Bio::SeqFeature::ToolsUserfContributed PBio::SeqFeature::Tools::Unflattener(3)NAMEBio::SeqFeature::Tools::Unflattener - turns flat list of
genbank-sourced features into a nested SeqFeatureI hierarchy
SYNOPSIS
# standard / generic use - unflatten a genbank record
use Bio::SeqIO;
use Bio::SeqFeature::Tools::Unflattener;
# generate an Unflattener object
$unflattener = Bio::SeqFeature::Tools::Unflattener->new;
# first fetch a genbank SeqI object
$seqio =
Bio::SeqIO->new(-file=>'AE003644.gbk',
-format=>'GenBank');
my $out =
Bio::SeqIO->new(-format=>'asciitree');
while ($seq = $seqio->next_seq()) {
# get top level unflattended SeqFeatureI objects
$unflattener->unflatten_seq(-seq=>$seq,
-use_magic=>1);
$out->write_seq($seq);
@top_sfs = $seq->get_SeqFeatures;
foreach my $sf (@top_sfs) {
# do something with top-level features (eg genes)
}
}
DESCRIPTION
Most GenBank entries for annotated genomic DNA contain a flat list of
features. These features can be parsed into an equivalent flat list of
Bio::SeqFeatureI objects using the standard Bio::SeqIO classes.
However, it is often desirable to unflatten this list into something
resembling actual gene models, in which genes, mRNAs and CDSs are
nested according to the nature of the gene model.
The BioPerl object model allows us to store these kind of associations
between SeqFeatures in containment hierarchies -- any SeqFeatureI
object can contain nested SeqFeatureI objects. The
Bio::SeqFeature::Tools::Unflattener object facilitates construction of
these hierarchies from the underlying GenBank flat-feature-list
representation.
For example, if you were to look at a typical GenBank DNA entry, say,
AE003644, you would see a flat list of features:
source
gene CG4491
mRNA CG4491-RA
CDS CG4491-PA
gene tRNA-Pro
tRNA tRNA-Pro
gene CG32954
mRNA CG32954-RA
mRNA CG32954-RC
mRNA CG32954-RB
CDS CG32954-PA
CDS CG32954-PB
CDS CG32954-PC
These features have sequence locations, but it is not immediately clear
how to write code such that each mRNA is linked to the appropriate CDS
(other than relying on IDs which is very bad)
We would like to convert the above list into the containment hierarchy,
shown below:
source
gene
mRNA CG4491-RA
CDS CG4491-PA
exon
exon
gene
tRNA tRNA-Pro
exon
gene
mRNA CG32954-RA
CDS CG32954-PA
exon
exon
mRNA CG32954-RC
CDS CG32954-PC
exon
exon
mRNA CG32954-RB
CDS CG32954-PB
exon
exon
Where each feature is nested underneath its container. Note that exons
have been automatically inferred (even for tRNA genes).
We do this using a call on a Bio::SeqFeature::Tools::Unflattener object
@sfs = $unflattener->unflatten_seq(-seq=>$seq);
This would return a list of the top level (i.e. container) SeqFeatureI
objects - in this case, genes. Other top level features are possible;
for instance, the source feature which is always present, and other
features such as variation or misc_feature types.
The containment hierarchy can be accessed using the get_SeqFeature()
call on any feature object - see Bio::SeqFeature::FeatureHolderI. The
following code will traverse the containment hierarchy for a feature:
sub traverse {
$sf = shift; # $sf isa Bio::SeqfeatureI
# ...do something with $sf!
# depth first traversal of containment tree
@contained_sfs = $sf->get_SeqFeatures;
traverse($_) foreach @contained_sfs;
}
Once you have built the hierarchy, you can do neat stuff like turn the
features into 'rich' feature objects (eg
Bio::SeqFeature::Gene::GeneStructure) or convert to a suitable format
such as GFF3 or chadoxml (after mapping to the Sequence Ontology); this
step is not described here.
USING MAGIC
Due to the quixotic nature of how features are stored in
GenBank/EMBL/DDBJ, there is no guarantee that the default behaviour of
this module will produce perfect results. Sometimes it is hard or
impossible to build a correct containment hierarchy if the information
provided is simply too lossy, as is often the case. If you care deeply
about your data, you should always manually inspect the resulting
containment hierarchy; you may have to customise the algorithm for
building the hierarchy, or even manually tweak the resulting hierarchy.
This is explained in more detail further on in the document.
However, if you are satisfied with the default behaviour, then you do
not need to read any further. Just make sure you set the parameter
use_magic - this will invoke incantations which will magically produce
good results no matter what the idiosyncracies of the particular
GenBank record in question.
For example
$unflattener->unflatten_seq(-seq=>$seq,
-use_magic=>1);
The success of this depends on the phase of the moon at the time the
entry was submitted to GenBank. Note that the magical recipe is being
constantly improved, so the results of invoking magic may vary
depending on the bioperl release.
If you are skeptical of magic, or you wish to exact fine grained
control over how the entry is unflattened, or you simply wish to
understand more about how this crazy stuff works, then read on!
PROBLEMATIC DATA AND INCONSISTENCIES
Occasionally the Unflattener will have problems with certain records.
For example, the record may contain inconsistent data - maybe there is
an exon entry that has no corresponding mRNA location.
The default behaviour is to throw an exception reporting the problem,
if the problem is relatively serious - for example, inconsistent data.
You can exert more fine grained control over this - perhaps you want
the Unflattener to do the best it can, and report any problems. This
can be done - refer to the methods.
error_threshold()get_problems()report_problems()ignore_problems()ALGORITHM
This is the default algorithm; you should be able to override any part
of it to customise.
The core of the algorithm is in two parts
Partitioning the flat feature list into groups
Resolving the feature containment hierarchy for each group
There are other optional steps after the completion of these two steps,
such as inferring exons; we now describe in more detail what is going
on.
Partitioning into groups
First of all the flat feature list is partitioned into groups.
The default way of doing this is to use the gene attribute; if we look
at two features from GenBank accession AE003644.3:
gene 20111..23268
/gene="noc"
/locus_tag="CG4491"
/note="last curated on Thu Dec 13 16:51:32 PST 2001"
/map="35B2-35B2"
/db_xref="FLYBASE:FBgn0005771"
mRNA join(20111..20584,20887..23268)
/gene="noc"
/locus_tag="CG4491"
/product="CG4491-RA"
/db_xref="FLYBASE:FBgn0005771"
Both these features share the same /gene tag which is "noc", so they
correspond to the same gene model (the CDS feature is not shown, but
this also has a tag-value /gene="noc").
Not all groups need to correspond to gene models, but this is the most
common use case; later on we shall describe how to customise the
grouping.
Sometimes other tags have to be used; for instance, if you look at the
entire record for AE003644.3 you will see you actually need the use the
/locus_tag attribute. This attribute is actually not present in most
records!
You can override this:
$collection->unflatten_seq(-seq=>$seq, -group_tag=>'locus_tag');
Alternatively, if you -use_magic, the object will try and make a guess
as to what the correct group_tag should be.
At the end of this step, we should have a list of groups - there is no
structure within a group; the group just serves to partition the flat
features. For the example data above, we would have the following
groups.
[ source ]
[ gene mRNA CDS ]
[ gene mRNA CDS ]
[ gene mRNA CDS ]
[ gene mRNA mRNA mRNA CDS CDS CDS ]
Multicopy Genes
Multicopy genes are usually rRNAs or tRNAs that are duplicated across
the genome. Because they are functionally equivalent, and usually have
the same sequence, they usually have the same group_tag (ie gene
symbol); they often have a /note tag giving copy number. This means
they will end up in the same group. This is undesirable, because they
are spatially disconnected.
There is another step, which involves splitting spatially disconnected
groups into distinct groups
this would turn this
[gene-rrn3 rRNA-rrn3 gene-rrn3 rRNA-rrn3]
into this
[gene-rrn3 rRNA-rrn3] [gene-rrn3 rRNA-rrn3]
based on the coordinates
What next?
The next step is to add some structure to each group, by making
containment hierarchies, trees that represent how the features
interrelate
Resolving the containment hierarchy
After the grouping is done, we end up with a list of groups which
probably contain features of type 'gene', 'mRNA', 'CDS' and so on.
Singleton groups (eg the 'source' feature) are ignored at this stage.
Each group is itself flat; we need to add an extra level of
organisation. Usually this is because different spliceforms
(represented by the 'mRNA' feature) can give rise to different protein
products (indicated by the 'CDS' feature). We want to correctly
associate mRNAs to CDSs.
We want to go from a group like this:
[ gene mRNA mRNA mRNA CDS CDS CDS ]
to a containment hierarchy like this:
gene
mRNA
CDS
mRNA
CDS
mRNA
CDS
In which each CDS is nested underneath the correct corresponding mRNA.
For entries that contain no alternate splicing, this is simple; we know
that the group
[ gene mRNA CDS ]
Must resolve to the tree
gene
mRNA
CDS
How can we do this in entries with alternate splicing? The bad news is
that there is no guaranteed way of doing this correctly for any GenBank
entry. Occasionally the submission will have been done in such a way as
to reconstruct the containment hierarchy. However, this is not
consistent across databank entries, so no generic solution can be
provided by this object. This module does provide the framework within
which you can customise a solution for the particular dataset you are
interested in - see later.
The good news is that there is an inference we can do that should
produce pretty good results the vast majority of the time. It uses
splice coordinate data - this is the default behaviour of this module,
and is described in detail below.
Using splice site coordinates to infer containment
If an mRNA is to be the container for a CDS, then the splice site
coordinates (or intron coordinates, depending on how you look at it) of
the CDS must fit inside the splice site coordinates of the mRNA.
Ambiguities can still arise, but the results produced should still be
reasonable and consistent at the sequence level. Look at this fake
example:
mRNA XXX---XX--XXXXXX--XXXX join(1..3,7..8,11..16,19..23)
mRNA XXX-------XXXXXX--XXXX join(1..3,11..16,19..23)
CDS XXXX--XX join(13..16,19..20)
CDS XXXX--XX join(13..16,19..20)
[obviously the positions have been scaled down]
We cannot unambiguously match mRNA with CDS based on splice sites,
since both CDS share the splice site locations 16^17 and 18^19.
However, the consequences of making a wrong match are probably not very
severe. Any annotation data attached to the first CDS is probably
identical to the seconds CDS, other than identifiers.
The default behaviour of this module is to make an arbitrary call where
it is ambiguous (the mapping will always be bijective; i.e. one mRNA ->
one CDS).
[TODO: NOTE: not tested on EMBL data, which may not be bijective; ie
two mRNAs can share the same CDS??]
This completes the building of the containment hierarchy; other
optional step follow
POST-GROUPING STEPS
Inferring exons from mRNAs
This step always occurs if -use_magic is invoked.
In a typical GenBank entry, the exons are implicit. That is they can be
inferred from the mRNA location.
For example:
mRNA join(20111..20584,20887..23268)
This tells us that this particular transcript has two exons. In
bioperl, the mRNA feature will have a 'split location'.
If we call
$unflattener->feature_from_splitloc(-seq=>$seq);
This will generate the necessary exon features, and nest them under the
appropriate mRNAs. Note that the mRNAs will no longer have split
locations - they will have simple locations spanning the extent of the
exons. This is intentional, to avoid redundancy.
Occasionally a GenBank entry will have both implicit exons (from the
mRNA location) and explicit exon features.
In this case, exons will still be transferred. Tag-value data from the
explicit exon will be transfered to the implicit exon. If exons are
shared between mRNAs these will be represented by different objects.
Any inconsistencies between implicit and explicit will be reported.
tRNAs and other noncoding RNAs
exons will also be generated from these features
Inferring mRNAs from CDS
Some GenBank entries represent gene models using features of type gene,
mRNA and CDS; some entries just use gene and CDS.
If we only have gene and CDS, then the containment hierarchies will
look like this:
gene
CDS
If we want the containment hierarchies to be uniform, like this
gene
mRNA
CDS
Then we must create an mRNA feature. This will have identical
coordinates to the CDS. The assumption is that there is either no
untranslated region, or it is unknown.
To do this, we can call
$unflattener->infer_mRNA_from_CDS(-seq=>$seq);
This is taken care of automatically, if -use_magic is invoked.
ADVANCED
Customising the grouping of features
The default behaviour is suited mostly to building models of protein
coding genes and noncoding genes from genbank genomic DNA submissions.
You can change the tag used to partition the feature by passing in a
different group_tag argument - see the unflatten_seq() method
Other behaviour may be desirable. For example, even though SNPs
(features of type 'variation' in GenBank) are not actually part of the
gene model, it may be desirable to group SNPs that overlap or are
nearby gene models.
It should certainly be possible to extend this module to do this.
However, I have yet to code this part!!! If anyone would find this
useful let me know.
In the meantime, you could write your own grouping subroutine, and feed
the results into unflatten_groups() [see the method documentation
below]
Customising the resolution of the containment hierarchy
Once the flat list of features has been partitioned into groups, the
method unflatten_group() is called on each group to build a tree.
The algorithm for doing this is described above; ambiguities are
resolved by using splice coordinates. As discussed, this can be
ambiguous.
Some submissions may contain information in tags/attributes that hint
as to the mapping that needs to be made between the features.
For example, with the Drosophila Melanogaster release 3 submission, we
see that CDS features in alternately spliced mRNAs have a form like
this:
CDS join(145588..145686,145752..146156,146227..146493)
/locus_tag="CG32954"
/note="CG32954 gene product from transcript CG32954-RA"
^^^^^^^^^^^^^^^^^^^^^^^^^^^
/codon_start=1
/product="CG32954-PA"
/protein_id="AAF53403.1"
/db_xref="GI:7298167"
/db_xref="FLYBASE:FBgn0052954"
/translation="MSFTLTNKNVIFVAGLGGIGLDTSKELLKRDLKNLVILDRIENP..."
Here the /note tag provides the clue we need to link CDS to mRNA
(highlighted with ^^^^). We just need to find the mRNA with the tag
/product="CG32954-RA"
I have no idea how consistent this practice is across submissions; it
is consistent for the fruitfly genome submission.
We can customise the behaviour of unflatten_group() by providing our
own resolver method. This obviously requires a bit of extra
programming, but there is no way to get around this.
Here is an example of how to pass in your own resolver; this example
basically checks the parent (container) /product tag to see if it
matches the required string in the child (contained) /note tag.
$unflattener->unflatten_seq(-seq=>$seq,
-group_tag=>'locus_tag',
-resolver_method=>sub {
my $self = shift;
my ($sf, @candidate_container_sfs) = @_;
if ($sf->has_tag('note')) {
my @notes = $sf->get_tag_values('note');
my @trnames = map {/from transcript\s+(.*)/;
$1} @notes;
@trnames = grep {$_} @trnames;
my $trname;
if (@trnames == 0) {
$self->throw("UNRESOLVABLE");
}
elsif (@trnames == 1) {
$trname = $trnames[0];
}
else {
$self->throw("AMBIGUOUS: @trnames");
}
my @container_sfs =
grep {
my ($product) =
$_->has_tag('product') ?
$_->get_tag_values('product') :
('');
$product eq $trname;
} @candidate_container_sfs;
if (@container_sfs == 0) {
$self->throw("UNRESOLVABLE");
}
elsif (@container_sfs == 1) {
# we got it!
return $container_sfs[0];
}
else {
$self->throw("AMBIGUOUS");
}
}
});
the resolver method is only called when there is more than one
spliceform.
Parsing mRNA records
Some of the entries in sequence databanks are for mRNA sequences as
well as genomic DNA. We may want to build models from these too.
NOT YET DONE - IN PROGRESS!!!
Open question - what would these look like?
Ideally we would like a way of combining a mRNA record with the
corresponding SeFeature entry from the appropriate genomic DNA record.
This could be problemmatic in some cases - for example, the mRNA
sequences may not match 100% (due to differences in strain, assembly
problems, sequencing problems, etc). What then...?
SEE ALSO
Feature table description
http://www.ebi.ac.uk/embl/Documentation/FT_definitions/feature_table.html
FEEDBACK
Mailing Lists
User feedback is an integral part of the evolution of this and other
Bioperl modules. Send your comments and suggestions preferably to the
Bioperl mailing lists Your participation is much appreciated.
bioperl-l@bioperl.org - General discussion
http://bioperl.org/wiki/Mailing_lists - About the mailing lists
Support
Please direct usage questions or support issues to the mailing list:
bioperl-l@bioperl.org
rather than to the module maintainer directly. Many experienced and
reponsive experts will be able look at the problem and quickly address
it. Please include a thorough description of the problem with code and
data examples if at all possible.
Reporting Bugs
report bugs to the Bioperl bug tracking system to help us keep track
the bugs and their resolution. Bug reports can be submitted via the
web:
http://bugzilla.open-bio.org/
AUTHOR - Chris Mungall
Email: cjm@fruitfly.org
APPENDIX
The rest of the documentation details each of the object methods.
Internal methods are usually preceded with a _
new
Title : new
Usage : $unflattener = Bio::SeqFeature::Tools::Unflattener->new();
$unflattener->unflatten_seq(-seq=>$seq);
Function: constructor
Example :
Returns : a new Bio::SeqFeature::Tools::Unflattener
Args : see below
Arguments
-seq : A L<Bio::SeqI> object (optional)
the sequence to unflatten; this can also be passed in
when we call unflatten_seq()-group_tag : a string representing the /tag used to partition flat features
(see discussion above)
seq
Title : seq
Usage : $unflattener->seq($newval)
Function:
Example :
Returns : value of seq (a Bio::SeqI)
Args : on set, new value (a Bio::SeqI, optional)
The Bio::SeqI object should hold a flat list of Bio::SeqFeatureI
objects; this is the list that will be unflattened.
The sequence object can also be set when we call unflatten_seq()
group_tag
Title : group_tag
Usage : $unflattener->group_tag($newval)
Function:
Example :
Returns : value of group_tag (a scalar)
Args : on set, new value (a scalar or undef, optional)
This is the tag that will be used to collect elements from the flat
feature list into groups; for instance, if we look at two typical
GenBank features:
gene 20111..23268
/gene="noc"
/locus_tag="CG4491"
/note="last curated on Thu Dec 13 16:51:32 PST 2001"
/map="35B2-35B2"
/db_xref="FLYBASE:FBgn0005771"
mRNA join(20111..20584,20887..23268)
/gene="noc"
/locus_tag="CG4491"
/product="CG4491-RA"
/db_xref="FLYBASE:FBgn0005771"
We can see that these comprise the same gene model because they share
the same /gene attribute; we want to collect these together in groups.
Setting group_tag is optional. The default is to use 'gene'. In the
example above, we could also use /locus_tag
partonomy
Title : partonomy
Usage : $unflattener->partonomy({mRNA=>'gene', CDS=>'mRNA')
Function:
Example :
Returns : value of partonomy (a scalar)
Args : on set, new value (a scalar or undef, optional)
A hash representing the containment structure that the seq_feature
nesting should conform to; each key represents the contained (child)
type; each value represents the container (parent) type.
structure_type
Title : structure_type
Usage : $unflattener->structure_type($newval)
Function:
Example :
Returns : value of structure_type (a scalar)
Args : on set, new value (an int or undef, optional)
GenBank entries conform to different flavours, or structure types. Some
have mRNAs, some do not.
Right now there are only two base structure types defined. If you set
the structure type, then appropriate unflattening action will be taken.
The presence or absence of explicit exons does not affect the structure
type.
If you invoke -use_magic then this will be set automatically, based on
the content of the record.
Type 0 (DEFAULT)
typically contains
source
gene
mRNA
CDS
with this structure type, we want the seq_features to be nested
like this
gene
mRNA
CDS
exon
exons and introns are implicit from the mRNA 'join' location
to get exons from the mRNAs, you will need this call (see below)
$unflattener->feature_from_splitloc(-seq=>$seq);
Type 1
typically contains
source
gene
CDS
exon [optional]
intron [optional]
there are no mRNA features
with this structure type, we want the seq_features to be nested
like this
gene
CDS
exon
intron
exon and intron may or may not be present; they may be implicit
from the CDS 'join' location
get_problems
Title : get_problems
Usage : @probs = get_problems()
Function: Get the list of problem(s) for this object.
Example :
Returns : An array of [severity, description] pairs
Args :
In the course of unflattening a record, problems may occur. Some of
these problems are non-fatal, and can be ignored.
Problems are represented as arrayrefs containing a pair [severity,
description]
severity is a number, the higher, the more severe the problem
the description is a text string
clear_problems
Title : clear_problems
Usage :
Function: resets the problem list to empty
Example :
Returns :
Args :
report_problems
Title : report_problems
Usage : $unflattener->report_problems(\*STDERR);
Function:
Example :
Returns :
Args : FileHandle (defaults to STDERR)
ignore_problems
Title : ignore_problems
Usage : $obj->ignore_problems();
Function:
Example :
Returns :
Args :
Unflattener is very particular about problems it finds along the way.
If you have set the error_threshold such that less severe problems do
not cause exceptions, Unflattener still expects you to
report_problems() at the end, so that the user of the module is aware
of any inconsistencies or problems with the data. In fact, a warning
will be produced if there are unreported problems. To silence, this
warning, call the ignore_problems() method before the Unflattener
object is destroyed.
error_threshold
Title : error_threshold
Usage : $obj->error_threshold($severity)
Function:
Example :
Returns : value of error_threshold (a scalar)
Args : on set, new value (an integer)
Sets the threshold above which errors cause this module to throw an
exception. The default is 0; all problems with a severity > 0 will
cause an exception.
If you raise the threshold to 1, then the unflattening process will be
more lax; problems of severity==1 are generally non-fatal, but may
indicate that the results should be inspected, for example, to make
sure there is no data loss.
unflatten_seq
Title : unflatten_seq
Usage : @sfs = $unflattener->unflatten_seq($seq);
Function: turns a flat list of features into a list of holder features
Example :
Returns : list of Bio::SeqFeatureI objects
Args : see below
partitions a list of features then arranges them in a nested tree; see
above for full explanation.
note - the Bio::SeqI object passed in will be modified
Arguments
-seq : a Bio::SeqI object; must contain Bio::SeqFeatureI objects
(this is optional if seq has already been set)
-use_magic: if TRUE (ie non-zero) then magic will be invoked;
see discussion above.
-resolver_method: a CODE reference
see the documentation above for an example of
a subroutine that can be used to resolve hierarchies
within groups.
this is optional - if nothing is supplied, a default
subroutine will be used (see below)
-group_tag: a string
[ see the group_tag() method ]
this overrides the default group_tag which is 'gene'
unflatten_groups
Title : unflatten_groups
Usage :
Function: iterates over groups, calling unflatten_group() [see below]
Example :
Returns : list of Bio::SeqFeatureI objects that are holders
Args : see below
Arguments
-groups: list of list references; inner list is of Bio::SeqFeatureI objects
e.g. ( [$sf1], [$sf2, $sf3, $sf4], [$sf5, ...], ...)
-resolver_method: a CODE reference
see the documentation above for an example of
a subroutine that can be used to resolve hierarchies
within groups.
this is optional - a default subroutine will be used
NOTE: You should not need to call this method, unless you want fine
grained control over how the unflattening process.
unflatten_group
Title : unflatten_group
Usage :
Function: nests a group of features into a feature containment hierarchy
Example :
Returns : Bio::SeqFeatureI objects that holds other features
Args : see below
Arguments
-group: reference to list of Bio::SeqFeatureI objects
-resolver_method: a CODE reference
see the documentation above for an example of
a subroutine that can be used to resolve hierarchies
within groups
this is optional - a default subroutine will be used
NOTE: You should not need to call this method, unless you want fine
grained control over how the unflattening process.
feature_from_splitloc
Title : feature_from_splitloc
Usage : $unflattener->feature_from_splitloc(-features=>$sfs);
Function:
Example :
Returns :
Args : see below
At this time all this method does is generate exons for mRNA or other
RNA features
Arguments:
-feature: a Bio::SeqFeatureI object (that conforms to Bio::FeatureHolderI)
-seq: a Bio::SeqI object that contains Bio::SeqFeatureI objects
-features: an arrayref of Bio::SeqFeatureI object
infer_mRNA_from_CDS
Title : infer_mRNA_from_CDS
Usage :
Function:
Example :
Returns :
Args :
given a "type 1" containment hierarchy
gene
CDS
exon
this will infer the uniform "type 0" containment hierarchy
gene
mRNA
CDS
exon
all the children of the CDS will be moved to the mRNA
a "type 2" containment hierarchy is mixed type "0" and "1" (for
example, see ftp.ncbi.nih.gov/genomes/Schizosaccharomyces_pombe/)
remove_types
Title : remove_types
Usage : $unf->remove_types(-seq=>$seq, -types=>["mRNA"]);
Function:
Example :
Returns :
Args :
removes features of a set type
useful for pre-filtering a genbank record; eg to get rid of STSs
also, there is no way to unflatten
ftp.ncbi.nih.gov/genomes/Schizosaccharomyces_pombe/ UNLESS the bogus
mRNAs in these records are removed (or changed to a different type) -
they just confuse things too much
perl v5.14.12011-0Bio::SeqFeature::Tools::Unflattener(3)
