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The morphology and frequency of B chromosomes (Bs) in Aegilops mutica were examined using materials collected from 16 natural populations in Turkey and Iran covering almost the entire distribution area of the species. Among 578 plants examined, 173 (29.9%) from 14 populations were found to have one to five Bs. Bs were distributed over the entire geographical area examined. All of the Bs detected were similar in morphology, suggesting their monophyletic origin. The plants with 2Bs were dominant among the B-carrying plants in most populations. Judging from these results, it was suggested that Bs originated, at the latest, at an early stage after the speciation of Ae. muticu, spread by mass migration of outbreeding Ae. mutica, and have been maintained in populations by directed non-disjunction during first pollen grain mitosis. The frequency of B-carrying plants and the mean number of Bs per plant were significantly heterogeneous among populations. No correlation was detected, however, between B frequency and the environmental factors of collection sites. Furthermore, the homogeneous proportion of plants with lB, 2Bs and more than 2Bs among populations suggested no difference in the directed non-disjunction rate among Bs. Based on these results, it was suggested that the significant difference in B frequency was due to the difference not in directed non-disjunction rate but in nonpaired B loss rate during meiosis. |
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