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
.
Mapping of paternal-sex-ratio deletion chromosomes localizes multiple regions involved in expression and transmission
