New DAPI and FISH findings on egg maturation processes in related hybridogenetic and parthenogenetic Bacillus hybrids (Insecta, Phasmatodea)

Bacillus stick insects have proved adequate for studying a wide array of re productive modes: sexual, parthenogenetic, hybridogenetic, androgenetic. Hy bridogenetic strains (B. rossius-grandii) were thought to discard the pater nal "grandii" haploset during first meiotic division and keep the "rossius" hemiclone, whereas the clonal B. whitei (=rossius/ grandii) would maintain its hybrid structure by fusing back two nonsister nuclei ach derived from previously segregated heterospecific complements-by the end of the 2nd meio tic division. New investigations on laid eggs and ovariole squashes, either DAR stained or FISH labeled, revealed that in hybridogens the "grandii" se t is excluded from the germ line prior to meiosis and that a DNA extra-synt hesis should occur to produce hemiclonal eggs after two cytologically norma l meiotic divisions. On the other hand, in B. whitei eggs no genome segrega tion appears to occur and an intrameiotic DNA extra-synthesis must take pla ce to produce 2n tetrachromatidic oocytes I; these divide twice and give un reduced clonal eggs. The new findings bring hybridogenetic oogenesis of Bac illus to be coincident with that of the known hemiclonal organisms and poin t to an independent onset of B. whitei from hemiclonal strains. In addition , B. whitei gains a closer resemblance to B. lynceorum owing to the sharing of a cytologically identical egg maturation mechanism, of the same materna l ancestor and of peculiar chromosomal features. It is here suggested that B. lynceorum originated from the incorporation of an "atticus" genome into a B. whitei egg, according to a pathway of repeated hybridization often occ urred with other polyploid hybrids. Mol. Reprod, Dev. 60: 270-276, 2001. (C ) 2001 Wiley-Liss, Inc.