In 1970, the Southern Corn Leaf Blight epidemic ravaged U.S. fields to great economic loss. The outbreak was caused by never-before-seen, supervirulent, Race T of the fungus Cochliobolus heterostrophus. The functional difference between Race T and O, the previously known, far less aggressive strain, is production of T-toxin, a host-selective polyketide. Supervirulence is associated with ~1 Mb of Race T-specific DNA; only a fraction encodes T-toxin biosynthetic genes (Tox1). Tox1 is genetically and physically complex, with unlinked loci (Tox1A, Tox1B) genetically inseparable from breakpoints of a Race O reciprocal translocation that generated hybrid Race T chromosomes. Previously, we identified 10 genes for T-toxin biosynthesis. Unfortunately, high-depth, short-read sequencing placed these genes on four small, unconnected scaffolds surrounded by repeated A+T rich sequence, concealing context. To sort out Tox1 topology and pinpoint the hypothetical Race O translocation breakpoints corresponding to Race T-specific insertions, we undertook PacBio long-read sequencing which revealed Tox1 gene arrangement and the breakpoints. Six Tox1A genes are arranged as three small islands in a Race T-specific sea (~634 kb) of repeats. Four Tox1B genes are linked, on a large loop of Race T-specific DNA (~210 kb). The race O breakpoints are short sequences of race O-specific DNA; corresponding positions in race T are large insertions of race T-specific, A+T rich DNA, often with similarity to transposable (predominantly Gypsy) elements. Nearby, are ‘Voyager Starship’ elements and DUF proteins. These elements may have facilitated Tox1 integration into progenitor Race O and promoted large scale recombination resulting in race T.
IMPORTANCE In 1970 a corn disease epidemic ravaged fields in the United States to great economic loss. The outbreak was caused by a never-before seen, supervirulent strain of the fungal pathogen Cochliobolus heterostrophus. This was a plant disease epidemic, however, the current COVID-19 pandemic of humans is a stark reminder that novel, highly virulent, pathogens evolve with devastating consequences, no matter what the host-animal, plant, or other organism. Long read DNA sequencing technology allowed in depth structural comparisons between the sole, previously known, much less aggressive, version of the pathogen and the supervirulent version and revealed, in meticulous detail, the structure of the unique virulence-causing DNA. These data are foundational for future analysis of mechanisms of DNA acquisition from a foreign source.


In 1970, an epidemic of Southern Corn Leaf Blight (SCLB) disease ravaged fields of corn on the eastern U.S. seaboard to great economic loss. The outbreak was caused by a novel, supervirulent strain, Race T, of the fungal maize pathogen Cochliobolus heterostrophus (Bipolaris maydis). Although this was a plant disease epidemic, the current COVID-19 pandemic of humans reminds us that evolution of previously unseen pathogens, mechanisms of dissemination, and principles of disease are universal. Moreover, C. heterostrophus is particularly amenable to genetic and genomic functional analyses (1, 2) making these overarching questions more easily addressed than with some pathogens of animals for which host virulence assays are more burdensome.
The essential difference between C. heterostrophus Race T and Race O, the only previously known, but much less aggressive strain, is production by Race T of T-toxin, a polyketide metabolite not produced by Race O (3). Notably, all genes responsible for T-toxin biosynthesis are encoded in approximately one Megabase (Mb) of DNA sequence of Race T-specific DNA and super virulence is entirely attributable to this additional DNA (collectively known as Tox1) (47). Unlike most gene cohorts required for biosynthesis of natural products in filamentous ascomycete fungi (8), the Tox1 genes are not tightly clustered (9). Rather, Tox1 is both genetically and physically multifaceted. Decades of research have shown that Tox1 genes are encoded at two unlinked loci (Tox1A, Tox1B) that are genetically inseparable from the breakpoints of a reciprocal translocation that formed a pair of hybrid Race T chromosomes (chromosomes 6;12, 12;6) when a pair of progenitor Race O chromosomes (chromosomes 6, 12) underwent a translocation (9, 10). The two Race T and two Race O chromosomes form a four-armed genetic linkage group with Tox1 mapping to the intersection of this cruciform structure (4). At meiosis, progeny of a cross between Race T and Race O strains segregate 1:1 for ability to produce T-toxin, indicative of a single genetic locus, although we know two unlinked loci on two different chromosomes are involved.
Previously, 10 Tox1 genes required for biosynthesis of T-toxin were identified (9, 11). Genetic mapping, functional genetics, and in depth, short-read sequencing revealed that six of these genes are located at Tox1A and four at Tox1B, on four small, physically unconnected, genomic scaffolds. Three of the four scaffolds map to Race T chromosome 12;6 and the other to chromosome 6;12 (9). The Tox1 genes are not found in Race O or in any of the more than 50 Cochliobolus species, however, as we reported recently (9), we have discovered orthologs of most Tox1 genes as single, compact gene clusters in the genomes of three unrelated Eurotiomycetes and seven unrelated Dothideomycetes of various lifestyles, including plant pathogens, a mycoparasite, and two aquatic fungi, leading us to hypothesize that the tight gene cluster arrangement is ancestral. Importantly, in Condon et al. (see Fig. 1 in [9]), we could not determine C. heterostrophus Tox1 gene order or the evolutionary progenitor. Thus, origin of the disjointed Tox1 genes in C. heterostrophus remains unknown.
FIG 1 Macrosynteny visualization of six strains of C. heterostrophus. Race O strains are shown in orange while Race T strains are shown in purple. The numbers above the scaffolds in Race O and below in Race T, are scaffold numbers in the assembly. A minus sign after the scaffold numbers indicates opposite orientation to the sequence in the genome assembly fasta file. The blue and green ribbons show the translocation between chromosomes 6 and 12 in the Race T strains. Image made using jcvi mcscan (33).
To fully characterize the physical topology of Tox1-associated genes and scaffolds and identify the hypothetical translocation breakpoints in Race O, and the actual breakpoints in Race T chromosomes, which should correspond to points of insertion of Race T-specific DNA, we undertook long read sequencing of a collection of Race T and O strains (Table S1). Long read sequencing successfully spanned the translocation breakpoints in most strains, allowing identification of the breakpoints, the insertion points of Race T-specific DNA, and precise physical arrangement of Tox1 genes. We found that, despite the fact that the six Tox1A genes are inseparable genetically, they are physically separated and arranged in tightly linked pairs at increasing distance from the genetic intersection of the four-armed linkage group. In total, Tox1A (known genes totaling 18.9 kb) consists of 634 kb of Race T-specific DNA in which three pairs of genes are islands in a sea of repetitive sequences. Four genes lie within 455 kb of Race T-specific sequence and two are found on 179 kb of Race T-specific sequence, separated from the former by 5.5 kb of sequence common to both Race T and Race O. The four genes at Tox1B (totaling ~11 kb) are tightly linked genetically and physically, but also on a large stretch (210 kb) of Race T- specific DNA.
The theoretical breakpoint positions on the Race O chromosomes, which correspond to large insertions of Race T-specific DNA on the hybrid Race T chromosomes, are short stretches of Race O-specific DNA (2.3 kb/315 bp and 1.6 kb) on chromosomes 12 and 6, respectively. At these positions, Race T carries large inserts of Race T-specific DNA (179 kb, 455 kb on chromosome 12;6, 210 kb on chromosome 6;12) encoding Tox1A and Tox1B genes, respectively. These inserts are flanked on one side, by sequences homologous to Race O chromosome 6, and on the other by sequences homologous to Race O chromosome 12. Thus, the complex topological arrangement of all Tox1 genes and the breakpoints of the reciprocal translocation are finally determined 50 years after the epidemic.
Additionally, the Tox1 genes are embedded in blocks of repetitive A+T rich DNA, with many fragments that have homology to retrotransposons, particularly members of the Gypsy group. Nearby these repetitive DNA blocks, we found the sequence AGAATACAGGGC (GCCCTGTATTCT), which has been reported to be the target site (Voyager target site, vts) for the Voyager Starship element mediated by proteins DUF3435 and DUF3723 (12). We evaluate whether these may have facilitated integration of Tox1 (source unknown) into a progenitor Race O strain.


PacBio assemblies.

While multiple efforts have been made to sequence C. heterostrophus Race O and Race T strains using both Sanger and Illumina short read technologies, the A+T rich and repetitive nature of the Tox1A and Tox1B loci severely limited efforts to assemble the region in its entirety. Although the majority of genic regions of the genome were captured, context was lost. Long read sequencing technologies significantly improved the fragmented nature of these assemblies, allowing complete construction of the Tox1 regions. This is exemplified by the assemblies of both C4 (Race T) and C5 (Race O) near isogenic strain genome sequences (1315), which improved from 207 to 70 and 68 to 53 scaffolds, respectively, and allowed more accurate gene coding and structural contextual comparisons (Fig. 1, Tables 1, S1-2). Assemblies generated from long read sequencing of field isolates of Race T and Race O strains have similar continuity. Most importantly, the new assemblies allow for deeper analysis of the multifaceted Tox1 locus.
TABLE 1 Genome statistics for PacBio assemblies
Assembly StatisticaRace ORace T
Assembly size (Mbp)38.4136.4837.7941.3245.2544.2
Number scaffolds113537067133111
N50 & L50 (Mbp)7 (2.27)6 (2.12)7 (2.06)7 (2.23)9 (2.13)9 (1.92)
% Repeats14.9711.4613.2318.3121.0523.63
Number genes11,14511,80811,32411,79212,84211,727

Identification of Tox1-associated scaffolds.

Because of the reciprocal translocation, flanking RFLP markers on the translocation-associated chromosomes differ between races T and O (Fig. 2, S1) (4). Race O chromosome 12 carries RFLP markers G395 and B88, which flank the theoretical breakpoint on this chromosome. Race O chromosome 6 carries G349 and B149 flanking the theoretical breakpoint. Race T hybrid chromosomes 12:6 and 6;12 carry G395/B149 and G349/B88, respectively. Using a BLAST-based approach with Tox1 genes and flanking RFLPs as queries we identified the relevant pairs of markers on large single scaffolds in both Race O and Race T and thus captured the Tox1 genes in Race T and reciprocal translocation breakpoints in both races (Fig. 2, S1, Table S3).
FIG 2 Diagram to scale of PacBio assembled Race O chromosomes 12 and 6 and Race T hybrid chromosomes 12;6 and 6;12 of all sequenced strains. Strains are listed on the left, color code for Tox1 genes, RFLPs, and repeated elements on the right. Coordinates for the relevant scaffolds, Tox1 genes and RFLPs listed in Table S3. Note Fig. 1 legend re a minus sign after a scaffold number. Automatically called genes are in purple beneath all chromosomes. Note absence of genes, except Tox1 genes, in the repeat dense regions. Scale in Mb on the bottom.
For Race O near-isogenic strain C5, flanking RFLP marker pairs G395/B88 and G349/B149 were identified on single scaffolds S12 (1.811 Mb, chromosome 12) and S8 (1.952 Mb, chromosome 6), respectively, indicating complete sequencing through the hypothetical breakpoints on chromosomes 12 and 6 (Fig. 2, Table 2). For Race O field strain PR9b, flanking RFLP marker pair G395/B88 was identified on scaffold S11 (1.849 Mb, chromosome 12), while marker pair G349/B149 was identified on scaffold S4 (2.601 Mb, chromosome 6).
TABLE 2 Mapping Tox1 genes and RFLPs to scaffolds associated with Race O and T reciprocally translocated chromosomes
StrainRaceChr 12Chr 12;6Length (Mb)Chr 6Chr 6;12Length (Mb)
PR9BOS11 1.85S4 2.60
C5OS12 1.81S8 1.95
C4T S42.62 S81.99
PR1x412T S42.79 S92.00
Hm338T S32.81 S13, S191.59, 0.49
HAW225T S42.76 S82.02
For Race T near-isogenic strain C4, the Tox1A genes, previously identified on three small unconnected scaffolds in the short read assemblies, were all on single scaffold S4 (2.625 Mb, chromosome 12;6) in the PacBio assembly, flanked by RFLP markers G395 and B149. In Race T field strains PR1x412, Hm338, and HAW225, Tox1A and the same flanking G395/B149 markers were also located on single scaffolds (PR1x412: S4, 2.791 Mb; Hm338: S3, 2.807 Mb; HAW225: S4, 2.757 Mb, chromosome 12;6) (Fig. 2, Table 2).
For Race T Tox1B genes, we resolved chromosome 6;12 for strains C4, PR1x412, and HAW225, where RFLP G349, Tox1B, and RFLP B88 map to single scaffolds (C4: S8, 1.987 Mb; PR1x412: S9 2.0 Mb, HAW225: S8, 2.021 Mb, chromosome 6;12) (Fig. 2, Table 2). For strain Hm338, Tox1B genes and RFLP marker B88 were located on single scaffold S19 (0.488 Mb), but RFLP G349 was on separate scaffold S13 (1.590 Mb).
Thus, the entire Tox1A and Tox1B loci and their flanking RFLP markers have been captured in all Race T (with one exception at Tox1B) and both Race O strains, allowing an in-depth examination of differences between the races at the Tox1 locus.

Mapping the Tox1 genes—tightly linked genetic distances, long physical distances.

In the short read assembly of strain C4 (v1), the 10 known Tox1 genes mapped to four very small, unconnected, scaffolds. Three scaffolds carry Tox1A genes that mapped in pairs on separate scaffolds: PKS1/TOX10 (S74, ~17 kb), PKS2/LAM1 (S83, ~11 kb), and OXI1/TOX9 (S102, ~3.5 kb). The fourth carries Tox1B genes in a tight cluster, also on a small scaffold (S82, ~13.5 kb). The PacBio assembly of four Race T strains, including inbred lab strain C4, indicates that all six genes at Tox1A are captured on single scaffolds, arranged in the order LAM1, PKS2, TOX10, PKS1, TOX9, OXI1, in tightly linked pairs of increasing distance, respectively, from the genetic intersection of the four-armed linkage group (Fig. 2, 3, S1). In total, Tox1A consists of 634 kb of Race T-specific DNA in which the three pairs of genes (totaling 18.9 kb) are islands in a sea of repetitive sequences. PKS2/LAM1, PKS1/TOX10 are on 455 kb of Race T-specific sequence and OXI1/TOX9 is on 179 kb of Race T-specific sequence, separated from the former by 5.5 kb of sequence common to both T and O races. The PKS1/TOX10 pair is 225 kb from OXI1/TOX9. The PKS2/LAM1 pair is 153 kb away from the PKS1/TOX10 pair (Fig. 3). Note, furthermore, although genetic mapping indicated that the six genes at Tox1A are genetically inseparable from the reciprocal translocation breakpoints, they are, in fact, physically separated from each other by hundreds of kilobases and not in a cluster like most secondary metabolite-associated genes. Interestingly, in all Race T strains, the two large loops of Race T-specific Tox1A DNA are separated by 5.5 kb of DNA common to both Race T and O.
FIG 3 Structural topography of the Tox1 region. The exact order and spatial arrangement of known Tox1 genes is shown. There are six genes at Tox1A, which are found as three tightly linked pairs of genes, separated by 100s of kb of A+T rich DNA. OXI1/TOX9 is on a 179 kb loop separated from a 455 kb loop carrying PKS1/TOX10 and PKS2/LAM1. 5.5 kb of DNA (light gray) in common to both Race T and Race O separates the Tox1A loops. OXI1 is furthest from the breakpoint while LAM1 is closest on chromosome 12:6. There are four genes at Tox1B clustered tightly together on chromosome 6;12 on a 210 kb loop of A+T rich DNA. All numbers are in kb except where indicated (not to scale). Double-headed arrows below each locus is distance between RFLP marker pairs G395/B149 (Tox1A) or G349/B88 (Tox1B), respectively. >1.15 Mb, >0.3 Mb, 288, and 106 are distances from the nearest RFLP marker to the beginning of the nearest loop.
As described above, the Tox1A genes are found in pairs separated from each other by vast stretches of Race T-specific DNA that is A+T rich, highly repeated, and devoid of genes apart from the Tox1 genes (Fig. 2, 3). Exceptions include two genes in the 5.5 kb DNA in common to both races, between the Race T Tox1A specific loops, which encode a predicted short-chain dehydrogenase and an alpha/beta hydrolase, corresponding to strain C5 (v3) JGI protein IDs 1196534 and 1226739, respectively. These two genes are highly conserved in most Dothideomycetes and therefore not unique to Race T, as are all known Tox1 genes. The two genes occur adjacent to each other in all Cochliobolus species for which we have sequence, suggesting conservation across the species. Furthermore, the genes flanking these two genes (C5 v3 JGI protein IDs 2337044 and 1142280) in race O, are found on two different chromosomes (S8, chromosome 12;6 and S4 chromosome 6;12, respectively) in race T as predicted, reflective of their change in position due to the translocation. This places the theoretical reciprocal translocation breakpoint in race O on chromosome 12 between genes encoding proteins 2337044 and 1226739 (or ~ within 1118 bp), which overlaps with the position of the 315 bp race O-specific DNA (Fig. S2).
The four Tox1B genes are tightly clustered, as noted earlier (9, 15, 16). Remarkably, no additional genes were predicted in the 210 kb Race T-specific loop (Fig. 2, 3).
Thus, the topographical organization and exact genomic position of the previously unconnected Tox1A genes were clarified by long read sequencing for the first time. This approach also extended the flanking A+T replete sequence surrounding Tox1A and Tox1B encoding genes and revealed these are sizeable expanses compared to the small tracts of Tox1 coding sequence. The A+T rich blocks on Tox1-associated scaffolds (Fig. 2) are replete with gypsy element fragments (most are ~75% A+T), a few of which are full open reading frames with high identity to multiple open reading frames in both Race T and Race O genomes suggesting these are native to C. heterostrophus. We speculate that their presence in Tox1-associated DNA is due to movement in Race T after the introduction of Tox1 DNA (still unknown donor) (9).

Tox1-associated scaffolds from geographically dispersed Race T and Race O strains are homologs.

Diagnostic RFLPs (G395/B149, G349/B88) and Tox1 genes map to chromosomes 12;6 and 6;12 in all Race T strains (Fig. 2, Table 2). Similarly, the diagnostic RFLP pairs (G395/B88, G349/B149) flanking the theoretical breakpoints in Race O map to chromosomes 12 or 6. This reflects conservation of the reciprocal translocation, the Race O theoretical breakpoints, and insertion points of Tox1 DNA in strains of wide geographical distribution. Furthermore, for Race T chromosome 12:6, the Tox1A genes are conserved in order, gene orientation, and position (Fig. 3). Distance differences between flanking RFLP markers G395 and B149, are likely due to variations in the number of repeat sequences encoding transposable elements associated with the Race T-specific loops (Fig. 2).
Our findings that Tox1-associated scaffolds from Race T strains are syntenic and that the Tox1 genes are identically arranged, support the hypothesis that Race T arose once from a single genetic event, at an unknown time, from an, as yet unknown source, in which Tox1 DNA integrated into a chromosome of a Race O strain. This strengthens earlier genetic and molecular karyotype data (17) which indicated all T strains carried the reciprocally translocated chromosomes.

The breakpoints of the reciprocal translocation.

Race O inbred strain C5 and Race T inbred strain C4 and Race O inbred strain C5 and Race T field isolate PR1x412 were chosen as representative isolates for further analysis (one Race O strain compared to one Race T inbred and one field strain). Relevant Tox1 genes and RFLP markers are on single continuous scaffolds in all (Fig. 2). Circos plots of strain C5 chromosomes 12 and 6 and strain PR1x412 or C4 chromosomes 12;6 and 6;12 support the reciprocal translocation hypothesis in showing that Race T chromosomes 12;6 and 6;12 each have homology to both Race O chromosomes 6 and 12, hence providing additional evidence at the nucleotide level for the genetically and physically documented reciprocal translocation (Fig. 4). For chromosome 12;6, the largest portion of identity is with Race O chromosome 12, while for chromosome 6;12, the largest portion of identity is with Race O chromosome 6. Both physical findings support the genetic convention of naming hybrid chromosomes according to the largest portion contributing to the hybrid, first.
FIG 4 Circos diagrams of C5 (Race O) chromosomes 6 and 12 and the reciprocally translocated PR1x412 or C4 (Race T) chromosomes 12;6 and 6;12 counterparts. Each Race O chromosome has a match to both Race T hybrid chromosomes. On Race O chromosome 6, the arrow points to 1.6 kb of Race O-specific DNA whose counterpart on chromosome 6;12 is the 210 kb Tox1B Race T-specific insert. On Race O chromosome 12, the arrow points to 0.315 and 2.3 kb of Race O-specific DNA, separated by 5.5 kb of DNA common to both races. The 0.315 Race O-specific DNA counterpart position in Race T is the 455 kb insert of Tox1A Race T-specific DNA carrying PKS2/LAM1 and PKS1/TOX10, while the 2.3 kb Race O counterpart is the 179 kb Tox1A insert carrying OXI1/TOX9, on chromosome 12;6.
The Circos plots also illustrate that both theoretical breakpoints in Race O correspond to large inserts of Race T-specific DNA on chromosomes 12;6 and 6;12 (Fig. 4, Table S4). Race O strain C5 chromosome 12 (scaffold 12) at the arrow in Fig. 4, actually corresponds to two Race O specific stretches of DNA (315 bp and 2.3 kb, respectively, Fig. 5), separated by 5.5 kb of DNA in common between the two races (Fig. S2). At these positions, Tox1A carries Race T-specific inserts of 179 kb encoding the OXI1/TOX9 gene island and 455 kb encoding the PKS1/TOX10 and PKS2/LAM1 islands, separated by 5.5 kb of DNA in common between the two races. We speculate that this 5.5 kb island of synteny between the O and T strains, which splits Tox1A is indicative of an additional recombination event, post initial insertion event. Chromosome 6 (scaffold 8) at the arrow in Fig. 4, corresponds to a Race T specific insert of 210 kb, carrying the tightly linked Tox1B genes (DEC1/RED1/RED2/RED3) (Fig. 4 and 5; Table S4).
FIG 5 Model of the complex genetic structure of the Tox1 locus and linkage map of chromosomes involved in the reciprocal translocation. Race O chromosome 12 in red, chromosome 6 in green. Race T hybrid chromosomes in red and green. Black loops indicate inserts of Race T-specific DNA corresponding to positions of Race O-specific DNA (dotted lines) as summarized in box insert. Figure is not to scale. All numbers in kb, except where indicated. Key RFLPs are shown (Fig. S1).
Remarkably, the theoretical Race O reciprocal translocation breakpoints are not sequences that are split and rearranged in Race T, but rather short stretches of Race O-specific DNA. These consist of 315 bp and 2.3 kb of sequence on chromosome 12 and 1.6 kb of sequence on chromosome 6 (Fig. 4 and 5; Table S4). As noted above, large inserts of Race T-specific DNA are found at the corresponding positions on Race T chromosomes 12;6 and 6;12 flanked by DNA from either Race O chromosome 6 or 12. The repeat islands around Tox1A locus are significantly larger than those around Tox1B.


Facilitated by long-read sequencing, megabase scale, nucleotide-level resolution has revealed the true organization of Tox1, the locus responsible for T-toxin production and the SCLB epidemic of 1970. Four Race T strains, collected shortly after the epidemic, but representing wide geographical distribution, were sequenced. We found that all Tox1-associated scaffolds/chromosomes were homologs, and the 10 known Tox1 gene positions, order, and direction were identical in all. We also determined, in every case, that the known unlinked Tox1A and Tox1B genes are small islands in a sea of Race T-specific repeats and that many sequences with similarity to A+T rich Gypsy transposons are found in this sea. These findings are strong verification not only for the reciprocal translocation but also the hypothesis that a one-time complex genetic event gave rise to Race T from a progenitor Race O strain.
As we reported in Condon et al. (9), orthologs of most Tox1 genes have been found as single, compact gene clusters in the genomes of three unrelated Eurotiomycetes and seven unrelated Dothideomycete species, which led us to hypothesize that the tight gene cluster arrangement is ancestral and that these species likely produce a metabolite that differs from T-toxin. Note that in Condon et al. (9), (see Fig. 1), we could not even determine C. heterostrophus Tox1 gene order because the genes were distributed on four very small scaffolds, which we could not connect by any means. Comparative analyses showed that only C. heterostrophus and Didymella zeae-maydis carry RED1, an NAD dependent oxidoreductase. Notably, D. zeae-maydis produces PM-toxin, a linear polyketide toxin, similar to T-toxin, and was also discovered around the time of the SCLB epidemic as it affects the same host and causes similar devastation of T-cms corn (1820). Phylogenetic analyses and similarity assays confirmed that neither of these taxa passed Tox1 to the other (9). In addition, most of the tight clusters in the other fungi encode an ABC transporter and an oxidoreductase in the middle of their clusters, neither of which are found anywhere in either the C. heterostrophus Race T or O genomes (Condon et al. [9], see Fig. 1). These genetic differences are likely key to functional differences, as not one of these Tox1-like carrying species produces a T-toxin like symptom using the T-toxin-specific microbial assay (9). The evolutionary progenitor of the disjointed Tox1 genes in C. heterostrophus Race T remains unknown and is not our focus here. Rather, here, with long read sequencing, we have determined that the C. heterostrophus Tox1 genes are in a different order from all other species known to carry Tox1-like genes, confirmed they are not clustered and revealed their context in large blocks of repeated A+T DNA.
It must be acknowledged that all Race T isolates in repositories, including ours, represent strains collected worldwide within a few years of the SCLB epidemic. In our case, these strains have been stored on silica gel or as glycerol stocks at -80C and revitalized for each experiment, thus they are “frozen in time.” Yet, as noted below, the ability of Race T strains to produce T-toxin is stable in culture and long-term storage suggesting it is stable in the population, as reported earlier (17) but that Race T strains are not as fit as race O strains, unless there’s strong selection (2123). Indeed, when T-cms corn, the specific host for supervirulent Race T and D. zeae-maydis was no longer planted, Race T/D. zeae-maydis disappeared from the field.

Reciprocal translocation breakpoints.

The Race O theoretical reciprocal translocation breakpoints were found not to be single nucleotide transition points, but rather stretches of Race O-specific DNA (315 bp and 2.3 kb on Race O chromosome 12, and 1.6 kb on chromosome 6). These breakpoints were evident in both Race O strains (field and inbred) that were sequenced by PacBio. Interestingly, the Race O-specific DNA and corresponding Race T inserts on chromosome 12 are interrupted by 5.5 kb of DNA common to both races. We speculate that one or more additional recombination events occurred in Race T after the initial Tox1 DNA insertion and reciprocal translocation events and that this resulted in separation of the two Tox1A loops.

Repeated regions.

The cultivation of monocultures of Race T-susceptible T-cms maize and complex DNA recombination events, including a reciprocal translocation are considered the primary causes of the SCLB epidemic that occurred in the 1970s (6). Were the vast blocks of associated repetitive DNA part of the original transfer? As reported earlier (24), ~50% of the TE sequences identified in the Race O genome belong to the Gypsy superfamily of retrotransposons and it is known that retrotransposons are the largest constituent of the repetitive fraction of the genome in phytopathogenic fungi (25). At least some of these regions correspond to centromeres which, although A+T rich and often associated with transposable elements, vary widely in structure and size in fungi (26). Indeed, PacBio sequencing has confirmed that the sea of repetitive DNA surrounding the Tox1 genes is Gypsy element replete. Some of these are full ORFs with high quality matches to multiple open reading frames in both Race T and Race O genomes which suggests they were already in the progenitor Race O genome before Race T appeared. It is possible they are/were active and perhaps were involved in proliferation of DNA within the Race T-specific Tox1 blocks after insertion of Tox1.
Although we’ve speculated that the original insert of Tox1 DNA was into a centromere, as these have characteristics similar to Tox1 regions (repeats, A+T rich, transposable elements), we currently have no conclusive evidence for this, as we did not perform experiments to specifically locate centromeres in our race T and O strains. Chromosomal rearrangements, associated with transposable elements have been demonstrated recently for Verticillium (27), and Aspergillus fumigatus (28), as well as plant genomes (29).

Are Starship elements involved?

We explored the possibility that Tox1 may be a giant Starship element (12) and looked for diagnostic features of these elements around Tox1. Flanking the B149 RFLP marker (Fig. 2, S1) in both the O and T strains are genes encoding proteins with conserved pfam domains DUF3435 and DUF3723 thought to mediate integration into the Voyager Starship element target site (vts). Approximately 300 kb upstream of B149 (270 kb-380 kb, depending on the assembly), we identified the vts sequence (AGAATACAGGGC) in Race O and also just outside the repeat block harboring the Tox1A locus in Race T (Fig. S3). In both races this is ~240 kb from DUF3435, whereas the average size of the region between vts and the DUF genes reported, originally, in (12) is ~100 kb. Note, the 12 bp vts sequence is rare in both race T and O genomes, seen only once on the scaffolds harboring B149.
We did not find an association with genes encoding 5S rDNA, into which Starship elements are reported to insert, or with other genes commonly localized with Starship elements (FRE, PLP, NLR), nor did we identify associated inverted terminal repeats (12). Therefore, although we found the vts, and two DUF proteins, we did not find other features commonly associated with Starship. We speculate, given the bizarrely complex organization of Tox1, that at least two massive recombination events occurred and that these events may have erased diagnostic signatures of Starships or other known mobile elements. We speculate that an investigation of the evolutionary history of Race O-specific gaps at the theoretical breakpoints will be revealing.

Evolutionary mechanism.

As reported in Condon et al. (9) the genetic source of C. heterostrophus Tox1, responsible for biosynthesis of T-toxin and the ravaging SCLB epidemic, remains unclear. In all cases where they are found, most Tox1 or Tox1-like genes have introns, thus, are likely of eukaryotic origin plus orthologs group together phylogenetically which suggests a common evolutionary origin. Tox1 clusters in non-T-toxin producing species are in compact linear array, which is likely the ancestral arrangement. Here, we have used PacBio sequencing to advance understanding of C. heterostrophus Tox1 gene topological organization and overall fine genetic structure >50 years after the epidemic. Evolutionary underpinnings defy understanding at this point. That said, we would argue that evolution of Tox1 included an initial insertion of foreign, likely eukaryotic DNA, a reciprocal translocation, and one or more recombination events promoted by mobile native gypsy elements that caused amplification of the A+T rich sea associated with Tox1 DNA.
The story of C. heterostrophus, T-cytoplasm corn and T-toxin is a timely reminder that natural selection drives both the rapid rise and decline of disease. One remarkable aspect of C. heterostrophus Tox1 is its high level of expression (30) and stability. We hypothesize that selective pressure from the widespread monoculture of T-cms maize either led to the acquisition of the biosynthetic cluster around the time of the epidemic or, alternatively, Race T might have been ‘lurking’ in the field and the T-cms maize monoculture made it an ideal ‘petri dish’ for selection for Race T. Strong selection provided ability to persist in the field amid a background population of Race O; the latter continues even in progeny of laboratory crosses with Race O. In either case, these properties made C. heterostrophus a “chassis” for synthetic biology driven by natural selection in the environment.



C. heterostrophus strains sequenced by PacBio (Table S1) included highly backcrossed lab strains C5 (ATCC 48332, Race O, MAT1-1, Tox1-) and C4 (ATCC48331, Race T, MAT1-2, Tox1+), expected to differ significantly only at Tox1 and MAT1 (mating type) after 6 backcrosses to a recurrent parent. The other strains were field isolates of wide geographic origin, collected around the time of the 1970 SCLB epidemic and included one Race O field isolate PR9b (Poza Rica, Mexico, MAT1-1) and three Race T field isolates as follows: Hm338 (New York, MAT1-2, Tox1+, ATCC 48317), PR1x412, (a progeny of a cross between PR1C from Poza Rica, Mexico, and strain 412, unknown geographical origin, Race T, MAT1-1, Tox1+), and HAW225 (Hawaii, MAT1-1, Tox1+). Aliquots of strains from −80°C glycerol stocks were plated on complete medium with xylose (CMX) for 1 week (16/8 h light, 23C) as previously described (31).

High molecular weight DNA preparation.

One thousand mls of liquid CM medium were inoculated with 106–107 spores collected from CMX plates and shaken at 150 to 200 rpm at room temperature for 2 days. Mycelium was harvested on Whatman number 4 filter paper using a Buchner funnel connected to a vacuum. The mycelium pad was peeled off the filter, placed in a 50 mL Falcon tube, and stored at −20°C or directly lyophilized for 25 h (cap loose) or until mycelium was dry and brittle. Lyophilized mycelia were pulverized with a prechilled pestle in a prechilled mortar to a fine powder. Mycelial powder (~0.2-0.5g) was suspended in 20 mL isolation buffer 150 mM EDTA pH 8.0, 50 mM Tris, pH 8.0, 1% (wt/vol) sarkosyl (n-lauroyl sarcosine), 300 mg/L Protease XI (Proteinase K) in disposable 30 mL polypropylene tubes (Sarstedt) and vortexed vigorously for 30 to 60 sec. Cell debris was pelleted by centrifugation in an SS-34 rotor (5 min, 5000 rpm, 4°C) and supernatant transferred to a clean tube. Twenty mL isolation buffer was added and centrifugation repeated. The recovered supernatant was gently mixed with an equal volume of Tris-saturated phenol and tubes centrifuged (10 min, 5,000 rpm, 4°C) to separate phases. The upper (aqueous) phase was transferred to a clean tube and mixed with an equal volume of 25:24:1 phenol:chloroform:isoamyl alcohol. Centrifugation and recovery of the top aqueous layer was repeated twice. Finally, the aqueous layers were mixed with an equal volume of 24:1 chloroform:isoamyl alcohol and centrifuged as before. DNA was extracted by ethanol precipitation (1/10 vol. of 3 M NaOAc plus, 2 vol. of cold absolute ethanol) for 10 min at −20°C and centrifuged as above. The pellet was transferred to a 1.5 mL tube, washed with 70% ethanol twice, centrifuged (5 min, 5K rpm, 4°C) and air dried. The DNA was dissolved in 2 × 200 μL TE (65°C 10 to 20 min to dissolve, check frequently, not longer than 30 min), 2 μL RNase A (10 μg/μL, Sigma R-6513) was added to each tube and incubated at 37°C for 1 h. A l.5 μL aliquot was run on a 0.7% agarose gel to check if RNA was still present.

Sequencing and annotation methods.

For all C. heterostrophus genomes (except C5), 10 μg of genomic DNA was sheared to >10 kb using Covaris g-Tubes. The sheared DNA was treated with exonuclease to remove single-stranded ends and DNA damage repair mix followed by end repair and ligation of blunt adapters using SMRTbell Template Prep kit 1.0 (Pacific Biosciences). The library was purified with AMPure PB beads and size selected with BluePippin (Sage Science) at >10 kb cutoff size. PacBio Sequencing primer was then annealed to the SMRTbell template library and sequencing polymerase was bound to them using Sequel Binding kit 3.0. The prepared SMRTbell template libraries were then sequenced on a Pacific Biosystems Sequel sequencer using v3 sequencing primer, 1 M v3 SMRT cells, and Version 3.0 sequencing chemistry with 1 × 600 sequencing movie run times. Filtered subread data were processed to remove artifacts and assembled with Falcon version pb-assembly = 0.0.2|falcon-kit = 1.2.3|pypeflow = 2.1.0 (https://github.com/PacificBiosciences/FALCON) to generate an initial assembly. Mitochondrial scaffolds were identified in the initial assembly and reads mapping to this were filtered out. A secondary Falcon assembly was generated using the mitochondria-filtered preads, improved with finisherSC version 2.1 (32), and polished with Arrow version SMRTLINK v7.0.1.66975 (https://www.pacb.com/support/software-downloads/).
Ten μg of C5 genomic DNA was sheared to 30 kb-50 kb using the Megaruptor 3 (Diagenode). The sheared DNA was treated with exonuclease to remove single-stranded ends, DNA damage repair enzyme mix, end-repair/A-tailing mix and ligated with overhang adapters using SMRTbell Express Template Prep kit 2.0 (PacBio) and purified with AMPure PB Beads (PacBio). Individual libraries were size-selected using the 0.75% agarose gel cassettes with Marker S1 and High Pass protocol on the BluePippin (Sage Science). PacBio Sequencing primer was then annealed to the SMRTbell template library and sequencing polymerase was bound to them using Sequel II Binding kit 2.0. The prepared SMRTbell template libraries were then sequenced on a Pacific Biosystems Sequel II sequencer using tbd-sample dependent sequencing primer, 8 M v1 SMRT cells, and Version 2.0 sequencing chemistry with 1 × 900 and 1 × 1800 sequencing movie run times. Filtered subread data were processed to remove artifacts, subsampled to 10% using BBtools (BBMap–Bushnell B; sourceforge.net/projects/bbmap/) version 38.87 [reformat.sh samplerate = 0.1] and assembled with Flye version 2.8.1-b1676 (https://github.com/fenderglass/Flye) to generate an assembly, which was polished with gcpp–algorithm arrow version SMRTLINK v8.0.0.80529 (https://www.pacb.com/support/software-downloads).
For all assemblies, contigs less than 1,000 bp were excluded. Further genome improvement was done using reads recruited to scaffold ends using bbtools (bbduk.sh), reassembled with Flye and polished with finisherSC. Some scaffolds were joined using reassembly of locally mapped reads.
Mitochondria was assembled separately from the Falcon preassembled reads (preads) with an in-house tool using the filtered preads and polished with Arrow version SMRTLink v7.0.1.66975. The C5 mitochondrial DNA was assembled separately with CS reads using an in-house tool and polished with gcpp –algorithm arrow version SMRTLink v8.0.0.80529.
Each genome was annotated using the JGI Annotation Pipeline (35).

Mapping of Tox1 genes and RFLP markers.

Nucleotide sequences for Tox1 genes and selected RFLP markers associated with the Tox1 locus at the intersection of the four-armed linkage group B43, B88, B149, B160, C60, G349, G395, (4) shared between Race O and Race T chromosomes were used to identify Tox1-associated scaffolds in the PacBio assemblies using blast. (Fig. S1, Table S3).

Data availability.

Genome assemblies and annotations for the organisms used in this study are available via the JGI fungal genome portal MycoCosm (35; http://mycocosm.jgi.doe.gov) and have been deposited to GenBank under accession numbers JAQRND010000000, JAQSSY010000000, JAQZZP010000000, JAQZZQ010000000, JARAAY010000000, and JARAAZ010000000.


B.G.T. would like to thank Olen Yoder for introducing her to the remarkable effects of T-toxin and all current and former lab members for their relentless enthusiasm for genetic and genomic discoveries. This work (proposal: https://doi.org/10.46936/10.25585/60001208) conducted by the U.S. Department of Energy Joint Genome Institute (https://ror.org/04xm1d337), a DOE Office of Science User Facility, is supported by the Office of Science of the U.S. Department of Energy operated under Contract No. DE-AC02-05CH11231. A portion of this work was part of the DOE Joint BioEnergy Institute (www.jbei.org) supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, through contract DE-AC02-05CH11231 between Lawrence Berkeley National Laboratory and the U.S. Department of Energy.


This article is a direct contribution from B. Gillian Turgeon, a Fellow of the American Academy of Microbiology, who arranged for and secured reviews by Nancy Keller, University of Wisconsin-Madison, and Thomas Wolpert, Oregon State Univ.

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Information & Contributors


Published In

cover image mBio
Volume 14Number 225 April 2023
eLocator: e00261-23
Editor: Alexander Idnurm, University of Melbourne
PubMed: 36883814


Received: 6 February 2023
Accepted: 9 February 2023
Published online: 8 March 2023


  1. fungi
  2. PacBio
  3. Cochliobolus
  4. secondary metabolite
  5. transposable element
  6. unlinked loci
  7. filamentous fungi
  8. repeats
  9. secondary metabolism
  10. Southern Corn Leaf Blight
  11. virulence determinants



United States Department of Energy Joint Genome Institute, Lawrence Berkeley National Laboratory, Berkeley, California, USA
Jennifer B. González
Section of Plant Pathology & Plant-Microbe Biology, School of Integrative Plant Science, Cornell University, Ithaca, New York, USA
Present address: Jennifer B. González, Biology Department, Nazareth College, Rochester, New York, 14618, USA.
Robert Riley
United States Department of Energy Joint Genome Institute, Lawrence Berkeley National Laboratory, Berkeley, California, USA
Maxim Koriabine
United States Department of Energy Joint Genome Institute, Lawrence Berkeley National Laboratory, Berkeley, California, USA
Mi Yan
United States Department of Energy Joint Genome Institute, Lawrence Berkeley National Laboratory, Berkeley, California, USA
Vivian Ng
United States Department of Energy Joint Genome Institute, Lawrence Berkeley National Laboratory, Berkeley, California, USA
Adriana Rightmyer
Section of Plant Pathology & Plant-Microbe Biology, School of Integrative Plant Science, Cornell University, Ithaca, New York, USA
United States Department of Energy Joint Genome Institute, Lawrence Berkeley National Laboratory, Berkeley, California, USA
Department of Plant and Microbial Biology, University of California Berkeley, Berkeley, California, USA
Functional and systems Biology Group, Environmental Molecular Sciences Division, Earth and Biological Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington, USA
DOE Joint BioEnergy Institute, Emeryville, California, USA
Section of Plant Pathology & Plant-Microbe Biology, School of Integrative Plant Science, Cornell University, Ithaca, New York, USA


Alexander Idnurm
University of Melbourne


Sajeet Haridas and Jennifer B. González contributed equally to the article. The author order was determined by the corresponding author after negotiation.
Scott E. Baker and B. Gillian Turgeon codirected the research.
The authors declare no conflict of interest.

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