A new analysis of the confusing Central Asian Lesser Whitethroat subspecies (some of which occur as vagrants in Europe) has helped more clearly define the limits of each.
Sylvia curruca seems to consist of six main clades, though these are only supported by the mitochondrial genes, which are inherited maternally. The clades correspond to the subspecies already named blythi, halimodendri, margelanica, althaea, minula and nominate, but most appear to possess largely clinal features and may not be definitively diagnosable in the field on current knowledge.
Whether to treat them as species or subspecies? The authors say that "this is largely a matter of species definition and is not resolved by our data".
Olsson, U, Leader, P J, Carey, G J, Khan, A A, Svensson, L, and Alström, P. 2013. New insights into the intricate taxonomy and phylogeny of the Sylvia curruca complex. Molecular Phylogenetics and Evolution 67: 72-85.