Karyotypes of <em>Chironomus</em> spp. (Diptera: Chironomidae) from the Bongshi River of Bangladesh

Abstract

Cytological studies of Chironomus spp. belonging to the salivary gland polytene chromosomes have been carried out for the first time in Bangladesh. Among the members of Diptera, the genus Chironomus is a very much valuable insect because of the large polytene chromosomes in their salivary glands and this insect is distributed in a broad range of ecological conditions. The objectives of this study are to investigate cytogenetic variation of the different Chironomus species in Bangladesh. Larvae were collected randomly from the Bongshi River, Savar, Dhaka and examined cytologically at the Cytology and Biocontrol Research (CBR) Lab of the Institute of Food and Radiation Biology (IFRB), AERE, Savar, Dhaka. Salivary glands of final (forth) instar larvae were used for polytene chromosome preparation following the aceto-orcein squash method. Salivary gland cells contained four pairs (2n=8) of polytene chromosomes with long precious banded structures, puffs, Balbiani rings (BR) and nucleolar organizers (NOR). Four polytene chromosomes are numbered with AB (1st), CD (2nd), EF (3rd) and G (4th). Initially, seven types of polytene chromosome banding structures (Type 1-7) were identified that most likely refers to seven different Chironomus species. Polytene complements of Type 1 has a sequence of banding in arms A-F and arm G contained one BR in the middle and one NOR of its both ends. Banding inversion is also observed in this arm. Clear banding sequences were found in the chromosomes of Type 2. Short arm G has a remarkable flattened funnel shaped banding structure at one terminal end. In the polytene chromosomes of Type 3, arm G is very short adorn with BR, NOR and banding inversion. A banding sequence with three dark bands and a puff near the tip of arm D are the landmarks of polytene chromosome in Type 4. Type 5 comprise of long polytene arm (A-F) and G has BR, NOR and banding inversion in the middle. Arm G of Type 6 has a specific triangle shape with three dotted bands at one end and other end possess a deeply stained thick band that likely to be the centromere of this arm. Polytene chromosome of arm G of Type 7 composed of one linear arm, no branching observed here. These cryptic polytene chromosome banding pattern and their arrangements provide significant cytotaxonomic information for identifying the Chironomus species. Further cytogenetic study of natural populations based on long-term sampling strategy from different ecological conditions could be very useful for testing the hypothesis of the role of chromosome inversion on local adaptation.