Sex chromosome evolution in the polytypic species Pycnogaster cucullata

Abstract

Pycnogaster cucullata (Charp.) is a polytypic species of Tettigonioidea having both X0 and neo XY populations. Moreover the neo XY populations are themselves of two distinct types referred to as neo XY1 and neo XY2. A comparison between the X0 state and the two different neo XY systems, using C-banding and silver staining techniques, has served to clarify the differentiation of the two neo XY systems. In both cases, a centric fusion between the progenitor M2 chromosome, the second largest autosome, and the X of the X0 form has been accompanied by loss of an active NOR in the neo Y, which is the direct derivative of the M2. In both categories of neo XY sex bivalents, a secondary construction appears in the distal region of the XL limb, which gives a silver reaction similar to that observed at the centromeres. The neo XY2 system is distinctive in two respects in comparison with the neo XY1. In the first place it has an additional C-positive block located distally in the neo Y. Additionally there is also a novel C-block in the proximal half of the XR limb of the neo X. The neo XY2 system thus provides the first unambiguous example of the “secondary heterochromatinization” of a sex chromosome system within a single species.