U-type exchanges in Muscari comosum L. (Liliaceae)

Some meiotic abnormalities, such as univalent formation, side-arm bridges, and bridges with fragments, were observed during both meiotic divisions in some individuals of Muscari comosum L. from one Spanish population. The latter aberrations were also detected during mitotic divisions. The majority of these abnormalities involved the first long pair of chromosomes. The exact nature of sister chromatid bridges has still not been elucidated. U-type exchanges can be explained on the basis of abnormal breakage and reunion occurring in the euchromatin that is near the heterochromatin present in the interstitial region of the long arm of the first bivalent. In accordance with the last aspect, there is no correlation between the distribution pattern of chiasmata within the first bivalent and the meiotic U-type exchanges. Spontaneous breakage and reunion events produce new combinations of chromosome segments in the first pair of chromosomes of M. comosum, and hence have been important from an evolutionary point of view in the origin of chromosome polymorphisms in this plant species.Key words: Muscari comosum, meiosis, side-arm bridges, U-type exchanges, chiasmata distribution.