Harold H. Smith

Recent Cytogenetic Studies in The Genus Nicotiana

Publisher Summary This chapter discusses the cytogenetic studies on Nicotiana. The genus Nicotiana has been a favored material for studies on inheritance and evolution in higher plants since the days of the pre-Mendelian hybridizers. The origins and evolution of species of the genus are presented and these relationships are summarized diagrammatically in the form of three phylogenetic arcs. The evidence shows that interspecific hybridization, with subsequent amphiploidy as well as genetic recombination, has played an important role in the evolution of the genus Nicotiana . More than 300 interspecific hybrids have been reported in the genus Nicotiana . Limits and consequences of multiple allopolyploidy are explored and the role of aneuploidy assessed. The use of introgressive hybridization to transfer resistance to various diseases into commercial tobacco from wild species is an important part of tobacco breeding efforts. Certain Nicotiana interspecific hybrids exhibit a striking somatic instability in forming spontaneous tumors. In certain interspecific combinations, evidence for a hereditary component in the cytoplasm has been found. This affects either pollen sterility or plastid-controlled chlorophyll variegation. Biometrical analyses of quantitative character inheritance in N . tabacum and in species crosses have shown preponderant additive gene effects and consequent effective response to selection.

Organogenesis from Callus Culture of Hordeum vulgare.

SummaryRapidly proliferating callus cultures from the apical meristem of Hordeum vulgare L., cultivars “Himalaya” and “Mari”, were established on a defined medium supplemented with auxin and cytokinin. A frequency of organ differentiation of ca. 85% was obtained by transfer to the same medium but without any growth regulators. All regenerated plants had the diploid chromosome number of 14 (2n).

Mutations induced by ethyl methanesulfonate in maize

Abstract “Single-locus mutations” were induced frequently and almost exclusively by treating seeds or young seedlings of I Sh Wx stocks of Zea mays with aqueous solutions of ethyl methanesulfonate, while X-irradiation of pollen produced a high proportion of “multiple-locus mutations”.

Partial Synchronization of Nuclear Divisions in Root Meristems with 5-Aminouracil.

Root tip cells of Vicia faba were partially synchronized in nuclear stages by treatment for 24 hours with 700 parts of 5-aminouracil per million. All division was suppressed by the analog treatment, and a peak in division stages ( up to 62.5 percent) was reached 14 hours after removal from the aminouracil. Populations of partially synchronized cells can be useful in experiments designed to study various intracellular reactions or responses at different stages in the nuclear cycle.

Radiation in the production of useful mutations

Applications of radiation in the productlon of genetic variants in plants are reviewed. Fundamental processes of gene mutation and chromosome breakage are discussed. The economic feasibility of employing radiation in plant improvement programs is considered. 90 references. (C.H.)

GENETIC CONTROL AND RADIATION EFFECTS IN NICOTIANA TUMORS

Genetic tumors are neoplastic growths tha t arise, without any apparent external cause, in organisms of certain particular genotypes. The genetic constitt~tion of the organism seemingly determines the potent iahty of the constituent cells to undergo spontaneous change from normal growth, with respect both to rate and differentiation, to abnormal, relatively undifferentiated proliferation. Evidence for genetic control of tumor formation ia Nicotiana has been obtained from three principal observations, namely, (1) tumors are confined to plants of particular interspecffic hybrid combinations, (2) reciprocal crosses give hybrids showing the same tumor expression, (3) tumors fail to form when only certain chromosomes are present in an interspecifie combination which with the full complements of chromosomes regularly forms tumors (KEHR and S~IT~ 1954). One purpose of the present paper is to present evidence of segregation and linkage for the tumorous condition, thus providing further confirmation of genetic control. A second objective is ~o report different effects of irradiation on a variety of hereditary tumor types in order to demonstrate interrelationships between genotype and radiation response in tumor formation.

Selection and differentiation of aminopterin resistant cells of Datura innoxia

Abstract Primary cultured pith cells of Datura innoxia were subjected to a stepwise selection of colonies resistant to the folic acid analog, aminopterin. The first two screenings were done at a concentration of 1 · 10−5 M, followed by 2 · 10−5 M and 4 · 10−5 M. The frequency of resistant colonies ranged from 5.4 · 10−6 to 10−7 during these steps. Resistance was maintained in cells through two subcultures (3 months) without aminopterin. Independently isolated resistant lines differed with respect to growth and ability to differentiate. Plants have been obtained from five resistant lines. One of these plants is fertile and has produced seeds.

Corn Seeds Affected by Heavy Cosmic Ray Particles

Corn seeds of a special genetic stock were recovered from two satellite flights and the plants grown from them were examined for abnor malities. Some evidence for a slight increase in chromosomal deletions was observed, which was predicted from the flux of heavy cosmic ray primary particles. Nothing unexpected was observed.

An analysis of H3-thymidine distribution throughout the vegetative meristem of Arabidopsis Thaliana (L.) heynh

Abstract The material was grown under aseptic culture conditions from seed through flowering under a 12-hr photoperiod. H 3 -thymidine was applied to the vegetative shoot meristem for a six-hr period, sixteen days after sowing. Samples of the material were collected, processed for histological and autoradiographic techniques. Chi-square analysis of the grain distribution in the shoot apex shows less H 3 -thymidine label in the most axial tunica 1, tunica 2 and subjacent outer corpus layer than the remaining portions of the shoot meristem.

The deuteron microbeam as a tool in botanical research

Abstract Maize kernels of a genetic stock heterozygous for Yg2/yg2 were irradiated at different dosages and positions with 22·5 meV deuteron beams, 25 μ and 250 μ in width, from a 60-in. cyclotron. The most striking morphological effects in seedlings grown from irradiated embryos were chlorotic marks that appeared in the first four leaves. The location of the area irradiated, as evidence by the position of the radiation mark, was useful in determining radiosensitivity of different regions of the embryo and in comparing relative growth in different regions of the early leaves. The 25 μ, compared to the 250 μ beam, required about twice the radiation dosage to produce the same amount of evident damage based on clarity of radiation marks on the leaves and incapacitation of the stem apex. Radiation of the seed embryo enhanced elongation of the primary root for from two to four days after hydration. This effect was most pronounced when the microbeam passed through the leaves only and was least evident when the embryonic root itself was irradiated. Cytogenetic effects of microbeam radiation were detected by the appearance of yellow-green sectors in the leaves, owing to loss or mutation of the dominant Yg2 allele. Maximum numbers of mutant sectors in seedling leaves were observed when the microbeam was directed toward the basal regions of leaves 1, 2 and 3 in the embryo. The size and position of mutant sectors can be used to study problems of cell lineage and histogenesis.