Pachytene analysis in the genus Oryza is the most fruitful line of approach to the elucidation of evolutionary problems. Cultivated species of this genus have more asymmetric karyotypes. The cell to cell variation in absolute lengths of the chromosomes in O. perennis var. barthii was shown to be due to general condensation differences and hence the data on relative lengths can be employed in the identification of the karyotype. Sterility In Japonica-indica rice hybrids is largely due to chromosome structural hybridity. Occurrence of 241 at metaphase I of the F1 hybrid O. sativa × O. officinalis is due to desynapsis rather than lack of homology between the constituent genomes. The F1 hybrid O. sativa × O. australiensis exhibits distinct timing imbalance in meiosis, the chromosomes of O. australiensis being earlier in condensation as well as migration. Pairing at pachytene is normal in the F1 hybrid O. perennis var. balunga × O. perennis var. cubensis. The F1 hybrid O. sativa × O. perennis exhibited several abnormalities in pairing at pachytene. The F1 hybird O. sativa (4x) × O. perennis var. longistalinata exhibited a high frequency of frying-pan-shaped tnvalents, indicatmg segmental homology between the genomes of the parents.

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