Reversion of XO to XY sex chromosome mechanism in a phasmid

Summary The sex chromosomes of the male phasmid Isagoras schraderi Rehn comprise an X and a Y, — each with a submedian kinetochore, and one euchromatic and one heterochromatic arm. At meiosis X and Y form an unequal sex bivalent in which the euchromatic arms are terminally associated. Relatively recent reversion from the XO-XX mechanism characteristic of the Phasmidae is indicated by the presence of the euchromatic arm in both X and Y. The diploid number of the male is 34.Unequal autosomal bivalents are found at meiosis in two other species of Isagoras — Isagoras subaquiles Rehn and Isagoras sp. — and in Pseudophasma menius Westwood. The chromosome complements of these species are described.