  Fission of atomic nuclei often produces mass asymmetric fragments. However,
the origin of this asymmetry was believed to be different in actinides and in
the sub-lead region [A. Andreyev {\it et al.}, Phys. Rev. Lett. {\bf 105},
252502 (2010)]. It has recently been argued that quantum shell effects
stabilising pear shapes of the fission fragments could explain the observed
asymmetries in fission of actinides[G. Scamps and C. Simenel, Nature {\bf 564},
382 (2018)]. This interpretation is tested in the sub-lead region using
microscopic mean-field calculations of fission based on the Hartree-Fock
approach with BCS pairing correlations. The evolution of the number of protons
and neutrons in asymmetric fragments of mercury isotope fissions is interpreted
in terms of deformed shell gaps in the fragments. A new method is proposed to
investigate the dominant shell effects in the pre-fragments at scission. We
conclude that the mechanisms responsible for asymmetric fissions in the
sub-lead region are the same as in the actinide region, which is a strong
indication of their universality.
