CYTOGEOGRAPHY AND PHYLOGENY OF THE NORTH AMERICAN SPECIES OF VERBENA

LEWIS, WALTER H., AND ROYCE L. OLIVER. (Stephen F. Austin State Coll., Nacogdoches, Texas.) Cytogeography and phylogeny of the North American species of Verbena. Amer. Jour. Bot. 48(7): 638-643. Illus. 1961.-Chromosome numbers of 26 Verbena species in North America, separable into 2 series, are reported. The x = 5 series has no diploid representative, few tetraploids, and numerous widespread hexaploid species, while the x = 7 series has many pandemic diploid species and few tetraploid and hexaploid taxa. When compared with the data available for the South American verbenas, these frequencies and distributions were found to be almost opposite. The origin of the 2 groups from an x = 6 prototype followed by ascending and descending aneuploidy is suggested. Migration via tropical land bridges is proposed to explain the occurrence of both groups in the 2 continents. The following taxa are proposed: subgenus Verbenaca (Schauer) Lewis, and subgenus Glandularia (Gmelin) Lewis. MOLDENKE (1959) lists 71 species of Verbena in North America, well over 100 in South America, and 2 native to Eurasia. Of the North American species, 7 are introduced, 23 are endemics principally to Mexico and often known from a single locality, and only 41 pandemic species are significant in the flora as a whole. Chromosomal data have been recorded for about 40 of these species, of which 12 are North American, giving in total chromosome numbers for less than 25% of the Verbena species (Junell, 1934; Derman, 1936; Noack, 1937; Beale, 1940; Schnack, 1942; Schnack and Covas, 1944, 1945, 1951). These studies have shown that the genus is separable into 2 series of x = 5 and 7 chromosomes with polyploid representatives in each and that they are characteristic of the sections Glandularia and Verbenaca, respectively (as modified by Schnack, 1942). Numerous attempts by Derman (1936) to produce intersectional hybrids were unsuccessful, whereas in several crosses, even between species of different chromosome numbers in the same section, he produced many hybrids. The objectives of this investigation were to determine the chromosome numbers of all available species in North America and to correlate these with their known distributions. The data were compared with the cytogeography of the South American entities to give a phylogenetic analysis of the genus. MATERIALS AND METHODS-The chromosomes of 42 verbenas have been studied from collections made in Arizona, New Mexico, Texas, and Mexico. Immature flower buds were fixed in the field in a solution! of 4 parts chloroform, 3 parts absolute ethyl alcohol, 1 part glacial acetic acid; as soon as possible after fixation, the buds were stored at I Received for publication January 30, 1961. This investigation was supported by the National Science Foundation G-9800 and by the Undergraduate Research Participation Program (NSF G-12059). refrigerator temperatures for periods up to 4 months. For 3 populations only, root tips were pretreated in a saturated solution of paradichlorobenzene for 3 hr before fixation (Fig. 1, 21, 23). Flower buds and root tips were squashed in 1% acetic-orcein and meiotic (PMCs) or more rarely mitotic plates were studied. Chromosome drawings were made with the aid of a camera lucida giving a total magnification of X 2300. Voucher specimens for each plant studied are filed in the Stephen F. Austin State College Her. barium (ASTC) and have been verified by Dr. Harold N. Moldenke, Trailside Museum, New Jersey. Distributional maps were made for all North American species chiefly from the records of Moldenke (1959 and supplements). RESULTs-Twenty-six Verbena species with their chromosome numbers, collection localities, and col. lectors are listed in Table 1. With respect to their chromosome numbers, the species fall into 2 basic groups of x 5 and x = 7, which, without exception, correspond to their classification in thesections Glandularia and Verbenaca as anticipated on the basis of earlier cytological studies. Chromosome counts are recorded previously for 5 of the species listed and agree with our results excepting 2n 30 for V. racemosa Eggert (Derman, 1936). Our number of n = 10 suggests 4x and 6x races for the taxon, or, alternately, that the binomial refers to 2 distinct species. Derman (1936) and Noack (1937) point out a correlation between chromosome size and section. They found small chromosomes for species in the x = 7 series and larger chromosomes (2-6 times) for those in the x 5 group. Our drawings illustrate a tendency for larger chromosomes in the Glandularia species (Fig. 1-12) than in the Verbenaca (Fig. 13-24) but as a number of these are equal in size (Fig. 1 vs 21, 23; 4 vs 15), no unqualified sectional separation on this basis is possible.