Partial Hybrids Research Articles

Evidence for gene transfer by the use of sublethally irradiated pollen in Zea mays and theory of occurrence by chromosome repair through somatic recombination and gene conversion

Gene transfer by the use of sublethally γ-irradiated pollen was investigated in Zea mays using three marker genes. Irradiations at 10 and 15 kr produced rare viable T1 seeds which gave rise, usually, to sterile plants. At 5 kr more viable T1 seeds were produced, the majority of which gave rise to phenotypically normal, fertile plants. About a quarter of the resulting (T1) plants showed recessive maternal characters, instead of being true hybrids showing dominant characters of the pollen parent. In effect, different, partial hybrids were produced in which only a portion, and not all, of the pollen genome was combined with the total egg genome. The above results are similar to those previously obtained in Nicotiana rustica (Jinks et al. 1981) where pollen irradiated at sublethal doses was used. A new hypothesis is proposed to explain gene transfer following use of sublethally irradiated pollen. It is suggested that in divisions during early embryogeny following normal fertilisation by pollen treated with near-lethal doses of radiation, there is selection of cells in which irradiation damaged segment of paternal chromosomes are repaired, through homologous somatic recombination and gene conversion, by segments from the normal maternal chromosomes. The chromosome repair may be specifically facilitated by irradiation-induced sub-chromatid breakage which occurs only in the prophase of mitotic cells—the usual phase of the generative and gametic nuclei in the pollen grains at the time of irradiation. The proposed mechanism highlights the dynamic role of single strands in chromosome recombination and repair and the use of ionising radiations and pollen grains in the timely production of such strands.

Genetic analysis of aspartate aminotransferase isozymes from hybrids between Drosophila melanogaster and Drosophila simulans and mutagen-induced isozyme variants.

The aspartate aminotransferases (designated GOT1 and GOT2) are two enzymes of Drosophila melanogaster for which naturally occurring electrophoretic variants were not found. There is an electrophoretic difference between D. melanogaster and D. simulans. Since the F1 hybrid offspring of these species are sterile, a genetic analysis of the ordinary type cannot be done on differences between the two species. A method was devised to make "partial hybrids" in which one chromosome arm is homozygous for melanogaster genes in an otherwise hybrid background. By using this method, Got1 was localized to 2R and Got2 to 2L. Once a gene can be assigned to a chromosome, it may be followed in crossing schemes and mutations from mutagen treatments may be looked for. At the locus of Got1 a mutation with low activity was recovered and designated Got1lo. It was located at a genetic map position of 75 on 2R. A Got2 mutant with a greater migration to the anode was recovered and designated Got2J. It was located at a genetic map position of 3.0, and in the salivary chromosome was between 22B1 and 22B4 inclusive.

An evaluation of F2 populations from a cross between Cucurbita pepo L. and C. moschata Dutch. for resistance to cucumber mosaic virus

Several varieties of Cucurbita moschata were found resistant to cucumber mosaic virus (CMV). Inter-specific hybridization between C. pepo and a CMV resistant variety of C. moschata resulted in partial fertile hybrids which set F2 seeds. Cotyledonary inoculation of F2 seedlings with CMV brought various results, depending on seasonal differences and developmental stages at which observations were made. Resistance to CMV generally declined under conditions of low temperature, short days and low light intensity.

Proliferating hybrid cells formed between rat spermatids and an established line of mouse fibroblasts.

ABSTRACT Proliferating hybrids were formed between rat spermatids and LM(TK−) cl 1 D, an established line of mouse fibroblasts. Spermatids were obtained from dissociated rat seminiferous tubules by differential attachment of testicular somatic cells to culture plates, followed by gravity velocity sedimentation into linear Ficoll gradients. Cells were fused with the aid of Sendai virus and hybrid selection was facilitated by the inability of cl 1 D cells to grow in HAT medium and the inability of spermatids to attach or proliferate in vitro. Three types of cells grew in fusion plates: (1) unfused contaminant rat testicular somatic cells; (2) rat testicular cell × cl 1 D hybrids and (3) rat spermatid × cl 1 D hybrids. Unfused rat testicular somatic cells were morphologically and karyologically different from either type of hybrid. The 2 hybrid classes were distinguished by the total number of chromosomes and the number of parental chromosome markers. Rat chromosomes are preferentially eliminated from haploid as well as diploid hybrids and at least 3 rat enzymes (lactate dehydrogenase, glucose phosphate isomerase and an unknown oxidase) are synthesized in hybrid clones of both types. The implications of partial haploid hybrids for genetic analysis are briefly discussed.

Inefficiency of genetic recombination in hybrids between Escherichia coli and Salmonella typhosa.

An Escherichia coli Hfr strain in which three negative chromosomal alleles (leu(-), arg(-), and mtl(-)) were closely linked to three positive alleles (ara(+), rha(+), and xyl(+), respectively) was employed in matings with a Salmonella typhosa recipient. The detected expression of the negative E. coli alleles in S. typhosa hybrids selected for receipt of an associated positive E. coli marker was used to determine the occurrence of haploid S. typhosa recombinants, as distinguished from stable partial diploid hybrids. At the same time, the inheritance patterns and segregation behavior of the positive alleles provided indicators of the occurrence of partial diploid hybrids. Examination of both positive and negative markers inherited by ara(+), rha(+), and xyl(-) selected S. typhosa hybrid classes indicated that relatively short E. coli chromosomal segments (generally about 4 min or less in length) were involved in recombination (haploidy), whereas rather extensive E. coli genetic segments were conserved in the diploid state. S. typhosa hybrids selected for receipt of the ara(+) marker showed a 52% incidence of leu(-) haploidy, which is probably close to being an accurate measure of recombination at the site of the ara(+) allele. S. typhosa hybrids selected for receipt of the rha(+) or xyl(+) markers showed only a 20% incidence of arg(-) or mtl(-) haploidy, respectively, but both of these hybrid classes exhibited a higher incidence of conservation of extensive E. coli diploid segments than did the ara(+) selected class. Remating of haploid S. typhosa hybrids with recombinant xyl(+)mtl(-) or rha(+)arg(-) regions resulted in higher frequencies of hybrid recovery than were observed in the initial matings. However, there was a higher incidence of partial diploidy and a lower incidence of haploidy among the hybrids obtained from these rematings.

Conservation and transfer of Escherichia coli genetic segments by partial diploid Hfr strains of Salmonella typhosa.

Heterozygous, partial diploid hybrids were obtained in a Salmonella typhosa Hfr strain by using it as the recipient in a mating with the Escherichia coli Hfr donor WR2004 (O...proA...leu). Three of these S. typhosa Hfr hybrids were observed to mobilize and transfer the diploid E. coli genes, at high frequencies, to an E. coli recipient. The gradient of transfer frequencies of E. coli markers from these S. typhosa Hfr hybrids was similar to that observed with E. coli Hfr WR2004, from which they were derived. Interrupted matings with one of these S. typhosa Hfr hybrids, designated WR4272, showed the entry times for the proA, thr(-)leu, and argB E. coli diploid markers to be identical to the times obtained for these markers with E. coli Hfr WR2004. Also, the pattern of unselected inheritance of the diploid E. coli markers of S. typhosa Hfr hybrid WR4272 was similar to that observed with the chromosomal markers of E. coli Hfr WR2004. It was concluded that S. typhosa Hfr hybrid WR4272 contains, in addition to its Salmonella genome, a physically continuous E. coli chromosomal segment which is genetically complete from proA to at least the strA locus. The two other S. typhosa Hfr hybrids, on the basis of transmission frequency gradients, appeared to contain a continuous E. coli diploid segment complete from proA through the fuc locus. Other classes of S. typhosa Hfr hybrids, derived from mating with E. coli Hfr WR2010 (O...tna...xyl), were also observed to transfer E. coli genes at high frequency.

Yield Reduction in Safflower Hybrids Caused by Female Selfs1

Fl hybrids of safflower, Carthamus tinctorius L., when grown in comparative yield trials with adapted varieties, outyielded ‘Ute,’ the highest yielding variety, by an average of 24%. Hybrids mixed with 16 to 43% female selfs (partial hybrids) failed to show the same superiority. Normal‐hull varieties containing mixtures of from 10 to 60% thin‐hull (females) failed to yield as much as the same varieties without the mixtures. As the percentage of thin‐hull plants increased in the mixture the yield decreased. Unless the competition resulting from the presence of low‐yielding selfs can be avoided, eliminated, or reduced, the maximum potential yield afforded by hybrids may not be realized.

P1-mediated transduction of a gene that controls radiation sensitivity and capsular polysaccharide synthesis from Shigella dysenteriae to Escherichia coli.

When Shigella dysenteriae strain 60 is used as a donor and Escherichia coli K-12 strains that are ultraviolet (UV)-sensitive, mucoid, and proline-requiring (Pro(-)) are employed as recipients, selection for Pro(+) yields 2 to 6% nonmucoid clones. All of the nonmucoid clones examined are UV-resistant. Most of the nonmucoid UV-resistant transductants are partial diploids for the genes being studied. When these Shigella-Escherichia hybrids are used as donors with the same E. coli recipients, the cotransduction of Pro(+) and nonmucoidness is greatly increased (59 to 94% cotransduction). All of these nonmucoid transductants examined were also UV-resistant. The results indicate that Shigella contains an allele (designated ShproC(+)) homologous to proC of E. coli and a second linked allele (designated ShcapR(+)) homologous to the capR allele of E. coli. The ShcapR(+) allele changes the phenotype of certain E. coli strains from mucoid UV-sensitive (capR6) or very sensitive (capR9) to nonmucoid and UV-resistant. Unanticipated capR allele interactions in the partial diploid hybrids are described.

Titration of the gene sites on DNA by DNA-RNA hybridization: II. The Escherichia coli chromosome

Escherichia coli RNA, having the same specific radioactivity in all molecules (long-labeled RNA), or labeled by exposing bacteria to [ 3H]uracil for 30 seconds (pulse-labeled RNA), was hybridized to E. coli DNA over a wide range of RNA/ DNA inputs † † Definitions. Gene, DNA site, RNA/DNA input and titration of DNA, as used here, are defined in paper I of this series (Kennell & Kotoulas, 1968). : from ratios so low (less than 1 1000 ) that not all copies of any DNA site were filled, to RNA/DNA inputs so high (greater than 1000) that no more RNA could be bound. RNA with radioactivity only in stable components was also hybridized over a wide range of RNA/DNA inputs. (a) The maximum fraction of input RNA detectable as authentic hybrid was about 75%; it is difficult to exceed this figure because of simultaneous formation of partial hybrids subsequently removed by ribonuclease. (b) The first DNA sites to become saturated did so at RNA/DNA inputs of about 1 1000 . All sites for stable RNA were occupied at an RNA/DNA input of about 1 160 . (c) At any RNA/DNA input between about 1 4 and 1 160 , virtually all unstable RNA is hybridized, and the size of this fraction equals the difference between the fraction of radioactivity hybridized from total RNA and from stable RNA. Such measurements suggested that unstable RNA is 2.5 to 3% of the bacterial RNA while 50% of the synthesized RNA is unstable. Competition experiments with excess unlabeled rRNA gave the same figures. (d) There is a wide variation in the numbers of molecules of different mRNA species per cell; 10% of the active genes are homologous to 70% of the mRNA mass. (e) Over the entire range of abundances, the same frequency distribution was found for the radioactivity of pulse-labeled mRNA. This suggests that variable life-times do not contribute to the abundance distribution of mRNA species. (f) Only 10% of the DNA was observed to be complementary to virtually all (more than 99.85%) of the label from both pulse-labeled or long-labeled mRNA; this suggests that in these cultures most of the potential E. coli gene sites make extremely little if any RNA.