We study the regularity of the De Gregorio (DG) model ωt + uωx = uxω on S 1 for initial data ω 0 with period π and in class X: ω 0 is odd and ω 0 ≤ 0 (or ω 0 ≥ 0) on [0, π/2]. These sign and symmetry properties are the same as those of the smooth initial data that lead to singularity formation of the De Gregorio model on R or the generalized Constantin-Lax-Majda (gCLM) model on R or S 1 with a positive parameter. Thus, to establish global regularity of the DG model for general smooth initial data, which is a conjecture on the DG model, an important step is to rule out potential finite time blowup from smooth initial data in X. We accomplish this by establishing a one-point blowup criterion and proving global well-posedness for C 1,α initial data with any α ∈ (0, 1). On the other hand, for any α ∈ (0, 1), we construct a finite time blowup solution from a class of initial data with ω 0 ∈ C α ∩ C ∞ (S 1 \{0}) ∩ X. Our results imply that singularities developed in the DG model and the gCLM model on S 1 can be prevented by stronger advection.