Orthology Inference is the process of detecting orthologous genes between a query organism and a set of subject organisms. Motivated by the discussion on which orthology inference method, paradigm or program is the most accurate or the fastest one, the orthologr package provides functions to perform genome wide orthology inference following two different paradigms:

  1. orthology inference based on 1-1 orthology relationship using BLAST reciprocal best hit

  2. orthology inference based on 1-many homology relationship using orthologous groups (OrthoFinder2)

To perform orthology inference you can start with the orthologs() function provided by orthologr. The orthologs() function takes nucleotide or protein sequences stored in fasta files for a set of organisms as input and performs orthology inference to detect orthologous genes or orthologous groups within the given organisms based on selected orthology inference programs.

The following interfaces are (yet) implemented in the orthologs() function:

BLAST based methods:

Advantages:

Disadvantages:

Orthogroup based methods:

Advantages:

Disadvantages:

Examples: BLAST based methods

Using a simple example stored in the package environment of orthologr you can get an impression on how to use the orthologs() function.

Note: it is assumed that when using orthologs() all corresponding programs you want to use are already installed on your machine and are executable via either the default execution PATH or you specifically define the location of the executable file via the path argument that can be passed to orthologs().

In the following examples, we will use toy fasta files stored within the orthologr package: system.file('seqs/ortho_thal_aa.fasta', package = 'orthologr').

Running blastp: 2.9.0+ ...


Building a new DB, current time: 07/06/2019 16:22:29
New DB name:   blastdb_ortho_lyra_aa.fasta_protein.fasta
New DB title:  blastdb_ortho_lyra_aa.fasta_protein.fasta
Sequence type: Protein
Deleted existing BLAST database with identical name.
Keep Linkouts: T
Keep MBits: T
Maximum file size: 1000000000B
Adding sequences from FASTA; added 20 sequences in 0.00440097 seconds.
Running blastp: 2.9.0+ ...


Building a new DB, current time: 07/06/2019 16:22:30
New DB name:   blastdb_ortho_thal_aa.fasta_protein.fasta
New DB title:  blastdb_ortho_thal_aa.fasta_protein.fasta
Sequence type: Protein
Deleted existing BLAST database with identical name.
Keep Linkouts: T
Keep MBits: T
Maximum file size: 1000000000B
Adding sequences from FASTA; added 20 sequences in 0.003829 seconds.
# A tibble: 20 x 21
# Groups:   query_id [20]
   query_id subject_id perc_identity num_ident_match... alig_length
   <chr>    <chr>              <dbl>            <int>       <int>
 1 AT1G010... 333554|PA...          74.0              347         469
 2 AT1G010... 470181|PA...          91.1              224         246
 3 AT1G010... 470180|PA...          95.5              343         359
 4 AT1G010... 333551|PA...          92.0             1812        1970
 5 AT1G010... 909874|PA...         100                213         213
 6 AT1G010... 470177|PA...          89.5              580         648
 7 AT1G010... 918864|PA...          95.1              348         366
 8 AT1G010... 909871|PA...          90.3              271         300
 9 AT1G010... 470171|PA...          96.8              420         434
10 AT1G011... 333544|PA...          93.6              494         528
11 AT1G011... 918858|PA...          99.2              525         529
12 AT1G011... 470161|PA...          98.4              446         453
13 AT1G011... 918855|PA...          72.6              207         285
14 AT1G011... 918854|PA...          84.9              152         179
15 AT1G011... 311317|PA...          85.6               83          97
16 AT1G011... 909860|PA...          92.6              287         310
17 AT1G011... 311315|PA...          94.2              502         533
18 AT1G012... 470156|PA...          95.8              228         238
19 AT1G012... 311313|PA...          95.3              102         107
20 AT1G012... 470155|PA...          96.7             1021        1056
# ... with 16 more variables: mismatches <int>, gap_openings <int>,
#   n_gaps <int>, pos_match <int>, ppos <dbl>, q_start <int>,
#   q_end <int>, q_len <int>, qcov <int>, qcovhsp <int>,
#   s_start <int>, s_end <dbl>, s_len <dbl>, evalue <dbl>,
#   bit_score <chr>, score_raw <dbl>
>

This small example returns 20 orthologous genes between Arabidopsis thaliana and Arabidopsis lyrata. As you can see, the query_file and subject_files arguments take the proteomes of Arabidopsis thaliana (query_file) and Arabidopsis lyrata (subject_files) stored in fasta files. The seq_type argument specifies that you will pass protein sequences (proteomes) to the orthologs() function. In case you only have either genomes (DNA sequences) or CDS files, you can modify the seq_type argument to seq_type = "dna" (when working with only genome data) or seq_type = "cds" (when working with CDS files). Internally the orthologs() function will perform a CDS prediction using predict_cds() and will furthermore translate the predicted CDS sequences into protein sequences. Analogously when seq_type = "cds" is specified, internally the orthologs() function will translate all CDS sequences into protein sequences to run orthology inference based on protein sequences.

The advantage of this type of output is that in addition to having the orthology relationship between genes from two different genomes, users also retrieve the BLAST results of the respective orthology relationship which allows them to perform subsequent filtering for either more conservative or more liberal orthology relationships.

Note: future versions of orthologr will allow to perform orthology inference using DNA sequences. Nevertheless, since most orthology inference methods or paradigms rely on protein sequences, the first version of orthologr will follow this paradigm.

In case you have to specify the path to the corresponding orthology inference method you can use the path argument as follows:

When you are working on a multicore machine, you can also specify the comp_cores argument that will allow you to run all analyses in parallel (to speed up computations).

# A tibble: 20 x 12
# Groups:   query_id [20]
   query_id subject_id perc_identity alig_length
   <chr>    <chr>              <dbl>       <dbl>
 1 AT1G010... 333554|PA...          74.0         469
 2 AT1G010... 470181|PA...          91.1         246
 3 AT1G010... 470180|PA...          95.5         359
 4 AT1G010... 333551|PA...          92.0        1970
 5 AT1G010... 909874|PA...         100           213
 6 AT1G010... 470177|PA...          89.5         648
 7 AT1G010... 918864|PA...          95.1         366
 8 AT1G010... 909871|PA...          90.3         300
 9 AT1G010... 470171|PA...          96.8         434
10 AT1G011... 333544|PA...          93.6         528
11 AT1G011... 918858|PA...          99.2         529
12 AT1G011... 470161|PA...          98.4         453
13 AT1G011... 918855|PA...          72.6         285
14 AT1G011... 918854|PA...          84.9         179
15 AT1G011... 311317|PA...          85.6          97
16 AT1G011... 909860|PA...          92.6         310
17 AT1G011... 311315|PA...          94.2         533
18 AT1G012... 470156|PA...          95.8         238
19 AT1G012... 311313|PA...          95.3         107
20 AT1G012... 470155|PA...          96.7        1056
# ... with 8 more variables: mismatches <dbl>,
#   gap_openings <dbl>, q_start <dbl>, q_end <dbl>,
#   s_start <dbl>, s_end <dbl>, evalue <dbl>,
#   bit_score <dbl>

In this case 2 cores are being used to perform parallel processing, the clean_folders argument specifies that all files returned by the corresponding orthology inference method are removed after analyses.

Program specific use of the orthologs() function

In this section small examples will illustrate the use of the orthologs() function for each orthology inference program.

BLASTp best hit

The BLASTp best hit method is a uni-directional BLAST best hit search of a query organism A against a subject organism B based on the e-value.

Orthology Inference using BLASTp best hit can be performed by specifying the argument ortho_detection = "BH" and one computing core comp_core = 1:

 # A tibble: 20 x 12
# Groups:   query_id [20]
   query_id subject_id perc_identity alig_length
   <chr>    <chr>              <dbl>       <dbl>
 1 AT1G010... 333554|PA...          74.0         469
 2 AT1G010... 470181|PA...          91.1         246
 3 AT1G010... 470180|PA...          95.5         359
 4 AT1G010... 333551|PA...          92.0        1970
 5 AT1G010... 909874|PA...         100           213
 6 AT1G010... 470177|PA...          89.5         648
 7 AT1G010... 918864|PA...          95.1         366
 8 AT1G010... 909871|PA...          90.3         300
 9 AT1G010... 470171|PA...          96.8         434
10 AT1G011... 333544|PA...          93.6         528
11 AT1G011... 918858|PA...          99.2         529
12 AT1G011... 470161|PA...          98.4         453
13 AT1G011... 918855|PA...          72.6         285
14 AT1G011... 918854|PA...          84.9         179
15 AT1G011... 311317|PA...          85.6          97
16 AT1G011... 909860|PA...          92.6         310
17 AT1G011... 311315|PA...          94.2         533
18 AT1G012... 470156|PA...          95.8         238
19 AT1G012... 311313|PA...          95.3         107
20 AT1G012... 470155|PA...          96.7        1056
# ... with 8 more variables: mismatches <dbl>,
#   gap_openings <dbl>, q_start <dbl>, q_end <dbl>,
#   s_start <dbl>, s_end <dbl>, evalue <dbl>,
#   bit_score <dbl>

The resulting table stores the orthologous gene pairs and the corresponding e-value of the best hit. By specifying detailed_output = TRUE more information of the BLAST result can be obtained.

BLASTp best reciprocal hit

The BLAST best reciprocal hit is a bi-directional BLAST best hit search of a query organism A against a subject organism B based on the e-value.

The Algorithm for BLAST best reciprocal hit runs as follows:

  1. bh_A <- best_hit(A,B)

  2. bh_B <- best_hit(B,A)

  3. join(bh_A,bh_B) by tupel (query_id, subject_id)

In other words, only in case the tuple (query_id, subject_id) is returned as best hit based on the e-value in both BLAST directions, the corresponding tupel (query_id, subject_id) is retained as orthologous gene pair.

As demonstrated before a simple call of orthologs() using ortho_detection = "RBH" and one computing core comp_core = 1 can be performed as follows:


      query_id            subject_id evalue
1  AT1G01010.1 333554|PACid:16033839  0e+00
2  AT1G01020.1 470181|PACid:16064328 7e-166
3  AT1G01030.1 470180|PACid:16054974  0e+00
4  AT1G01040.1 333551|PACid:16057793  0e+00
5  AT1G01050.1 909874|PACid:16064489 2e-160
6  AT1G01060.3 470177|PACid:16043374  0e+00
7  AT1G01070.1 918864|PACid:16052578  0e+00
8  AT1G01080.1 909871|PACid:16053217 1e-178
9  AT1G01090.1 470171|PACid:16052860  0e+00
10 AT1G01110.2 333544|PACid:16034284  0e+00
11 AT1G01120.1 918858|PACid:16049140  0e+00
12 AT1G01140.3 470161|PACid:16036015  0e+00
13 AT1G01150.1 918855|PACid:16037307 3e-150
14 AT1G01160.1 918854|PACid:16044153  1e-93
15 AT1G01170.2 311317|PACid:16052302  3e-54
16 AT1G01180.1 909860|PACid:16056125  0e+00
17 AT1G01190.1 311315|PACid:16059488  0e+00
18 AT1G01200.1 470156|PACid:16041002 3e-172
19 AT1G01210.1 311313|PACid:16057125  7e-76
20 AT1G01220.1 470155|PACid:16047984  0e+00

When you need more parameters returned by RBH, you can specify the detailed_output = TRUE argument to obtain all BLAST parameters.

Variables:
$ query_id      (chr) "AT1G01010.1", "AT1G01020.1", "AT1G01030.1", "AT1G01040.1"...
$ subject_id    (chr) "333554|PACid:16033839", "470181|PACid:16064328", "470180|...
$ perc_identity (dbl) 73.99, 91.06, 95.54, 91.98, 100.00, 89.51, 95.08, 90.33, 9...
$ alig_length   (dbl) 469, 246, 359, 1970, 213, 648, 366, 300, 434, 528, 529, 45...
$ mismatches    (dbl) 80, 22, 12, 85, 0, 58, 14, 22, 8, 34, 4, 6, 68, 19, 0, 20,...
$ gap_openings  (dbl) 8, 0, 2, 10, 0, 5, 2, 2, 3, 0, 0, 1, 3, 2, 1, 1, 1, 0, 0, 0
$ q_start       (dbl) 1, 1, 1, 6, 1, 1, 1, 1, 1, 1, 1, 1, 5, 4, 2, 1, 1, 1, 1, 1
$ q_end         (dbl) 430, 246, 359, 1910, 213, 646, 366, 294, 429, 528, 529, 45...
$ s_start       (dbl) 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 16, 2, 4, 1, 6, 1, 1, 1
$ s_end         (dbl) 466, 246, 355, 1963, 213, 640, 362, 299, 433, 528, 529, 45...
$ evalue        (dbl) 0e+00, 7e-166, 0e+00, 0e+00, 2e-160, 0e+00, 0e+00, 1e-178,...
$ bit_score     (dbl) 627, 454, 698, 3704, 437, 1037, 696, 491, 859, 972, 1092, ...

In case you would like to store the corresponding hit tables returned by BLAST for subsequent analyses, you can specify the clean_folders = FALSE argument. The corresponding BLAST hit table can then be found in file.path(tempdir(),"_blast_db").

A detailed overview of further analyses that can be done with the corresponding BLAST output can be found in the BLAST vignette.

Examples: Orthogroup based methods