Strong Reproductive Isolation Research Articles

THE STRUCTURE OF A POLYSPECIES HYBRID SWARM IN LIATRIS.

Biotically sympatric flocks containing more than two species afford special media for complex hybridization because of the multitude and broad array of recombinants which may be produced and because of the many avenues available for gene flow. The potential for hybridization in polyspecies populations, however, is limited, for species weakly differentiated in their ecological demands often are strongly isolated by incompatibility and sterility barriers (Stebbins, 1950). The relationship between close spatial association and strong reproductive isolation is readily seen in the genus Liatris. The genus, which extends throughout much of the eastern United States east of the Rocky Mountains and in southern Canada and northern Miexico, forms extensive areas of polyspecies sympatry, and in many such areas three or more species coexist. In accordance with the general rule referred to above, hybridization within such flocks is practically nonexistent. An exception to the rule, found in a sand prairie in northern Illinois, provided an opportunity to study polyspecies hybridization. The flock in question is composed of L. aspera Michx., L. cylindracea Michx., L. spicata Willd. and L. ligulistylis (Nels) K. Sch. The present paper represents an interpretation of this assemblage. The specific goals of this investigation were (1) to determine the extent and direction of gene flow and (2) to establish the identity of F1, advanced generation and backcross hybrids.

CYTOGENETIC STUDIES ON THE ORIGIN OF TAGETES PATULA. I. MEIOSIS AND MORPHOLOGY OF DIPLOID AND ALLOTETRAPLOID T. ERECTA × T. TENUIFOLIA

Towner, Joseph W. (U. California, Los Angeles.) Cytogenetic studies on the origin of Tagetes patula. I. Meiosis and morphology of diploid and allotetraploid T. erecta × T. tenuifolia. Amer. Jour. Bot. 48 (9): 743–751. Illus. 1961.—Cytological and morphological evidence is presented to show that Tagetes patula L. is an allotetraploid species (2n = 48) which probably originated by hybridization between the diploids T. erecta L. and T. tenuifolia Cav., or species closely related to them. The putative parent species are separated by strong reproductive isolation barriers, and controlled crosses between them yielded very few good seed. Most of the hybrid seedlings died before reproductive age, and the few survivors were sterile and extremely weak. At meiosis in the diploid hybrid, bivalent frequency per cell was 0–11 and averaged 4.4. Although 43.6% of the quartet stage consisted of restitution dyads, pollen was essentially all non‐viable and seed set was less than 1%. A true‐breeding, fertile, allotetraploid T. erecta‐tenuifolia was synthesized by colchicine‐doubling of the chromosome number of the F1 hybrid. In the synthetic allotetraploid, average chromosomal association was 23.97 bivalents plus 0.05 univalents, and 97.4% of MI cells had 24 bivalents; there were no multivalents, bridges, or fragments. Tagetes erecta‐tenuifolia showed a striking morphological similarity to T. patula. Both the natural and synthetic allotetraploids were intermediate between T. erecta and T. tenuifolia for nearly every character. Of 10 species of Tagetes whose chromosome numbers are known, 5 are tetraploid (2n = 48).