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  • Research article
  • Open Access

Comparative genomics of plant pathogenic Botrytis species with distinct host specificity

BMC Genomics201920:203

https://doi.org/10.1186/s12864-019-5580-x

  • Received: 31 August 2018
  • Accepted: 3 March 2019
  • Published:

Abstract

Background

Fungi of the genus Botrytis (presently containing ~ 35 species) are able to infect more than 1400 different plant species and cause losses in a wide range of crops of economic importance. The best studied species is B. cinerea, which has a broad host range and is one of the best studied necrotrophic plant pathogenic fungi. Most other Botrytis spp. have a narrow host range and have been studied in less detail. To characterize genomic variation among different representatives of Botrytis spp., we sequenced and annotated the draft genomes of nine Botrytis species: B. calthae, B. convoluta, B. elliptica, B. galanthina, B. hyacinthi, B. narcissicola, B. paeoniae, B. porri and B. tulipae.

Results

Bioinformatics and comparative genomics tools were applied to determine a core of 7668 shared protein families in all Botrytis species, which grouped them in two distinct phylogenetic clades. The secretome of all nine Botrytis spp. was similar in number (ranging from 716 to 784 predicted proteins). A detailed analysis of the molecular functions of the secretome revealed that shared activities were highly similar. Orthologs to effectors functionally studied in B. cinerea were also present in the other Botrytis species. A complex pattern of presence/absence of secondary metabolite biosynthetic key enzymes was observed.

Conclusions

Comparative genomics of Botrytis show that overall, species share the main signatures and protein families in the secreted proteins, and of known effectors. Our study provides leads to study host range determinants in the genus Botrytis and provides a stepping stone to elucidate the roles of effector candidates in the infection process of these species.

Keywords

  • Effector
  • Grey mould
  • Necrotroph
  • Secondary metabolite
  • Secretome

Background

The fungal genus Botrytis comprises ~ 35 species, which all interact with plants [1, 2]. With one exception, they are notorious pathogens with a necrotrophic infection behaviour, i.e. they kill host cells and invade the dead cells to acquire nutrients. The single exception, B. deweyae, can grow as a symptomless endophyte inside its host plant Hemerocallis, and occasionally switches lifestyle into a necrotizing infection destroying the young foliage, thereby causing ‘spring sickness’ [3]. The two most extensively studied species with a necrotrophic lifestyle, B. cinerea and B. pseudocinerea, are morphologically indistinguishable and cause grey mould on a broad range of (> 1400) host plant species [4]. Other Botrytis species are considered to be restricted to a single host species or a small number of taxonomically related hosts [5]. Phylogenetic analysis has separated the genus Botrytis into two distinct clades, one of which comprises B. cinerea and B. pseudocinerea along with a small number of species pathogenic on dicot plants, while the second (large) clade comprises mainly species pathogenic on monocot host plants, and especially on bulb-producing monocot species [1, 2, 5]. Many ornamental bulb flower crops of economic relevance (tulip, lily, Narcissus, snowdrop, Gladiolus) are each infected by a separate specialized Botrytis species [5].

Fungi with a necrotrophic lifestyle achieve plant cell death by the induction of an apoptotic programmed cell death in the host [6]. Rather than brutally killing the host cells, these pathogens co-opt the plant cell death machinery to their own benefit. Evidence is accumulating that many plant pathogenic fungi with a necrotrophic lifestyle secrete effector molecules (proteins and/or secondary metabolites) that trigger the host cell death machinery, following effector recognition by a cognate receptor protein in the host plant [7]. In interactions of Parastagonospora nodorum with its host (wheat), several cases have been described of inverse gene-for-gene interactions, in which a P. nodorum effector protein genetically interacts with a single genetic locus in the host that confers response to the effector and susceptibility to a fungal isolate producing this effector [8]. The cyclic non-ribosomal peptide victorin from Cochliobolus victoriae also triggers apoptotic programmed cell death in plants carrying the genetic locus for sensitivity [9, 10].

Several plant cell death-inducing proteins and metabolites were identified in B. cinerea [1114], all of which act indiscriminately on all dicot plants that were tested. None of these necrotrophic effectors was essential for virulence of B. cinerea and none of them is considered to play a role in host specificity. Yet, there is evidence for the production of necrotrophic effectors in some specialised Botrytis species. The lily pathogen B. elliptica produced in liquid culture a proteinaceous effector activity that caused apoptotic cell death exclusively in lily, and conferred upon the non-pathogen B. cinerea the capacity to infect lily following effector infiltration [15]. Also the broad bean pathogen B. fabae was reported to produce a host-specific effector activity [16]. In order to understand the mechanisms of infection of host-specific Botrytis species, as compared to the broad host range pathogen B. cinerea, we generated draft genome sequences of nine Botrytis species, mostly selected from the phylogenetic clade of monocot-infecting pathogens, with emphasis on species infecting ornamental flower bulb crops. The genomes were analysed with special attention for the presence of genes that potentially contribute to the host specificity, such as genes encoding effector proteins, enzymes involved in secondary metabolite biosynthesis or enzymes involved in the degradation of monocot versus dicot cell wall polysaccharides.

Results and discussion

Sequencing and assembly

The genome assembly sizes of the nine Botrytis species ranged from 43 Mb to 55 Mb (Table 1). The genomes of eight species are similar in size to the previously described genome of B. cinerea (43.5 Mb; [17]), while the genome of B. narcissicola was ~ 10 Mb larger than the other species analysed here. Contigs that contained mitochondrial genes were identified with Blast and removed from the assemblies. To estimate the completeness of the assembled genomes, Benchmarking Universal Single-Copy Orthologs (BUSCO) was used. This analysis indicated that all genomes had a high level of completeness (96.3–99.2%), with the B. elliptica genome being the most complete. The assemblies are predicted to encode between 12,033 and 12,663 protein-coding genes.
Table 1

Assembly and gene prediction information of Botrytis spp. genomes

Species

Scaffolds

Assembly Size

Largest Scaffold

N50

BUSCO complete/partial

Predicted genes

Secretome size

% of secreted proteins

B. calthae

3985

47.56 Mb

293,700

56,053

97.5 (98.5)

12,492

745

5.96

B. convoluta

2054

45.40 Mb

436,056

97,955

98.7 (99.1)

12,532

752

6.00

B. elliptica

5594

47.68 Mb

230,072

36,976

99.2 (99.3)

12,663

762

6.02

B. galanthina

3422

43.97 Mb

354,402

65,043

98.9 (99.0)

12,575

784

6.23

B. hyacinthi

2509

43.91 Mb

532,180

115,520

99.0 (99.2)

12,197

752

6.17

B. narcissicola

8392

54.85 Mb

289,279

43,203

96.3 (98.1)

12,341

730

5.92

B. paeoniae

1833

46.36 Mb

901,367

125,168

99.0 (99.2)

12,138

746

6.15

B. porri

4737

43.11 Mb

181,158

35,774

97.4 (97.9)

12,033

716

5.95

B. tulipae

653

45.45 Mb

738,496

172,411

98.8 (99.0)

12,656

750

5.93

Phylogenetics and phylogenomics

A phylogenetic tree was constructed of 7668 conserved core genes based on the amino acid alignment of ~ 3.7 million positions, and using Sclerotinia sclerotiorum as outgroup (Fig. 1). The relationship among the Botrytis species is in accordance with previous studies by [5], which divided the genus in two clades based on three protein-coding genes (G3PDH, HSP60 and RPB2). Clade 1 contains species that mainly infect eudicot plants, while species in Clade 2 infect either eudicot or monocot plants (but mainly monocots). A pan-genome analysis for the 9 Botrytis species sequenced in this study in combination with the previously sequenced B. cinerea B05.10 [17], indicated that the core genome of Botrytis spp. consists of 7617 orthologous gene clusters (Fig. 2a). On the other hand, the pan-genome consists of 12,245 orthologous gene clusters (Fig. 2b).
Fig. 1
Fig. 1

Phylogenetic tree based on single-copy orthologous genes of different Botrytis species, with S. sclerotiorum as the outgroup to root the tree. All branches have a high bootstrap support (ML > 90). Two clades previously reported in the genus Botrytis are highlighted. The bulb plant symbols next to the species names indicate species that infect monocotyledonous plants, species without the symbol infect dicotyledonous host plants

Fig. 2
Fig. 2

Pan-genome analysis of Botrytis spp. a Estimation of the Botrytis spp. core genome, in which the number of shared genes is plotted as a function of the number of species sequentially added. b Estimation of Botrytis spp. pan-genome size, in which the numbers of all genes are plotted as a function of the number of species sequentially added.

GC content distribution

Previous studies have revealed that some fungal plant pathogens have distinct patterns in their genome, a so called two-speed genome, which consist of an alternation of repeat-rich and gene sparse regions with GC-equilibrated and gene dense regions [18, 19]. Moreover, it has been reported that repeat-rich regions can display an enrichment of rapidly evolving genes [20]. Analysis of the GC content in Botrytis spp. showed the presence of a bimodal distribution of GC content. The proportion of AT-rich regions in the genomes ranged from 4.86% in B. cinerea up to 27% in B. narcissicola (Table 2). The difference in genome sizes of the nine species, as compared to B. cinerea, is correlated with the proportion of AT-rich regions, as shown in B. narcissicola, the largest among the Botrytis spp. sequenced up to date (Table 2). Illumina-based sequence assemblies tend to lack repeats, especially AT-rich repeats [21]. Therefore it can be assumed that a more complete assembly of the nine genomes (based on long read technology) would have an even higher content of AT-rich repeats. Interestingly, B. cinerea has the lowest proportion of AT-rich regions in its genome even though it was sequenced with long read technology, which would encompass more difficult to assemble regions such as repetitive regions.
Table 2

GC content distribution among Botrytis spp.

 

BCAL

BCIN

BCON

BELL

BGAL

BHYA

BNAR

BPAE

BPOR

BTUL

Genome Size

47.4

42.6

45.3

47.6

43.9

43.8

54.8

46.2

43

45.3

%AT rich regions

15.6

4.86

12.6

12.7

6.45

9.79

27

15.9

7.6

9.95

GC peak in AT rich regions

32.5

18.2

26.4

18.1

18.1

25

27

30.5

19.7

16.2

Genes in AT rich regions

99

0

8

4

1

6

17

33

4

2

Gene density in AT rich regions

13.3

0

1.4

0.66

0.35

1.4

1.15

4.49

1.22

0.44

Range of GC content in AT rich regions

0–37

0–27.4

0–33.6

0–28.6

0–27.7

0–32.6

0–35.4

0–36

0–28.7

0–27.5

The number of genes located in these AT-rich regions differed per species, with B. calthae having the highest number of genes (99) and B. cinerea the lowest (0). In order to elucidate if the AT-rich regions resulted from Repeat-Induced Point mutation (RIP), a meiosis-specific mechanism that affects duplicated sequence regions [22, 23], we compared the frequencies of dinucleotides between AT-rich regions and GC-equilibrated regions (Additional file 1). The AT-rich regions contained a strongly elevated frequency of TpA, which is the primary product of RIP in fungi, and is a strong indicator of RIP activity in these genomes [22]. Furthermore, RIP indices (ratio of TpA/ApT dinucleotide frequency) indicate RIP activity in all Botrytis spp. analysed here (Additional file 2).

Secretome functions and effector proteins

The secretome of all nine Botrytis spp. was similar in number (ranging from 716 proteins in B. porri to 784 in B. galanthina), representing 5 to 6% of the total proteome (Table 1), in line with what was reported for B. cinerea [17, 24]. The molecular function of the secreted proteins could be annotated for more than 50% of the secretome of each species (Fig. 3). There were no significant differences between the species in any of the Gene Ontology categories (error bars in Fig. 3 denote variability between species). Hydrolase activity (GO:0016787) was the most common molecular function of the secretome for all Botrytis species (around 25% of the total secretome), similar to what was reported previously for B. cinerea [25]. B. galanthina, B. hyacinthi and B. cinerea have the highest number of genes with hydrolase activity (approx. 230), while B. porri and B. calthae have the lowest number (approx. 200 genes). Half of the genes with hydrolase activity are plant cell wall degrading enzymes (PCWDEs, Table 3) with a potential role in host tissue decomposition and pathogenesis. Oxidoreductase activity (GO:0016491) was present in the secretome of all Botrytis species, representing 10% of the total secretome. This activity was more abundant in B. galanthina, B. cinerea and B. tulipae with approx. 70 proteins, while it was less abundant in B. porri (56 proteins). Transferase activity (GO:0016740) was present in similar levels as the oxidoreductase activity (10% of the total secretome) and B. porri was among the species with a higher number of proteins (approx. 70) along with B. elliptica and B. cinerea, while B. convoluta and B. calthae had a lower number of proteins with this activity (approx. 60). The final fairly abundant molecular function was isomerase activity (GO:0016853), accounting for 5% of the total secretome in all the species. The annotations of the secreted proteins in the GO Biological Process domain were mainly related to carbohydrate metabolic processes (GO:0005976, GO:0000272, GO:0044262), cell wall (GO:0071554) and pigment processes (GO:0042440), while annotations in the GO Cellular Component domain were mainly the cell periphery (GO:0071944) and cell wall (GO:0005618), as is expected from secreted proteins (Additional file 3). The 10 Botrytis species did not show significant differences in the number of genes in any of the categories of these GO domains.
Fig. 3
Fig. 3

Number of genes encoding secreted proteins in Botrytis species grouped by GO annotation for the Molecular Function domain. The average of all species is shown. The error bars indicate the deviation in number of genes between the species

Table 3

Comparison of plant cell wall degrading enzymes (PCWDEs) among Botrytis spp.

Species

Cellulose

Hemicellulose

Hemicellulose or pectin side chains

Pectin

Plant or Fungal CWDEs

Total

B. calthae

17

30

12

32

21

112

B. cinerea

20

34

13

38

27

132

B. convoluta

18

31

12

34

25

120

B. elliptica

19

36

11

35

22

123

B. galanthina

19

35

12

36

23

125

B. hyacinthi

20

34

13

35

23

125

B. narcissicola

20

31

11

28

21

111

B. paeoniae

19

31

12

32

22

116

B. porri

19

30

12

33

22

116

B. tulipae

18

32

11

29

22

112

All secreted proteins of the nine newly sequenced species and B. cinerea were clustered using Orthofinder and out of 7521 proteins, 7189 were assigned to 901 orthologous groups. A total of 393 orthologous groups were shared among all 10 Botrytis species (Fig. 4). Orthologs to known effectors or virulence factors from B. cinerea were present in this subset. For instance, the Bcxyn11A gene which encodes a necrosis-inducing xylanase [26], and Bcpg1 and Bcpg2, encoding endopolygalacturonases that are required for full virulence [27, 28], are present in all Botrytis spp. Furthermore, all Botrytis spp. contained an ortholog to the Bcpls1 gene, which encodes an integral membrane tetraspanin protein that participates in B. cinerea appressorium function and is required for full virulence [29]. Orthologs of the Bcspl1 gene, encoding a protein with a cerato-platanin domain that elicits plant defences and is required for full virulence [11] are also present in all Botrytis spp. The nep1 and nep2 genes, encoding phytotoxic effector proteins that induce necrosis and the synthesis of ethylene in plant leaves, are also present in all Botrytis spp., as previously reported [30, 31]. There are proteins from other plant pathogenic fungi that are involved in pathogenesis and contain a CFEM domain, which contains eight cysteines [3234]. The mechanism by which CFEM domain-containing proteins act is unknown but a recent study reported that B. cinerea lacking BcCFEM1 is less virulent [35]. Three orthogroups in the secretome contain a CFEM domain, of which the orthogroup of BcCFEM1 is present in all Botrytis spp., another is shared between B. calthae, B. cinerea, B. elliptica, B. hyacinthi, B. paeoniae and B. porri, and the third orthogroup is only present in B. porri. Only the orthogroup shared between all Botrytis spp. is considered an effector based on EffectorP.
Fig. 4
Fig. 4

Presence/absence of orthologous groups of secreted proteins among Botrytis species. Species are listed in the order defined by the phylogeny in Fig. 1. Dark/white circles indicate the presence/absence of different subgroups in each species. The number of orthologous groups for each subgroup is shown above the bars. The total number of secreted proteins for each species is indicated in the left black box. The black line represents the number of predicted effectors per species in each subgroup. Two subgroups on the far right hand size represent proteins that are present only in species from Clade 1, or in species from Clade 2, respectively

Besides the orthogroups shared by all ten Botrytis spp., an additional 332 orthologous groups are unique to only one single species (columns 2–11 in Fig. 4) and 102 groups (columns 12–18 in Fig. 4) are common to all species but one. Nine orthologous groups are present only in the two species from clade 1 (B. cinerea and B. calthae) and seven are present exclusively in the species from clade 2 (last two columns in Fig. 4) [5]. The nine orthologous groups unique to B. cinerea and B. calthae contain 5 candidate effectors of 9–15 kDa. Additionally, a hydrolase (hydrolyzing O-glycosyl compounds), a protein kinase and an oxidoreductase were present in orthologous groups unique to B. cinerea and B. calthae. The seven orthologous groups unique to the other eight species contain mainly hydrolytic enzymes (one serine-type peptidase (GO:0008236), one hydrolase acting on glycosyl bonds (GO:0004553), one asparaginase (GO:0004067) and one G1 endopeptidase (GO:0004190)) but no effectors.

In order to elucidate whether putative effector-encoding genes were clustered near repeats, and thus associated with rapidly evolving regions of the genome, we calculated the distances to the nearest repeat for all putative effector-encoding genes and compared them to the distances of a random subset of non-effector-encoding genes to the nearest repeat. Among the 10 species analysed, putative effector genes were on average significantly closer to repeats only in B. cinerea and B. galanthina but not in the other species (Additional file 4). The biological repercussion of this observation for B. cinerea and B. galanthina is unclear, however, there is no general tendency in Botrytis of proximity of effector genes to repeats, as was also observed in S. sclerotiorum, a close relative of Botrytis [36].

The orthologous groups of proteins that are unique for one of the Botrytis species are shown in columns 2–11 of Fig. 4 (for a complete list, see Additional file 5). The different GO molecular functions and effector predictions for these species-specific proteins are shown in Fig. 5. There are 8–19 unique effectors in each species. Whether such species-specific effectors serve as host range determinants requires further functional studies, including the analysis of gene expression during host plant infection, the construction of targeted knockout mutants and the use of effector proteins (produced in heterologous expression systems) in assays for host-specific cell death induction.
Fig. 5
Fig. 5

Unique proteins in the secretome for each Botrytis species. “Effector” proteins have been predicted with EffectorP. “Unknown protein” category refers to proteins for which no conserved domain has been found. “Unknown function” comprises proteins for which conserved domains have been found but no molecular function has been associated with the domain. The categories “Hydrolase activity”, “Ion binding”, “Protein binding”, “Transferase activity” and “Lyase activity” include proteins with a conserved domain associated with the GO molecular functions GO:0016787, GO:0043167, GO:0005515, GO:0016740, GO:0016829 or any of their subcategories, respectively

Furthermore, each of the 10 species has its own set of unique hydrolases, however, only few of these hydrolase activities are unique for a single species: only B. cinerea possesses a unique lyase and B. calthae possesses a unique transferase activity, for which orthologs are undetected in the other nine species. Among the secreted enzymes that are unique to single Botrytis species could be enzymes that detoxify antifungal secondary metabolites produced by host plants as defense against pathogens, collectively known as phytoanticipins or phytoalexins. In order to effectively colonize its host, a pathogenic fungus must be able to inactivate the antifungal compound. Such enzymatic inactivation can involve oxidation or hydrolysis. There is ample evidence for the capacity of several Botrytis species to detoxify host antifungal compounds: B. cinerea can degrade resveratrol [37] from grapes and α- tomatine [38] in tomato, B. tulipae degrades tulipalin [39] from tulips, and B. fabae degrades wyerone acid [40] in broad bean. The genes encoding such enzymes have not been identified with the exception of the B. cinerea laccase gene Bclcc2 [41]. For most Botrytis species, however, even the phytoalexins in their host plants remain to be chemically characterized, and it remains elusive which types of detoxifying enzymes could play a role in pathogenicity.

We further analysed a subset of the secretome that is related to the degradation of plant cell wall carbohydrates, as this is an important process during host plant infection. The genomes of the nine Botrytis spp. contain between 111 and 125 plant cell wall degrading enzymes (PCWDEs), slightly less than in B. cinerea (132; see Table 3). The PCWDEs can be further subdivided depending on the substrate that they degrade: cellulose, hemicellulose, and pectin. Overall, the number of secreted enzymes capable of degrading cellulose, hemicellulose and pectin is very similar among Botrytis spp., except for B. narcissicola and B. tulipae, where the number of pectin degrading enzymes is lower (28 and 29, respectively). The pectin content of plants can differ significantly, especially between monocot and dicot hosts [42]. Despite the fact that seven of the ten Botrytis spp. analysed here infect monocots, the content of PCWDE encoding genes does not differ. This may be correlated with the fact that the monocot hosts of these seven species are members of the Asparagales and Liliales, known to have a relatively high levels of pectin [42] as compared to Gramineae. The large number of genes encoding pectin degrading enzymes in the Botrytis species infecting bulb flower crops therefore should not be considered unusual.

Secondary metabolite gene clusters

Fungi are able to produce a wide array of compounds, defined as secondary metabolites (SM), that help them adapt and survive in different environments and compete with other organisms [43]. In B. cinerea, more than 40 secondary metabolite gene clusters have been identified, but only a few of the metabolites have been completely characterized, such as the well-characterized phytotoxins botrydial and botcinic acid (reviewed by [44]). The nine draft genomes were examined for the presence of secondary metabolite clusters present in B. cinerea by homology to the B. cinerea reference genome (Fig. 6). The two key enzymes related to the production of melanin (Bcpks12 and Bcpks13), a key enzyme related to carotenoid synthesis, retinal (Bcphs1), and a key enzyme putatively involved in synthesis of pyrones, resorcylic acids and resorcinols (Bcchs1), are present in all nine species. Also key enzymes for the synthesis of coprogene siderophore (Bcnrps6) and other siderophores (Bcnrps2, Bcnrps3), that are found across all ascomycetes [45], are shared between Botrytis species. For other SM key enzyme-encoding genes, the distribution was heterogeneous. The key enzymes for the production of botcinic acid (Bcboa6/Bcpks6 and Bcboa9/Bcpks9) turned out to both be present in B. calthae, B. convoluta, B. narcissicola and B. porri, whereas B. elliptica, B. galanthina and B. tulipae only contained Bcboa9/Bcpks9 but not Bcboa6/Bcpks6. The SM key enzymes of the botcinic acid cluster were totally absent in B. hyacinthi and B. paeoniae. The key enzyme for botrydial synthesis (Bcbot2/Bcstc1) was present in B. elliptica, B. paeoniae and B. porri but not in the other species. The key enzyme for production of abscisic acid (Bcaba4) was present in B. calthae, B. galanthina, B. narcissicola and B. porri but not in other species.
Fig. 6
Fig. 6

Visualization of the presence/absence of the secondary metabolite biosynthetic key enzymes of B. cinerea in nine other Botrytis species and S. sclerotiorum. The gene symbols for each key enzyme are along the perimeter of the outer circle depicting the 18 B. cinerea chromosomes (not to scale). The inner tracks with coloured blocks represent the presence of the secondary metabolite biosynthetic key enzymes for each species

The remainder of the B. cinerea key enzymes are involved in the synthesis of metabolites with an unknown function, but they can be classified based on the chemical nature of the metabolite synthesized by the gene cluster: peptides (non-ribosomal peptide synthetase), polyketides (polyketide synthase), terpenes (terpene synthase) and alkaloids (dimethylallyl tryptophane synthase). Of those, orthologs of some key enzymes are shared between all species (five polyketides, four peptides, and one alkaloid), some are only present in B. cinerea (indole-terpene Bcpax1 and peptide-polyketide Bcpks7), and for the rest of the key enzymes, the presence/absence varies per species (Additional file 6). In total, of the 43 key enzymes present in B. cinerea, B. calthae and B. galanthina shared the highest number of orthologs (31 key enzymes), while B. paeoniae and B. porri shared the lowest number (24 key enzymes) with B. cinerea, respectively. In total, there are 17 SM key enzymes that are present in all Botrytis species, which is noticeably low, when considering that there are 16 SM key enzymes shared between the more distantly related S. sclerotiorum and B. cinerea (Fig. 6). The SM key enzymes that are shared between S. sclerotiorum and B. cinerea, and B. cinerea and the rest of the Botrytis spp. are not the same, and only 11 key enzymes overlap between all Botrytis species and S. sclerotiorum.

The result of this comparison differs from a previous study which reported that 19 SM key enzymes were shared between S. sclerotiorum and B. cinerea [24]. This difference results from improvements of the assemblies and annotations in each species [17, 36]. For instance, the gene sizes of pks1 and pks18, which were previously reported to be shared between S. sclerotiorum and B. cinerea, have changed considerably, around 10% in the case of Bcpks1, and 64 and 300%, for Bcpks18 and Sspks18, respectively. Also, Ssdmats1 which had homology to Bcdmats1, is now more closely related to Bcdmats2. Interestingly, the updated annotation of B. cinerea B05.10 no longer contains a gene that encodes the diterpene cyclase Bcdtc2, which highlights the importance of having a good genome assembly and annotation.

The distribution of SM key enzyme genes among the ten Botrytis species analysed here is very patchy. The chemical structure of metabolites produced by most Botrytis SM gene clusters remain to be characterized, and there is no information under which conditions these gene clusters are expressed. Altogether, it is difficult to pinpoint SM gene clusters that may be interesting candidates for functional studies that aim to identify host range determinants.

Conclusions

In this study we present the draft genome of nine species of Botrytis along with a comparative analysis with the previously sequenced B. cinerea. Our results show that overall Botrytis species share the main signatures and protein families in the secreted proteins. Furthermore, all effectors that were previously functionally characterized in B. cinerea are also present in other members of the genus. Analysis of the secondary metabolite biosynthetic gene clusters also gave insights in the complexity of presence/absence of these clusters. These genome data will provide leads to design hypotheses about candidate host range determinants in the Botrytis genus, either in the effector repertoire or in genes that participate in the tolerance to antimicrobial compounds of the host plant species. In the present study we especially focused on Botrytis species infecting ornamental flower bulb crops that are dispersed through the phylogeny of the species. The analysis of genomes of additional Botrytis species that are phylogenetically more closely related to each other will enable to zoom in on a further characterization of secreted proteins with potential roles in the infection process.

Methods

Strains and culture conditions

The fungal species used for this study are listed in Table 4. All fungal species were kept as conidial suspensions in 15% glycerol at − 80°C for long storage and grown on malt extract plates at 21°C before DNA extraction. DNA extraction was done as described by [46].
Table 4

Information about the strains used in this study

Botrytis sp.

Acronym

Isolate

Main host

B. calthae

BCAL

MUCL2830

Caltha pallustris

B. cinerea

BCIN

B05.10

> 1000 host species

B. convoluta

BCON

MUCL11595

Iris

B. elliptica

BELL

Be9601

Lilium

B. galanthina

BGAL

MUCL435

Galanthus

B. hyacinthi

BHYA

Bh0001

Hyacinthus

B. narcissicola

BNAR

M2120

Narcissus

B. paeoniae

BPAE

Bp 0003

Paeonia section paeoniae

B. porri

BPOR

MUCL3349

Allium porrum

B. tulipae

BTUL

Bt9001

Tulipa

Sequencing and assembly

All sequences were sequenced with a paired-end library of 150 bp insert size, with a read length of 2 × 91 bp, except for B. tulipae, in which a paired-end library of 400 bp insert size, and an additional mate pair library of 3.5 kb were constructed. DNA from B. tulipae was sequenced by Macrogen (Korea) while DNA from the other eight species was sequenced by DNAVision (Liège, Belgium). De novo assembly was performed using the improved A5 pipeline [47], and further improved with Gapcloser from the Soapdenovo assembler [48]. For B. tulipae, assembly was performed using Spades assembler v.3.10.1 [49]. Completeness of the genome assemblies was assessed by the Benchmarking Universal Single-Copy Orthologs (BUSCO) v.2.0.1 software tool [50]. The GC content distribution of the sequenced genomes was examined using OcculterCut with default settings [19], and the analysis of repeat induced point mutations (RIP) was performed with RIPCAL [22].

Genome annotation

The assembled genomes were annotated using the MAKER (v.2.31.9) pipeline [51]. Before annotation, a species-specific repeat library was constructed using RepeatModeler (v.1.0.8) in order to mask repeats [52]. Gene models were predicted with AUGUSTUS [53], SNAP [54], and GeneMark-ES [55] ab initio gene predictors. The gene models of the manually curated genome of B. cinerea, and all the fungal proteins available in the Swissprot database were provided as evidence for gene prediction. The predicted proteins were functionally annotated using BLASTp [56] against the non-redundant database of the National Center of Biotechnology Information (NCBI) and classified by InterProScan and Pfam analysis [57].

Phylogenetic and phylogenomic analysis

The phylogenetic relationships were determined between all species sequenced, including the previously sequenced B. cinerea B05.10 [17], and using S. sclerotiorum [36] as the outgroup of the tree. The tree was constructed using 7668 single-copy orthologue genes, identified with Orthofinder [58]. The protein sequence for each gene was aligned and concatenated into a single matrix using MAFFT [59], and a maximum likehood phylogenetic tree was inferred with RAxML v.8.2.10 [60] using a generalized time reversible (GTR) plus GAMMA amino acid substitution model with 100 rapid bootstraps. A pan-genome analysis was done to calculate the number of core genes and was estimated using OrthoMCL [61] implemented in GET_HOMOLOGUES-EST [62] with e-value 1e− 5 and 75% coverage. For the pangenome analysis, only the orthogroups present in at least two species were included.

Secretome, GOterm and effector prediction

Genes encoding putatively secreted proteins were identified for each Botrytis genome using several prediction tools. Signal-P v4.1 [63] was initially used to screen for a signal peptide, followed by TMHMM v.2.0 [64] to identify putative transmembrane domains. Proteins that did not have a signal peptide, or that had a transmembrane domain (a single transmembrane domain in the first 60 amino acids was allowed) were discarded. WoLF PSORT was used to predict protein localization [65]. Effectors were predicted using the EffectorP tool [66]. The GO enrichment in molecular functions was produced with the dcGO:database [67] and InterPro v66.0 [68]. To determine whether there was a relation between putative effector proteins and repeats, the distance to the nearest repeats was measured. Likewise, the distance between non-effector proteins and repeats was measured, using a random subset containing the same number of proteins as the set of effector proteins for that same species. The distances for the putative effector proteins were compared to the distances for the random non-effector proteins using Wilcoxon’s test in R (version 3.5.2).

Secondary metabolite analysis

Identification of genes involved in the biosynthesis of secondary metabolites were based on homology to previously described genes in Botrytis cinerea and S. sclerotiorum [24, 44]. Sequences of the key enzymes were subjected to a BLASTn analysis to all Botrytis sp. genomes using an e-value of 1e− 5 and a sequence identity of 70%. Moreover, putative gene clusters that are predicted to be involved in biosynthesis of secondary metabolites were identified using antiSMASH (Antibiotics and Secondary Metabolite Analysis SHell) version 4.0.1 [69].

Abbreviations

BUSCO: 

Benchmarking Universal Single-Copy Orthologs

G3PDH: 

Glyceraldehyde 3-phosphate dehydrogenase protein

GO: 

Gene Ontology

HSP60: 

Heat shock protein 60

PCWDE: 

Plant cell wall degrading enzyme

RIP: 

Repeat-Induced Point mutation

RPB2: 

RNA polymerase II protein

SM: 

Secondary Metabolites

Declarations

Acknowledgements

The authors acknowledge the contribution of Alicia Abarca, Si Qin, Michele Malvestiti, Guido Puccetti, and Maikel Steentjes in the annotation of the nine genomes.

Funding

The research of Claudio A. Valero-Jiménez is supported by the Dutch Technology Foundation STW, which is part of the Netherlands Organisation for Scientific Research (NWO), and which is partly funded by the Ministry of Economic Affairs (project 15003). Javier Veloso was funded by the Postdoctoral program of the Xunta de Galicia (Spain). The funding organizations were neither involved in the design of the study and collection, analysis, and interpretation of data, nor in writing the manuscript.

Availability of data and materials

The Whole Genome Shotgun project has been deposited in GenBank under the Bioproject number PRJNA401386. The genomes are deposited under the genome accession numbers: PHWZ00000000 (B. calthae), PQXN00000000 (B. convoluta), PQXM00000000 (B. elliptica), PQXL00000000 (B. galanthina), PQXK00000000 (B.hyacinthi), PQXJ00000000 (B. narcissicola), PQXI00000000 (B. peaoniae), PQXO00000000 (B. porri), and PQXH00000000 (B. tulipae).

Authors’ contributions

JvK and MS conceived the study. MS performed DNA extractions and performed preliminary assemblies. CAVJ performed the final assemblies and gene predictions. CAVJ, JV and JvK analyzed the data. CAVJ, JvK and JV prepared the figures and the manuscript. All authors read and approved the final manuscript.

Ethics approval and consent to participate

Not applicable

Consent for publication

Not applicable

Competing interests

The authors declare that they have no competing interests.

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Authors’ Affiliations

(1)
Laboratory of Phytopathology, Wageningen University, 6708PB Wageningen, the Netherlands
(2)
Department of Biology, Faculty of Sciences, University of A Coruña, A Coruña, Spain
(3)
Biosystematics Group, Wageningen University, 6708PB Wageningen, the Netherlands
(4)
Present address: RIKILT Wageningen University and Research, 6708WB Wageningen, the Netherlands

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© The Author(s). 2019

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