Supernumerary Chromosomes in Haplopappus gracilis

Abstract

Since reporting the chromosome number of Haplopapputs gracilis as n=2 (Jackson, 1957), I have found other chromosome numbers in this species. In a collection of plants grown from seed obtained at Bandelier National Monument in New Mexico, chromosome numbers of n=2, n=2+1, n=3, and n=3+1 were counted. Plants having more than the normal two pairs of chromosomes were found to contain supernumerary chromosomes in the usual sense in which this term is used. In all of the New Mexican plants studied, the supernumerary chromosomes were morphologically identical. Each chromosome possessed a near-median centromere and was approximately two microns in length at mitotic metaphase. This is about one-half the length of chromosome B and one-third the length of chromosome A of H. gracilis. Studies of diakinesis failed to show any pairing of the supernumeraries with the A and B chromosomes. In the few plants grown thus far, there have been certain morphological characteristics associated with plants carrying the supernumerary chromosomes. In some plants the type of pubescence was slightly altered and the branches were thicker, especially near the tips, giving a polyploid appearance. In one plant, there was a greater distance between the spines on the leaves than is found in normal individuals. In typical plants of H. gracilis the achenes are brown or reddish, but in plants carrying the supernumerary chromosomes the achenes were a dark purple. It thus appears that the supernumerary chromosomes may exert a decided genetic effect, depending, perhaps, upon the genotypes with which they are associated. The characteristics associated with the supernumerary chromosomes were noticed first, and a later cytological study revealed the presence of the extra chromosomes. In addition to the supernumeraries found in plants from New Mexico, others were found recently in a population of plants from southwestern Arizona. Some plants from this population contained up to four supernumerary chromosomes that were much shorter than those in the New Mexican material. Seeds from this population are being grown in order to further analyze the material cytologically and genetically. Various explanations have been offered for the origin of supernumerary chromosomes in plants (Lewis, 1951; Swanson, 1943). In H. gracilis, I believe that two methods of origin may be possible. One would be the introduction of extra chromosomes into populations of H. gracilis as a result of hybridization and backcrossing with a species having a larger chromosome number. The second possibility might be that the extra chromosomes represent the centromeres and adj acent chromatin resulting from the aneuploid reduction process that may have given rise to the species. The mechanisms of this process have been outlined by Darlington (1937) and Stebbins (1950). It is generally assumed, however, that the centromeres and adjacent chromatin will be lost if they are genetically inert. Nevertheless, it may be possible that the centromere and adj acent chromatin are retained in some populations of the species even though they are inert and unnecessary for normal development.