Chromosome Chiasmata Research Articles

Comparative molecular and cytogenetic methods can clarify meiotic incongruities inAvenaallopolyploid hybrids

Avena longiglumis is a diploid species that has been proposed as the A-genome donor of allopolyploid oats. In order to clarify its participation in the evolution of oats, a triploid A. magna × A. longiglumis hybrid was synthesized and studied, via a concurrent combination of cytogenetic and molecular techniques. The analysis of pollen mother cells revealed that synapsis percentage and chromosome chiasmata per cell were low; still they were the highest recorded among the diploid species used as pollinators of AC-genome tetraploids. In addition, genomic in situ hybridization revealed extensive chromosomal translocations among the allopolyploids that may have contributed to the low chromosomal affinity. Furthermore, amplified fragment length polymorphisms revealed the extensive genomic modifications that followed the interspecific hybridization, depicted mainly as the loss of parental bands (51.29%) and the creation of novel bands in the progeny (15.73%). As a result, the combination of cytogenetic and molecular techniques can elucidate meiotic incongruities that can make the unraveling of oat evolution challenging.

Effect of B chromosomes on meiosis in tetraploid pearl millet.

In order to study the meiotic effects of B chromosomes when they are present in a tetraploid background, B chromosomes were introduced into autotetraploid pearl millet by crossing tetraploids with B-carrying triploids. A majority of the eutetraploids in the second generation progeny of this cross had two types of Bs viz., standards and isos, both in varying numbers and present separately or in com-bination. As it is not possible to fix a model number to a plant, cell level analyses were carried out. The mean A chromosome chiasmata per cell was higher in the 4n B cell population compared to controls. Within the 4n B cell population those with Bs had more chiasmata than those without Bs. Independent of this effect on A chromosome chiasmata, B chromosomes had a quadrivalent enhancing effect as was evident from regression analyses of variance. It is pointed out that the chro-mosome pairing pattern in 4n B plants was as may be predicted. on the basis of information about the effect of Bs on meiosis in diploids.

B chromosomes in tetraploid pearl millet

B chromosomes were found in a triploid which had no seedset in selfing, or in a cross with a highly fertile tetraploid (3nB×4n). From the reciprocal cross (4n×3nB) 8 progeny plants were obtained which contained B chromosomes. These plants had very low seedset and yielded only 24 eutetraploids (4n=28) in the next generation. All except one of these plants had B chromosomes. The 4nB plants showed high frequencies of A chromosome chiasmata and multivalents, including complex configurations.

Chromosome Behavior in the Pollen Mother Cells of “<i>Shogoin-daikon</i>”and“<i>Nerima-daikon</i>, ”the Horticultural Varieties of <i>Raphanus sativus</i>, L

Chromosome behavior in the pollen mother cells of two horticultural varieties of Raphanus sativus, L.“Shogoin-daikon”and“Nerima-daikon, ” was examined.1. In the diakinesis nucleus, a tetravalent or a hexavalent chromosome is often found besides the ordinary bivalent chromosomes. These polyvalent chromosomes consist of four or six univalent chromosomes attached end-to-end successively in the form of a chain (Figs. 1, 2). A ring was not found.2. Frequency of occurrence of nuclei at diakinesis with polyvalent chromosome was examined in“Shogoin-daikon”and“Nerima-daikon”. The results are given in Table 1.3. In“Shogoin-daikon, ” the number of interstitial and terminal chiasmata in chromosomes was counted in mid- and late diakinesis nuclei in which no polyvalent chromosome was found, The results are given in Table 2 A, B.4. At the beginning of metaphase, the tetravalent chromosome is resolved into two bivalent chromosomes and the hexavalent chromosome into three bivalent chromosomes. The polyvalent chromosomes are found very rarely in the metaphase equatorial plate (Fig. 4). From metaphase onward, all chromosomes are found to behave themselves after the ordinary manner.5. The chiasmata in metaphase chromosomes are all terminal and no interstitial ones are found. Table 2 shows the number of chromosomes with one and two chiasmata counted in ten cells of“Shogoin-daikon”at metaphase.