Populational and Karyological Analysis of the Phytophilous Chironomid Endochironomus tendens F. (Diptera, Chironomidae). 2. Chromosomal Polymorphism and Cytogenetic Differentiation in Populations

Chromosomal polymorphism was studied in samples of the phytophilous larvae of Endochironomus tendens (Fabricius, 1794) collected in different water bodies of Saratov and Volgograd provinces within the Volga (the Tereshka and Volga rivers) and Don (the Don, Medveditsa, and Khoper rivers) basins. The larvae were obtained from tissues of four species of littoral aquatic plants: Sparganum erectum L., Sagittaria sagittifolia L., Butomus umbellatus L., and Typha angustifolia L. In total, 40 zygotic combinations of chromosomal sequences were recorded in the karyotypes of E. tendens. Seven combinations of sequences were discovered and described for the first time: tendA1.3, tendE4.5, tendE5.5, tend(DG)2.4, tend(DG)3.4, tend(DG)4.5, and tend(DG)4.6. Heterozygous individuals comprised 66.7–100% (on average 99.8%), with 1.3–3.2 heterozygous inversions per individual (on average 2.2). Altogether, 398 genotypic combinations of sequences were recorded in chromosomal arms of the larvae. Statistical analysis of the samples of chironomids living in the same substrate but in different water bodies revealed intraspecific cytogenetic differentiation. For example, the greatest Mahalanobis distance equal to 6.70 was observed between the samples of larvae from B. umbellatus L., collected from the Tereshka and Don rivers belonging to different river basins. Analysis of chromosomal differentiation of the larval samples collected from different substrates in the same water body (the Medveditsa River) revealed the greatest Mahalanobis distance (2.63) between the samples from S. erectum and B. umbellatus and the smallest distance (0.35) between those from B. umbellatus and S. sagittifolia. Intraspecific karyotypic differentiation associated with geographic variability was the most pronounced in the phytophilous E. tendens larvae. Chromosomal differentiation in E. tendens is also related to the development of larval adaptations to mining different macrophytes, during which a variety of genomic combinations must have been formed, allowing this species to most fully exploit its habitats.