CHROMOSOMAL EVIDENCE OF HEMICLONAL AND ALL-PATERNAL OFFSPRING PRODUCTION IN BACILLUS-ROSSIUS-GRANDII-BENAZZII (INSECTA, PHASMATODEA)

The stick insects Bacillus rossius-grandii benazzii and B. rossius-gra ndii grandii naturally reproduce by hybridogenesis and androgenesis. T he hybrid karyotype of the former (2n=35, XX female; 34, X0 male) clea rly sums up a B. rossius haploset (r) with n=18 and a B. grandii benaz zii one (gb) with n=17. The two sets keep the parental features for C- heterochromatin amount (much larger in the gb complement) and satellit es/NORs (nuclear organizer regions) (more numerous and variably locate d in the r set); hybridogenetically produced males always show severel y impaired gametogenesis and are therefore sterile, whereas hybridogen etic females are fertile. Reproductive, karyological and cytogenetical properties of the hybridogenetic system have been exploited to obtain the chromosomal evidence of whole haploset exchanges. In progenies ob tained by crossing B. rossius-grandii benazzii females to B. rossius m ales with either standard or repatterned (with Robertsonian fusions) k aryotypes, there has always been complete agreement between electropho retically genotyped and karyologically analyzed hybridogenetic offspri ng: the unassorted maternal r haploset (r(m)) is transmitted and the g b(m) haploset replaced by that of the fathering male (r(p)), thus evi dencing the hemiclonal reproduction and the new r(m)-r(p) chromosomal constitution. New karyotype traits of the offspring relate to chromoso me number (2n=36, female; 35, male), C-heterochromatin pattern (the he terochromatin-rich gb haploset completely disappears) and satellite/NO R features (corresponding to r(m) plus r(p) locations). The same cross es also produce genetically and chromosomally all-paternal descendants (androgenetics), of both sexes and fully fertile, with an r(p) r(p) s tructure. These androgenetic progeny show segregation for alleles and chromosomes at which fathering males are heterozygous: it was therefor e possible to demonstrate that androgenetics can derive from syngamy o f two sperm nuclei, of the several present in the polyspermic hybridog enetic egg. The production of androgenetics from field fertilized fema les of B. rossius-grandii benazzii, B. rossius-grandii grandii and par thenogenetic Bacillus whitei (= B. rossius/grandii grandii) suggests t he occurrence of unsuspected relationships between hybrids and their p arental species, so that the hybrids cannot be simply considered as 's exual parasites'. Furthermore, there is a suggestion of evolution of p arthenogenetic clonal species from selection of initially hybridogenet ic strains. The ability to produce uniparental progeny naturally from the spermatic genome may open a new field of investigation on genomic imprinting.