1. ¹Department of Biological Sciences, 138 Biology Building, University of Iowa, Iowa City, IA 52242, USA
2. ²Centre for Ecological and Evolutionary Studies, University of Groningen, The Netherlands
3. ³Department of Biology, University of Rochester, Rochester, NY, USA

Correspondence: BF McAllister, Department of Biological Sciences, 138 Biology Building, University of Iowa, Iowa City, IA 52242, USA. E-mail: bryant-mcallister@uiowa.edu

Received 7 August 2002; Accepted 7 July 2003; Published online 24 September 2003.

Abstract

The paternal-sex-ratio (PSR) chromosome in the parasitic wasp Nasonia vitripennis is a submetacentric supernumerary (B chromosome). Males transmit PSR, but after fertilization it causes the loss of the paternal autosomes. Paternal genome loss caused by PSR results in the conversion of a female (diploid) zygote into a male (haploid) under haplodiploid sex determination. In this study, site-specific markers were developed to assay deletion derivatives of PSR. Both polymerase chain reaction and Southern hybridization were used to detect the presence/absence of 16 single-site markers on a set of 20 functional and nine nonfunctional deletion chromosomes. Based on the pattern of marker loss on the deletion chromosomes, the basic organization of PSR was revealed. Two sets of markers were deleted independently, apparently representing the two arms of the submetacentric chromosome. The presence or absence of specific regions was examined in relation to phenotypic characteristics of the deletion chromosomes; ability to cause paternal genome loss, and stability in mitotic cell divisions. Rather than identifying a single region on PSR as being responsible for PSR function, the results suggest that the retention of one of two chromosomal regions is sufficient for causing paternal genome loss. Furthermore, a region was identified that is tightly correlated with mitotic stability, as measured from chromosomal transmission rates. Functional chromosomes with short-arm deletions had high (100%) transmission rates, whereas functional chromosomes with long-arm deletions had low (85%) transmission rates.