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The nucleotide sequences of the mtDNA cytochrome oxidase I (COI) gene fragment 535 bp long (pos. 100 to 634) in 14 midge species (Diptera, Chironomidae): Dicrotendipes nervosus, Endochironomus albipennis, E. tendens, Glyptotendipes barbipes, G. glaucus, G. gripekoveni, G. imbecillis, G. mancunianus, Polypedilum sordens, Stenochironomus gibbus, Synendotendipes kaluginae, Xenochironomus xenolabus, Xenochironomussp., Cricotopus glacialis, and those of the mtDNA COIIgene fragment 561 bp long (pos. 22 to 583) in 3 midge species: Chironomus plumosus, Chironomus balatonicus, and Baeotendipes noctivagawere determined. The time of gene COIdivergence in the phytophilous midges of various taxa was estimated. The wood-mining species Stenochironomus gibbusbelongs to the most ancient midges which diverged together with the ancestor form of two subfamilies, Chironominae and Orthocladiinae, about 104 Mya. Phytophily evolved in the midge larvae at different times in different taxa: among Chironominae, about 47.2 Mya in P. sordens, not earlier than 20 Mya in Glyptotendipes, Kiefferulus, and Dicrotendipes, and not earlier than 13 Mya in Endochironomus albipennis; among Orthocladiinae, 19 Mya in Cricotopus. The divergence of the genera containing phytophilous and fouling species, both in Chironominae (Dicrotendipes, Glyptotendipes, Kiefferulus) and Orthocladiinae (Orthocladiusand Cricotopus), occurred approximately 20 Mya, and coincided with the beginning of formation of the recent-type limnofaunas 23 Mya.

Hybridogenesis as a possible way of speciation in Chironomidae was considered with special reference to the species Chironomus usenicusresulting from hybridization between C. plumosusand C. behningi.The three species had 2n= 8 and belonged to the thummicytocomplex with chromosome arm combinations AB, CD, EF, and G. Arm G had a marker chromosome disk sequence (CDS) and was used to demonstrate the hybrid origin of C. usenicus.Most C. usenicuslarvae were heterozygous in CDS of arm G. CDS useG2 proved to be identical to CDS behG1 ofC. behningiand CDS useG1, to CDS pluG1 of C. plumosus.It was assumed that C. usenicusresults from hybridization between eurybiont C. plumosusand stenobiont C. behnigniat the boundary of their species areas, in freshwater or brackish water bodies of the southern Saratov oblast and northern Kazakhstan. Morphologically and karyotypically, the hybrid was probably similar to C. plumosus.Crosses with C. plumosuseliminated virtually all C. behningichromosome sequences from the karyotype of the hybrid. Further chromosome divergence resulting in C. usenicusinvolved a number of chromosome rearrangements, including duplication of pericentric heterochromatin and other chromosome regions; inversion, which occurred in arm F (regions 13–16) and was fixed in the karyotype; and other paracentric inversions and deletions accumulated in heterozygote in the karyotype pool of the species. Since C. behningiwas eliminated from the introgression zone and its species area reduced, the assimilation character was assumed for introgressive hybridization of C. behningiand C. plumosus.