Cytogenetical Studies of Oryza sativa L. and its Related Species: 5. Differential condensation and chromosome pairing in the hybrid O. sativa×O. australiensis

The chromosomes of O. australiensis were made up partly of heterochromatin whereas those of O. sativa were almost of euchromatin. In the F1 hybrid of O. sativa×O. australiensis, there was differential condensation in these two morphologically different types of chromosomes. The ones with partly heterochromatin and partly euchromatin condensed early starting off presumably from pachynema on till diakinesis. Whereas the ones with only euchromatin seemingly started their condensation later but condensed more complete at first metaphase. Thus before diakinesis the australiensis chromosomes were darker in staining and were 2-4 times the size of the sativa chromosomes at MI-AI. At either diakinesis or MI-AI, about two bivalents could be found per PMC. These bivalents could be separated into two types at MI-AI by size, or by difference in taken up the stain at diakinesis: 1. autosyndetic 2. allosyndetic (multivalents were also found only very rarely) All these bivalents were proved to be authentically true bivalents. The evidences were: 1. There were loosely paired chromosomal segment observed repeatedly in many PMC's at pachynema. 2. At diplonema, allosyndetically paired bivalents were found to have one chiasma mostly, or two or more chiasmata in some PMC's. 3. At diakinesis, these allosyndetically paired bivalents were found to be ring-shaped as well as the end-to-end ones. 4. Closed allosyndetic bivalents with two chiasmata were frequently observed at MI-AI. Pairing of the homologous segments in these allosyndetic pairs was assumed to be carried out at the euchromatic regions of the two chromosomes concerned. Presumably, these euchromatic regions of the chromosomes from two different species might have same rate of condensation at various stages of meiosis.