Karyotype structure, supernumerary chromosomes and heterochromatin distribution suggest a pathway of karyotype evolution in Dactylorhiza (Orchidaceae)

The relationships between Dactylorhiza romana and D. saccifera from
Southern Italy were analysed. These two species, both with 2n = 2x = 40
chromosomes and belonging to different sections of the genus, were
distinguishable on the basis of karyotype structure and heterochromatin
amounts and distribution. Their C-banded karyotypes differed considerably.
D. saccifera showed most chromosomes with banded regions in the short arms,
whereas in D. romana the bands were located mostly at telomeric regions of
longer arms. Several individuals of D. romana had one or two very large
heterochromatic supernumerary chromosomes. Based on evidence resulting from
karyotype structure and heterochromatin distribution in the two species and
on the genetic distances derived from the comparison of ITS sequences, it
is suggested that D. romana represents a primitive form with respect to D.
saccifera and is a possible intermediate step in the evolution of the genus
Dactylorhiza from the 42-chromosomes Orchis group.