SATELLITE SEQUENCE TURNOVER IN PARTHENOGENETIC SYSTEMS : THE APOMICTIC TRIPLOID HYBRID BACILLUS LYNCEORUM (INSECTA, PHASMATODEA)

: In the genus Bacillus (Insecta, Phasmatodea) the Bag320 satellite DNA family is present in the bisexual B. grandii and in the related automictic nonhybrid B. atticus; it is lacking in the other bisexual taxon of the genus, B. rossius. This family of highly repeated sequences was analyzed for 11 populations of the apomictic triploid hybrid B. lynceorum. In the neighbor-joining dendrogram, B. lynceorum nucleotide sequences distribute, regardless of geographical origin, among two clusters, one also including all clones of the three B. atticus races, and the other including sequences of the B. grandii grandii subspecies. Thus, B. lynceorum is a trihybrid taxon: as the molecular approach definitively demonstrates, it embodies one haploid complement each of both B. grandii grandii and B. atticus, which must be added to that of B. rossius. The contribution of the latter species has already been assessed on karyological and allozymic grounds. A statistical analysis performed on p-distances shows that for the parental taxa, nucleotide substitution values are of comparable magnitudes at the population level but differ at the subspecific level, being higher for the bisexual taxon. In the apomictic hybrid, atticus- and grandii grandii- like sequences coexist with significantly different p-distance values. For three clones, the nucleotide compositions at the diagnostic loci suggest that gene conversion can occur between atticus- and grandii grandii-like monomers. On the whole, this supports bisexuality as a driving force in variant fixation and suggests that in Bacillus, different gametogenetic processes and different origins of the unisexuals are mirrored in genomic turnover rates of satellite DNA.