The trihybrid genome constitution of Bacillus lynceorum (Insecta Phasmatodea) and its karyotypic variations.

The standard karyotype and a wide array of repatterned cytotypes from 21 demes of the double-allotriploid thelytokous Bacillus lynceorum have been analyzed by means of Giemsa, C-banding, and silver-staining techniques. The present study substantially amends the first karyotype description and also analyzes in detail the chromosomal rearrangements to trace their most likely derivation. Bacillus lynceorum cytotypes also provide a well-documented instance of an intraspecific gain of centromeric function. The contribution of three different specific haplosets is particularly evidenced from centromeric heterochromatin pattern and satellite/Ag-NOR locations. In stick insects, both hybridogenetic and parthenogenetic Bacillus hybrids, including B. lynceorum, can utilize the rDNA of all available parental haplosets, although a hierarchical role of the B. rossius genome seems to emerge. Satellite/Ag-NOR patterns, besides clearly allowing the recognition of ancestral parental genomes, also suggest a polyphyletic origin for B. lynceorum, which, to our knowledge, represents the only described karyotype of a trihybrid invertebrate.