The short PARs of platypus chromosomes X2–X5 have a significantly
higher GC content (one-sided Wilcoxon rank-sum test, P < 0.01) than
the SDRs or the longer PARs (Extended Data Fig. 4c), [which probably
reflects strong GC-biased gene conversion that is caused by a high
So I take it that you're asking if the bracketed relative clause should be a defining one or a non-defining one.
Despite the highly technical nature of the sentence, I'd say it's non-defining, since it appears to refer to the whole of the clause that precedes it, as opposed to one of the preceding noun phrases.
Note that non-defining relatives are not modifiers, but supplements, i.e. additional non-integrated content set apart by punctuation and a slight pause in speech.