Homologous banding patterns in the polytene chromosomes from the larval salivary glands and ovarian nurse cells of Anopheles stephensi Liston (Culicidae)

Diverse cellular morphologies during lumen maturation in Anopheles gambiae larval salivary glands

Mosquitoes are the greatest animal threat to human health, causing hundreds of millions of infections and around 1 million deaths each year. All mosquito-borne pathogens must traverse the salivary glands (SGs) to be transmitted to the next host, making this organ an ideal target for interventions. The adult SG develops from precursor cells located in the larval SG duct bud. Characterization of the larval SG has been limited. We sought to better understand larval SG architecture, secretion and gene expression. We developed an optimized method for larval SG staining and surveyed hundreds of larval stage 4 (L4) SGs using fluorescence confocal microscopy. Remarkable variation in SG cell and chromatin organization differed among individuals and across the L4 stage. Lumen formation occurred during L4 stage through secretion likely involving a coincident cellular apical lipid enrichment and extracellular vesicle-like structures. Meta-analysis of microarray data showed that larval SG gene expression is divergent from adult SGs, more similar to larval gastric cecae, but different from other larval gut compartments. This work highlights the variable cell architecture of larval Anopheles gambiae SGs and provides candidate targets for genetic strategies aiming to disrupt SGs and transmission of mosquito-borne pathogens.

Polytene Chromosomes: 70 Years of Genetic Research

Polytene chromosomes were described in 1881 and since 1934 they have served as an outstanding model for a variety of genetic experiments. Using the polytene chromosomes, numerous biological phenomena were discovered. First the polytene chromosomes served as a model of the interphase chromosomes in general. In polytene chromosomes, condensed (bands), decondensed (interbands), genetically active (puffs), and silent (pericentric and intercalary heterochromatin as well as regions subject to position effect variegation) regions were found and their features were described in detail. Analysis of the general organization of replication and transcription at the cytological level has become possible using polytene chromosomes. In studies of sequential puff formation it was found for the first time that the steroid hormone (ecdysone) exerts its action through gene activation, and that the process of gene activation upon ecdysone proceeds as a cascade. Namely on the polytene chromosomes a new phenomenon of cellular stress response (heat shock) was discovered. Subsequently chromatin boundaries (insulators) were discovered to flank the heat shock puffs. Major progress in solving the problems of dosage compensation and position effect variegation phenomena was mainly related to studies on polytene chromosomes. This review summarizes the current status of studies of polytene chromosomes and of various phenomena described using this successful model.

Nurse cell polytene chromosomes of Drosophila melanogaster otu mutants: morphological changes accompanying interallelic complementation and position effect variegation

Combinations of certain mutant alleles of the ovarian tumor gene permit the production of viable eggs. Two alleles that behave in this way are otu7 and otu11. Females homozygous for either allele are sterile, and their ovarian nurse cells (NC) contain giant polytene chromosomes of various morphologies. Fertile flies (otu+/otu+, otu+/otu7, otu+/otu11) have endopolyploid nurse cells with typical dispersed chromosomes. Fertile hybrids (otu7/otu11) produce large numbers of polytene chromosomes comparable to, and often larger than, classic salivary gland (SG) chromosomes. Therefore, these otu hybrids provide a unique system for studying, at the chromosomal level, the activation and expression of genes functioning during oogenesis. The otu gene encodes a long and a short isoform. The normal long isoform appears to be responsible for the dispersion of chromosomes during the endomitolic DNA replications occurring in ovarian NCs. The genetic inactivation of euchromatic genes placed next to pericentric heterochromatin by a chromosomal rearrangement is accompanied by the compaction of corresponding chromosome regions. A comparative study of the manifestation of position-effect variegation for the polytene chromosomes of SG cells and NCs was made using the Dp(1;1)pn 2b and Dp(1;f)1337 rearrangements. The percentage frequencies of block formation in the SG and NC nuclei for Dp(1;1)pn 2b rearrangement were 92.6% vs. 15.8%, respectively; for Dp(1;f) 1337, these values were 56.8% vs. 9.7%. Therefore heterochromatin belonging to germ line chromosomes is in a configuration that is far less likely to inactivate inserted segments of euchromatin than is heterochromatin from somatic chromosomes.

General characteristics of the polytene chromosome from ovarian pseudonurse cells of the Drosophila melanogaster otu11 and fs(2)B mutants

Polytene chromosomes of good cytological quality from pseudonurse cells (PNCs) of fs(2)B and otu11 mutants were obtained, photomaps for otu11 mutants were constructed and the general characteristics of polytene chromosomes from salivary glands (SGs) and PNCs were compared. Three conditions were found to improve the cytological quality of PNC chromosomes: temperature below 18 degrees C, a protein-rich medium and presence of the Y-chromosome. Detailed comparison of the chromosome banding pattern from SGs and PNCs has shown only minor differences between them. The frequency of asynapsis appeared to be 10 times higher for PNC chromosomes. Despite previous reports, features such as breaks and ectopic contacts turned out to be also typical for PNC chromosomes, but with remarkably lower frequencies.

Polytene chromosomes from ovarian pseudonurse cells of the Drosophila melanogaster otu mutant. II. Photographic map of the X chromosome

The banding pattern of the polytene chromosomes of the ovarian pseudonurse cells (PNC) of the Drosophila melanogaster otu mutant were compared with larval salivary gland (SG) polytene chromosomes. The X chromosome was studied and no significant differences were found in the banding pattern between these functionally very different tissues. Most of the differences result from differential puffing activity. In situ hybridisation with five different DNA probes located along the X chromosome was used to cross-check the results obtained by morphological mapping. The constrictions present in the SG chromosomes were found to be absent in the germ line derived PNC chromosomes. There are prominent puffs in the PNC chromosomes at certain locations where genes known to be transcriptionally active in the germ line reside. This suggests that at least some of the genes active in the wild-type nurse cells may also be active in the PNC cells.

Comparison of the polytene chromosomes of the salivary gland, the fat body and the midgut nuclei of Drosophila auraria

Photo-maps of the fat body and midgut polytene chromosomes of Drosophila auraria were constructed. These photo-maps are compared with a new, more detailed photo-map of the salivary gland chromosomes of the same species. Seven, not previously described inverted tandem-duplications were detected, raising the number of such structures found in this species to 31. The constancy of the banding pattern based on the analysis of the above chromosomes is discussed.

Polytene chromosomes from ovarian pseudonurse cells of the Drosophila melanogaster otu mutant. I. Photographic map of chromosome 3

Certain mutant alleles of the otu locus in Drosophila melanogaster produce abnormal nurse cells in the ovaries. These cells are called pseudonurse cells (PNC), since they generate polytene chromosomes instead of endopolyploid ones and do not normally have an oocyte to nurse. The banding pattern of polytene chromosome 3 from the salivary glands (SG) and from PNCs of homozygous otu1 females was compared and a detailed photomap of PNC chromosomes with different degrees of polyteny is presented. The banding pattern was found to be strikingly similiar in the two tissues. The puffing pattern of the PNC chromosomes is discussed. No constrictions or breaks were found in the PNC chromosomes which seems to indicate that these sites, which are known to be underreplicated in the SG chromosomes, are equally replicated along with the rest of the chromosomes in the PNC nuclei.

Developmental changes in midgut chromosomes of Drosophila gibberosa

In Drosophila gibberosa, differences between midgut and salivary gland chromosomes fall into two categories: tissue-specific band modulations which persist throughout the 90 h developmental period that we studied and tissue-specific puffs. Puffs that are common to both tissues tend to appear earlier in the midgut. Some major early ecdysteroid-induced puffs appear simultaneously in both tissues at the end of the third larval instar; however, the many late puffs that follow in the salivary glands are absent from the midgut. Intense puff activity in the early third larval instar midgut declines at the time of the hormonal pulse that initiates intense gene and secretory activity in salivary glands; the sloughing of midgut cells ensues.

The fourth chromosome of Chironomus tentans Malpighian tubules: an ultrastructural study

Morphology and banding pattern of the 4th chromosome in Chironomus tentans Malpighian tubules have been investigated by electron microscopy, using the squash and selection technique. The map we composed from our observations shows a remarkable increase (75%) in band numbers as compared to the map previously presented by Beermann for the 4th chromosome from salivary glands. Extrapolation of this increase to the entire genome would result in a total band number of about 3,500. The mean DNA content of bands can thus be calculated to be about 50 kb. Many bands show a complex structure, including the BR2 band. Some bands seem to result from fusion of smaller components. "Minibands" have also been observed. Some interbands contain RNP particles. In our material the interbands appeared to be made up of fibrils with a diameter of about 120 A. On the basis of these results we estimate the DNA in the interbands as amounting to 2% of the entire genome. The results are discussed with respect to the organization of the polytene chromosomes and the functional significance of the banding pattern.