Breakdown of Tetraploidy in Colchicine-Induced Autotetraploid Eruca Sativa LAM. Rajan S. S., Hardas M. W., Parthasarathy N. Division of Botany, Indian Agricultural Research Institute, New Delhi Summary (i) Colchicine-induced autotetraploids of Eruca sativa were found to give rise to triploids and aneuploids under natural conditions. (ii) The meiosis in the P.M.C. of the autotetraploids was studied. Gametes with unbalanced number of chromosomes were produced to an extent of 37.7 per cent. of these 27.8 per cent of the total, were found to be due to unequal separation of the multivalents and 9.9 per cent of the total due to meiotic irregularities like lagging of chromosomes in anaphase I and their subsequent elimination. Range of variability of the number of chromosomes in the gametes produced was from 18 to 24. (iii) 4n X 2n cross was successful only with 4n as the mother parent. The progeny from this cross indicated that the numerically unbalanced gametes are functional. The range of variation in the number of chromosomes of functional gametes was found to be from 18 to 24. The functioning of unbalanced gametes from the microspore side also was indicated. Since Eruca sativa is predominantly a cross-pollinated crop the tetraploids were found to cross with diploids even under natural conditions, giving rise to mostly triploids and a few aneuploids. (iv) The selfed progeny of a tetraploid (Tl0) was found to show plants giving a high variability in chromosome number ranging from 28 (= 2n+6) to 44(=4n). In the mitosis of the root tips of this progeny spindle abnormalities were found to lead to chromosome reduction. (v) The breakdown of tetraploidy in Eruca sativa is attributed to the formation and functioning of gametes with unbalanced chromosome number from both micro- and megasporogenesis. Natural crossing of the tetraploids with their diploids resulting in triploids and aneuploids also contributes towards the breakdown. However, rapid chromosome reduction results from the disturbances of the spindle mechanism leading to chromosome elimination. The probable causes and significance of such abnormalities of the spindle are discussed. Top |