CHROMOSOME NUMBERS AND EVOLUTION IN NORTH AMERICAN SPECIES OF LINUM

Among the North American species of Linum theIe are three basic chromosome numbers representing invasions from the Old World of three distinct evolutionary lines. NT = 9 is foulnd only in the blue-flowered group represented in North America by two species, L. Iewisii and L. pratense which are closely related to and may be conspecific with the Old World L. perenne. The basic number for the yellow-flowered species is n = 18 which is characteristic of the Scabrelia and Virginiana subgroups. The loss of chromosomes in the NATeo-mexicana (n = 13) anid Sulcata-Rigida (n = 15) subgroups suggests that the basic haploid number of 18 might be a polyploid derivative of an Old World ancestor with n = 9. The incidence of n = 9 among Old World species of L num may indicate that this represents an ancestral condition. Linum catharticum has n = 8; this number and features of morphology and distribution suggest that it is not directly related to either the blue-flowered or yellow-flowered groups in North America but represenits a separate introduction on this continent. THE LAST comprehensive listings of known chromosome numbers of the genus Linum were done by Ray (1944) and Darlington and Wylie (1955) in which a'lmost all of the citations were of Old World species, very little investigation having been made on the New World segment of the genus. Since that time the North American plants have been studied somewhat more intenisively (Osborlne and Lewis, 1962; Rogers and Harris, 1966; IN/1osquin and Hayley, 1967) with the result that chromosome numbers have been established for over 80% of the 45 wild species and varieties found in North America. Osborne and Lewis (1962) reported chromosome numbers for 10 species and varieties from Mlexico and the southerni United States and pointed out the possibility that those species which combined a chromosome count of n = 15 with certain morphological similarities might form one natural group. This approach was not extended to the other groups and it should, therefore, be worthwhile to determine whether the chromosome complements of the various North American species correlate with differences and similarities in morphology and geographic distribution. This paper, then, will be a discussion of the chromosome numbers, ranges and possible derivations based on cytological evidence of all the known American taxa, including 14 taxa reported here for the first time. New chromosome counts were made on pollen mother cells from immature flower buds collected IReceived for publication 12 April 1968. Contribution No. 209 from the Biology Department, Wayne State University, Detroit. This investigation was supported in part by National Science Foundation Grant GB-3209.. either in the field or from plants grown from seeds collected from wild populations. All citations for the chromosome numbers of North American Linurm are given in Table 1 where newly examined taxa are indicated by their collection numbers. Observations of chromosome pairing during meiosis showed that chromosomal behavior was, with the exception of L. mediumn var. medium, regular. Counts were made during second division since the low chiasmata frequency in first prophase permitted separation of bivalents and made accurate counting difficult. This situation was discussed by Osborne and Lewis (1962) and may be a fairly regular characteristic. Where material was available, measnrements were also made of chromosomes in dividing root tip cells to check the possibility of correlation among chromosome size, chromosome number and taxonomic groups. Size groups had been previously established by Ray (1944), primarily among Old World taxa, and were designated as small, 1-1.5 ,1; medium, 3-4 ,u; and large, up to 6 ,u although there was no absolute correlation between length and chromosome number. During the course of the present study, measurements of chromosomes of the New World taxa failed to reveal any significant correlation between the length of the chromosomes and the taxonomic groupings to which the species belong. While species of the Riqida (n = 15) group tend to have somewhat larger chromosomes (2-2.5 ,) than those of the Scabrella (n = 18) group (1.5-2 ,u), the differences in length do not lend themselves to assignment to the established size groups. Evidently the chromosome length here is not sharply defined but tends to gradate slightly from one size to another, and none were found whicl