Tissue-specific schedule of selective replication in Drosophila nasutoides

Hawaiian Drosophila genomes: size variation and evolutionary expansions

This paper reports genome sizes of one Hawaiian Scaptomyza and 16 endemic Hawaiian Drosophila species that include five members of the antopocerus species group, one member of the modified mouthpart group, and ten members of the picture wing clade. Genome size expansions have occurred independently multiple times among Hawaiian Drosophila lineages, and have resulted in an over 2.3-fold range of genome sizes among species, with the largest observed in Drosophila cyrtoloma (1C = 0.41 pg). We find evidence that these repeated genome size expansions were likely driven by the addition of significant amounts of heterochromatin and satellite DNA. For example, our data reveal that the addition of seven heterochromatic chromosome arms to the ancestral haploid karyotype, and a remarkable proportion of ~70 % satellite DNA, account for the greatly expanded size of the D. cyrtoloma genome. Moreover, the genomes of 13/17 Hawaiian picture wing species are composed of substantial proportions (22-70 %) of detectable satellites (all but one of which are AT-rich). Our results suggest that in this tightly knit group of recently evolved species, genomes have expanded, in large part, via evolutionary amplifications of satellite DNA sequences in centric and pericentric domains (especially of the X and dot chromosomes), which have resulted in longer acrocentric chromosomes or metacentrics with an added heterochromatic chromosome arm. We discuss possible evolutionary mechanisms that may have shaped these patterns, including rapid fixation of novel expanded genomes during founder-effect speciation.

Organ growth without cell division: somatic polyploidy in a moth, Ephestia kuehniella

Organ growth depends on cell division and (or) cell growth. Here, we present a study on two organs whose growth depends entirely on cell growth, once they are formed in the embryo: Malpighian tubules and silk glands of the flour moth, Ephestia kuehniella . Between first and last larval instar, the volume of Malpighian tubule cells increases by a factor of ∼1800 and that of silk gland cells by a factor of ∼3100. We determined the number of endocyles required to reach these stages by Feulgen cytometry. Cells of Malpighian tubules were in the 2C stage in first instar larvae and reached 1024C after 9 endocycles in last instar larvae (1C = 0.45 pg DNA). Silk gland cells already reached a DNA content of 8C-16C in first instar larvae and attained up to 8192C in last instar larvae after a total of 12 endocycles. The nuclei were small and more or less spherical in first instar larvae, but they were huge, flat, and bizarrely branched in last instar larvae. We consider branching as a compensatory adaptation to improve molecular traffic between nucleus and cytoplasm in these excessively large and highly polyploid cells (i) by reducing the mean distance between nucleus and cytoplasm and (ii) by enlarging the surface-to-volume ratio of these nuclei.

Endoreplication of zygotic nuclei in the larger foraminifer Heterostegina depressa (Nummulitidae)

A single gamont of Heterostegina depressa released cells thought to be gametes. These sexual products were Feulgen stained and their nuclei were scanned with a microphotometer. Their mean absorption values (DNA contents) fell into 4 distinct classes: 0.71 AU (absorption units), 1.34 AU, 2.89 AU and 5.73 AU. The consistant increase by a factor of 2 indicated a definite nuclear status in each class. We propose that the respective DNA-contents belong to haploid isogametes, to zygotes immediately after fertilization, to zygotes after one replication, and to individuals after a second endocycle. The nuclei in agamonts rapidly became polyploid. This endoreplication is in contrast to cell cycles of other foraminifera, in which zygotic nuclei undergo metagamic divisions resulting in multinuclear agamonts. A 2 C-value of 0.19 pg DNA was obtained for H. depressa. The general morphology of the gametes resembles that of biflagellate gametes of other foraminifera. The gametes were 2.7 um long, contained a 1.5 μm-diameter nucleus, conspicuous lipid droplets and a vacuole of undetermined content. The two flagella were acronematic.

Analysis of Drosophila species genome size and satellite DNA content reveals significant differences among strains as well as between species

The size of eukaryotic genomes can vary by several orders of magnitude, yet genome size does not correlate with the number of genes nor with the size or complexity of the organism. Although "whole"-genome sequences, such as those now available for 12 Drosophila species, provide information about euchromatic DNA content, they cannot give an accurate estimate of genome sizes that include heterochromatin or repetitive DNA content. Moreover, genome sequences typically represent only one strain or isolate of a single species that does not reflect intraspecies variation. To more accurately estimate whole-genome DNA content and compare these estimates to newly assembled genomes, we used flow cytometry to measure the 2C genome values, relative to Drosophila melanogaster. We estimated genome sizes for the 12 sequenced Drosophila species as well as 91 different strains of 38 species of Drosophilidae. Significant differences in intra- and interspecific 2C genome values exist within the Drosophilidae. Furthermore, by measuring polyploid 16C ovarian follicle cell underreplication we estimated the amount of satellite DNA in each of these species. We found a strong correlation between genome size and amount of satellite underreplication. Addition and loss of heterochromatin satellite repeat elements appear to have made major contributions to the large differences in genome size observed in the Drosophilidae.

Karyotypes, constitutive heterochromatin, and genomic DNA values in the blowfly genera Chrysomya, Lucilia, and Protophormia (Diptera: Calliphoridae)

The karyotypes and C-banding patterns of Chrysomya species C. marginalis, C. phaonis, C. pinguis, C. saffranea, C. megacephala (New Guinean strain), Lucilia sericata, and Protophormia terraenovae are described. All species are amphogenic and have similar chromosome complements (2n = 12), including an XY-XX sex-chromosome pair varying in size and morphology between species. Additionally, the C-banding pattern of the monogenic species Chrysomya albiceps is presented. The DNA contents of these and of further species Chrysomya rufifacies, Chrysomya varipes, and Chrysomya putoria were assessed on mitotic metaphases by Feulgen cytophotometry. The average 2C DNA value of the male genomes ranged from 1.04 pg in C. varipes to 2.31 pg in C. pinguis. The DNA content of metaphase X chromosomes varied from 0.013 pg (= 1.23% of the total genome) in C. varipes to 0.277 pg (12.20%) in L. sericata; that of Y chromosomes ranged from 0.003 pg (0.27%) in C. varipes to 0.104 pg (5.59%) in L. sericata. In most species, the corresponding 5 large chromosome pairs showed similar relative DNA contents. The data suggest that the interspecific DNA differences in most species are mainly due to quantitative variation of (repetitive) sequences lying outside the centromeric heterochromatin blocks of the large chromosomes. The results are also discussed with regard to phylogenetic relationships of some species.

Heteropycnosis of an underreplicating chromosome

Drosophila nasutoides has an extraordinary genome since 62% of its DNA resides in chromosome 4. This element mainly consists of constitutive heterochromatin which does not polytenize. Earlier studies of heterochromatin attributed little attention to the fact that "condensed" chromosomes often vary in condensation. This paper reports that chromosomes of the same complement display different degrees and kinetics of condensation. In D. nasutoides, even sex specific differences can be observed. The results of a comparative microphotometric study on neuroblast metaphases in both sexes revealed the following picture. The process of chromosome condensation is not restricted to mitotic prophase but continues into the metaphase. The mean condensation is not equal for all chromosomes. In the metaphase of the female, Feulgen density increases from the X chromosome, via 3 and 2, to chromosome 4. In the male, the order is X, 2, 3, Y, and 4. During the metaphase of the male, chromosomes condense with similar kinetics. In contrast, chromosomes of the female display asynchrony as monitored by area and length determinations. The X chromosomes of the female probably have enhanced shortening during prophase. This would explain the metaphase of the female where the X chromosomes shorten less than the autosomes, and why each of the X chromosomes is 15% shorter than the X chromosome in the metaphase of the male. Further differences were observed in the longitudinal and lateral compaction of the chromosomes in males and females. The sex chromosomes and chromosome 3 condense by shortening, while chromosome 2 and 4 preferentially reduce their diameter. The large amount of DNA engaged in heteropycnosis and the isochromosome nature allow the identification of chromosome 4 during interphase. At this stage, a new category of extreme DNA packaging was detected. The interphase density of chromosome 4 can exceed that of metaphase by a factor of up to 8. Two events account for this high degree of condensation: (1) the homologues are particularly associated due to somatic pairing and (2) the arms are further tightened as a result of pericentric folding. The features of the isochromosome suggest that the interaction of chromatids during interphase is essentially caused by specific DNA sequences. The data confirm that heteropycnosis not only interferes with gene expression but also strongly inhibits DNA synthesis in endocycles.