The breakup into drops of complex-fluid filaments - usually jets of polymeric and surfactant solutions plays a central role in many engineering applications ranging from crop spraying to food processing and propulsion systems. To assess the influence of interfacial perturbations in the breakup process, we followed the dynamics of the initial wavelength of surface instability on complex-fluid filaments using direct numerical simulation. We found a threshold wavelength at low Reynolds numbers corresponding to a change on the filament's configuration near breakup from large primary drops connected by slender liquid threads for wavelengths below the threshold to clearly defined satellite drops in between the primary drops for wavelengths larger than the threshold. We also found that shear-thinning effects, by reducing the viscous resistance to the Marangoni stress responsible for the formation of the satellites, cause the threshold to appear at shorter wavelengths. (C) 2011 Elsevier Ltd. All rights reserved.