M M Jiménez

Genetic control of the rate of transmission of rye B chromosomes. I. Effects in 2B × 0B crosses

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 ? 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.

Maternal imprinting effect on B chromosome transmission in rye

A new feature affecting the transmission rate of rye B chromosomes is described. Progenies of 0B × 2B, 2B × 0B and 2B × 2B crosses were studied, considering both the B chromosome number of the plants crossed and that of their maternal parents. A remarkably large variation within these progenies was observed. The variation found in 0B × 2B crosses was mainly due to the effect of the maternal parent of the males and females involved: pollen grains formed in 2B plants whose maternal parent had 2Bs transmitted B chromosomes at high frequency; 0B plants whose maternal parent had 2B, accepted pollen with Bs at high frequency. Conversely, 2B males and 0B females with 0B maternal parent, formed progeny with B chromosomes at a much lower frequency.A very large variation in B chromosome frequency was also observed in progenies of 2B × 0B crosses although in this case the maternal parent of 2B females does not seem to be the cause of such variation.The observed progenies of 2B x 2B crosses fitted with expectations only when the effect of the maternal parent of the plants used for crosses was considered. The interaction between females having a 2B maternal parent with 2B pollen, and the effect of the male maternal parent was also evident in this case.It seems that 2B chromosomes produce an imprinting in plants, in such a way that all their descendants can be recognised whether they carry Bs or not. The frequency of transmission of B chromosomes is increased through the plants with a 2B maternal parent. A 0B maternal parent causes the transmission frequency to decrease. We believe that these interactions have an important effect at the population level on the maintenance of the polymorphism for B chromosomes.

B-chromosomes in inbred lines of rye (Secale cereale L.)

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.

Genetic control of the rate of transmission of rye B chromosomes. III. Male meiosis and gametogenesis

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.

Genetic control of the rate of transmission of rye B chromosomes. IV. Localization of the genes controlling B transmission rate

Crosses between rye plants from selected lines for high and low B chromosome transmission rate were carried out to determine the location of the genes controlling B transmission rate. Our results show that they are located on the Bs and we hypothesize that such ‘genes’ are sites for chiasma formation. Our results also suggest that rye B chromosome polymorphism is controlled mainly by the Bs.

Factors controlling the dynamics of the B chromosome polymorphism in Korean rye

In order to test if a particular rye population has a specific frequency of B chromosomes or, on the contrary, if it can tolerate Bs over a wide range of frequencies, we have studied the transmission of Bs in two Korean varieties which differ in their natural B frequencies. Synthetic populations were made for each variety which carried Bs at both the natural frequency as well as at the frequency of the alternative population. The populations were open pollinated, and the frequencies of the Bs were studied over three generations. The experiment was duplicated in two different environments and the variables of viability and fertility were also compared.Fertility was lower in the 2B than in the 0B plants, and it was much lower again in the 4B ones. Viability was not dependent on the number of Bs. The results were similar for both environments and this finding argues against an adaptive role for B chromosomes.The frequency of Bs in the populations sown at their natural frequencies remained constant over the generations in both environments, while the Bs of both varieties tended to move towards their natural frequencies after they were experimentally changed.There was a strong tendency for the proportion of plants with Bs in the variety with a high natural frequency to increase, while the tendency for the proportion of plants with Bs in the variety with a low natural frequency to decrease was less. These results indicate just how strong are the forces which maintain Bs in populations in the face of their harmful effects upon fertility.In our opinion the genes that control B transmission rates are the main factors that determine the range of frequencies at which a natural population can carry B chromosomes, while the tendency to recover natural frequencies rapidly following a disturbance is due to frequency-dependent selection and to a maternal imprinting effect.

Rye B chromosome transmission depends on the B, the carrier of the B and the mother of the carrier

B chromosomes are not required for the normal processes of growth and development, they lack homology with any members of the basic set, and show irregular and non-Mendelian modes of inheritance. There are two main reasons for our interest in Bs. Firstly they are a widespread phenomenon in nature and they are known to occur in many species. Secondly, they raise questions about genome organisation and evolution. They provide us with avenues for exploring the significance of “selfish” genetic elements which are promoted by processes of drive, and which transgress our normal experience of evolutionary forces which operate by the processes currently known for Mendelian populations.