HYBRIDOGENESIS AND ANDROGENESIS IN THE STICK-INSECT BACILLUS ROSSIUS-GRANDII BENAZZII (INSECTA, PHASMATODEA)

In northwestern Sicily interspecific hybrid females between Bacillus rossius and B. grandii benazzii (Insecta, Phasmatodea) are sympatric with facultatively parthenogenetic demes of the former and bisexual populations of the latter. Preliminary observations suggested that hybrid females are maintained by hybridogenetic reproduction, not by current F₁ hybrid production nor through parthenogenesis. Being hybridogens, a complex of hemiclonal lineages, we informally refer to them as B. rossius-grandii benazzii, according to Schultz's proposal. In this study B. rossius-g. benazzii females were crossed with males of B. g. benazzii, B. g. grandii, B. g. maretimi, and B. rossius. Allozyme analysis of the progeny showed that the great majority of them were actually produced by hybridogenesis with a hemiclonal inheritance of the maternal B. rossius genotype (Br^m ) and actual syngamy with a sperm from the fathering male, so that Br^m -gb^p , Br^m -gg^p , Br^m -gm^p , and Br^m -r^p offspring were obtained in the respective crosses. All-paternal progeny (androgenetics) were also produced (Bgb^p gb^p , Bgm^p gm^p , Br^p r^p ) and two gynogenetic descendants were observed. Cytological investigations on virgin eggs that failed to hatch revealed in most of them a haploid-diploid blocked blastoderm; this rudimentary parthenogenesis appears to be an important prerequisite for further evolution of this hybridogen. Reproductive modes of descendants were also analyzed; although Br^m -g^p hybrids are still able to reproduce by hybridogenesis, a progressive disruption of the hybridogenetic-androgenetic system takes place in synthetic B. rossius (Br^m -r^p , Br^p r^p ) and abundant thelytokous parthenogenetic offspring are obtained from females of androgenetic origin. The evolutionary role of these hybridogens appears to be linked to their shift towards parthenogenesis; this has apparently occurred in the southeastern Sicilian hybrid B. whitei (=B. rossius/g. grandii), which exhibits both hybridogenesis and parthenogenesis.