Cytological and molecular analysis of centromere misdivision in maize

B chromosome derivatives suffering from breaks within their centromere were examined cytologically and molecularly. We showed by high resolution FISH that misdivision of the centromere of a univalent chromosome can occur during meiosis. The breaks divide the centromere repeat sequence cluster. A telocentric chromosome formed by misdivision was found to have the addition of telomeric repeats to the broken centromere. A ring chromosome formed after misdivision occurred by fusion of the broken centromere to the telomere. Pulsed-field electrophoresis analyses were performed on the telocentric and ring chromosomes to identify fragments that hybridize to both the telomeric repeat and the B-specific centromeric repeat. We conclude that healing of broken maize centromeres can be achieved through the mechanisms of addition or fusion of telomeric repeat sequences to the broken centromere.Key words: centromere, telomere, meiosis, chromosome healing, B chromosome, Zea mays.

THE ULTRASTRUCTURE OF KINETOCHORES OF UNPAIRED CHROMOSOMES IN A WHEAT HYBRID

The kinetochore region of unpaired chromosomes (univalents) consists of two kinetochores, each belonging to a sister chromatid, that are located adjacent to one another on the surface of the univalent chromosome. This condition results in a movement by the univalent towards one of the polar regions at the onset of metaphase I. Once arrived in this region, one of the sister kinetochores obtains attachments of microtubules from the opposite pole. This results in a gradual return of the univalent to the equatorial plate, where it reaches an equilibrium. The sister kinetochores remain adjacent during the movement, but once arrived at the metaphase plate they develop a typical mitotic appearance, in which the sister kinetochores have opposite positions on the chromosomes.

CYTOGENETICS OF A UNIVALENT CHROMOSOME IN BS DROSOPHILA MELANOGASTER MALES

Bs males bearing a modified X:4 translocation in which one of the members, XPYL∙YS, is an invariable univalent show that the univalent-bearing gametes from males raised at 26.0 °C are recovered only half as frequently as from those raised at 18.0 °C (Zimmering, 1963).Such males were raised at 26.0 °C (Experiments I and II) and at 18.0 °C (Experiment III) and upon eclosion were mated to three yellow free-X females for 3 days at 26.0 °C. Testes of sib males were squashed and examined with phase optics to follow the meiotic behavior of the univalent. All tests were carried out over a 3-year period.The genetic results confirm those of Zimmering (1963) and show that the univalent is recovered with a frequency of 22.5 ± 0.9% and 18.5 ± 0.9% in Experiments I and II, whereas it is recovered with a frequency of 36.7 ± 1.6% in Experiment III. However, the cytological results show that while the univalent lags at A I with a frequency of 9.4, 28.7, and 32.2%, respectively, cells with and without XPYL∙YS are observed with equal frequency at M II and A II in all experiments. The post meiotic stages also appear to be normal at the level of the light microscope. Although negligible loss of the univalent is not excluded, it is clear that the frequency of XPYL∙YS lag is unrelated to temperature or to the distorted genetic ratios from males raised at 26.0 °C. Degeneration of XPYL∙YS-bearing sperm beyond the level of the light microscope remains as a possible explanation for the observed genetic distortion.

EARLY GENERATION TESTING AS A MEANS OF PREDICTING THE VALUE OF SPECIFIC CHROMOSOME SUBSTITUTIONS INTO COMMON WHEAT

Two experimental methods were used in an attempt to establish tests by which the value of whole chromosomes for substitution could be determined. The first of these, the F1 method, depended on the comparison of the performance of reciprocal F1 monosomic × disomic hybrid combinations, in which the univalent chromosome from the two different sources was tested against the uniform genetic background provided by the F1 hybrid. The second method, the F3 bulk method, involved the establishment of the chromosome pair under test in a pure state in a F3 hybrid bulk by the use of the appropriate monosomic line. This material was tested against the F3 hybrid bulk of the parental varieties in which all the chromosomes were freely segregating, and also against existing substitution lines.The F1 method demonstrated marked differences between homologous chromosomes of different varieties when they were tested in the univalent condition for the characters investigated. From these differences the homologous chromosomes of the varieties could, in some cases, be arranged in a linear order of performance, an order which may indicate their relative contribution if they were used in the production of substitution lines.The F3 bulk method did not reveal appreciable differences between the chromosome lines studied and therefore could not be used as a predictive method.