Chi is a sequence of eight nucleotide pairs which stimulate recBC-mediated recombination (Smith, 1983a, b). The effect of two linked Chis on recBC-mediated recombination was tested in bacteriophage lambda. It was noticed that the Chi element located on the right side of the phage chromosome is epistatic on the other Chi. These findings support a model proposed by Stahl et al. (1983) which suggests that the recombination machinery moves unidirectionaly in the phage chromosome from right to left. The results also suggest that in the presence of more than one Chi only the rightmost one stimulates recombination.

I describe here a new screening procedure to isolate ribosomal suppressors in Podospora anserina. I have used the sporulation defect displayed by an antisuppressor mutation AS7–2. The revertants able to sporulate are due to either true reversions or external mutations. The mutations which restore most efficiently the sporulation show all the properties of ribosomal suppressors and are localized in two new suppressor loci su11 and su12.

Haploid amoebae of Physarum polycephalum may form plasmodia by crossing, a sexual process that involves cellular and nuclear fusions, or by selfing, an asexual process in which the development of a single amoeba into a plasmodium may involve neither cellular nor nuclear fusion. Mutant strains (npf) in which selfing is suppressed were previously assigned to several functional groups on the basis of their ability to cross with one another in certain combinations. In the present study hybrid, diploid amoebae were isolated from both crossing-compatible and incompatible mixtures of npf mutants. The diploid amoebae from mixtures of compatible strains readily formed plasmodia by selfing, but selfing was suppressed in the diploids from incompatible mixtures. Thus the crossing tests between npf mutants may be viewed as complementation tests: their results reflect the differing selfing abilities of the hybrid, diploid amoebae that formed in each mixture. Genetical and environmental factors affecting the efficiency of formation of diploid amoebae were studied, and the diploids were shown to be stable during repeated subcultures. Although diploid amoebae carrying complementing npf mutations readily formed plasmodia by selfing at 26 δC, they could be cultured without plasmodium formation at 30 δC, a temperature that also inhibited selfing of the haploid npf+ strains. Ways are discussed of exploiting this combination of properties in a general procedure for isolating and testing diploids for dominance and complementation of amoebal mutations in P. polycephalum.

Inferences on the mechanism of chiasma maintenance can be drawn from study of the distribution and frequency of chiasma-like associations between bridges and fragments and between normal chromatids and fragments in meiotic material heterozygous for paracentric inversions. These bridges and fragments are the result of crossing over within the inverted region so that differing predictions for the associations are generated by the various models for chiasma maintenance mechanism. Results of such a study in material heterozygous for a large paracentric inversion in the long arm of chromosome 1 in maize are reported here. Findings are generally consistent with predictions of the ‘generalized sister chromatid cohesiveness model’, but are markedly at odds with the ‘binder only at specific crossover sites model’, and with the ‘late effective doubling of telomeres model’. Some of the results do not conform quantitatively to predictions of the ‘generalized sister chromatid cohesiveness model’ for a linear relationship between potential extent of sister chromatid cohesiveness and frequency of maintained association, suggesting additional complexity.

A special screening procedure for the detection of induced Y-autosome translocations with centric breakpoints was applied. A series of Experimental stocks was constructed, each containing a different half of one of the induced T(Y; 2)'s (T element). The three other elements that were involved in the segregation experiments in each stock were a sex chromosome (X element), an inverted chromosome 2 (A element), and a free arm of chromosome 2 (F element). It is not feasible to determine the relative frequencies of all the 16 possible gamete types by mating an Experimental stock to one tester, nor to different testers that have each at least one class of progeny of the same genotype. Each Experimental stock was mated to four different Tester stocks and the data were calibrated so that a coherent segregation pattern could be obtained.

Segregation patterns in meiosis of males from 15 Experimental stocks, each with a different T element were studied. In most Experimental stocks the T element was of the left autosomal arm, while the F element was of the right autosomal arm. In four Experimental stocks the X element segregated independently of the A, F and T elements. In these Group 1 stocks, both the F and the T elements disjoined regularly from the A element. It was concluded that the T element of these stocks had no sex-chromosome disjunction determinants (‘S-determinants’) to interact with the determinants on the X element. Both the T elements and the F elements carried autosomal disjunction determinants (‘H-determinants’) that secured the segregation of the autosomal elements. The H-determinants of the left autosomal arm were qualitatively different from those of the right arm.

In the remaining 11 Group-2 Experimental stocks the X and T elements disjoined regularly, indicating the presence of S-determinants on the T elements of these stocks. The segregation of the T and the A elements in these stocks varied from nearly complete dependence to complete independence. It was concluded that this gradation reflected differences in the quantity of H-determinants present on the T elements of these Experimental stocks. It was impossible to discriminate between a model of continuous H determinants activity and one of a finite discrete number of determinants. The results do not agree with the claim that there are no autosomal disjunction determinants in the proximal heterochromatin of chromosome 2.

The S-determinants on the BsYy+ chromosome were located both adjacent to the centromere and distally on the long arm. The latter were probably translocated to the Y chromosome together with the Bs marker.

This is a study of the chromosomal segregation patterns in females of 15 Experimental stocks of Drosophila melanogaster, each carrying one element of a T (Y; 2) with a centric break-point. In each Experimental stock the relative frequency of all eight possible meiotic configurations of four relevant chromosomal elements was followed: an attached-X chromosome, a multiply-inverted chromosome 2, a free arm of chromosome 2, and a half-translocation element. Although the 15 translocation elements were broken at different sites, there were no basic differences among the Experimental stocks in their segregation patterns. The three two-by-two configurations were the most common. Comparison of this pattern with that of the segregation pattern of stocks similar but for an inversion-free chromosome 2, showed that in the Experimental stocks exchange pairing did not play a significant role in the determination of the segregation pattern.

The results of these experiments, together with the analysis of results from other published studies provide evidence against the model that had been forwarded by Grell. According to this model, chromosomes that did not participate in exchange pairing undergo another pairing cycle, in which total chromosome length is a factor in the determination of segregation.

We support a modified version of Novitski's model of premetaphase chromocenter-like chromosome aggregation. Disjunction of non-exchange chromosomes is regulated by determinants located in the proximal heterochromatin of the sex chromosomes and the autosomes. However, the specificity, especially that of the autosomal determinants, is not high. Thus, if an autosome and a sex chromosome are available, their determinants may interact-to-disjoin by default. More frequently, the determinants of the left-arm autosomal element may interact-to-disjoin with those of the right-arm chromosomal element.

Using mice that were mosaics for both Xce and phosphoglycerate kinase (Pgk-1) alleles, we have established that the parental source of the Xce gene may affect the probability with which the X chromosome carrying it will remain active. This effect was seen in one allelic combination of Xce but not in another. The relationship between these effects and other phenomena of maternal ‘imprinting’ is discussed.

Lactate dehydrogenase regulator gene Ldr-1 controls the amount of the lactate dehydrogenase A subunit in rat serum and the amount of lactate dehydrogenase B subunit in rat erythrocytes because no recombinant rats were found among 295 pups produced in the BDIV × (ACI × BDIV)F1 and (ACI × DI) Fl × DI backcrosses. Two alleles regulate the high (Ldr-1a) and low (Ldr-1b) activity enzyme. The Ldr-1 is not linked to hooded coat pattern or glyoxalase-1. Sex and Ldr-1 assorted independently; hence the gene is autosomal.

We report here the occurrence of a new spontaneous mutation at the tufted locus present within the crossover-suppressed region of the complete t haplotype fwLub1 This new tufted allele shows complete expressivity of the tufted phenotype without any apparent effect on viability or reproduction. The presence of a tufted mutation within a t haplotype that is cytologically marked by a Robertsonian translocation is particularly useful in genetic experiments aimed at understanding the structure and function of t haplotypes.