MATE RECOGNITION AND GAMETE CYTOLOGY FEATURES ALLOW HYBRID SPECIES PRODUCTION AND EVOLUTION IN BACILLUS STICK INSECTS

The increasing number of recognized hybrid unisexual complexes among i nvertebrate and vertebrate animals has promoted investigations about t heir composition and origin. Morphological, karyological and genetic ( protein and DNA) analyses clearly show that, owing to their persistenc e and incomplete reproductive isolation from ancestors, several all-fe male complexes are much more diversified than generally assumed and th at they may also have an evolutionary role. Here the case of the stick -insects of the genus Bacillus is reported in some detail. This holome diterranean genus comprises three well differentiated species that in Sicily have hybridized repeatedly. The Bacillus mate-recognition syste m has not followed the species-specific differentiation of the allozym e-coding loci, allowing interspecific crosses to occur in areas of spe cies sympatry with the production of two hybridogens, a corresponding allodiploid parthenogen and a trihybrid triploid parthenogenetic speci es. Hybridogenetic females eliminate the paternal haploset (grandii) w hile passing the unassorted rossius hemiclone to offspring, which will be again of F-1 hybrid structure through a real fertilization by host male sperm. The polyspermic eggs of the hybridogens can also produce full-paternal fertile progeny of both sexes (androgenetics), when mixi s occurs between two sperm heads. The parthenogenetic mechanism of the corresponding hybrid B. whitei is very similar to the hybridogenetic one, excepting the automictic re-use of the segregated grandii haplose t; therefore B. whitei offspring clonally maintain the maternal hybrid structure. The trihybrid B. lynceorum produces clonal descendants thr ough an apomictic mechanism undergoing two seemingly normal meiotic di visions. Each Bacillus hybrid actually realizes a different egg matura tion process; however, the three share one important feature: an intra meiotic DNA extra-doubling, leading to the formation of four-stranded chromosomes, and enabling the meiotic system to produce balanced gamet es even under different ploidy level and hybrid structure. The extra-r ound of DNA synthesis seems to be triggered by the hybrid condition im pairing the synaptic process. Also the parthenogenetic B. whitei produ ces androgenetics and it is even capable of incorporating a third geno me into its automictic but clonal eggs, following fertilization by B. grandii or B. rossius males with the production of fertile ''synthetic '' triploids. These findings are evidence of clonal unisexuals reprodu ctively interacting with related bisexuals and also suggest that evolu tionary pathways have been undertaken by Bacillus. Also other unisexua l complexes seem to have undergone similar microevolutionary steps and their reproductive traits and persistence, longer than commonly assum ed, make increasingly difficult to think of the whole of unisexuals as ''dead ends'' or ''blind alleys''.