Abstract Genome size and base composition in 16 spe- cies and subspecies of the Hydrangea, a woody orna- mental genus of Hydrangeaceae, were evaluated by flow cytometry in relation to their chromosome number. This is the first such study concerning the genome size of these species together with a karyotype study of the most important species, Hydrangea macrophylla subsp. mac- rophylla (Hortensia), from an economical point of view. The 2C DNA content ranged from 1.95 pg in Hydrangea quercifolia to 5.00 pg in Hydrangea involucrata. The base composition ranged from 39.9% GC in Hydrangea aspera subsp. sargentiana to 41.1% in Hydrangea scan- dens subsp. scandens (significant difference at p < 0.05). The smallest genome sizes were those of the three spe- cies originating from North or South America. Most of the species studied presented a chromosome number of 2n = 2x = 36, except for those of the section Aspereae which showed 2n = 30, 34 and 36. A primary karyotype has been made for the first time for H. macrophylla su- bsp. macrophylla. Phylogenetic relationships between species, the origin of chromosome number and an explo- ration of the genetic diversity within the genus are dis- cussed. Keywords Hydrangea · Hydrangeaceae · DNA content · Chromosome number · Karyotype · Phylogeny Introduction The great ornamental value of Hydrangea has made this genus famous. Since the introduction of the most-culti- vated species, Hydrangea macrophylla, from Japan to England, garderners have improved this species with em- pirical crosses and selection in England, France, Germa- ny and Switzerland. Compared with wild varieties, culti- vars tend towards more-colored flowers, more-rigid and dwarf stems. However, future progress will depend upon a genetic understanding of the genus and on more-elabo- rate breeding programs to explore genetic resources. Nevertheless, since the extensive study of McClintock in 1957, few systematic studies have been devoted to Hydrangea, a woody genus of the Saxifraga- ceae family. This genus occurs in the temperate regions of eastern Asia and eastern North America and extends southward into the tropics of both hemispheres (McClintock 1957). From a systematic point of view, the Saxifragaceae sensu lato is a very large family composed of 15 to 17 sub-families, one of which is the Hydrangeoideae. However, a molecular phylogeny based on 18S rRNA and rbcL sequences places the Hydrangeoideae sub-fam- ily far from the general clade of the Saxifragaceae, which can be more-narrowly defined (Soltis and Soltis 1997). Indeed, Cronquist (1981) had already proposed a family, the Hydrangeaceae, distinct from the Saxifraga- ceae. Phylogenetic relationships between species within the genus Hydrangea have not yet been studied. According to McClintock’s classification (1957) the genus Hydrangea is divided into two sections, Hydran- gea McClint. and Cornidia Engl., mainly on the basis of plant habit, leaf texture, and the presence and morpholo- gy of floral bracts. These sections comprise several sub- sections, species and subspecies, essentially and respec- tively based on the position of the ovary and on seed morphology, the shape of the leaves and the type of in- florescence. Furthermore, there are many varieties creat- ed from these species, which have often been named as Communicated by H.C. Becker M. Cerbah · E. Mortreau · H. Bertrand · C. Lambert Institut National d’Horticulture, 2, rue Le nôtre, 49045 Angers, France M. Cerbah ( ✉ ) · S. Siljak-Yakovlev Laboratoire Evolution et Systématique, ESE, Université Paris XI/CNRS UPRESA - 8079, F-91405 Orsay cedex, France e-mail: cerbah@esv.u-psud.fr Tel.: +33-1-69156730, Fax: +33-1-69154697 S. Brown Institut des Sciences Végétales, Cytométrie, CNRS UP 40, 91198 Gif-sur-Yvette, France Theor Appl Genet (2001) 103:45–51 © Springer-Verlag 2001 ORIGINAL PAPER M. Cerbah · E. Mortreau · S. Brown S. Siljak-Yakovlev · H. Bertrand · C. Lambert Genome size variation and species relationships in the genus Hydrangea Received: 24 July 2000 / Accepted: 31 October 2000