Structure, mitotic and meiotic behaviour, and stability of centromere-like elements devoid of chromosome arms in the fly Megaselia scalaris (Phoridae)

Minute elements detected in Megaselia scalaris (Phoridae, Diptera) lack chromosome arms but carry centromeres and possess kinetochore microtubules in mitosis as well as in meiosis. These centromere-like elements (CLEs) were present in two geographically independent strains of the fly. This indicates that their origin is not a recent event in the karyotype evolution of M. scalaris and that they are rather stable constituents of the karyotype. Most often, two CLEs were found in gonial and somatic mitosis. Spermatocytes contained one CLE. Two individuals examined deviated from this rule in that a metaphase spermatogonium showed three and an anaphase spermatogonium eight CLEs. These animals are believed to have been aneuploid relative to the CLEs. An analysis of spermatogonial division revealed that the CLEs behave like the centromeres of the regular chromosomes but seem to separate precociously, since they were closer to the spindle poles in late anaphase cells. Whereas the size of the CLEs was not significantly different between mitotic cells and secondary spermatocytes, the CLEs in primary spermatocytes were larger in volume by a factor of about 4.5 than those in mitosis and meiosis II. The additional material is interpreted as a glue that holds two CLEs together. This, in turn, is a prerequisite for orderly segregation. The function of the CLEs is not known. They are considered as B chromosomes reduced to the minimum required for segregation, the centromere.