The interspecific origin of B chromosomes: Experimental evidence

A centric fragment was generated during the introgression of a chromosome r egion from Nasonia giraulti into N. vitripennis. This neo B chromosome carr ies the N. giraulti or 123(+) gene for wild-type eye color. Using this phen otypic effect, the transmission of this chromosome was analyzed. The supern umerary chromosome showed less than Mendelian segregation rate in meiosis a nd some mitotic instability manifested as mosaic phenotype for eye color. H owever, transmission rate and mitotic stability increased over successive g enerations. The transmission rate through male gametogenesis was nearly 100 %. These results support the interspecific hybridization model for B chromo some origin and reveal that problems in chromosome stability can persist fo r several generations after "foreign chromosomes" are introduced into a dif ferent species. We suggest that hybrid zones should be investigated as poss ible sites for neo-B chromosome generation.