The behavior of tetravalent chromosomes and its bearing on sterility in Aegilops Heldreichii×Ae. comosa and its offsprings

Abstract

1. F₁ hydrids between Aegilops Heldreichii Holzm. (n=7) and Ae. comosa Sibth. et Sm. (n=7) have in most cases 1_IV+5_II at the 1st meiotic metaphase in PMC. The tetravalent complex is arranged in an N- or U- configuration (90.20%). Metaphase plates with 7_II or 1_III+5_II+1_I are also observed (9.78%).
2. In diakinesis and in early metaphase the N- and U-configurations could not be recognized (fig. 1, 3, _{I, II}). The arrangement of chromosomes are determined during the stages from early metaphase to metaphase.
In F₂ two chromosome types, the one with 7_II and the other with 1_IV+5_II were observed in a ratio of 1:1. F₃ offsprings from F₂ with 1_IV+5_II gave also these two types in the same ratio.
3. Two types of tetravalent chromosome configuration (N and U) were counted most extensively with F₃-heterozygotes (Table 1).
4. In such tetrapartite chromosomes regular disjunction of chromosomes occur when adjacent chromosomes separate at metaphase. This type of separation is expected from N-configuration.
Deficiency and duplication of chromosomes segments occur simultaneously, when adjacent chromosomes pass to the same pole (U-configuration).
The average of regular disjunction of tetravalents in F₁, F₂ and F₃ was about 75.8% and the amount of good pollen was 75.1% (see fig. 5, I). In F₂- and F₃-Plants having only 7 bivalents regular disjunction was observed and showed more than 90% good pollen grains. The percentage of regular disjunction in this material is therefore closely related to the degree of pollen fertility.
5. The percentage of ovule fertility observed in F₁- and F₃-plants having one tetravalent was 82.58 and 82.27 respectively. This fertility is a little higher than that of good pollen grains and that calculated from chromosome arrangement. However germination test showed that such difference between ovule fertility and percentage of regular disjunction is due to the vitality of a part of female gametes having incomplete genom resulted from irregular disjunction.
The percentage of ovule fertility in plants having 7 bivalents in F₂ and F₃ was more than 90%.
6. From these cytological results it can be said that this tetravalent complex is resulted from a simple translocation.
7. From our observations it was assumed that the types of chromosome disjunction are in the main attributed to the repulsion between the homologous chromosome segments. Position of the kinetochore, number and position of chiasmata and environmental conditions affect also the chromosome arrangement.