Single-larva study confirms Chironomus nuditarsis in Armenia’s Sevan Lake

A single fourth-instar larva collected from Armenia’s Sevan Lake was identified as Chironomus nuditarsis Keyl, 1961 through combined morphological, cytogenetic and mitochondrial cytochrome c oxidase subunit I (COI) sequence analyses. The record brings the number of Chironomus species known from the lake to eight.

The specimen was recovered on October 10, 2019, from a macrozoobenthos sample in Artanish Bay, Gegharkunik Province. The sampling site was 1.3 metres deep and had black silty gravel sediments. Because no additional specimens of the species were collected, the researchers used the preserved larva for all three lines of evidence.

For the morphological examination, the head capsule was mounted in Fora-Berlese solution and assessed using established terminology for Chironomus larvae. The larva was about 15 mm long and had a yellow or light-brown head capsule with blurred gular-spot borders. Its basal antennal segment was cone-shaped, and the antenna blade extended beyond the fifth antennal segment. These features and the accompanying measurements corresponded to published descriptions of C. nuditarsis and differed from its sibling species C. curabilis.

Cytogenetic analysis identified a diploid chromosome number of 2n = 8 and the chromosome-arm combination AB CD EF G, designated the “thummi” cytocomplex. Balbiani rings were located in arms B and G, while the nucleolus occurred in arm G. The larva carried one zygotic combination of banding sequences, including ndtA1, ndtA2, ndtG1 and ndtG2, which the study described as characteristic of C. nuditarsis.

DNA extracted from the specimen produced a 658-base-pair COI sequence, which was deposited in GenBank as OR652398. Its estimated Kimura two-parameter distance from a C. nuditarsis sequence from the United Kingdom was 0.98%. The smallest distances were to sequences labelled C. curabilis: 0.49% for an Iranian sample and 0.73% for a sample from Russia’s Saratov region.

Bayesian analysis placed the Sevan sequence in a cluster containing C. nuditarsis and C. curabilis, associated with the Ch. plumosus group. A median-joining network based on 25 GenBank COI sequences for the two species contained 21 haplotypes shared by both named species. The Sevan haplotype was separated by three mutational steps from a C. curabilis sequence from Iran and a C. nuditarsis sequence from Germany.

The combined morphological, chromosome and molecular evidence supported identifying the Lake Sevan larva as C. nuditarsis. However, COI alone did not reliably separate C. nuditarsis from C. curabilis in the analyzed dataset, because sequences assigned to both species occurred in the same cluster and shared haplotypes. The authors also proposed that interspecific hybridization or mitochondrial introgression could contribute to this pattern.

The study was limited by its reliance on one larva, which cannot establish the species’ abundance, wider distribution or population structure within Sevan Lake. The chromosome data provided only a limited basis for tentatively comparing the specimen with northwestern Caucasian populations. The COI analysis also lacked sequences from Siberian populations, limiting comparisons across the species’ full range.

Despite those limitations, the taxonomic study demonstrated that a single chironomid larva can provide substantial identification evidence when morphology, cytogenetics and molecular analysis are combined. The authors noted that the inventory of Chironomus species in Sevan Lake remains incomplete and called for continued long-term investigation of the lake as environmental conditions change through warming and human activity.

Subject of Research: Taxonomic identification of a single Chironomus nuditarsis larva from Sevan Lake using morphology, karyotype and COI sequencing

Article Title: The first record of Chironomus nuditarsis Keyl, 1961 from Sevan Lake (Armenia) confirmed by morphology, karyotype and COI gene sequence

Article References: Bolshakov, V., Prokin, A., Ivanova, E., & Movergoz, E. (2024). The first record of Chironomus nuditarsis Keyl, 1961 from Sevan Lake (Armenia) confirmed by morphology, karyotype and COI gene sequence. Comparative Cytogenetics, 18, 123-141. https://doi.org/10.3897/compcytogen.18.126130

Image Credits: AI Generated

DOI: 10.3897/compcytogen.18.126130

Keywords: Chironomus nuditarsis, Sevan Lake, Armenia, chironomid taxonomy, polytene chromosomes, COI sequencing, phylogenetics, species identification, first, record, Chironomus, nuditarsis