I don't understand the genetics well enough to answer with any degree of
certainty, but the pattern does remind me of a common pattern that
came from crossing I. pallida with I. variegata long ago. In that case it
seems perhaps the I. variegata pattern (not the "variegata" coloring we refer to
now, but the striped falls and nearly unmarked standards of I.
variegata) was combining with and/or being modified by the plicata pattern of I.
pallida. It expressed itself in those diploids in varying degrees from
barely striped (with the strongest expression usually at the shoulders) to
covering the entire falls.
I've been referring to the ones with mostly just shoulder striping as
the "Flavescens" pattern, and it is seen in cultivars like 'Joyance',
'Flavescens', 'G.P.Baker', 'Mrs. Horace Darwin', 'Mrs. George Darwin', 'Queen of
May', etc. However, in all of those there are fewer veins in smaller and
narrower petals, so the look of the overall pattern is somewhat less
It seems to me that this may be an expression of similar genetics, but the
tetraploid compliments and the influence from additional ancestors and
developments, could mean that it is totally unrelated too.