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Introduction Within the Simuliidae, cytological analysis of populations using polytene chromosome commonly reveals sibling species complexes within morphotaxonomic groups on a worldwide basis (Rothfels 1979; Adler and Crosskey 2009). The [...], Cytological descriptions are given for two sympatric sibling species, Simulium praelargum IIIL-st and Simulium praelargum IIIL-1.2, from Darjeeling, West Bengal, India. Sibling IIIL-1.2 differs from IIIL-st by a two step fixed included inversion (IIIL-1.2) found on the long arm of chromosome III. Both siblings possess heterochromatinized IIIS polytene chromosome ends, while IIIL-1.2 is unique within the Simuliidae in that it exhibits a very large enhanced chromocentre that persists in normal somatic tissue, contrary to other chromocentre-containing taxa. In IIIL-1.2, the chromocentre occurs as a positively allocyclic heteropycnotic body in normal mitotic interphase cells of neuroblast ganglia, oogonia, and spermatogonia. In mitotic metaphase chromosomes, the chromocentre forms large pronounced primary (centromere) constrictions and appears to be associated with nonhomologous pairing. Initial cytological studies on taxa within the feuerborni group have revealed heterochromatic chromosomal polymorphisms. Progressive fixation of these polymorphisms in different taxa within the group opens up the possibility of using comparative molecular or genomic approaches to begin to define the functional and structural aspects of the epigenome and to further characterize mitosis and meiosis in S. praelargum IIIL-1.2. Key words: heterochromatin, epigenome, mitosis, sibling species, Simulium (feuerborni group). Des descriptions cytologiques sont produites pour deux especes sreurs sympatriques, Simulium praelargum IIIL-st et Simulium praelargum IIIL-1.2, provenant de Darjeeling, dans l'etat du Bengale-Occidental en Inde. L'espece IIIL-1.2 differe de l'espece sreur IIIL-st par une inversion fixee en deux etapes (IIIL-1.2) sur le bras long du chromosome III. Les deux especes sreurs presentent des extremites des chromosomes polytenes IIIS heterochromatiques tandis que le IIIL-1.2 est unique au sein des Simuliidae en ce qu'il presente un tres grand chromocentre elargi qui persiste au sein des tissus somatiques normaux, contrairement a ce qui est observe chez les autres taxons qui presentent des chromocentres. Chez IIIL-1.2, le chromocentre prend la forme d'un corps affichant une heteropycnose positive et allocyclique dans les cellules mitotiques normales a l'interphase au sein des neuroblastes ganglionnaires, des oogones et des spermatogones. Chez les chromosomes en metaphase mitotique, ce chromocentre forme de grandes constrictions primaires prononcees (centromere) et semble associe a l'appariement entre chromosomes non-homologues. Les premieres etudes cytogenetiques sur des taxons au sein du groupe feuerborni ont revele du polymorphisme chromosomique heterochromatique. Une fixation progressive de ces polymorphismes chez differents taxons au sein de ce groupe rend possible l'utilisation d'approches moleculaires ou genomiques comparatives pour commencer a definir les aspects fonctionnels ou structuraux de l'epigenome et de caracteriser plus a fond la mitose et la meiose chez le S. praelargum MIIIL-1.2. [Traduit par la Redaction] Mots-cles: heterochromatine, epigenome, mitose, especes sreurs, Simulium (groupe feuerborni).

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 [ABSTRACT FROM AUTHOR]