Descrição
In genetic hybrids, nucleolus formation on chromosomes inherited
from only one parent is the epigenetic phenomenon, nucleolar
dominance. By using Arabidopsis suecica, the allotetraploid hybrid
of Arabidopsis thaliana and Arabidopsis arenosa, natural variation
in nucleolar dominance was found to occur, providing a unique
opportunity to examine homologous nucleolus organizer regions
(NORs) in their active and inactive states. In A. suecica strain LC1,
NORs derived from A. arenosa are active, whereas A. thalianaderived
NORs are silenced. In A. suecica strain 9502, NORs of both
parental species are active. When active, NORs are partially, but not
fully, decondensed. Both active and inactive LC1 NORs colocalize
with the nucleolus, contradicting the long-standing assumption
that rRNA gene transcription drives nucleolus association. Collectively,
these observations clarify the relationships among NOR
chromatin topology, rRNA gene transcription, and NOR–nucleolus
associations. A. suecica strains LC1 and 9502 have each lost one pair
of A. thaliana NORs during evolution, and amplified fragmentlength
polymorphism analysis further indicates that these strains
are genetically very similar. These data suggest that nucleolar
dominance can result from subtle genetic or epigenetic variation
but is not a trait fundamental to a given interspecies hybrid combination
; Viegas, Wanda 
