Non banded sex chromosome elements have been identified in polytene trichogen cells of Lucilia cuprina using Y-autosome translocations, C-banding and Quinacrine fluorescence. The X chromosome is an irregular granular structure while the much smaller Y chromosome has both a dense darkly stained and a loosely organised segment. The X and Y chromosomes are under-replicated in polytene cells but comparison of C- and Q-banding characteristics of sex chromosomes in diploid and polytene tissues indicates that selective replication of non C-banding material occurs in both the sex chromosomes. Brightly fluorescing material in the Y chromosome is replicated to such an extent that it consists of half the polytene element, while the C-banding material, which makes up most of the diploid X chromosome, is virtually unreplicated. Differential replication also occurs in autosomes. In XXY males, and in males carrying a duplication of the X euchromatic region, a short uniquely banded polytene chromosome is formed. It is suggested that in males carrying two doses of X euchromatin a dosage compensation mechanism operates in which genes in one copy are silenced by forming a banded polytene chromosome.

Mitotic and meiotic chromosomes of the Old World screwworm fly, Chrysomya bezziana, were studied using C-banding and quinacrine and counterstain-enhanced fluorescence techniques. The five autosomes in the karyotype are evenly graded in size, with somewhat variable arm ratios. Distinguishing all autosomes on these features alone can be difficult. C-banding produces small centromeric bands in the autosomes, whereas the much longer X and Y chromosomes have extensive dark C-band blocks with intermediate background staining. Most bright fluorescence occurs in the sex chromosomes, particularly the X chromosome, which has remarkable banding detail. Band resolution is greatly increased in mitotic metaphase cells from embryos. Quinacrine staining of mitotic chromosomes produces bright fluorescence at the centromere regions of chromosomes 2, 3, and 4, assisting in their identification. Meiotic chromosomes have distinctly reduced brightness and resolution of fluorescent bands and show marked chromatid asynapsis in the brighter regions of the sex chromosomes. Fluorochromes staining A.T-rich DNA (quinacrine and 4,6-diamidino-2-phenylindole (DAPI) produce bright staining in a large proportion of the sex chromosomes. By contrast chromomycin, which binds preferentially to G.C-rich DNA, stains a much smaller proportion of the sex chromosomes than expected from reciprocal staining. Together with the asynapsis data this indicates that much of the heterochromatin in the sex chromosomes has unusual structural properties.

The existence of sibling species in the Old World screwworm fly Chrysomya bezziana would raise serious problems in eradicating this pest if it entered Australia. Cytogenetic variation in C. bezziana was investigated by analyzing pupal trichogen polytene chromosomes. Natural populations of C. bezziana spanning its range from southern Africa to Papua New Guinea were examined as well as hybrids between a New Guinea laboratory strain and natural populations. No evidence of sibling species was found. All populations exhibited the same basic banding pattern as the standard sequence established from a Papua New Guinea strain. Extensive asynapsis of chromosome homologues was found in some hybrid crosses and was therefore measured in all populations and hybrids to detect systematic variation. Asynapsis levels in most hybrids could not be statistically distinguished from those present in the parent populations except for crosses between populations at the ends of the range. This result does not permit asynapsis levels to be used in establishing the origin of introduced flies by estimating their distance from known populations. One inversion polymorphism and six band polymorphisms spread over three chromosomes were analyzed. Populations in each sampled region had characteristic combinations of band polymorphisms. This may offer a diagnostic method for determining the origin of flies accidentally introduced to Australia.

The location of genes coding for 18S and 28S ribosomal RNA in mitotic and polytene cells of Lucilia cuprina and Chrysomya bezziana was investigated using in situ hybridization of an 18 + 28S ribosomal gene probe and silver staining. In both species ribosomal genes were localized to secondary constriction regions in sex chromosome heterochromatin. In L. cuprina mitotic cells the probe hybridizes to a distal secondary constriction region in the short arms of the X and Y chromosomes. In C. bezziana mitotic chromosomes ribosomal genes were located in distal secondary constriction regions in the long arms of the X and Y chromosomes. In polytene trichogen cells of both species, hybridization results varied with the level of polyteny. Cells of low polyteny have a single hybridization site, but with greater polytenization, increasing numbers of extrachromosomal fragments strongly hybridize to the ribosomal gene probe. No hybridization occurs in structures representing the sex chromosomes or in the autosomes. These results indicate that fragmentation and dispersal of the nucleolus occurs during polytenization. Silver staining of both unsquashed and squashed polytene nuclei show identical behaviour of multiple, varied-sized nucleolar bodies, thus confirming the in situ hybridization results. Uridine incorporation studies in L. cuprina indicated that transcription occurs in extrachromosomal bodies similar to nucleolar fragments. Nucleolar fragmentation is more pronounced in L. cuprina males, particularly in those with the translocation T(Y;2)540. Chromosomally normal C. bezziana show nucleolar fragmentation levels similar to that in L. cuprina males. Ribosomal genes are disproportionately replicated in trichogen cells to a much greater extent than surrounding heterochromatin. Nucleolar fragmentation may be a gene amplification system, but it is not known to what degree, relative to diploid amounts, ribosomal genes replicate in trichogen cells.

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