The mechanisms behind the regular segregation of autosomal univalents in Calocoris quadripunctatus (Vil.) (Miridae, Hemiptera)

Abstract

One autosomal chromosome pair was found to appear as univalents in about 13.5% of late prophase I and metaphase I nuclei in Calocoris quadripunctatus males, providing a possibility to analyze the segregational behaviour of univalent chromosomes. It was found, that either syntelically or amphitelically orientated at metaphase I, the univalents segregated regularly from each other at the first meiotic division. On the basis of the behaviour of univalents, it is Suggested that syntelically orientated univalents utilized a mechanism operating during meiotic prophase, and amphitelically orientated, a mechanism operating during anaphase I for their regular segregation. Details involved in these mechanisms are present in all normal meioses. Further, it is suggested, that although the existence of such mechanisms can be considered beneficial, their disadvantage lies in the fact that they reduce selection for the existence of at least one chiasma per bivalent, thus leading to the occurrence of a small amount of nondisjunction in meiosis.