Cytogenetics of the parthenogenetic grasshopper Warramaba virgo and its bisexual relatives

Abstract

A study of the chromosomal location and genomic organization of the ribosomal RNA cistrons in the genus Warramaba, involving in situ hybridization and restriction enzyme analysis as well as C- and N-banding and silver staining, has confirmed that the parthenogenetic species W. virgo has two phylads. These phylads appear to have originated independently by hybridization between the precursors of the present day bisexual species “P169” and “P196”. The clones of the Standard phylad of W. virgo have their 18S+26S rDNA cistrons located in C-bands 4, 44 and 49, while those of the Boulder-Zanthus phylad have them in C-bands 50, 74 and 87.5. The relative numbers of the ribosomal genes at the different sites vary greatly from clone to clone and are closely correlated with the width of the corresponding C- and N-bands. Site 49 of the ribosomal cistrons is present as a separate band in the eastern race “A” of P196 but has been incorporated into band 50 in the western race “B” of this species. The former race is assumed to be ancestral to the Standard phylad of W. virgo, the latter to the Boulder-Zanthus phylad, but there has been loss of the 74 and 87.5 sites in the the Standard phylad and the 4 and 44 sites in the Boulder-Zanthus clones. The ribosomal cistrons in W. picta, a species with a primitive karyotype, occur in several sites, only some of which have counterparts in P169 and P196. The 5S rDNA cistrons are located in bands 59.5, 69 and 72.5 in the Standard phylad of W. virgo. — The genomic organization of the 18S+26S rDNA cistrons, as shown by restriction enzyme analysis, is different in the two W. virgo phylads and there are also differences in organization between P196A and P196B. The pattern in P196B and that in the Boulder-Zanthus phylad suggest that they are related. As in the in situ analyses, the genomic organizations of the ribosomal cistrons in both W. virgo phylads are not simply the additive products of those in any known populations of P169 and P196. New repeat lengths indicative of segmental amplification events occur in particular clones of W. virgo. — Throughout the genus Warramaba the N-banding technique stains all bands containing 18S+26S and 5S rDNA cistrons. The Olert silver technique stains band 72.5 in the Standard phylad, but does not correlate with the locations of 18S+26S ribosomal genes.