SYNTHETIC HYBRIDS OF NEW WORLD AND OLD WORLD AGROPYRONS: III. AGROPYRON REPENS × TETRAPLOID AGROPYRON SPICATUM

Three hybrids of A. repens, 2n = 42, × A. spicatum, 2n = 28, and two reciprocal hybrids were obtained from emasculated and unemasculated crosses, respectively. The 35-chromosome hybrids tended to be morphologically intermediate between the parent species but resembled A. repens more closely than A. spicatum. A. repens behaved cytologically as a segmental autoallohexaploid, and A. spicatum acted cytologically as an autotetraploid. Mean chromosome associations of 8.04 I, 12.72 II, 0.41 III, 0.06 IV, and 0.009 V were observed in 116 hybrid cells at metaphase I. Most chromosome pairing in the hybrids was attributed to autosyndesis. A. spicatum, A. repens, and their hybrids were represented by genome formulas of SSSS, R₁R₁X₁X₁X₂X₂, and SSR₁X₁X₂, respectively. Hybrid fertility ranged from 0.02 to 0.69 seeds per spikelet.     

SYNTHETIC AGROPYRON-ELYMUS HYBRIDS. I. ELYMUS CANADENSIS × AGROPYRON SUBSECUNDUM

Twenty-four seeds were formed in 34 hand-emasculated E. canadensis florets exposed to A. subsecundum pollen. Fifteen of the 24 seeds germinated, and 12 plants were raised to maturity. Each of the 12 plants proved to be a hybrid. The hybrids were morphologically intermediate between the parents, although they favored A. subsecundum in general appearance. The parent plants were meiotically regular and behaved cytologically as strict allotetraploids, 2n = 28. Fourteen bivalents were formed in 12.6% of the hybrid cells interpreted at metaphase I. All other hybrid cells contained various combinations of univalents, bivalents, and multivalents. Mean chromosome associations of 0.22 univalents, 11.70 bivalents, 0.11 trivalente, 0.23 quadrivalents, 0.01 pentavalents, and 0.55 hexavalents were observed in 135 hybrid cells. All hybrid pollen examined was shriveled and non-staining. Five hybrids produced eight seeds, and seven hybrids were completely sterile. The ready cross-compatibility of E. canadensis and A. subsecundum and the relatively good chromosome pairing in their hybrids suggest a much closer relationship between the parent species than is implied by the prevailing taxonomic treatment. Structural rearrangements appear to be responsible for the relatively few differences between E. canadensis and A. subsecundum genomes and for the sterility of the hybrids. Cytological data from this and other investigations indicate a close relationship between many self-fertilizing Agropyron, Elymus, and Sitanion species. It is postulated that this composite of self-fertilizing Agropyron, Elymus, and Sitanion species originated by hybridization between A. spicatum and an unidentified Hordeum species.     

GENOME ANALYSIS OF AGROPYRON REPENS × AGROPYRON CRISTATUM SYNTHETIC HYBRIDS

Hexaploid A. repens, 2n = 42, and diploid A. cristatum, 2n = 14, were hybridized and gave rise to two 28-chromosome reciprocal hybrids. Approximately 1% of hand-emasculated florets of both parent species produced viable hybrid seed following controlled pollination. Early embryo abortion prevented greater hybrid seed set on A. repens, whereas failure of fertilization appeared to be the major cause of poor hybrid seed set on A. cristatum. Reciprocal differences in hybrid vegetative and spike morphology were striking. The A. repens × A. cristatum hybrid was vigorous, highly rhizomatous, and bore abundant spikes whose morphology was intermediate between that of the parent species. A. cristatum × A. repens hybrids were weak, non-rhizomatous with frequently-malformed spikes. Mean chromosome associations of 0.10 I, 20.10 II, and 0.43 IV were observed in 134 metaphase-I cells of A. repens. Subsequent meiotic stages were regular except for occasional laggards and bridges at anaphase I and II. Metaphase-I chromosome associations averaged 0.07 I and 6.97 II in 124 A. cristatum cells. Chromosome pairing in the hybrids was highly variable and averaged 11.45 I, 7.58 II, 0.44 III, and 0.02 IV per cell in 187 cells interpreted. From 5 to 14 laggards appeared in every hybrid cell at anaphase I. Bridges were observed in approximately 25% of the anaphase-I cells. Similar irregularities were observed at anaphase II. Pollen viability was estimated as 3%, and the hybrids failed to set viable seed. On the basis of chromosome pairing in the species itself and in the hybrids, A. repens was designated as a segmental autoallohexaploid with a genome formula of the type A₁A₁A₂A₂BB. Although A. repens and A. cristatum chromosomes paired occasionally, the genomes of the 2 species were essentially non-homologous. Some of the interpretational difficulties of genome analysis were discussed.     

SEGREGATION IN NEW ALLOPOLYPLOIDS OF GOSSYPIUM. IV. SEGREGATION IN NEW WORLD × ASIATIC AND NEW WORLD × WILD AMERICAN HEXAPLOIDS

Phillips , Lyle L. (North Carolina State Coll., Raleigh.) Segregation in new allopolyploids of Gossypium. IV. Segregation in New World × Asiatic and New World × wild American hexaploids. Amer. Jour. Bot. 49(1): 51–57. 1962.—The New World tetraploid cottons, G. hirsutum and G. barbadense, are natural amphidiploids (genome formula, 2 [AD]) combining species of the cultivated Asiatic (2A) and wild American (2D) groups of diploid cottons. Genetic segregation for marker alleles in New World × Asiatic and New World × wild American synthetic hexaploids have been determined. Average segregation for several loci in New World × Asiatic hexaploids is close to the autoploid 5:1 ratio, ranging from 5.1 to 6.8:1. Average segregation for 3 loci (L, Rd, and R₁) common to a series of New World × wild American hexaploids is: New World × G. raimondii, 9.3:1;–× G. harknessii, 16.4:1;–× G. armourianum, 17.4:1;–× G. aridum, 20.3:1;–× G. lobatum, 21.4:1–× G. thurberi, 32.9:1;–× G. gossypioides, 66.5:1. These data are discussed, and the method by which they were derived is compared with other cytogenetical means of discerning phylelic interrelationships among Gossypium species.     

R. D. Fitzgerald's F. L. S. „Australian Orchids“

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