Factors controlling the dynamics of the b chromosome polymorphism in korean rye

Abstract

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 Bfrequencies. 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 OB 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. © 1991 The Genetical Society of Great Britain.