CHROMOSOME STUDIES IN THE ALPINE AND SUBALPINE FLORAS OF MEXICO AND GUATEMALA

BEAMAN, J. H. (Michigan State U., East Lansing), D. C. D. DE JONG, and W. P. STOUTAMIRE. Chromosome studies in the alpine and subalpine floras of Mexico and Guatemala. Amer. Jour. Bot. 49(1): 41-50. Illus. 1962.-Chromosome counts of 101 collections representing 69 species in 42 genera and 22 families have been obtained. Counts are reported for the first time in 59 species and 2 genera (Oreobroma and Villadia). Counts establishing new basic numbers were found in Draba, Lithospermum, and possibly in Arenaria. Where pertinent, the chromosomal data are related to previous taxonomic or chromosomal investigations. Chromosome numbers are now known in nearly half of the species in the alpine flora of Mexico. THE CHROMOSOMAL data reported here are part of a taxonomic, phytogeographic, and ecological investigation being conducted on the alpine2 flora of Mexico and Guatemala. Counts in 16 species were published in a preliminary paper by Stoutamire and Beaman (1960). The present report includes counts of 101 collections representing 69 species in 42 genera and 22 families; 59 species have been examined for the first time. The primary objective of the present paper is to record chromosome numbers in alpine and subalpine species and in some cases to briefly consider the taxonomic or cytological significance of the numbers. A high incidence of polyploids has been found in the alpine flora, and this matter will be considered in a later paper. Most of the bud collections for melotic chromosome counts were obtained from plants growing in their native habitats during the summers of 1959 and 1960. Additional bud and root-tip material (collection numbers prefixed by L in Table 1) was obtained from specimens transplanted from their native habitat to a greenhouse at Michigan State University. Cotida pygmaea and Phacelia. platycarpa (3200) were grown from seeds collected in the field. Fixation and slide-preparation procedures employing the aceto-carmine squash technique were similar to those outlined by Beaman (1954). Permanent slides are available for most counts. The chromosome configurations were drawn at an initial magnification of 4000 XAll counts are documented by specimens filed in the Herbarium of Michigan State University. The species determinations and chromosowe counts are our own, except where otherwise indicated. A complete listing of species studied, source of 1 Received for publication June 9, 1961. This investigation has been supported by National Science Foundation Grant G-9045. 2The term "alpine" denotes areas above timberline. In Mexico and Guatemala the average elevation of timberline is about 4000 m. Subalpine vegetation extends from timberline down to a poorly defined lower limit at between 2500 and 3000 m. material, habitat (i.e., alpine or subalpine), life. duration, chromosome number, and level of ploidy is given in Table 1. Levels of ploidy are sometimes followed by a question mark, indicating uncertainty about the basic chromosome number. Not all species listed in Table 1 are discussed in the text. Col. lection numbers are by Beaman except where other. wise indicated. Altitudes, approximate within 100 m or less, will be discussed in detail in a subsequent paper by Beaman. Thorough taxonomic studies may eventually necessitate some nomenclatural changes because relatively few of the species have been included in satisfactory monographic or revisional treatments. GRAMINEAE-Nielsen (1939) reported Agrostis exarata to be tetraploid with 2n = 28. Our hexa. ploid (n = 21) count for this species agrees with the findings of Stebbins and Love (1941). Poa annua (n = 14) has been previously counted by several investigators (cf. Hovin, 1958). Chromosome numbers in the Trisetum spicatuim complex have been summarized by Morrison (1959). IRIDACEAE-Our material of Sisrinchium quad. rangulatum was found to have n 18, but Lewis and Oliver (1961) reported a count of n = 16 in S. cf. quadrangulatum. Their collection probably represents a different species, since it was obtained from a locality at least 500 m below the usual lower altitudinal limit of this species. PORTULACACEAE-Chromosome counts have not been reported previously for Oreobroma. However, some authors (e.g., Pax and Hoffmann, 1934) have considered the genus not to be distinct from Calandrinia. Darlington and Wylie (1955) list basic numbers of x = 8 10, 11, and 12 for this genus. The relatively high number of chromosomes in 0. megarhizum (n = 34) is probably the result of both polyploid and aneuploid processes. We have arbitrarily considered this species to be an approximate hexaploid. CARYOPHYLLACEAE-Arenairia-On a base of either x = 10 or 11, A. bryoides (2n ca. 54)