HYBRIDIZATION BETWEEN ASTER AND MACHAERANTHERA AND ITS TAXONOMIC SIGNIFICANCE

A controlled crossing program resulted in the production of four interspecific and three intergeneric hybrids among species of Aster sect. Oxytripolium and Machaeranthera. Meiotic analysis revealed stronger degrees of genome homology in some of the intergeneric hybrids than in some of the interspecific hybrids. The significance of these results to the taxonomy of Aster and Machaeranthera was considered and the result was the transfer of Aster sonorae to Machaeranthera. AT THE TIME Asa Gray (1849) originally described Psilactis, he implied that it might be systematically intermediate between Dieteria (Machaeranthera) and Aster. Since then, other investigators have proposed different interpretations of the relationships among Psilactis, Aster, and Machaeranthera. Notable among these interpretations are those of Gray (1852), Brandegee (1889), Jackson (1962), and Turner and Horne (1964) which suggest an affinity between Psilactis and Machaeranthera. Aster boltoniae (Greene, 1897) as well as P. asteriodes (Gray 1849) and P. lepta (Shinners, 1949) were considered as synonyms of Machaeranthera boltoniae by Turner and Horne (1964). Shinners (1949) commented on the resemblance between P. lepta and A. pauciflorus of section Oxytripolium. These previous judgments concerning the taxonomic relationships among Machaeranthera sects. Machaeranthera and Psilactis and Aster sect. Oxytripolium have been based primarily on comparative morphology and chromosome number. For instance, Turner and Horne (1964) in their treatment of Psilactis as a section of Machaeranthera, referred mainly to comparative morphology and chromosomes but only incidentally to apparent genetic affinities, as revealed by hybridization studies (Jackson, 1962). The investigation reported here utilized hybridization and cytological analysis to reexamine the relative degree of similarity between selected species in the Machaeranthera-Psilactis-Oxytripolium complex. 1 Received for publication 14 May 1976; revision accepted 1 September 1977. This paper is adapted from a Ph.D. dissertation submitted to the Department of Biological Science, Texas Tech University, 1974. The author thanks Dr. R. C. Jackson for his valuable advice and assistance. Paper No. 4981 of the Journal Series of the North Carolina State University Agricultural Experiment Station, Raleigh. MATERIALS AND METHODS-A controlled crossing program involving the species in Table 1 was conducted. Seeds collected in the field and from greenhouse crosses were germinated and the seedlings were raised under greenhouse conditions. Root tips newly emerging from the seedlings were harvested and treated with 0.01% 1bromonaphthalene for 2 hr, fixed in 95% ethanol-propionic acid (4: 1) for 24 hr, and stored in 70% ethanol at 5 C. The root tip squash technique using FLP orcein stain (Jackson, 1973) was used to prepare chromosomes for karyotype analysis. Selected metaphase karyotypes were photographed at x800. Juvenile heads were harvested from at least three greenhouse parental individuals and from varying numbers of hybrid individuals and were fixed in 95% ethanol-propionic acid (3:1) for at least 8 hr, then transferred to 70% ethanol and stored at 5 C. Microsporocyte squashes were made using FLP orcein. Chromosome pairing configurations and minimum chiasma frequencies were recorded from diakinesis cells. Standardized chiasma frequencies (mean minimum chiasma frequencies per cell divided by the number of paired chromosomes in the modal diakinesis configuration) and cytological affinity indices (standardized chiasma frequencies multiplied by the percent of chromosomes paired in the modal diakinesis configuration) were calculated for each parental species and hybrid. Cytological difference indices (the average difference in cytological affinity index between the hybrid and its two parental species) were calculated for the hybrids. Expression of the cytological data as cytological difference indices compensated for genic control of chiasma frequency or chromosome pairing in the parental species and presumably yielded an accurate reflection of the degree of homology between the genomes of the species. The number of microspores in indi-