Underreplication of a polytene chromosome arm in the chironomid Prodiamesa olivacea

Extremely small genomes in two unrelated dipteran insects with shared early developmental traits

We discovered extremely small genomes (1C ~100 Mb) in the dipteran insects Coboldia fuscipes (Scatopsidae) and Psychoda cinerea (Psychodidae). The small genomes of these species cannot be explained by a fast developmental rate, which has been shown to correlate with small genome sizes in animals and plants but might accommodate the combined effects of other developmental traits, including small egg size, thin blastoderm layer, and long-germ development.

The Drosophila endocycle is controlled by Cyclin E and lacks a checkpoint ensuring S-phase completion

Early during Drosophila oogenesis the 16 interconnected cells of each germ-line cyst choose between two alternative fates. The single future oocyte enters meiosis, arrests, and becomes transcriptionally quiescent. The remaining 15 cells initiate a series of polyploid cell cycles to prepare for their role as nurse cells. Like many other polyploid and polytene cells, during nurse cell growth the major satellite DNAs become highly under-represented by a mechanism that has remained obscure. We implicate the cell-cycle regulator cyclin E in DNA under-representation by identifying a hypomorphic, female sterile cycE mutation, cycE01672, that increases the amount of satellite DNA propagated in nurse cells. In mutant but not wild-type endomitotic nurse cells, "late S" patterns of bromodeoxyuridine incorporation are observed similar to those in mitotic cells. CycE protein still cycles in cycE01672 germ-line cysts but at reduced levels, and it is found throughout a longer fraction of the cell cycle. Our experiments support the view that oscillating levels of CycE control the polyploid S phase. Moreover, they indicate that a checkpoint linking the presence of unreplicated DNA to the CycE oscillator is lacking, leading to incomplete replication of late-replicating sequences such as satellite DNAs. Unexpectedly, two to three of the 16 cells in cycE01672 cysts frequently differentiate as oocytes, implicating cell-cycle programming in oocyte determination.

Differential replication of heterochromatin in polytene chromosomes of the Old World screw-worm fly, Chrysomya bezziana (Diptera: Calliphoridae), analysed by fluorescence staining

The distribution and replication of heterochromatin in polytene trichogen chromosomes of the Old World screw-worm fly, Chrysomya bezziana, were studied using fluorescent staining techniques. Quinacrine and distamycin-DAPI, which selectively stain AT-rich DNA, and chromomycin, specific for GC-rich sequences, were used. Bright quinacrine and DA-DAPI fluorescence was found in the sex chromosome body and in all autosomal centromere regions. Chromomycin (CMA) staining results in very little bright fluorescence of the sex chromosome body and autosomal centromeric regions, but many bright bands of varying morphology are distributed in autosomal arms. The expected negative CMA staining of quinacrine and DA-DAPI bright regions was not found. The lack of reciprocal staining patterns may result from changes in the higher order chromatin structure of polytene chromosomes, or intercalation of divergent heterochromatic sequences. Comparison of the different staining techniques in mitotic and polytene cells shows that heterochromatin is differentially under-replicated, so that the proportions of the distinct fluorescent-specific chromatin changes during polytenization. CMA staining within autosomal arms suggests that repeated sequences intercalated in euchromatin are co-replicated during polytenization. The numerous fluorescent markers described also provide further morphological features for use in comparative cytological analysis of C. bezziana.

3B55: a repetitious sequence family which is transcribed and proportionately replicated in germ-line polyploid nuclei of Calliphora erythrocephala

The chromosomes of dipteran polyploid nurse cell nuclei are functionally analogous to the oocyte lampbrush chromosomes of the Amphibia. In investigating the transcriptional and replicative activity of these nuclei in Calliphora erythrocephala we have identified a cloned highly repetitious DNA fragment which shows enhanced transcriptional activity in these cells and different replicative behavior in these germ-line polyploid nuclei as opposed to somatic polytene nuclei. The clone, 3B55, contains a 6.8-kb insert which consists primarily of tandemly repeated 200-bp sequences defined by RsaI sites. From Southern hybridizations to diploid (embryonic) genomic DNA, 3B55-related DNA was calculated to represent a significant fraction of the haploid genome (0.8% or 5000 kb). In situ hybridizations established that these sequences are present in the pericentric regions of four of the six chromosomes. Thus the 3B55 sequences have the properties of a satellite-type DNA family. Quantitation of the 3B55 200-bp monomer (which represents approximately 60% of the genomic 3B55 DNA sequences in all tissues examined) revealed that in somatic polytene salivary gland nuclei, 3B55 DNA is highly under-replicated to give a monomer representation of only 48 +/- 11 kb per haploid genome. However, in germ-line nurse cell nuclei, 3B55 DNA is proportionately replicated to give a monomer genomic representation (3560 +/- 344 kb) equivalent to that of diploid DNA (3011 +/- 202 kb). Transcripts complementary to 3B55 sequences are at least 25 times more abundant in total nurse cell nuclear RNA than in total embryonic nuclear RNA. These findings suggest an association of the 3B55 sequence family with some germ-line specific function.

Chromosome structure and DNA replication in nurse and follicle cells of Drosophila melanogaster

In the nurse cells of Drosophila, nuclear DNA is replicated many times without nuclear division. Nurse cells differ from salivary gland cells, another type of endoreplicated Drosophila cell, in that banded polytene chromosomes are not seen in large nurse cells. Cytophotometry of Feulgen stained nurse cell nuclei that have also been labeled with 3H-thymidine shows that the DNA contents between S-phases are not doublings of the diploid value. In situ hybridization of cloned probes for 28S + 18S ribosomal RNA, 5S RNA, and histone genes, and for satellite, copia, and telomere sequences shows that satellite and histone sequences replicate only partially during nurse cell growth, while 5S sequences fully replicate. However, during the last nurse cell endoreplication cycle, all sequences including the previously under-replicated satellite sequences replicate fully. In situ hybridization experiments also demonstrate that the loci for the multiple copies of histone and 5S RNA genes are clustered into a small number of sites. In contrast, 28S + 18S rRNA genes are dispersed. We discuss the implications of the observed distribution of sequences within nurse cell nuclei for interphase nuclear organization. In the ovarian follicle cells, which undergo only two or three endoreplication cycles, satellite, histone and ribosomal DNA sequences are also found by in situ hybridization to be underrepresented; satellite sequences may not replicate beyond their level in 2C cells. Hence the pathways of endoreplication in three cell types, salivary gland, nurse, and follicle cells, share basic features of DNA replication, and differ primarily in the extent of association of the duplicated chromatids.

Allocyclic behaviour and underreplication of the nucleolus chromosome in Pseudodiamesa (Chironomidae)

The chironomid midge Pseudodiamesa branickii shows three polytene, banded elements in salivary gland and Malpighian tubule cells, none of which carry a nucleolar organizer. The present investigation revealed four chromosome pairs in mitotic metaphases, the nucleolus was localized in the tiny chromosome 4. Microphotometric determinations of the DNA contents in mitotic and salivary gland nuclei of Ps. branickii yielded a 2C value of 0.23 pg DNA and a maximum level of 12 endoreplications. In both salivary glands and Malpighian tubules, chromosome 4 appeared as a nonbanded network of chromatin in Feulgen preparations. Indirect immunofluorescent staining with antibodies specific for RNA X DNA hybrids indicated that this structure was transcriptionally active. Its reaction to heavy metal staining suggested that it included the nucleolar organizer, and this was conclusively demonstrated by in situ hybridization with 125I-rRNA. Chromosome pair 4 comprises about 11.4% of the total DNA in metaphase, but only 3.7% in the highly polytene salivary gland complement. According to a numerical simulation, the degree of underrepresentation suggests that about 70% of chromosome 4 is precluded from polytenization. The allocyclic aspect is probably due to this underreplication, with the nucleolar activity causing a structural loosening of the rest of the chromosome. The possible nature of the underreplicated sequences is discussed in the light of recent work on molecular aspects of rDNA structure in various Diptera.

Cytophotometric DNA determinations and autoradiographic studies in salivary gland nuclei from larvae with different karyotypes in Drosophila melanogaster

Cytophotometric DNA determinations in Feulgen stained mitotic diploid chromosome sets of neuroblasts from larvae of Drosophila melanogaster stocks, which possess different karyotypes, show significant differences between the 4C values, caused by an additional or deficient X- and Y-chromosome depending on the karyotype. The ranges of polytenic DNA size classes are theoretically expected to be doublings of the corresponding 4C mean value of each karyotype. The extinction integral data of nuclei with completely duplicated 4C quantities exclusively fall into the range of the expected size classes. Not all data falling into the range of a size class necessarily originate from duplicated nuclei, because the limits of the DNA size classes cannot be determined by measurements, but must be estimated from the confidence limits of the corresponding 4C mean value. The validity of the mitotic 4C values of the karyotypes X/X and X/Y is tested using data from non-labeled interphase nuclei, where extinction integral data accumulate in two groups. The larger values (= G2-nuclei) confirm the 4C values of mitotic chromosome sets, and the lower values (= G1-nuclei) are just half of these. Extinction integrals from individual, 3H-thymidine non-incorporating polytene salivary gland nuclei accumulate in distinct, non-overlapping groups which are always complete doublings of the preceding smaller group. In each karyotype, the most frequent data of each group are in accord with the 4C doublings. The data from labeled nuclei alternate with those from unlabeled nuclei. The measured DNA values of individual polytene nuclei that did not incorporate any 3H-thymidine, demonstrate that all chromosomal DNA replicates completely during polytenization of the chromosomes in the larval salivary gland nuclei of Drosophila melanogaster. Specifically, this would mean that the heterochromatic Y-chromosome replicates as well as the partially heterochromatic X-chromosome along with the autosomes. There is no indication of underreplicating heterochromatin.

Sex-linked difference in DNA content of a polytene chromosome in Prodiamesa (Chironomidae)

The chironomid Prodiamesa olivacea possesses 3 pairs of chromosomes. The underreplication of the right arm in the polytene 3rd was presented earlier and is here shown to affect in females the cell nuclei of salivary glands. Malpighian tubules and the hindgut. The same somatic nucleic in males exhibit also a polytene 3rd with specific characteristics inasmuch as it contains half the relative DNA value of the female nuclei (3% instead of 6% complemental portion). - In male meiocytes the two partners in each of the 3 diakinetic bivalents were recognized to be homomorphic. In view of this morphological criterium the polytene situation in males must be due either to an unknown elimination process or to the underreplication of both right arms and further one left arm of the 3rd chromosomes. The latter model interprets the elements of the 3rd pair as differently polytenizing heterosomes in P. olivacea.

Satellite DNA of Anopheles stephensi Liston (Diptera: Culicidae). Chromosomal location and under-replication in polytene nuclei

Four satellite DNAs in the Anopheles stephensi genome have been defined on the basis of their banding properties in Hoechst 33258-CsCl density gradients. Two of these satellites, satellites I and II, are visible on neutral CsCl density gradients as a light density peak forming approximately 15% of total cellular DNA. Hoechst-CsCl density gradient profiles of DNA extracted from polytene tissues indicates that these satellites are underreplicated in larval salivary gland cells and adult female Malpighian tubules and possibly also in ovarian nurse cells. The chromosomal location of satellite I on mitotic and polytene chromosomes has been determined by in situ hybridisation. Sequences complementary to satellite I are present in approximately equal amounts on a heterochromatic arm of the X and Y chromosomes and are also present, in smaller amounts, at the centromere of chromosome 3. A quantitative analysis of the in situ hybridisation experiments indicates that sequences complementary to satellite I at these two sites differ in their replicative behaviour during polytenisation: heterosomal satellite I sequences are under-replicated relative to chromosome 3 sequences in polytene larval salivary gland and ovarian nurse cell nuclei.

Two AT-rich satellite DNAs in the chironomid Glyptotendipes barbipes (Staeger): isolation and localization in polytene chromosomes of G. barbipes and Chironomus thummi

Two AT-rich satellite DNAs are present in the genome of Glyptotendipes barbipes. The two satellites have densities of 1,680 g/cm3 (=21% GC) and of 1.673 g/cm3 (=13% GC) in neutral CsCl-density gradients. The main band DNA has a density of 1.691 g/cm# (= 32% GC). This value is in agreement with the 33% GC=content of G. barbipes DNA calculated from thermal denaturation (TM=83 degrees C). - In brain DNA as well as in salivary gland DNA the two satellite sequences together comprise 12-15% of the total G. barbipes DNA. Comparisons of the density profiles of DNA extracted from polytene and non-polytene larval tissue gave no hints for under-replication of the satellite DNAs during polytenization. - The two satellite DNAs have been isolated from total DNA by Hoechst 33258-CsCl density centrifugation and then localized in the polytene salivary gland chromosomes by in situ hybridization. Both satellite sequences hybridize to all heterochromatic centromer bands of all four chromosomes of G. barbipes. Satellite I (1.673 g/cm3) hybridizes mainly with the middle of the heterochromatin, satellite II (1.680 g/cm3) hybridizes with two bands at the margin of the heterochromatin. In situ hybridization with polytene chromosomes of Chironomus thummi revealed the presence of G. barbipes satellite sequences also in the Ch. thummi genome at varios locations, mainly the centromere regions.