Cytogenetics and seed-set of autotetraploid rye, secale cereale L.
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Twenty years after chromosome doubling an unselected population of tetraploid rye appeared to have reached an "equilibrium state" in chromosome pairing behaviour. Cytogenetically the population was highly heterogeneous. Compared to early C-generations, meiotic behaviour in the population improved by an increase in quadrivalent frequency, mainly at the expense of trivalents and univalents. Quadrivalent frequency, however, failed to correlate with other meiotic features; instead bivalent frequency had significant positive correlations with the features of meiotic regularity including chiasma frequency. Furthermore, the average quadrivalent frequency in the population was considerably less than that of inbred lines (Hazarika & Rees, 1967). These facts led to the conclusion that disomic pairing dominated the chromosome behariour in this random mating population. No meiotic features ner morphological characters were correlated with seed-set in the population. A simultaneous selection for seed-set and regular tetrads was effective in increasing the bivalent frequency and reducing the frequencies of quadrivalents and aneuploids. A comparative study of three populations ("high", "low" and unselected) indicated the variable relationship between chiasma frequency and the frequencies of quadrivalents and bivalents. This and the higher frequency of qudrivalents in inbred materials was explained on the basis of "free" and "restricted" pairing of the four homologous chromosomes of an autotetraploid. It was concluded that the pairing pattern in inbred materials is predominantly tetrasomic whereas in outbred materials this may vary from tetrasomic to disomic depending on the chromosomal differentiation within the homologous sets. In the unselected population the lack of correlation of seed-set with meictic features and morphological characters proved to be due to a supplementary interaction between the cytological and the so-called physiclogical factors in determining the fertility of a plant. It was demonstrated that the genetical control of the cytological factors is independent from that of the so-called physiological factors. Once the interaction due to the latter was reduced by selection pressure, the effects of the cytological factors on seed-set became evident. But depending on whether the pairing pattern is predominantly tetrasomic or disomic, seed-set is correlated with quadrivalent or bivalent frequency. Quadrivalents were found to be sensitive to environmental changes whereas bivalents remained relatively stable. Furthermore, quadrivalent formations are restricted with 2/3 of the chromosomes. It is, therefore, concluded that the only way of ensuring meiotic stability in an autotetraploid is to induce disomic pairing. The possible ways of achieving this are outlined. Several chromosomal aberrations detected during the investigation are illustrated and discussed.
Thesis, PhD Doctor of Philosophy
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