The Chromosome Complement of the Hybrid Bacillus whitei Complex (Insecta Phasmatodea) II. The repatterned cytotypes

Abstract

The hybrid Bacillus whitei complex clearly derives from two different interspecific hybridization events between B. rossius and B. grandii grandii. The older one gave origin to nowadays parthenogenetic clones with a “metacentric” “paleo”-karyotype, while from the more recent one originated the presently hemiclonal hybridogenetic strains, which exhibit the “acrocentric” “neo”-karyotype. These two 35-chromosome sets have been referred to as standard karyotypes. Extensive chromosomal analysis of both central and peripheral populations showed a variety of differently repatterned cytotypes (2n=35-37), all clearly derived from the “paleo”-standard karyotype. This study analyzes these cytotypes and suggests their derivation from the standard “paleo”-karyotype mainly through Robertsonian fissions of the largest chromosomes and a few heterozygous translocations. Chromosome repatterning appears to increase with the increasing distance from the Canicattini Bagni area which therefore appears to be the hybrid radiation center. The “fissionist” versus “fusionist” trend in the phasmatodean karyotypes is discussed and the particularly convincing evidence of intraspecific Robertsonian fissions of the largest chromosomes of the B. whitei set is stressed. The utilization of such fissions as unambiguous models for the molecular analysis of centromeric regions is suggested and the synthetic rDNA activity in the hybrid of both parental sets is also pointed out. © 1992, Japan Mendel Society, International Society of Cytology. All rights reserved.