The origin of a selfish B chromosome triggering paternal sex ratio in the parasitoid wasp Trichogramma kaykai

Abstract

This study uses molecular and cytogenetic methods to determine the origin of a B chromosome in some males of the wasp Trichogramma kaykai. This so-called paternal sex ratio (PSR) chromosome transmits only through sperm and shortly after fertilization triggers degeneration of the paternal genome, while keeping itself intact. The resulting embryos develop into haploid B-chromosome-carrying males. Another PSR chromosome with a very similar mode of action is found in the distantly related wasp Nasonia vitripennis and its origin was traced by transposon similarity to the genus Trichomalopsis, which is closely related to Nasonia. To determine whether both PSR chromosomes have a similar origin we aimed to reveal the origin of the Trichogramma PSR chromosome. Using fluorescent in situ hybridization, we discovered a major satellite repeat on the PSR chromosome, the 45S ribosomal DNA. Analysis of the internal transcribed spacer 2 (ITS2) of this repeat showed the presence of multiple ITS2 sequences on the PSR chromosome resembling either the ITS2 of T. oleae or of T. kaykai. We therefore conclude that the Trichogramma PSR chromosome originates from T. oleae or a T. oleae-like species. Our results are consistent with different origins for the PSR chromosomes in Trichogramma and Nasonia.

Figure 1.

Karyogram of T. kaykai: (a) labelled with 45S rDNA from wheat; (b) labelled with ITS2 from T. kaykai.

Figure 2.

Alignment of Trichogramma ITS2 sequences. Identities are denoted by dots. A dash denotes a gapped position. The T. pretiosum ITS2 sequence is from strain PRV4 from Riverside, CA (accession number U76226; Pinto et al. 1997). The T. oleae ITS2 sequence is from strain ‘Tunisia’ (accession number U74601; Schilthuizen & Stouthamer 1997). Restriction enzyme digestion sites: diamond is DraI (TTT|AAA), arrow is EcoRI (G|AATTC), asterisk is XmnI (GAANN|NNTTC).