Cyto-Taxonomy and Phylogeny of the Oleaceae

Abstract

The Oleaceae are composed of more than 20 genera with over 400 species of temperate and tropical shrubs and trees (Rehder 1940). Many of these, such as Syringa, Forsythia, Jasminum, Ligustru?m, and Fraxinus, are cultivated extensively as ornamentals. Wood of Fraxinus and Olea has a number of commercial uses. The most important member of the family, however, is the olive, which has been grown since antiquity for its fruit and oil. Botanically, the group is particularly interesting as a natural family of plants with a wide geographical distribution, ranging from the tropics through the temperate regions and represented on almost every important land mass in the world except those of the polar regions. Although rather easily delimited as a family, the group contains a great variety of forms, some of which are stable enough to be easily recognized as species, while others, particularly in the genus Fraxinus, are a problem for the taxonomist. Some genetic work has been done in the Oleaceae, where one generic cross is known and several interspecific crosses have been made among the species of Osmanthus, Jasminum, Ligustrum, Forsythia, and especially Syringa, which has produced a multitude of striking forms. Evolutionary trends and phylogenetic relations within the family, as well as its position among related families, offer interesting possibilities for investigation. Although over a thousand species distributed among twenty-four or more genera are listed in Index Kewensis, most taxonomic treatments estimate fewer species, for example, 390-525 species (Johnson 1931), 390 (Engler & Gilg 1924), 370-390 (Knoblauch 1895), 525 (Small 1933), and over 500 (Bailey 1938). These estimates are probably nearer the correct number, since, as is shown in the groups of the family which have been monographed, considerable synonymy occurs (see Gray 1860; Decaisne 1878.; Hill 1910; Wesmael 1892; Lingelseheim 1907, 1920; McKelvey 1928; Steyermark 1932; Knoblauch 1933; Kobuski 1932, 1939a, 1939b, 1940; Rehder 1917; Mansfeld 1924; Gilg 1901; Weber 1928; and Newberry 1937). The first extensive cytological study in the Oleaceae was that of Sax (1930) on the genus Syringa. He found most of the pure species to have either 23 or 24 pairs of chromosomes. From the meiotic behavior of the hybrid S. chinensis (S. persica lacitiata x S. vulgaris), in which he saw about 12 paired and 12 single chromosomes at the first reduction division, he believed the basic number in the genus was probably 12. O'Mara (1930) found 28 chromosomes in all of the species and varieties of Forsythia examined. In the hybrid Forsythia intermedia (P. suspensa x F. viridissima) he found all fourteen chromosomes homologous enough to permit regular pairing and normal behavior at the first and second meiotic divisions, indicating a rather close relationship of the species in this genus. Representative species from several other genera were counted in a study of the chromosomes and anatomy of the secondary xylem of this family by Sax and Abbe (1932). These studies, aside from chromosome numbers of a few species deter-