natrialba form definitely separate clusters in neighbour-joining trees, suggesting that these two clusters could be separated into two genera. In an attempt to solve these problems, the RNA polymerase B′ subunit has been used as an additional target molecule for phylogenetic analysis, using partial sequences of 1305 bp. In this work, a primer set was designed that consistently amplified the full-length RNA polymerase B′ subunit gene (rpoB′) (1827–1842 bp) from 85 strains in 27 genera of the Halobacteriaceae. Differences in sequence length were found within the first 15 to 31 nt, and their downstream sequences (1812 bp) were aligned unambiguously without any gaps or deletions. Phylogenetic trees reconstructed from nucleotide sequences and deduced amino acid sequences by the maximum-likelihood method demonstrated that multiple species/strains in most genera individually formed cohesive clusters. Two discrepancies were observed: (i) the two species of Natronolimnobius were placed in definitely different positions, in that Natronolimnobius innermongolicus was placed in the Haloterrigena/Natrinema cluster, while Natronolimnobius baerhuensis was closely related to Halostagnicola larsenii, and (ii) Natronorubrum tibetense was segregated from the three other Natronorubrum species in the protein tree, while all four species formed a cluster in the gene tree, although supported by a bootstrap value of less than 50 %. The six Haloterrigena species/strains and the five species of Natrinema formed a large cluster in both trees, with Halopiger xanaduensis and Nln. innermongolicus located in the cluster in the protein tree and Nln. innermongolicus in the gene tree. Hpg. xanaduensis broke into the cluster of the genus Halobiforma, instead of the Haloterrigena/Natrinema cluster, in the gene tree. The six Natrialba species formed a tight cluster with two subclusters, of neutrophilic species and alkaliphilic species, in both trees. Overall, our data strongly suggest that (i) Nln. innermongolicus is a member of Haloterrigena/Natrinema, (ii) Nrr. tibetense might represent a new genus and (iii) the two genera Haloterrigena and Natrinema might constitute a single genus. As more and more novel species and genera are proposed in the family Halobacteriaceae, the full sequence of the rpoB′ gene may provide a supplementary tool for determining the phylogenetic position of new isolates.