Frequency Of Chromosome Pairing Research Articles

Fine mapping of wheat powdery mildew resistance gene Pm6 using 2B/2G homoeologous recombinants induced by the ph1b mutant

Using the ph1b mutant, the recombination frequency between the homoeologous region of 2B and 2G was significantly increased. By this, we narrowed Pm6 to a 0.9Mb physical region. The powdery mildew (Pm) resistance gene Pm6 from Triticum timopheevii (2n = 48, AAGG) was mapped to the long arm of chromosome 2G and introduced into common wheat in the form of 2B-2G introgressions. The introgression line IGV1-465 has the shortest 2G segment, which is estimated 37Mb in size when referring to 2BL genome reference of Chinese Spring (CS). The further fine mapping of Pm6 was impeded by the inhibition of allogeneic chromosome recombination between 2B and 2G in the Pm6 region. In the present study, to overcome 2B/2G recombination suppression, a ph1b-based strategy was employed to produce introgressions with reduced 2G fragments for the fine mapping of Pm6. IGV1-465 was crossed and backcrossed to the CSph1b mutant to produce plants with increased 2B/2G chromosome pairing frequency at the Pm6 region. A total of 182 allogeneic recombinants were obtained through two-round screening, i.e., first round of screening of 820 BC1F2:3 progenies using the flanking markers CIT02g-14/CIT02g-19 and second round of screening of 642 BC1F2:4 progenies using the flanking markers CIT02g-13/CIT02g-18, respectively. Through marker analysis using 30 chromosome 2G-specific markers located in the Pm6 region, the identified recombinants were divided into 14 haplotypes. Pm resistance evaluation of these haplotypes enabled us to narrow Pm6 to a 0.9Mb physical region of 2BL, flanked by markers CIT02g-20 and CIT02g-18. Six wheat varieties containing Pm6 were identified from a natural population, and they showed increased Pm resistance. This implied Pm6 is still effective, especially when used in combination with other Pm resistance genes.

Genetics of inbred Drosophila melanogaster. VI. Crossing-over in secondary non-disjunction exceptionals.

Although Bridges (1916, Genetics 1: 1–52 and 107–183) found recombinants among exceptional XXY females in tests of secondary non-disjunction, he misinterpreted their origin and claimed that true secondary non-disjunction resulted in XX eggs with non-crossover chromosomes. Preliminary tests of inter-strain crosses of inbred lines indicated different frequencies of recombinant exceptional females in reciprocal crosses. Large scale tests confirmed this observation. However, they indicated that, as judged by recombination in the most distal end (y-w interval), there was presumably nearly the same frequency of chromosome pairing in XX females as in exceptional females from XXY mothers. In the more proximal regions there was a decreasing proportion of recombinants among exceptional females as compared to regular females. The decrease was more pronounced in one direction of crosses than in the reciprocal indicating a maternal influence. The use of inbred lines made it possible to compare the frequencies of recombinant X-chromosomes in females of primary and secondary non-disjunctional origin in the same genetic and cytoplasmic background. It was shown that the absolute frequency of recombinants was much higher in secondary non-disjunction than in primary. Some complex phenodeviants among the exceptional females revealed that their recombination could occur between euchromatic parts of the X-chromosomes and between X and either Y-arm indicating trivalent formation, however, of type different from that suggested by Cooper (1948, Proc. Nat. Acad. Sci. 34,5: 179–187).

Development of high-gossypol cotton plants with low-gossypol seeds using trispecies bridge crosses and in vitro culture of seed embryos

The objective of this work is to develop an upland cotton, Gossypium hirsutum L. [2n = 4x =52, 2(AD)h], having a reduced level of gossypol in the seeds for food and feed uses, and a high level of gossypol in the remaining organs to limit pest incidence. Using G. sturtianum Willis (2n = 2x = 26, 2C1) as donor and G. thurberi Torado (2n = 2x = 26, 2D1) or G. raimondii Ulbrich (2n = 2x = 26, 2D5) as bridge species, two trispecies hybrids G. thurberi– G. sturtianum– G. hirsutum and G. hirsutum– G. raimondii– G. sturtianum were synthesized. Both trispecies hybrids were male sterile. Recurrent backcrossing to G. hirsutum as pollinator and selfing of the second backcross (BC) progenies resulted in seeds which were rescued by in vitro culture. In total, 1208 flowers of the trispecies hybrids and their BC progenies yielded 192 seed embryos from which 62 plants were obtained. Cytogenetic analyses indicated a relatively high frequency of chromosome pairing and chiasmata. The gland levels in backcross seeds ranged from glandless seeds to normally glanded seeds. All vegetative parts of those hybrids were glanded, but a wide range of variability for gland density was observed on leaf, stem, bract and calyx. Plants derived from seeds having a reduced level of gossypol constitute very interesting germplasm to develop a cultivated glanded cotton with low-gossypol seeds.

Cytogenetic evidence for cryptic structural hybridity causing intraspecific hybrid sterility in Aegilops caudata L..

To elucidate the mechanism causing the intraspecific hybrid sterility in Aegilops caudata, chromosome pairing and segregation at meiosis and fertility were examined in sterile F1 hybrids, a tetraploid derivative induced from one of the sterile hybrids, and their parental lines. The F1 hybrids showed a normal configuration and frequency of chromosome pairing at MI but was completely sterile. At AI, chromosomes consisting of two sister chromatids different in length were observed. The induced tetraploid was shown to be an autotetraploid based on the configuration and frequency of chromosome pairing at MI, and it showed incomplete but obvious restoration of fertility. From the results, it was strongly suggested that the intraspecific hybrid sterility observed in this species is a chromosomal sterility caused by cryptic structural hybridity defined by Stebbins (1945, 1950). However, the differences in chromosomal structure between parental lines are presumably not great enough to cause preferential pairing in the induced tetraploid. Ae. caudata, thus, most probably is considered to be just on the way to speciation due to cryptic chromosomal rearrangements.

Effect of chromosome 1B, chromosome 6B, and low temperature on the frequency of chromosome pairing at first meiotic metaphase in hexaploid triticale

Chromosome 1B in 'Rosner' and chromosome 6B in line 125 both reduced the frequency with which chromosomes were paired at first meiotic metaphase of hexaploid triticale. On the other hand, chromosome 6B in 'Rosner' and chromosomes 1B and 6B in line 110 had no such effect. The 1B pairing suppressor in 'Rosner' was located on the short arm of the chromosome (1Bs). Between 10 and 30 °C, pairing frequency was quite stable in 'Rosner' triticale in comparison with common wheat, although the level was consistently lower in the triticale. Some reduction of pairing frequency was noted at 10 °C in 'Rosner'. This effect of low temperature did not interact with 1B dosage to cause a disproportionate decrease in pairing frequency when plants with high 1B dosage were grown at 10 °C.

HYBRIDIZATION IN THE GENUS GLYCINE SUBGENUS GLYCINE WILLD. (LEGUMINOSAE, PAPILIONOIDEAE)

The genus Glycine is composed of two subgenera, Glycine and Soja. Soja includes the cultivated soybean, G. max, and its wild annual counterpart G. soja, while Glycine includes seven wild perennial species. Hybridization was carried out within and between wild perennial species of the subgenus Glycine. The success rate (pods set/flowers crossed) was 11% for intraspecific and 8% for interspecific crosses. A total of 220 F1 hybrids was examined morphologically and cytologically where possible. Hybrids within G. canescens (2n = 40) and G. latifolia (2n = 40) were fertile as expected. Glycine clandestina (2n = 40) was morphologically separable into at least three groups, which produced fertile hybrids within each group. One cross between two groups gave vegetatively vigorous but sterile hybrids. The majority of crosses within G. tabacina (2n = 80) were fertile, except that extremely narrow‐leaved forms gave sterile hybrids in combination with more usual forms. Sterility was also encountered in G. tomentella when aneuploids (2n = 78) from New South Wales, Australia, were crossed with tetraploids (2n = 80) from either Queensland, Australia, or Taiwan; crosses between the latter two populations resulted in seedling lethality. Cytological behavior of sterile hybrids followed a similar pattern, whether at the diploid or tetraploid level. The frequency of chromosome pairing was approximately half that expected if genomes showed full pairing homology. Bivalent disjunction at anaphase I was usually followed by precocious division of the majority of univalents. Telophase I and II were characterized by lagging chromosomes and micronuclei, so that resulting pollen was misshapen and sterile. Chromosome pairing data from sterile intraspecific hybrids at the tetraploid level may indicate a polyphyletic origin of tetraploids, whereby different diploid populations were involved in their formation. Similarly, chromosome pairing in sterile intraspecific diploid hybrids may indicate that the various diploid groups arose independently of one another. Both 40‐ and 80‐chromosome forms are fully diploidized, however, and if they are of ancient origin, divergence since that time could have resulted in the chromosomal differentiation which becomes apparent when intraspecific hybridization is effected.Diploid (2n = 40) interspecific hybrids G. falcata × G. canescens, and G. falcata × G. tomentella grew poorly and did not reach flowering stage. Diploid (2n = 40) crosses between G. latifolia and G. tomentella produced inviable seedlings. Tetraploid (2n = 80) hybrids between G. tomentella and G. tabacina were vegetatively vigorous but sterile owing to low chromosome pairing at meiosis, indicating little pairing homology between the two species. Diploid hybrids between G. canescens and G. clandestina, however, showed almost complete chromosome pairing at diakinesis and partial fertility. Although morphologically distinct, these two species have not diverged sufficiently to prevent hybridization and possible gene exchange through recombination.Self compatibility, perennial growth habit, and geographic isolation have favored divergence among Glycine populations to the point that gene exchange appears no longer possible in many cases. Internal isolating mechanisms have been shown to operate at various levels of plant development from hybrid lethality at seedling stage, to failure of seed‐set in sterile but vegetatively vigorous hybrids.

Nonhomologous pairing in oogonia and ganglia of Drosophila melanogaster.

A high frequency of chromosome pairing between an X-duplication (Dp) and a compound 4 (−44), each of which lacks its homologue in the genome, has been observed in somatic ganglia cells in female and male larvae as well as in oogonia of adult Drosophila melanogaster. Three Dp's of different sizes were used. Pairing was determined both by inspection and by precise measurement of the distances between chromosomes; the two methods agreed in virtually every case. Nonhomologous pairing frequencies ranged from 23.0 to 62.6%, showing a strong dependency on size. The Dp that was closest in size to −44 produced the greatest frequency of pairing in each of the three tissues. Homologues in the same cells paired with a frequency of 89% or greater. The X and Y in the male ganglia showed a pairing frequency of only 45.7%, thus resembling nonhomologues in their pairing behavior. Genetic studies were also carried out on the three genotypes. Segregation data for the Dp's and −44 established that the same pattern of pairing observed in the mitotic cells was present in the oocytes — the Dp which was closest in size to the −44 again producing the greatest frequency of pairing. In the oocytes the pairing frequencies were much higher than those observed in the mitotic cells.