Supernumerary chromosomes in Saxifraga virginiensis (Saxifragaceae)

Acetocarmine squashes of root tips have demonstrated that 2n = 20 and 38 in Saxifraga virginiensis. These contrast with the earlier reported count of 2n = 28 for this species. In several populations supernumerary chromosomes were detected. Both intrapopulational and interpopulational variation in supernumerary chromosome number were detected, with the largest number of supernumerary chromosomes observed being six. Because these supernumerary chromosomes are equal in size to many of the smaller A chromosomes during mitotic metaphase, the presence of supernumerary chromosomes in this species could not be ascertained by analysis of mitotic metaphase preparations alone. During mitotic prophase, however, the supernumerary chromosomes of S. virginiensis are highly heterochromatic, appearing more densely coiled and darkly stained than the A chromosomes. This characteristic facilitated the recognition of supernumerary chromosomes in this species. The similarity in size of A and supernumerary chromosomes during mitotic metaphase and the observation of six supernumerary chromosomes in one population suggest that the count of 2n =28 reported earlier for S. virginiensis may actually be a misinterpretation of 2n = 20 plus 8 supernumerary chromosomes. Furthermore, these findings and the observation of this same supernumerary chromosome phenomenon in other species of Saxifraga raise the possibility that some of the many disparate chromosome counts attributed to aneuploidy in the large genus Saxifraga may also be the result of misinterpretations of supernumerary chromosomes as A chromosomes. SAXIFRAGA is a large genus of about 300 species distributed throughout the Arctic and North temperate zones (Spongberg, 1972). Chromosome numbers reported for species of Saxifraga range from 2n= 12 to 220, and aneuploidy is reputedly common in the genus (Fedorov, 1969; Spongberg, 1972). As part of an ongoing cytological investigation of tribe Saxifrageae, a chromosomal study of Saxifraga virginiensis Michaux was undertaken. This species is distributed from New Brunswick westward to Manitoba and southward to Georgia, Arkansas, and Oklahoma. The purpose of the present work is to: 1) report two new chromosome numbers for S. virginiensis; 2) report the presence of supernumerary chromosomes in S. virginiensis and to discuss the difficulties caused by these supernumerary chromosomes in accurately determining chromosome numbers for this species; and 3) discuss the implications of these findings with regard to the cytology of Saxifraga in general. I Received for publication 14 August 1982; revision accepted 9 November 1982. This work was supported in part by NSF grant DEB 8109817. I thank Jeff Doyle, Charles Werth, Dave Darrow, Georgia Hammond-Soltis, Ken Reading, and Tim Storbeck for assistance in obtaining collections. MATERIALS AND METHODS -Living plants of Saxifraga virginiensis were collected in the field (Table 1) and subsequently maintained in greenhouse culture. Chromosomal preparations were made from root tips using techniques discussed previously (Soltis, 1980). Photographs were made with a Zeiss Photomicroscope III utilizing phase contrast optics and Kodak technical pan film 2415. RESULTS AND DISCUSSION-Acetocarmine squashes of root tips showed 2n = 20 in plants of Saxifraga virginiensis representing one population each from Massachusetts, Missouri, and Tennessee, and 2n = 38 for plants from the populations analyzed from North Carolina and Virginia (Fig. 1-4). The chromosome numbers reported here contrast with the earlier reported count of 2n = 28 for S. Oirginiensis (Fedorov, 1969; Love and Ritchie, 1966). In several of the analyzed populations, supernumerary chromosomes were detected; six supernumerary chromosomes were observed in plants from the Tennessee population (Fig. 3, 5), and 1-4 supernumerary chromosomes were sometimes observed in root tips from plants representing the Yorktown, Virginia and Halifax, North Carolina populations (Fig. 4, 6). As is discussed in detail below, however, the supernumerary chromosomes observed in the above noted populations are indistinguish-