SPECIATION BY HYBRIDIZATION IN PHASMIDS AND OTHER INSECTS

Speciation by hybridization in insects has been recently recognized on the basis of isozyme and chromosome studies showing that several species, either diploid or polyploid, have genomes that combine the genes and chromosome sets of 2 (or more) bisexual species. Until this evidence became available, thelytokous invertebrates were all considered uniparental derivatives of bisexual species. The authors review examples including the stick insects Bacillus whitei, B. atticus, B. lynceorum, Leptynia hispanica D, Clonopsis gallica, Carausius morosus; the grasshopper Warramaba virgo; some Otiorrhynchus weevils; the planthopper Muellerianella 2-fairmairei-brevipennis; and black flies of the genera Gymnopais and Prosimulium. Insect hybrid species may differ in their mode of reproduction (apomictic or automictic thelytokous parthenogenesis, gynogenesis), degree of ploidy, and genetic structure (level of heterozygosity, clonal variation). The main problems involved in the origin and evolution of hybrid species are discussed, with particular regard to changes in the maturation divisions allowing the transmission of the hybrid genome to the next generation, and their successful adaptation. An origin of hybrid species from occasional parthenogenetic development of hybrid eggs produced in areas of extensive interspecific hybridization (eg disturbed habitats) is suggested. -from Authors