A cytological description of 10 taxa in Metacnephia (Diptera: Simuliidae)

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Abstract
Cytological descriptions and phylogenetic relationships are presented for 10 blackfly taxa in Metacnephia [M. pallipes, M. freytagi, Metacnephia Crete, M. tredecimata, M. sommermane, M. borealis, M. saskatchewana, M. amphora]. These descriptions are given in terms of standard polytene chromosome maps based on M. pallipes. All members are male chiasmate and differ from related Cnephia by a whole arm interchange between chromosomes I and II. Sex chromosome differentiation varies from undefined changes resulting in a short, nonpairing centromeric segment in X0Y1 males of M. amphora to a complex system in M. borealis. The latter system involves Y chromosome differentiation by associated inversions and sex linkage of the nucleolar organizer (NO). M. borealis exhibits a sex bias in NO expression with excess expression in females and an apparent conversion phenomenon which dosage compensates males. The closest members of Metacnephia differ only in their sex chromosomes and share floating inversions; more distant taxa differ by fixed inversions as well as sex chromosomes. IIIS-1, IIIS-2 and IIIS-3 may be polymorphic, sex linked or fixed in different species of the group. Nonhomology of sex chromosomes of different species, association of the sex locus and NO, and presence of an identical supernumerary block on nonhomologous chromosomes indicate a mobility phenomenon for certain loci.