B-chromosomes from an experimental population of the Japanese JNK strain of rye, isogenic for its Bs, have been backcrossed into twelve different inbred lines. The experiment provides a way to study the effects of the Bs against a range of homozygous A-chromosome backgrounds. This publication deals with vigour and fertility: it shows that the rye Bs fit a parasitic model, and that they interact in their effects with the A-chromosome background genotype.

Selection for plants showing low or high transmission rates of B chromosomes demonstrates the existence of genotypes which affect this character. The results suggest that the gene or genes involved in this control are located in the A chromosomes. It is shown that plants from the low transmission rate class tend to lose B chromosomes, while plants from the high transmission rate class tend to accumulate Bs. It is therefore concluded that these genotypes can influence the frequency of B chromosomes in different populations.

Twenty 9 OB x d 2B crosses were made in which a single male was crossed with four different females, and the character 'mean number of Bs per plant in the progeny' was analysed. The 2B plants used as males and the OB females belonged either to low or to high transmission rate lines, previously selected. The genetic basis of the difference between the high and low transmission rate lines was statistically tested. The male group (high or low class of the 2B male) has a significant effect on the progeny, while the female group (high or low class of the 013 female) has no significant effect. The genes controlling the transmission rate of B chromosomes affect the proportion of OB vs. 2B plants in the OB X 2B cross, but this is not a result of any effect on the nondisjunction mechanism, since the proportion of lB plants is low in all cases. Structural variants of B chromosomes appeared de novo at a high frequency.

Male meiosis and gametogenesis were studied at metaphase I, metaphase and anaphase of the first pollen grain mitosis, and bicellular and tricellular pollen grain stages in 2B rye plants belonging to the low (L) and high (H) B transmission rate lines previously selected. Our results show that B chromosome behaviour significantly differs in both lines whereas the behaviour of the normal complement does not differ. In the L line the Bs form univalents in 81.07 per cent of the metaphase I cells, and are conserved in 44.14 per cent of the pollen grains at first metaphase whereas the remaining Bs are eliminated as micronuclei. In the H line the Bs form bivalents in 87.71 per cent of the metaphase I cells, and are present in 82.48 per cent of pollen grains at first metaphase. The Bs of the L and H lines do not differ in their ability to undergo nondisjunction at first pollen grain anaphase. This indicates that the different B transmission in the L and H lines results from their differential ability to form uni-or bivalents at metaphase I, which determines their loss or conservation in the pollen grains. The L and H lines also differ in pollen viability at the tricellular stage because 19.75 per cent of pollen grains of the L line and only 1.2 per cent of the H line are inviable.

Extensive synaptonemal complex formation was found at prophase I in whole mount spread preparations of a spontaneous haploid rye, Secale cereale, with values of up to 87.8 per cent of the chromosome complement synapsed. Pairing-partner switches were frequent, giving rise to multiple associations in which all or most of the chromosomes were involved. However, the distribution of synaptonemal complex stretches suggests that synapsis does not occur at random. The frequency of multivalents and the mean frequency of bonded arms at metaphase I were 0.03 and 0.39, respectively. Associations between chromosome arms without heterochromatin were more frequent than between the remaining arms. The observation of recombinant chromosomes for telomeric C-bands at anaphase I indicates that metaphase I bonds are true chiasmata. The correspondence between the location of pairing initiation sites and chiasmata indicates that early synapsis could be confined to homologous regions.