The systematic value of nuclear genome size for “all” species of Tulipa L. (Liliaceae)

Nuclear genome size, as measured by flow cytometry with propidium iodide, was used to investigate the relationships within the genus Tulipa L. (Liliaceae). More than 400 accessions representing 123 taxa from mainly wild-collected plants were investigated. Most species of Tulipa have the same basic chromosome number, 2n = 2x = 24. However, the somatic DNA 2C value (2C) is shown to range from 32 to 69 pg for the diploids. The largest genome contains roughly 3.4 x 10¹⁰ more base pairs than the smallest and has chromosomes that are more than twice as large. These large differences in the amount of nuclear DNA predict that the hybrids, if any arise, are usually sterile. Depending on the size of the total genome, 1 pg amounts to several thousand genes. Moreover, genome sizes are evaluated here in combination with available morphological, geographical, and molecular data. Therefore, the taxonomy proposed here is not a single-character taxonomy based on genome size alone. The genus Tulipa, as here determined, has 87 species, 29 more than accepted by van Raamsdonk et al. [Acta Hort (ISHS) 430:821-828, 1997], but including 25 species that were not available to them. Of these 87 species, 28 were not seen by Hall (The genus Tulipa, The Royal Horticultural Society, London, 1940) in a living state and placed by him in an addendum. Species of the subgenus Clusianae (Baker) Zonn. differ strongly in nuclear DNA content (DNA 2C value), 32 versus 40-68 pg for all other tulips, and are placed here in a separate subgenus. Also Orithyia, the only group with a style and with only 38-39 pg is placed in a separate subgenus. Therefore, all tulips are attributed to four subgenera, Clusianae (Baker) Zonn., Tulipa, Eriostemones Raamsd., and Orithyia (D. Don) Baker and divided further into 12 sections. Seven of the eight series of section Eichleres (A.D. Hall) Raamsd. are now placed in four sections: (1) section Lanatae (Raamsd.) Zonn., mainly confined to species from the Pamir-Alay and including series Lanatae Raamsd., (2) section Multiflorae (Raamsd.) Zonn. (including series Glabrae Raamsd.), (3) section Vinistriatae (Raamsd.) Zonn. (including series Undulatae Raamsd.), and (4) section Spiranthera Vved. ex Zonn. and Veldk. Triploids, tetraploids, and pentaploids were found in several species. DNA content confirmed the close relationships of the species within the different sections. The rather similar looking and therefore often confused T. armena Boiss. (51.8 pg), T. systola Stapf (56.3 pg), and T. julia K., Koch (61.6 pg) could be clearly distinguished. The same is true for T. biebersteiniana Schult. f. (56.9 pg), T. sylvestris ssp. australis (Link) Pamp. (62.0 pg), and T. primulina Baker (64.6 pg). T. doerfleri Gand. and T. whittalli (Dykes) Hall could be placed as polyploid forms of T. orphanidea Boiss. ex Heldr. On the basis of DNA content, a systematic association between T. julia K. Koch and the triploid T. aleppensis Boiss. and between T. systola Stapf and the triploid T. praecox Tenore was suggested. The new species T. lemmersii Zonn., Peterse, and de Groot is described, and four possible new species are indicated. Genome size as measured by using flow cytometry may conveniently be used to produce systematic data. It is applicable even in the case of dormant bulbs or sterile plants for monitoring the trade in bulbous species.