Rearrangement hotspots in the sex chromosome of the Palearctic black fly Simulium bergi (Diptera, Simuliidae)

The biology of Simulium noelleri and Simulium dolini: morphological, ecological and molecular data

Molecular genetic research has revolutionized the taxonomy and systematics of the Simuliidae family. Simulium noelleri Friederichs, 1920 is a species of blackfly, common in the Holarctic, reported for 33 countries. In 1954, Topchiev recorded it in Ukraine for the first time. Simulium dolini Usova et Sukhomlin, 1989 has been recorded at the borders of Ukraine and Belarus. It was described for the first time by Usova and Sukhomlin in 1989 from the collection from the territory of Volyn region in 1985. Usova and Sukhomlin, Yankovsky, Adler state that S. noelleri and S. dolini are different species by the morphological characteristics that differ in all phases of development. Adults differ in the structure of the genital appendages, palps, the margin and shape of the face and forehead, the colour of the legs; the larva – in the pattern on the frontal capsule, the number of rays in the fans, mandibular teeth and the hypostoma, the structure of the hind organ of attachment; pupae – in the branching way of gills. Molecular data are becoming an increasingly important tool in insect taxonomy. Therefore, we had to check that these two closely related species also have genetic difference. The development of S. noelleri and S. dolini was studied in four small rivers of Volyn region, Ukraine (Chornohuzka, Konopelka, Putylivka, Omelyanivka) in the period from 2017 to 2019. During initial processing of insect samples, we used the standard protocols EPPO PM7/129. We obtained the nucleotide sequence of S. dolini. It was proved that the populations of S. noelleri and S. dolini from medium and small rivers of Volyn differ in biological, morphological, behavioural and genetic characteristics. Comparison of the species S. noelleri with the data of the GenBank confirms the identification of three distinct morphotypes from Volyn, Great Britain and Canada. As a result of the conducted researches, it was confirmed that two close species of S. dolini and S. noelleri from the noelleri species group differ in the structure of mitochondrial DNA, which confirms their independent taxonomic status. Additional studies comprising more individuals from larger areas of Europe are required to verify the taxonomic position of these two species.

When and how do sex-linked regions become sex chromosomes?

The attention given to heteromorphism and genetic degeneration of "classical sex chromosomes" (Y chromosomes in XY systems, and the W in ZW systems that were studied first and are best described) has perhaps created the impression that the absence of recombination between sex chromosomes is inevitable. I here argue that continued recombination is often to be expected, that absence of recombination is surprising and demands further study, and that the involvement of selection in reduced recombination is not yet well understood. Despite a long history of investigations of sex chromosome pairs, there is a need for more quantitative approaches to studying sex-linked regions. I describe a scheme to help understand the relationships between different properties of sex-linked regions. Specifically, I focus on their sizes (differentiating between small regions and extensive fully sex-linked ones), the times when they evolved, and their differentiation, and review studies using DNA sequencing in nonmodel organisms that are providing information about the processes causing these properties.

Molecular characterization of black flies (Diptera: Simuliidae) in areas with pest outbreaks and simuliotoxicosis in Northeast Anatolia Region, Turkey

Accurate species identification provides the foundation for successful pest management and vector control of black flies. Accordingly, we examined the mitochondrial DNA cytochrome oxidase I (COI) gene sequences of four morphologically and chromosomally identified species of black flies (Simulium vernumgroup sp., S. bergi Rubtsov, S. bezzii (Corti), and S. kiritshenkoi Rubtsov) in Northeast Anatolia Region of Turkey where simuliid pest problems and simuliotoxicosis cases have been reported among cattle. COI gene sequences of these species and closely related species available in GenBank were used to provide species-level diagnoses and infer relationships. Both subgenera (Nevermannia and Simulium) were monophyletic, and subclades generally corresponded with species groups. Intraspecific genetic divergence was 0.2-1.6%, whereas the mean interspecific genetic divergence among the four species was 11.2-14.5%. The COI analysis produced results congruent with morphological concepts of the nominal species S. bergi and S. bezzii. Probable misidentifications in GenBank were revealed, especially for species in the S. ornatum and S. vernum groups, complicating identification capability. Sequence variation in the COI barcode region also might not be adequate for species delineation and identification among some species in these two species groups.

Sympatric speciation in the Simulium arcticum s. l. complex (Diptera: Simuliidae): The Rothfels model updated

ABSTRACT

We tested the Rothfels sympatric speciation model for black flies by comparing all available data for sex-chromosome diversity with the geographic locations of larval collection sites within the Simulium arcticum complex of black flies (Diptera: Simuliidae). Five separate data sets equaling about 20,000 larvae were included from throughout the geographic range of this complex. We record a total of 31 taxa having unique sex chromosomes, all of which demonstrate linkage disequilibrium with most taxa sharing autosomal polymorphisms. All siblings share portions of their distributions with S. negativum, the presumed oldest member of the complex. Twenty-one of 22 cytotypes have distributions within the ranges of siblings thus supporting the sympatric speciation model of Rothfels. Chromosomally diverse sites may require analysis of as many as 200 larvae to be properly described. There is no effect of any inversions influencing the occurrence of other inversions. Finally, we report a new cytotype, Simulium arcticum IIL-6, which we originally discovered in Alaska. Aspects of future genomic research are discussed as they relate to the main chromosomal structural/functional tenants of the model.

OPEN RESEARCH BADGE

This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data are available at https://doi.org/10.6084/m9.figshare.7719398.

Evolutionary and biogeographic history of the black fly Simulium wayani (Diptera: Simuliidae) on the island of Timor

A recently described species of black fly, Simulium wayani Takaoka and Chen, from the island of Timor was chromosomally mapped to provide insights into its evolutionary and biogeographic history. The morphologically based species status of S. wayani is supported by a suite of fixed chromosomal rearrangements and unique sex chromosomes derived primarily from a large pool of polymorphisms in the S. ornatipes complex in Australia. The banding patterns of its polytene chromosomes indicate that S. wayani is closely related to a pair of homosequential cryptic species (S. norfolkense Dumbleton and S. ornatipes cytoform A2) in the S. ornatipes Skuse complex on mainland Australia; all three species uniquely share the same amplified band in their chromosomal complement. The low level of polymorphism and heterozygosity in S. wayani, relative to Australian populations of the S. ornatipes complex, suggests few colonization events from the larger land mass.

Biodiversity of Simulium metallicum sensu lato (Diptera: Simuliidae), a complex of Neotropical vectors associated with human onchocerciasis

The polytene chromosomes of 130 larvae of the Neotropical Simulium metallicum complex from Brazil, Costa Rica, and Ecuador revealed five cytoforms, including three ('M', 'N', and 'O') that are new and two ('B' and 'J') that represent range extensions of up to 850km. The discovery of three new cytoforms brings the total number in the complex to 17. Cytoforms 'B', 'J', and 'N' are reproductively isolated from one another, and their species status is corroborated by morphological evidence. None of the three new cytoforms is known from current or historical onchocerciasis foci, although 'M' inhabits the periphery of the former Ecuadorian Santiago onchocerciasis focus a mere 30km to the west. The number of fixed chromosomal differences, as many as 24, separating some members of the S. metallicum complex far exceeds that known between members of any other simuliid species complex. Two distinct groupings can be diagnosed within the S. metallicum complex, based on at least eight fixed chromosomal rearrangements and structural characters in the larval stage. Consequently, a recommendation is made to recognize the S. horacioi complex and the S. metallicum complex sensu stricto. Recognition of two separate complexes provides potential phylogenetic content with predictive power for understanding biological phenomena such as vector potential.