NEW EVIDENCE CONCERNING THE ORIGINAL BASIC CHROMOSOME NUMBER OF ANGIOSPERMS

Abstract

The Annonales (woody Ranales; a portion of Polycarpicae) generally have been conceded to include the living angiosperms with the most impressive array of unspecialized characteristics (see Thorne, in Carlquist, 1964, p. 433, for a definition of this group). Because of this, they have been studied in some detail morphologically, but, with a few exceptions (e.g., Whitaker, 1933), insufficiently cytologically. An analysis of chromosome numbers in Annonales, however, might be expected to shed some light on the original basic chromosome number of angiosperms, and is therefore of especial interest. The purpose of this communication is to record several newly determined chromosome numbers in the group and to comment on their possible phylogenetic significance. One of the most interesting families in this alliance is Winteraceae, with six genera and more than 100 species. All Winteraceae lack vessels in their xylem; they are found mostly in the eastern tropics and Southern Hemisphere of the Old World. The largest genus, Drimys, is the only one with representatives in the New World. It is divided into two sections. In the larger, exclusively Old World section Tasmannia, fo-ur Australian species have a gametic chromosome number of n = 13 (Hotchkiss, 1955). One of these, Drimys lanceolata (Poir.) Baill. (D. aromatica F. v. Muell.), has also been reported to have a somatic chromosome number of 2n = 28 (Gulline unp., in Darlington and XVylie, 1955). In studying pistillate material of this species cultivated in Golden Gate Park in San Francisco (Walther in 1949, CAS), however, we found that each member of one pair of chromosomes has a satellite which often is widely separated from the pair early in mitosis, giving the impression that there are 28 chromosomes. Later in the course of mitosis, when the chromosomes were more contracted, it was a simple matter to determine the somatic number 2n -26, also reported for this species by Stebbins (1958). Another chromosome number reported for the same species, 2n = 38? (Janaki Ammal unp., in Darlington and Wylie, 1955), was probably from a triploid individual with 2n = 3x = 39. To summarize the available information, n = 13 is the only chromosome number reliably reported for Drimys sect. Tasmannia. For the New World section, Drimys sect. Drimys (Wintera), only the commonest of the four species, D. winteri J. R. & G. Forst., has been studied cytologically. We examined a plant of this species cultivated in the botanic garden, University of California, Los Angeles (Raven 18738, DS), and another from the Strybing Arboretum, San Francisco (Menzies in 1960, CAS). Both of these plants formed 43 bivalents at meiotic metaphase I (Fig. 1). The bivalents were conspicuously variable in size. Both plants are probably from the same original introduction from plants cultivated in England (Brandt, 1951), as was also, probably, the plant reported earlier as having 2n = about 76 (Whitaker, 1933). The San Francisco plant had a small fragment in addition to its normal chromosome complement. Thus the chromosome numbers determined for the Old and New World sections of Drimys bear no obvious numerical relationship to one another. The New World species are neither polyploids based on x = 13, as suggested by Hotchkiss (1955), nor polyploids based on x = 19, as suggested by Whitaker (1933) and many others. The differences in chromosome