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

摘要

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