Germ line restricted B chromosomes in grasshoppers

Abstract

Decades ago, the presence of extra chromosomes restricted to the male germ line in several grasshopper species was interpreted as recurrent polysomy, as experimental crosses suggested that the extra chromosomes were not transmitted from adult male parents to their embryo offspring. Under this hypothesis, polysomy was generated de novo through a nondisjunction for some chromosomes of the standard karyotype. In the current study, I test this hypothesis by analysing 17 families of tandem repeats (TRs) in two males of the grasshopper Chorthippus parallelus, which displays mosaicism for this kind of extra chromosome. According to the de novo polysomy hypothesis, the extra chromosomes should show the same FISH pattern for the TRs analysed as at least one of the A chromosomes. However, three TR families displayed patterns of FISH bands on the standard and extra chromosomes that ruled out the former as a possible source for the latter. Therefore, these extra chromosomes are best interpreted as B chromosomes restricted to the germ line, presumably present in both sexes, which are inherited as such and are not recurrently generated de novo from the A chromosomes.