Simulium reptans (Linnaeus, 1758)

Vaulin, Oleg V., Petrozhitskaya, Liudmila V. & Novgorodova, Tatiana A., 2024, Genetic diversity of populations of Simulium reptans s. l. (Diptera: Simuliidae) in the territory of Russia and Northern Kazakhstan, Zootaxa 5543 (3), pp. 329-342 : 336-338

The features of the genetic structure of population of black flies of the Simulium reptans s.l.

Our results, information from other researchers ( Kúdela et al. 2014; Đuknić 2023) and the DNA database indicate significant spatial heterogeneity in the distribution of S. reptans . The S. reptans A form is distributed relatively locally in northern Europe. The S. reptans B form is widespread throughout Eurasia : from the British Isles to Eastern Siberia and Northern Kazakhstan. Based on the sequence of the standard fragment of the COI gene for S. reptans B, three branches can be distinguished— Eastern European ( Slovakia, Slovenia, Latvia, Lithuania), Eurasian (general for Northern European and Asian habitats) and Asian ( Western and Eastern Siberia , Northern Kazakhstan). The expected restriction spectra of the COI gene fragment by the BstF5 I enzyme differ between the Eurasian (100+250+350 bp) and Asian (600+100 bp) branches of S. reptans B. Accordingly , based on the frequencies of these variants ( Figure 4 ), it is possible to estimate the predominant mtDNA variants in populations. In Eastern Siberia (Ust`- Kut ), both variants occur with similar frequencies (55 and 45%, respectively), while in the Ob-Irtysh basin ( Novosibirsk region , Krasnoyarsk region and Pavlodar region of Kazakhstan), the Asian branch of mtDNA predominates (88%). Consequently , S. reptans B populations show heterogeneity. Branches within S. reptans B differ in this DNA region by few nucleotide substitutions. The Asian branch differs from the Eurasian branch by two substitutions, and each of them differs from the East European branch by three. These distances are comparable to the degree of divergence between the A and B forms of S. reptans . The distance between the nearest tree nodes belonging to S. reptans A and B is only three substitutions. Of particular interest is the actual relationship between forms A and B of S. reptans . Are these forms reproductively isolated, having their own gene pools and ecological niches, i.e., are they cryptic species? Or is mtDNA divergence just a peculiar variant of neutral intraspecific genetic diversity? An indication of genomic divergence between forms A and B could be divergence of nuclear DNA markers. Sequences of fragments of nuclear rRNA genes with minimal polymorphism do not demonstrate subdivision between A and B forms. Thus, according to ITS2, the samples of S. reptans A had the same nucleotide sequence as some of the individuals of S. reptans B. According to D 2 28S, only two Asian individuals had variability in the form of superposition in a single position of sequence. The absence of differences in rRNA gene fragments between the studied branches A and B of S. reptans does not confirm the hypothesis about the divergence of the genomes of A and B forms. It is most likely that these forms are only neutral genetic variants of mtDNA that do not mark separate, reproductively isolated groups of varying degrees. However, in order to clarify the relationships between these forms, it is necessary to conduct additional detailed studies, including an extensive multilocus analysis covering many regions of the genome and carried out for localities where the A and B forms of S. reptans exist sympatrically.

The habitats of forms A and B of S. reptans are well defined. The features of their distribution can be explained by the difficulties of expansion/mixing of conspecific individuals with different mtDNA variants. At the same time, another possible explanation could be the actual genetic isolation of forms A and B and their adaptation to different environmental conditions. Thus , S. reptans A is found in small and medium-sized rivers in northern Europe –from Great Britain to the northeast of the European part of Russia, which is within the boundaries of 54– 69°N and 2– 30°E, in conditions from subarctic to temperate continental climate of wet type, with an average annual precipitation of 500–600 mm, mainly on hilly plains and low mountains (50–250 m above sea level) with forest vegetation. Simulium reptans B is distributed much more widely, mainly on flat, less often low-mountain, forest and steppe landscapes, with a temperate to sharply continental climate, with an annual precipitation of 300–500 mm. The development of S. reptans B occurs in both large, medium and small rivers.

Unfortunately, we were not able to study material from mid-mountain forest landscapes of Siberia, which somewhat limits the possibilities of comparative analysis with European data on Simulium reptans s.l. Due to the lack of information from the mountainous regions of Southwestern Europe, Siberia and Northern China, it is currently impossible to evaluate borders of distribution of forms of S. reptans A and B, taking into account elevational zonation. Provided that the mitochondrial DNA variants corresponding to the A and B forms of S. reptans mark reproductively isolated and ecologically isolated groups, one would expect that limited populations of Simulium reptans A could be found on the upper border of the middle mountains, by analogy with the zonal foresttundra.

At this stage of the study, the black fly Simulium reptantoides was not found in the territory of Russia and Northern Kazakhstan. However, it is too early to make conclusions about the pattern of distribution of this species. According to Kúdela et al. (2014), the range of S. reptantoides is not limited to Europe. To obtain a more complete understanding of the distribution of this species and answer the question of whether S. reptantoides is present in the Asian part of Eurasia, additional studies, including areas with mountain taiga landscapes, are required.

According to Adler (2022), the southern part of the range of S. reptans covers Greece, Italy, Portugal, Spain, Tunisia, Turkey, Uzbekistan, North West and north East of China. More detailed study in the territory of these countries may expand our understanding of the phylogeography of S. reptans s.l.

Figures

FIGURE 4. Proportions of two restriction spectra among the studied material of Simulium reptans s.l. collected in different localities. North Europe (MR)–Murmansk Region (Russia); Western Siberia (NR+PR)—Novosibirsk Region (Russia) and Pavlodar Region (North Kazakhstan); Eastern Siberia (IR)–Irkutsk Region (Russia). The data marked with different letters are significantly different (Fisher’s exact test with Bonferroni correction, p <0.017).