Chromosome Numbers in Bromeliaceae

Abstract

Eighty-three chromosome counts are reported for 72 taxa of the Bromeliaceae. Fifty-eight of these counts are the first known chromosome number reports for their respective taxa. A model of chromosomal evolution in the Bromeliaceae (n = 25) is presented. The model is parsimonious and consistent with existing data on meiotic chromosome numbers within the family and in the closely related Velloziaceae (n = 9). Two hypothesized paleodiploids (n = 8 and n = 9) hybridized to form a tetraploid that in turn hybridized with the n = 8 lineage. The resultant n = 25 is the extant base number for the family. Two alternative hypotheses could explain the unique extant base number (n = 17) for Cryptanthus: 1) Cryptanthus represents the paleotetraploid level, i.e., prior to the second round of hybridization, or 2) the lower number represents the result of a more recent series of aneuploid reductions from n = 25. Given the existence of intergeneric hybrids involving Cryptanthus, aneuploid reduction is the more likely interpretation. RECENT RESEARCH concerning Bromeliaceae systematics and evolution (e.g., Brown and Gilmartin, 1984, 1986; Gilmartin and Brown, 1985, 1986b) has sparked renewed interest in the study of Bromeliaceae chromosomes and chromosome evolution. Past chromosome number surveys in the family (i.e., Lindschau, 1933; Gauthe, 1965; Weiss, 1965; Sharma and Ghosh, 1971; Till, 1984) have relied mostly on mitotic material. The only major exception to this was Marchant (1967) who utilized meiotically active microsporocytes. There is great variability in reported mitotic chromosome numbers (Brown and Gilmartin, 1986), and lack of concordance between mitotic and meiotic numbers for some taxa within the family. This variability in mitotic number is reflected in the variable interpretations of chromosome base numbers for the family. Brown and Gilmartin (1986) summarized the previous controversy over base number determination for Bromeliaceae, and discussed the current level of knowledge concerning poly' Received for publication 13 October 1987; revision accepted 28 October 1988. We thank a dedicated group of field collaborators, without whom this project would not have been possible: James Ackerman, Puerto Rico; Stephan Beck, Bolivia; Olga Benavides, Colombia; Elizabeth Bravo, Ecuador; David Brunner, Paraguay; I. Chacon, Costa Rica; Hermes Cuadros, Colombia; Linda Escobar, Colombia; Gert Hatschbach, Brazil; Stephen Koch, Mexico; Gustavo Martinelli, Brazil; Fernando Ortiz, Ecuador; Isidoro Sanchez Vega, Peru; and Rosa Subils, Argentina. Expert technical help was supplied by Carol Annable. We thank Ron Hartman, Don Hauber, and two anonymous reviewers for their comments. This work was supported by collaborative research grants BSR-8607 187 (GKB) and BSR-8407573 (AJG) from the National Science Foundation. 3 Deceased 10 February 1989. ploidy, chromosome size bimodality, and the correlation of nonconcordance in meiotic and mitotic chromosome numbers with the epiphytic mode of growth. The purpose of this paper is to describe results of an ongoing meiotic chromosome number survey within the Bromeliaceae, and especially subfamily Tillandsioideae. We also present a model for chromosome base number evolution for the family that is consistent and parsimonious with existing data. METHODS AND MATERIALSFloral buds were collected in the field, or obtained from cultivated material at Marie Selby Botanical Gardens, Sarasota, Florida (SEL). Buds were fixed in field-mixed Farmer's solution (100% EtOH: glacial acetic acid; 3: l/v:v) to which a drop of saturated aqueous ferric chloride (FeCl3-6H20) had been added. The latter enhances chromosome stainability. After a minimum of 24 hr, fixed buds are transferred to 70% EtOH. See Gilmartin and Brown (1986a) for a complete description of the field collaborator network and its operation. For chromosome squash preparations, individual anthers were removed from the bud in 70% EtOH and transferred to a pool of 1% acetic carmine on a microscope slide. While in the stain, the anther is cut transversely in half. Using ultrafine-tipped needle and forceps, the sporogenous masses are squeezed from each microsporangium through the median transverse cut. The sporogenous masses are positioned toward the center of the stain pool and a coverslip and gentle finger pressure are applied. The preparation is further flattened by passing the slide through an alcohol flame sev-