The salivary gland chromosomes of seven species of Prosimulium (Diptera, Simuliidae) in the mixtum (IIIL-1) group

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Abstract
The paper has two aspects: (1) It describes the salivary gland chromosomes of Prosimulium mixtum, P. saltus, P. approximatum, P. mysticum, and P. rhizophorum of the eastern North American IIIL-1 or mixtum group. The chromosomes of P. fontanum (CIt, IIIL-1) have been described previously. Description of the complement of P. fuscum (IIIL) is included because of the apparent close relationship of that species and P. mixtum. The cytological criteria provided should make possible identification of last-instar larvae of all known species of the IIIL-1 group. (2) The study leads to some inferences regarding evolutionary mechanisms. The centromere region of each of the three chromosomes exists in at least two alternate forms (CIs/CIt, CIIs/CIIi, CIIIs/CIIIi); sex chromosomes may be based on one or the other of the two alternative sequences in CIII, and extra heterochromatin and chromocenters may be present or absent. Contrary to a priori concepts, which visualize restriction of new chromosomal traits to single lineages, virtually all possible combinations of chromosomal traits are observed. This indicates that in this group of black flies long-lived polymorphisms provided the basis for subsequent interspecific chromosomal differentiation.

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