Pairing and segregation of the sex chromosomes in X1X2-males of Dysdercus intermedius with a note on the kinetic organization of heteropteran chromosomes.

The existence of an X1X2-mode of sex determination is confirmed by a study of all meiotic stages in the male cotton stainer (X1X2) and pertinent stages in the female (X1X1X2X2). In the male, the X-chromosomes are heterochromatic and pair end-to-end in early meiotic prophase. At diakinesis, they disjoin and align side-by-side in the center of the spindle, forming a pseudotetrad. Anaphase I is equational for the sex chromosomes. At late anaphase or telephase, X1 and X2 join end-to-end but form spindle fiber connections to only one of the poles of the metaphase II spindle, leading to one daughter cell without X chromosomes and one with both X1 and X2. An attempt is made to explain sex chromosome pairing and orientation on the basis of a telocentric organization of meiotic chromosomes. The apparent differences in the kinetic organization of mitotic and meiotic chromosomes in Heteroptera are discussed.