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Fungi in which the haploid nuclei resulting from meiosis are linearly arranged in asci provide unique opportunities to analyse abnormal segregation. Any meiotic drive system in such fungi will be observed in a cross between a driving and a sensitive strain as spore killing: the degeneration of half the ascospores in a certain proportion of the asci. In a sample of some 100 strains isolated from a single natural population we have discovered at least six different meiotic drive elements (van der Gaag et al., 2000). Here we report results of research that was aimed at elucidating a possible correlation between meiotic drive and vegetative incompatibility in eight different Spore killer strains from this population. We show that there is a strong correlation between these two phenotypes, although the precise genetic nature of the correlation is not yet clear. We discuss the implications of our results for the understanding of the population genetics of meiotic drive in Podospora

Meiotic configurations and segregation were studied in tetraploid hybrids (4n=28) between Secale cereale (2n=14) carrying a reciprocal translocation between chromosomes II and VI and a derivative of Secale montanum (2n=14), naturally differing from S.cereale in a translocation involving chromosomes I, III and V. Multivalent frequencies of the translocation complexes were reduced compared to expected but of still appreciable magnitude, which was ascribed to incomplete preferential pairing. Marker segregation (one of the S. cereale translocation chromosomes) also suggested partial preferential pairing.

In a previous paper it was shown that chiasmata are involved in the metaphase-I associations of haploid rye. Ring univalents and associations of two and more chromosomes were assumed to be formed by chiasma formation in several sets of homologous segments. An attempt is made to deduce the number of these segments and their location from the different (C-banded) metaphase-I associations. Ten of the twelve sets of homologous segments found to be present have been approximately localized but it was not possible to work out one consistent model for localizing all.

In pearl millet, chain trivalents composed of two telocentric and one metacentric chromosomes, showed an excess of linear orientation over the 1/3 expected with random centromere activation and inactivity of a central centromere stretched between the two outer centromeres. Chain trivalents composed of two metacentrics and one telo or of three metacentrics behaved as predicted. The difference was explained by assuming precocious activation of completely terminal centromeres as opposed to median centromeres. This early activity was reflected in precocious separation at late metaphase. In rye, all trivalents composed of two telos and one metacentric showed alternate orientation and anaphase separation did not precede that of metacentric chromosomes. It is concluded that in rye terminal centromeres are not precocious and that the spindle at meiosis is not long enough to permit stretching of the central centromere, which consequently always has the opportunity to orient and to induce the other centromeres to choose the opposite pole either directly or after reorientation, accumulating the most stable (alternate) orientation type.

For 4 different telotrisomics of Arabidopsis thaliana segregation ratios are given for a number of genes located on the extra telocentric chromosome. These data are discussed in relation to the meiotic behaviour expected with telotrisomics. By relating the frequencies of recombinants in the progeny of heterozygous telotrisomics with the map position of the loci, the centromere position may be determined (not only in respect to order, but also in respect to distance). Chromosome arms may differ with respect to univalent loss and the amount of recombination suppression near the centromere in telotrisomics.

The kinetic activity of chromosomes is either distributed evenly along the length of the chromosome or localized in a single centromere. Within the group of organisms with evenly distributed (holokinetic) activity great variation occurs, which has its consequences for chromatid segregation. Three major types, with some overlap, can be distinguished. They form a gradient of increasing specialization. Arguments are presented for the hypothesis that the monokinetic chromosomes are nothing but the end point in the same gradient and do not form a fundamentally different category. These arguments include: concentration of kinetic activity at meiosis in some types of holokinetic chromosomes, and ‘atavistic’ traits in monokinetic chromosomes pointing to a holokinetic origin. Possible reasons for the wide distribution of monokinetic chromosomes in the more evolved taxa are given.

This report reviews and evaluates the various methods and techniques used for measuring the incidence of spontaneous meiotic non-disjunction in mammals generally and particularly in mouse and man. It also gives the principal results obtained with these methods and techniques and consequently, in a sense, is also a review of the incidence of meiotic non-disjunction in these species. The incidence of non-disjunction is only given for normal meiosis. Studies involving chromosomal aberrations, e.g. of translocation carriers, have not been included.