The paternal sex ratio chromosome induces chromosome loss independently of Wolbachia in the wasp Nasonia vitripennis

Abstract

 The paternal sex ratio chromosome (PSR) is a paternally-inherited supernumerary chromosome found in some males of Nasonia vitripennis. PSR induces the loss of N. vitripennis’s paternal autosomes in early fertilized embryos. Previous examinations have not directly addressed the complication of PSR’s co-occurrence with Wolbachia. Wolbachia is the name assigned to a group of cytoplasmic bacteria which induce numerous reproductive alterations in their hosts. In Nasonia, Wolbachia cause cytoplasmic incompatibility (CI) which also results in paternal chromosome loss. Here we address the question of whether PSR’s function (i.e. PSR’s transmission and/or ability to induce chromosome loss) depends upon or interacts with Wolbachia. A strain of PSR males is artificially cleared of Wolbachia. Test crosses and cytological observations of this strain demonstrate that PSR’s transmission and ability to induce chromosome loss is not dependent upon Wolbachia. Comparisons suggest an absence of interactions between PSR and Wolbachia when they co-occur. Fluorescent and confocal microscopy are used to examine and compare early embryos. Observations demonstrate that microtubule interactions with chromatin do not appear to cause the initial loss of the paternal chromosomes. Cytological observations presented here also differ from previous reports of PSR- and Wolbachia-induced chromosome loss.