In some eukaryotes, mitosis is unorthodox and involves unusual mechanisms of chromosome movement. On this basis, we can speculate on the evolution of mitosis. For other atypical divisions, we do not know if similar explanations and interpretations are required to account for unusual chromosome behavior or if these are simply atypical expressions of the conventional mitotic process. We would examine the ultrastructure of the mitotic spindle and patterns of chromosome distribution in Plasmodium spp., parasitic protozoa (sporozoa) in which chromosomes have a peculiar relationship with the nuclear membrane during mitosis. We hope to determine if the membrane-chromosome relationship results in unusual non-random distribution of chromosomes. We will study meiosis in males of the dipteran Sciara and of Hemiptera in the family Reduviidae where non-random chromosome distribution is known to occur. By ultrastructural examination of spindle structure, including combined living cell-electron microscopic studies, we will determine if these unusual patterns of chromosome segregation can be rationalized in terms of conventional mitosis or if, instead, unusual chromosome behavior involves fundamentally different mechanisms of chromosome orientation and movement. These studies will help define the requirements for a comprehensive theory of mitosis; they will therefore contribute to an understanding of congenital diseases such as Down's syndrome which result from anomalous mitotic processes.
