Chiasma frequency and the disjunction of interchange associations in rye

Chromosomal identification and meiotic behavior of reciprocal translocations in a rye polymorphic population. Evolutionary implications

The spontaneous interchange polymorphism of rye cultivar "Ailés" is composed, as can be deduced from the chromosomal identification of the interchanges analyzed, of several different reciprocal translocations in which the chromosomes of its haploid complement are involved with a similar frequency, except for chromosomes 4R and 6R. Several features of chromosome behavior at metaphase I, such as configuration and orientation of quadrivalents and frequency of chiasmata, were analyzed in structural heterozygotes for different interchanges. The two main factors affecting the orientation of quadrivalents at metaphase I proved to be the morphology of these chromosome associations at metaphase I and, in particular, the frequency of bound chromosome arms that they showed. A genotypic control of alternate orientation of quadrivalents independent of chiasmata frequency was not detected. In addition, the frequency of alternate orientation shows no relation to the fitness. Possible evolutionary implications of the results obtained are discussed.

Modification of the properties of a translocation in the German cockroach by selection

A stock of Blattella germanica bearing the interchange T(3; 12)/3;12 was subjected to close inbreeding with selection for random disjunction at metaphase I. After 3-4 generations of selection, interchange quadrivalent chiasma frequency decreased, variability in free bivalent chiasma frequency increased sharply, and individuals with either random or directed disjunction were present in the stock. Random disjunction was modified from a ratio of 2∶1∶1∶2 (adj.-1; alt.-1; adj.-2; alt.-2) to a ratio of 1∶1∶1∶1. After 7-8 generations of selection, chiasma frequency appeared to stabilize at lower than normal levels and variability decreased for both quadrivalents and free bivalents. Directed disjunction was modified from a ratio of 2∶1∶1∶4 to 1∶1∶1∶2, and no individuals with the original high level of directed disjunction were detected. Chains-of-four tended to orient randomly, especially in individuals where the ring quadrivalents showed directed disjunction. Relaxation of inbreeding, but not selection, produced an increase in chiasma frequency and variability in both free bivalents and quadrivalents, but the modified ratios for both random and directed disjunction were retained. These results are discussed with respect to inbreeding effects and genetic control of chiasma frequency and metaphase I disjunction in interchange quadrivalents.

Structural differences in reciprocal translocations. Potential for a model of risk in Rcp

Interchange segment sizes and the sizes of chromosome imbalance arising from the different modes of meiotic segregation were measured in a selected sample of 20 reciprocal translocations (Rep). The Rep were selected by two modes of ascertainment: (I) neonates with an unbalanced form of the translocation, and (II) couples with recurrent spontaneous abortions without evidence of full-term translocation aneuploid offspring. The measurements (% of haploid autosomal length: %HAL) were plotted as the observed or potential chromosomal imbalance with monosomy (abscissa) and trisomy (ordinate). It was found that (a) the interchange segments were larger in the spontaneous abortion Rcp, (b) that all of the imbalances observed in full-term neonates plotted close to the origin and to the left of the line joining 4% trisomy to 2% monosomy, and (c) the imbalances observed in the neonates in each individual Rcp were of the smallest size possible arising by any segregation mode. It was concluded that a major factor in the survival to term of aneuploid conceptuses is the size (proportion of genome) of the chromosome abnormality, irrespective of the origin of the chromosome regions. These results are discussed in relation to their use as a model to evaluate the risk of abnormal offspring in the progeny of translocation heterozygotes (the Chromosome Imbalance Size-Viability Model).

Induction of multiple chromosome interchanges in pearl millet, Pennisetum typhoides

With a view to building up a series of translocation stocks and also, eventually, a complex heterozygote involving all the 14 chromosomes of pearl millet, its seeds were exposed to gamma rays from a (60)Co Gamma Source. Seeds from the meiotically established interchanges were given several cycles of gamma irradiation. Frequency of the incidence of interchange heterozygotes in different generations was scored. As well as receiving recurrent irradiation, different translocation stocks were intercrossed to build up higher interchange configurations and also to identify the translocated chromosomes. Interchanges involving more than 12 of the 14 chromosomes were not obtained, presumably because the somatic and gametic sieves operate more rigorously when more chromosomes are involved in interchanges. It is nevertheless suggested that the karyomorphological features of pearl millet, including its symmetrical karyotype, are favourable for the induction, and also perhaps the maintenance, of translocation heterozygosity. Possible use of translocation heterozygotes in building aneuploids and certain duplications of value in breeding is suggested.