The effects of derived B-chromosomes on meiosis in pearl millet Pennisetum typhoides

The cytological behaviour of Standard, Deficient and Iso-Bs and their effects on the endophenotype were studied in Pennisetum typhoides. The Standard and Deficient Bs were acrocentric with the short arms consisting entirely of heterochromatin. The Deficient B was derived from the Standard B by spontaneous loss of short segment extending over two chromomeres in the euchromatic long arm. The Iso-B was derived from the Deficient by centromere misdivision, the long arm forming the Iso-B. The Iso-Bs were, therefore, less heterochromatic than either the Standard or the Deficient Bs. These three types of Bs had different effects on the mean cell A-chiasma frequency. The Deficients had a depressing effect, the Iso-Bs had an enhancing effect, while the Standard Bs had no effect on the mean A-chiasma frequency but did affect its variance. When the Standard and Deficient Bs were present in combination with Iso-Bs in different numbers, the values of mean A-chiasma frequencies were intermediate between the values of Standard, Deficient and Iso-Bs present separately in corresponding numbers. It is suggested that a balance between the amount of heterochromatin and euchromatin present in the B-chromosomes is responsible for the differential effects of these three types of Bs on mean A-chiasma frequency. In this species, the extra euchromatin present in the form of Iso-Bs seems to have an enhancing effect on the mean A-chromosome chiasma frequency whereas the extra heterochromatin has a repressing effect.