B-A interchanges are an unlikely pathway for B chromosome integration into the standard genome.

One of the conceivable evolutionary pathways of a parasitic B chromosome is its integration into the host genome through translocation to an A chromosome. To investigate this possibility, we analyze here the nature, meiotic behavior and genetic effects of a spontaneous interchange between a medium-sized autosome and a B chromosome, found in one male caught in a Moroccan population of the grasshopper Eyprepocnemis plorans and the offspring of controlled crosses with six different females. Most metaphase I cells analyzed (115 out of 118) showed a trivalent with chiasmata in both the interstitial and pairing segments, which predicted about half of genetically unbalanced spermatozoa. The analysis of 234 embryos sired by this male on six females showed the lethality of some meiotic products paralleled to a decrease in egg fertility (0.541 +/- 0.051, compared to the 0.879 +/- 0.017 shown by females mated to standard males). These results suggest that the cost of the B-A interchange on host fitness, in terms of gametic inviability, highly diminishes the possibility of frequency increase for the interchange to reach a polymorphic status, which is the first and indispensable step to reach fixation and thus integration of the B chromosome DNA into the host genome.