Developmental genetics of the Drosophila gut: specification of primordia, subdivision and overt-differentiation.

The Drosophila gut is composed of three major parts, the foregut, midgut and hindgut, which arise from anterior and posterior invaginations of the early blastoderm. We review the process of the specification of the gut primordia, subsequent subdivision and region-specific cell differentiation in terms of developmental genetics. Graded activities of maternal signals at anterior and posterior terminal domains of the blastoderm, being mediated by activities of two zygotic gap genes, tailless and huckebein, lead to the activation of key genes that determine the gut primordia: serpent (GATA factor gene) for the endodermal midgut; brachyenteron (Brachyury homolog) for the ectodermal hindgut. fork head (HNF-3 homolog) and caudal (Cdx homolog) are also essential for the development of all gut primordia or hindgut primordium, respectively. Subdivision of the midgut epithelium is regulated by inductive signals emanating from the visceral mesoderm, which is under the control of HOM-C genes. In contrast, pattern formation of the ectodermal foregut and hindgut is regulated by secreted signaling molecules, such as Wingless (Wnt homolog), Hedgehog and Decapentaplegic (Bmp-4 homolog), as in the case of segmented structures and imaginal discs. Finally, the gut is subdivided into at least 36 compartments that are recognized asminimum tissue units of regional differentiation. A few genes that are responsible for determining and maintaining the state of overt-differentiation of the compartments have also been reported. A marked feature of the genetic mechanism of the gut development is the unexpectedly wide spectrum of the similarities of relevant genes and regulatory pathways of gene expression between Drosophila and vertebrates, which may imply a prototypic style of body plan common to protostomes and deuterostomes.     

A Role of Polycomb Group Genes in the Regulation of Gap Gene Expression in Drosophila

Anteroposterior polarity of the Drosophila embryo is initiated by the localized activities of the maternal genes, bicoid and nanos, which establish a gradient of the hunchback (hb) morphogen. nanos determines the distribution of the maternal Hb protein by regulating its translation. To identify further components of this pathway we isolated suppressors of nanos. In the absence of nanos high levels of Hb protein repress the abdomen-specific genes knirps and giant. In suppressor-of-nanos mutants, knirps and giant are expressed in spite of high Hb levels. The suppressors are alleles of Enhancer of zeste (E(z)) a member of the Polycomb group (Pc-G) of genes. We show that E(z), and likely other Pc-G genes, are required for maintaining the expression domains of knirps and giant initiated by the maternal Hb protein gradient. We have identified a small region of the knirps promoter that mediates the regulation by E(z) and hb. Because Pc-G genes are thought to control gene expression by regulating chromatin, we propose that imprinting at the chromatin level underlies the determination of anteroposterior polarity in the early embryo.     

Three hormone receptor-like Drosophila genes encode an identical DNA-binding finger.

The putative finger domain of knirps (kni), a member of the gap class of segmentation genes, was used to isolate two sequence-related genes of Drosophila melanogaster under reduced stringency hybridization conditions. The two kni homologous genes map close to kni in the proximal portion of the third chromosome. One of them is the previously identified gene knirps-related (knrl), kni and knrl are spatially co-regulated in both early and late stages of embryogenesis. Their posterior domains of expression at blastoderm stage are under the control of the maternal pattern organizer gene nanos. In contrast, the expression of the second kni homologous gene is restricted to the late embryonic gonads. Due to its site of expression, we termed this gene 'embryonic gonad' (egon). In addition to the conserved DNA-binding domain, these three genes share an additional sequence of 19 amino acids, the kni-box, adjacent to the finger region. The identical N-terminal Cys/Cys finger encoded by each of the three genes suggests that they code for DNA-binding proteins which might bind to similar (or even identical) target sequences.     

扩头蔡白蚁肠道蛋白的鉴定

【目的】本研究旨在分析比较扩头蔡白蚁Tsaitermes ampliceps工蚁前中肠和后肠及其内容物的蛋白构成和表达差异,挖掘降解木质纤维素的相关酶和蛋白。【方法】通过扩头蔡白蚁工蚁的前中肠和后肠及其内容物蛋白的双向电泳,对高表达或高差异表达的47个蛋白点进行MALDI-TOF/MS测序,并进行生物信息学分析。【结果】测序分析发现,扩头蔡白蚁肠道及其内容物蛋白中有结构蛋白13个、调节蛋白9个、白蚁代谢相关蛋白10个、微生物代谢相关蛋白7个。经PD Quest分析发现,在前中肠和后肠有11个蛋白均高表达;仅在前中肠表达的蛋白有12个,主要是白蚁代谢相关蛋白和调节蛋白;仅在后肠表达的蛋白有8个,主要是微生物代谢相关蛋白。整个肠道内参与木质纤维素降解的相关酶有5个,分别是白蚁自身分泌的内源性纤维素酶,细菌产生的内切-β-1,4-葡聚糖酶和过氧化物歧化酶以及原生动物产生的GH11。【结论】白蚁对木质纤维素食物的降解主要在前中肠,后肠对降解产物进一步降解并进行微生物生长代谢。这些降解产物和微生物菌体蛋白为白蚁的肛哺提供营养成分。     

Conservation of epigenetic regulation, ORC binding and developmental timing of DNA replication origins in the genus Drosophila

There is much interest in how DNA replication origins are regulated so that the genome is completely duplicated each cell division cycle and in how the division of cells is spatially and temporally integrated with development. In the Drosophila melanogaster ovary, the cell cycle of somatic follicle cells is modified at precise times in oogenesis. Follicle cells first proliferate via a canonical mitotic division cycle and then enter an endocycle, resulting in their polyploidization. They subsequently enter a specialized amplification phase during which only a few, select origins repeatedly initiate DNA replication, resulting in gene copy number increases at several loci important for eggshell synthesis. Here we investigate the importance of these modified cell cycles for oogenesis by determining whether they have been conserved in evolution. We find that their developmental timing has been strictly conserved among Drosophila species that have been separate for approximately 40 million years of evolution and provide evidence that additional gene loci may be amplified in some species. Further, we find that the acetylation of nucleosomes and Orc2 protein binding at active amplification origins is conserved. Conservation of DNA subsequences within amplification origins from the 12 recently sequenced Drosophila species genomes implicates members of a Myb protein complex in recruiting acetylases to the origin. Our findings suggest that conserved developmental mechanisms integrate egg chamber morphogenesis with cell cycle modifications and the epigenetic regulation of origins.     

Ultrastructure of the digestive tract in Acarus siro (Acari: Acaridida)

The gut of the mite Acarus siro is characterized on the ultrastructural level. It consists of the foregut (pharynx, esophagus), midgut (ventriculus, caeca, colon, intercolon, postcolonic diverticula, postcolon), and hindgut (anal atrium). The gut wall is formed by a single-layered epithelium; only regenerative cells are located basally and these have no contact with the lumen. Eight cell types form the whole gut: (i) simple epithelial cells forming fore- and hindgut; (ii) cells that probably produce the peritrophic membrane; (iii) regenerative cells occurring in the ventriculus, caeca, colon, and intercolon; (iv) spherite cells and (v) digestive cells forming the ventriculus and caeca; (vi) colonic cells and (vii) intercolonic cells; and (viii) cells forming the walls of postcolonic diverticula and postcolon. Spherite and digestive cells change in structure during secretory cycles, which are described and discussed. The cycle of spherite, colonic, and intercolonic cells is terminated by apoptosis. Ingested food is packed into a food bolus surrounded by a single homogeneous peritrophic membrane formed by addition of lamellae that subsequently fuse together. The postcolonic diverticula serve as a shelter for filamentous bacteria, which also are abundant in the intercolon.     

Spatial expression of Drosophila glutathione S-transferase-D1 in the alimentary canal is regulated by the overlying visceral mesoderm

Regional gene expression within Drosophila gut epithelium is regulated by the homeotic genes expressed in the overlying visceral mesoderm. Here it is reported that Glutathione S-transferase-D1 (Gst-D1) had three distinctive expression domains in the gut epithelia: the inner epithelium of the proventriculus, the anterior border of the hindgut epithelium, and the midgut epithelium. Gst-D1 expression in the midgut epithelium became restricted to the region that later formed the third midgut constriction. This spatial restriction within the midgut epithelium required abdominal-A activity in the overlying visceral mesoderm, suggesting that Gst-D1 will be a useful marker to analyze the mechanism of gene regulation across the mesoderm and endoderm.     

Ultrastructure of the hindgut of Drosophila larvae, with special reference to the domains identified by specific gene expression patterns

The hindgut of Drosophila larvae consists of nine domains that have been distinguished by specific gene expression patterns. In the present study, we examined the ultrastructure of the hindgut of Drosophila larvae, with special reference to the domains, in order to determine whether or not the domains are morphologically distinct functional units. Each domain showed specific ultrastructural features that suggested specific corresponding functions. According to the morphological features, terms are proposed for each domain: the imaginal ring; the "pylorus," which has a thick cuticular layer and well-developed sphincter muscles; the "large intestine," which occupies a major middle portion of the hindgut and has a unique dorsal and ventral subdivision; "border cells," which delineate the anterior and posterior borders of the large intestine and the border between the dorsal and ventral domains of the large intestine; and the "rectum," which is situated at the posterior end of the hindgut and has a thick cuticular layer and sphincter muscles. The morphological features indicate that the large intestine has active absorptive activities. The domains, which have been distinguished by gene expressions, were demonstrated to be functional tissue units of the gut.     

Drosophila minichromosome maintenance 6 is required for chorion gene amplification and genomic replication

Duplication of the eukaryotic genome initiates from multiple origins of DNA replication whose activity is coordinated with the cell cycle. We have been studying the origins of DNA replication that control amplification of eggshell (chorion) genes during Drosophila oogenesis. Mutation of genes required for amplification results in a thin eggshell phenotype, allowing a genetic dissection of origin regulation. Herein, we show that one mutation corresponds to a subunit of the minichromosome maintenance (MCM) complex of proteins, MCM6. The binding of the MCM complex to origins in G1 as part of a prereplicative complex is critical for the cell cycle regulation of origin licensing. We find that MCM6 associates with other MCM subunits during amplification. These results suggest that chorion origins are bound by an amplification complex that contains MCM proteins and therefore resembles the prereplicative complex. Lethal alleles of MCM6 reveal it is essential for mitotic cycles and endocycles, and suggest that its function is mediated by ATP. We discuss the implications of these findings for the role of MCMs in the coordination of DNA replication during the cell cycle.