REPRODUCTIVE FEATURES OF HOMOSPECIFIC HYBRIDOGENETICALLY-DERIVED STICK INSECTS SUGGEST HOW UNISEXUALS CAN EVOLVE

Hybridogenetic reproduction has been demonstrated in both vertebrate a nd invertebrate unisexual hybrids. Its most peculiar feature is the tr ansmission to the progeny of one invariant genome (hemiclone) through the egg and the replacement of the other by host fathering males. Baci llus hybridogens are the only known example of hemiclonal invertebrate s; their comparison to Poeciliopsis and Rana systems helps in understa nding peculiar and shared features of vertebrate and insect hybridogen esis. In P. monacha-lucida, the experimental production of non-hybrid progeny through the reunion of the maternal hemiclone with a homospeci fic paternal genome provided by males of the maternal ancestor leads t o inviable or severely impaired sterile specimens, whereas in Rana esc ulenta viable offspring are the rule. The comparable synthetic B. ross ius progeny (Rr) embodying the maternal R hemiclone and a paternal r h aploset, appear perfectly viable and fertile, clearly demonstrating co mpatibility between the two homospecific genomes, and also supporting a lack of deterioration of the R hemiclone. This condition can be ascr ibed to the recent origin of the hemiclones, and also to the absence o f lethal recessives, owing to their most likely derivation from an aut omictic doubling in the parthenogenetic mechanisms of the maternal anc estor. However, the hybridogenetic system breaks down in the gamete pr oduction of the majority of Rr females, since normal allele segregatio n also occurs in their progeny. These reproductive modes suggest a lik ely evolutionary dynamic for newly originated hybridogens: to achieve stability, an interruption of reproductive interactions with the mater nal ancestor seems necessary. In stick insects, this constraint appear s to be fulfilled in both areas of sympatry. The microevolutionary pat hway suggested by the ecological scenario also supports the possibilit y that a shift of hemiclonal stick insect strains to clonality has occ urred.