A KARYOSYSTEMATIC STUDY OF THE GENUS AGAVE

THE PRESENT investigation of the karyotypes of the genus Agave has had several objectives in view. The widespread distribution of some 300 species, the fact that the plants require from fifteen to twenty years to mature and that they hybridize very easily, make the taxonomic and phylogenetic study of the genus extremely complicated and very difficult to attack. It was hoped that chromosome studies of the individual species might throw some light on the present state of confusion in the classification of the genus and on our knowledge of its evolution. An understanding of the distribution of the karyotype of this group of plants may aid in the clarification of generic relationships. The "Yucca-Agave" karyotype consists of a pattern of ten large chromosomes on the periphery of the metaphase plate and fifty small ones differing slightly in size grouped in the center. This karyotype is also found in Yucca, Furcraea and Hosta. Several workers have suggested that a karyotype so unique in the plant kingdom must be of significance and have proposed using it as a basis for the revision of several related genera of the Liliaceae and Amaryllidaceae. Watkins (1936) objected to this suggestion, one of his reasons being that in the large genus Agave only four counts had been reported, those not in agreement. The data presented in this paper together with those of recent workers overcome this objection. The occurrence of polyploidy in Agave raises the question as to whether the correlations between polyploidy and vegetative structure common in other genera are also to be found in Agave and whether the higher polyploids are more useful, as is generally true in plant genera of economic importance. It is also interesting to note whether any correlation can be made between polyploidy and geographical distribution and whether polyploidy is more predominant in one or another section of the genus. REVIEW OF LITERATURE. -Much work has been done on the genera containing the "Yucca-Agave" karyotype. This unique chromosome complex was early noted by Strasburger (1882), who described the diversity in length of chromosomes in Funkia (Hosta). The early workers, among them Strasburger (1900) on Funkia, Koernicke (1901) on Yucca, Schaffner (1909) on Agave, Muller (1910) 1 Received for publication February 14, 1944. Paper No. 736 from the Department of Botany and the Botanical Garden of the University of Michigan. This material was taken from a dissertation submitted for the degree of Doctor of Philosophy in 1938. Photographs of all the plants used, a detailed summary of the literature on the cytology of Agave, and various cytological methods tested may be found in the dissertation filed in the University of Michigan Library. The writer wishes to express her gratitude to Prof. W. R. Taylor under whom this work was pursued, for his kind suggestions and criticism. The writer also wishes to express her appreciation to Prof. H. H. Bartlett for the material which made this study possible and for his continued interest in this work. on Yucca, and Woycicki ( 1911 ) on Yucca, were concerned mainly with the number and size differences of the chromosomes and the fact that the large chromosomes show a definite tendency to pair in the somatic nuclei. The description by Muller (1912) of the chromosome complex of Agave americana, having twenty instead of ten large chromosomes arranged around the periphery of the metaphase plate, gave the first suggestion of a polyploid series in Agave. From about 1925 on, workers paid more attention to chromosome morphology and to the phylogenetic relationships that might be indicated from karyotype similarities. In the work of Taylor (1925) and Morinaga et al. (1929) on Yucca filamentosa; Imai and Kanna (1934), Akemine (1935) and Yasui (1935) on Hosta; Matsuura and Suto (1935) on Hosta and Yucca; Sato (1935), Doughty (1936) and Vignoli (1936, 1937) on Agave, relative size differences were brought out and terminal spindle fiber attachments indicated in the large chromosomes. A number of workers, including Koernicke (1901), Miyake (1905), Muller (1912), have stressed the similarities between the karyotypes of Funkia, Beschorneria, Yucca and Agave. Heitz (1926) was the first to suggest a correlation between taxonomy and cytology in the Agavoideae. More recently the papers of McKelvey and Sax (1933), Whitaker (1934) and Sato (1935) have pointed out the significance of the "Yucca-Agave" karyotype in classification and have also tried to find the ancestors of the "Yucca-Agave" group by studies of the karyotypes of the more primitive monocotyledons. A survey of the literature to date (table 3) indicates that the basic chromosome number of the genera Agave, Yucca and their allies is thirty and that the various karyotypes are identical or very similar in general morphology. MATERIAL AND METHODS. -The material for this study came from plants growing in the University of Michigan Botanical Garden. These plants cover a large portion of the normal distribution range of the genus and contain representatives of the most important groups of the genus except those of the West Indies. Many of the collections were made on a number of botanical expeditions conducted or sponsored by Professor H. H. Bartlett. Some of the plants were collected by Professor Bartlett in the Maya region of Yucatan, others by Dr. C. L. Lundell and A. Whiting in Central Mexico, by H. S. Gentry in Mexico and by 0. Grassl in New Mexico. The largest number of representatives from the United States were collected by Dr. E. U. Clover in Texas. The remainder of the assortment is derived from miscellaneous sources, such as gifts from individuals or seeds from other botanical gardens and from the U. S. Department of Agriculture. Unfor-