  Until recently, relativistic jets were ubiquitously found to be launched from
giant elliptical galaxies. However, the detection by the Fermi-LAT of gamma-ray
emission from radio-loud narrow-line Seyfert 1 (RL-NLSy1) galaxies raised
doubts on this relation. Here, we morphologically characterize a sample of 29
RL-NLSy1s (including 12 gamma-ray emitters, gamma-NLSy1s) in order to find
clues on the conditions needed by AGN to produce relativistic jets. We use deep
near-infrared images from the Nordic Optical Telescope and the ESO VLT to
analyze the surface brightness distribution of the galaxies in the sample. We
detected 72% of the hosts (24% classified as gamma-NLSy1s). Although we cannot
rule out that some RL-NLSy1s are hosted by dispersion supported systems, our
findings strongly indicate that RL-NLSy1s hosts are preferentially disc
galaxies. 52% of the resolved hosts (77% non-gamma-emitters and 20%
gamma-emitters) show bars with morphological properties (long and weak)
consistent with models that promote gas inflows, which might trigger nuclear
activity. The extremely red bulges of the gamma-NLSy1s, and features that
suggest minor mergers in 75% of their hosts might hint to the necessary
conditions for gamma-rays to be produced. Among the features that suggest
mergers in our sample, we find 6 galaxies that show offset stellar bulges with
respect to their AGN. When we plot the nuclear versus the bulge magnitude,
RL-NLSy1s locate in the low-luminosity end of flat-spectrum radio quasars
(FSRQs), suggesting a similar accretion mode between these two AGN types.
