A MOLECULAR CYTOGENETIC COMPARISON BETWEEN RHYNCHOSCIARA-AMERICANA AND RHYNCHOSCIARA-HOLLAENDERI (DIPTERA, SCIARIDAE)

The polytene chromosomes of Rhynchosciara americana and R. hollaenderi , a pair of sibling species in the americana-like group of Rhynchoscia ra, were compared using a number of techniques, including in situ hybr idization. With classical cytological techniques, the only differences observed were in the morphology of centromeric and telomeric heteroch romatin, in the size of a DNA and RNA puff, and in the presence of an inversion polymorphism in R. hollaenderi. However, after in situ hybri dization with rDNA and poly-r(A) probes, differences between the two s pecies appeared at a number of sites. Differences in poly-r(A) sites w ere especially informative in establishing phylogenetic relationships between these two species and a third species currently being examined from this group. Chromosomal evolution between these species appears to have occurred mainly through differential amplification and transpo sition of repetitive sequence DNA, of which dA:dT tracts are an import ant component. The R. hollaenderi karyotype is tentatively considered more ancestral than that of R. americana because it has features prese nt in the third Rhynchosciara species. Explanations for the monomorphi sms observed in Rhynchosciara species and mechanisms of speciation in the group are considered within the context of the species' complex be havior.