Meiosis without Chiasmata in Periplaneta americana

Abstract

THAT meiosis in Periplaneta americana is characterized by the absence of any visible chiasma has already been reported by Morse¹ and Suomalainen². However, this absence of visible chiasmata with the consequent parallel disposition of the homologues in the bivalents from the zygotene to the pro-metaphase I, in this species, differs from that in coccids³ and dragon-flies⁴, but resembles Callimantis⁵ inasmuch as the meiosis is pre-reductional. A thorough examination of the chromosomes during the prophase of the first meiotic division reveals that this anomalous type of meiosis is closely related to the persistence of the single thread of each homologue right from the leptotene to the pro-metaphase, and it is only during the pro-metaphase stage, which is of a little longer duration than usual, that the latter is duplicated. In fact there is very little difference between the bivalents at the late zygotene and those at the early pro-metaphase, and it is the orientation of the bivalents at the metaphase plate which results in the regular segregation (pre-reduction) during anaphase of the first division. Thus, like Callimantis, the diplotene and diakinesis which are of practically universal occurrence in the typical pre-reduction meiosis are altogether absent in this species.