Cytological characterisation of heterochromatin in mitotic and meiotic chromosomes of the Old World screwworm fly, Chrysomya bezziana (Diptera: Calliphoridae)

Abstract

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.

Comparative cytogenetic study in Muscidae flies

“…Most dipteran species have heterochromatin, which can extend from pericentromeric regions to include whole arms of some chromosomes or even the entire sex chromosomes (Kaul et al, 1978;Wallace and Newton, 1987;Marchi and Mezzanotte, 1990;Bedo, 1991;El Agose et al, 1992). While in many species only the Y chromosome is heterochromatic, in the examined species both sex chromosomes were totally heterochromatic.…”

Sex-Linked Expression of a Sexually Selected Trait in the Stalk-Eyed Fly, Cyrtodiopsis Dalmanni

“…the housefly M. domestica (El Agoze et al 1992), the olive fruit fly Bactrocerca oleae (Mavragani-Tsipidou 2002), and several blow fly species e.g. Chrysomya megacephala and C. putoria (Ullerich 1976;Parise-Maltempi and Avancini 2001), C. chloropyga, C. varipes (Ullerich 1976) and C. bezziana (Bedo 1991). While all these species have the same chromosome number, there exists striking diversity in the length of the sex chromosomes.…”

“…Sex chromosomes are thought to evolve very rapidly under antagonistic sexual selection (Van Doorn and Kirkpatrick 2007), and it has been proposed that the influence of sex-linked genes on polygenic sexual dimorphic traits is approximately proportional to the length of the X-chromosome (Lande 1987). However, the X chromosomes are partially or largely heterochromatic in all of these species (Ullerich 1976;Bedo 1991;Hediger et. al.…”

Comparative study of mitotic chromosomes in two blowflies, Lucilia sericata and L. cluvia (Diptera, Calliphoridae), by C- and G-like banding patterns and rRNA loci, and implications for karyotype evolution

“…In Calliphoridae, cytogenetic data are scarce and refer almost exclusively to the karyotype, C-banding and/or the C value of a very few species (Boyes and van Brink 1965, Boyes and Shewell 1975, Bedo 1980, 1991, Parise-Maltempi and Avancini 2001, El-Bassiony 2006, Ullerich and Schöttke 2006). This family shows a remarkably uniform karyotype (2n = 12), generally comprising five pairs of large euchromatic autosomes and a pair of heteromorphic sex chromosomes.…”