PhiX libraries A/T composition

[luc]

The A bias of PhiX forward reads (please see the attachment) has made me curious about the libraries. Would anybody have an idea how these are generated by Illumina?

PhiX has a single-stranded DNA genome with is enriched for As (1684 As, 1291 Ts, 1254 Gs, 1157 Cs).

[Brian Bushnell]

I suppose an unamplified library of a single-stranded organism will strongly reflect the organism's own composition bias. So, perhaps it is unamplified? That's probably not a very good decision for something used for calibration, but I'm just speculating.

[Martin_Ranik]

looks like the phiX library is directional, which is interesting. Could be that the orignal material used to make the lib was ssDNA and the library was made using a strand-specific protocol...