Input data files¶
ipyrad can be used to assemble any type of data that is generated using a restriction digest method (RAD, ddRAD, GBS) or related amplification-based process (e.g., NextRAD, RApture), both of which yield data that is anchored on at least one side so that reads are expected to align fairly closely. ipyrad is not intended for constructing long contigs from many partially overlapping (i.e., shotgun) sequences. It can, however, accomodate paired-end reads, and has methods for detecting and merging overlaps. ipyrad can also combine reads of various lengths, so that older data is easily combined with newer data of different lengths.
Depending how and where your sequence data were generated you may receive data as one giant file, or in many smaller files. The files may contain data from all of your individuals mixed up together, or as separate files for each Sample. If they are mixed up then the data need to be demultiplexed based on barcodes or indices. Input files to ipyrad can be either demultiplexed or not:
multiplexed (raw) sequence files – If your data are not yet sorted
among individuals/samples then you will need to have barcode
information organized into a barcodes file.
Sample names are taken from the barcodes file.
The raw data file path(s) will need to be entered in the
raw_fastq_path parameter of the params file.
demultiplexed (sorted) sequence files – If your data are already sorted
then you simply have to enter the path to the data files in the
sorted_fastq_path parameter. The cookbook recipes
section provides more complex methods for combining data from multiple
sequencing runs into the same individual, or for using multiple barcodes file.
Should you pre-filter your data?¶
We would recommend you start by running the program (fastqc)[fastqc] on your data to produce output files with information about the distribution of quality scores, and the occurrence of Illumina adapter sequences. If your data contain adapters, then it is very important that these are removed, especially for paired-end data. There is a variety of software available to filter your data before starting an ipyrad analysis, such as trimmomatic or cutadapt. We prefer cutadapt. However, you can also filter your data directly in ipyrad, which uses cutadapt. Step 2 of the ipyrad analysis will apply different filters depending on the parameter settings you enter, to filter and trim data based on quality scores, and the occurrence of barcode+adapter combinations. Further, for paired-end data ipyrad uses vsearch to merge trimmed paired-end reads, which uses an algorithm similar to that in PEAR. The merged and non-merged reads are combined into a single downstream analysis. For more details on how quality and adapter trimming are performed in ipyrad (using cutadapt) [see here (link coming soon)]().
If demultiplexing, then Sample names will be extracted from
the barcodes files. Whereas if your data are already
sorted demultiplexed then Sample names are extracted from the file names
directly. Do not include spaces in file names. For paired-end data we need
to be able to identify which R1 and R2 files go together, and so we require that
every read1 file name contains the string
_R1_ (with underscores before
and after), and every R2 file name must match exactly the R1 file
except that it has
_R2_ in place of
See the example data for an example.
Pay careful attention to file names at the very beginning of an analysis since these names, and any included typos, will be perpetuated through all the resulting data files. Do not include spaces in file names.
The barcodes file is a simple table linking barcodes to samples. Barcodes can be of varying lengths. Each line should have one name and then one barcode, separated by whitespace (a tab or spaces).
sample1 ACAGG sample2 ATTCA sample3 CGGCATA sample4 AAGAACA
The parameter input file, which typically includes
params.txt in its name,
can be created with the
-n option from the ipyrad command line. This file
lists all of the parameter settings
necessary to complete an assembly. A description of how to create and use a
params file can be found in the introductory tutorial.
Supported data types¶
There is increasingly a large variety of ways to generate reduced representation genomic data sets using either restriction digestion or primer sets, and ipyrad aims to be flexible enough to handle all of these types. Because it is difficult to keep up with all of the names, we use our own terminology, described below, to group together data types that can be analyzed using the same bioinformatic methods. If you have a data type that is not described below and you’re not sure if it can be analyzed in ipyrad let us know here.
rad – This category includes data types which use a single cutter to generate DNA fragments for sequencing based on a single cut site. e.g., RAD-seq, NextRAD.
ddrad – This category is very similar data types which select fragments that were digested by two different restriction enzymes which cut the fragment on either end. During assembly this type of data is analyzed differently from the rad data type by more stringent filtering that looks for occurrences of the second (usually more common) cutter. e.g., double-digest RAD-seq.
gbs – This category includes any data type which selects fragments that were digested by a single enzyme that cuts both ends of DNA fragments. This data type requires reverse-complement clustering because the forward vs reverse adapters can attach to either end of each fragment, and thus when shorter fragments are sequenced from either end the resulting reads often overlap partially or completely. When analyzing GBS data we strongly recommend using a stringent setting for the filters_adapters parameter. e.g., genotyping-by-sequencing (Elshire et al.), EZ-RAD (Toonin et al.).
pairddrad – This category is for paired-end data from fragments that were generated through restriction digestion using two different enzymes. During step 3 the paired-reads will be tested for paired read merging if they overlap partially. Because two different cutters are used reverse-complement clustering is not necessary. e.g., double-digest RAD-seq (w/ paired-end sequencing).
pairgbs – This category is for paired-end data from fragments that were generated by digestion with a single enzyme that cuts both ends of the fragment. Because the forward adapter might bind to either end of these fragments,approximately half of the matches are expected to be reverse- complemented with perfect overlap. Paired reads are checked for merging before clustering/mapping. e.g., genotyping-by-sequencing, EZ-RAD, (w/ paired-end sequencing).
2brad – This category is for a special class of sequenced fragments generated using a type IIb restriction enzyme. The reads are usually very short in length, and are treated slightly differently in steps 2 and 7. (We are looking for people to do more testing of this method on empirical data).
pair3rad – This category is for 3Rad/RadCap data that uses multiplexed barcodes. 3Rad/RadCap can use up to four restriction enzymes, and also uses a suite of custom adapters to control for PCR duplicates. This data is always paired end, since one barcode is ligated to each read. PCR clones are removed in step 3, after merging but before dereplication. The pair3rad datatype is used for both 3Rad and RadCap types because these datatypes only differ in how they are generated, not how they are demultiplexed and filtered. See Glenn et al 2016, and Hoffberg et al 2016