Klaus Sander

Segmentation gene expression in the mothmidge Clogmia albipunctata (Diptera, psychodidae) and other primitive dipterans.

Abstract To obtain a clearer understanding of the evolutionary transition between short- and long-germ modes of embryogenesis in insects, we studied the expression of two gap genes hunchback (hb) and Krüppel (Kr) as well as the pair-rule gene even-skipped (eve) in the dipteran Clogmia albipunctata (Nematocera, Psychodidae). This species has features of both short- and long-germ mode of embryogenesis. In Clogmiahb expression deviates from that known in Drosophila in two main respects: (1) it shows an extended dorsal domain that is linked to the large serosa anlage, and (2) it shows a terminal expression in the proctodeal region. These expression patterns are reminiscent of the hb expression pattern in the beetle Tribolium, which has a short germ mode of embryogenesis. Krüppel expression, on the other hand, was found to be rather similar to the Drosophila expression, both at early and late stages. eve expression starts with six stripes formed at blastoderm stage, while the seventh is only formed after the onset of gastrulation and germband extension. Surprisingly, no segmental secondary Eve stripes could be observed in Clogmia although such segmental stripes are known from higher dipterans, beetles and hymenopterans. We therefore also studied another nematoceran, Coboldia, to address this question and found that some segmental stripes form by intercalation as in Drosophila, although belatedly. Our results suggest that Clogmia embryogenesis, both with respect to morphological and molecular characteristics represents an intermediate between the long-germ mode known from higher dipterans such as Drosophila, and the short-germ mode found in more ancestral insects.

Localized ultraviolet laser microbeam irradiation of early Drosophila embryos: Fate maps based on location and frequency of adult defects

Drosophila embryos were locally irradiated with a 257-nm laser microbeam during blastoderm and germ band stages. Depending on stage and beam diameter (10–30 μm), from 0 to 45 nuclei were exposed to the uv radiation. The doses used, 5 or 10 erg, did not eliminate nuclei or cells at once, but up to 50% of the adult survivors from irradiated eggs carried defects in the thorax. These were scored with reference to the imaginal discs from which the affected structures derive. For each thoracic disc a “target center” was calculated as the weighted mean value of all beam locations affecting the respective adult derivatives. The target centers for the germ band stage map within the respective germ band segments. The pattern of target centers for the blastoderm stage is comparable to the thoracic region of published fate maps, and the distances between adjacent leg centers (approximately three cell diameters) agree with recent evidence based on mosaic flies. We discuss the question whether the target centers mark the position of the respective disc progenitor cells at the stages of irradiation and conclude that these positions are rendered rather correctly at least with reference to the longitudinal egg axis.

The Cytoplasmic Architecture of the Insect Egg Cell

Investigations on pterygote insect oogenesis have yielded a multitude of data on the architecture and development of early and late oocytes (for reviews see King, R. C., 1970; Telfer and Smith, 1970; Mahowald, 1972; Anderson, 1974; Telfer, 1975; see also Miya et al., 1969; Truckenbrodt, 1970; Buning, 1972). Oogenesis has also been studied in some Apterygota (Cone and Scalzi, 1967; Cantacuzene and Martoja, 1972; Palevody, 1972, 1973; Matsuzaki, 1973; Bilinsky, 1976, 1977, 1979; Klag, 1977, 1978). Yet the end product of oogenesis was usually exempted from investigation; the newly laid insect egg thus remains largely unknown with respect to ultrastructure. This may mainly be due to technical reasons: the prevalence of yolk and the extremely resistant egg covers of insect eggs provide considerable handicaps for fixation and sectioning. Descriptions of the egg shell are therefore much more frequent than publications dealing with the egg cell.

Pair-rule patterning in the honeybee Apis mellifera : Expression of even-skipped combines traits known from beetles and fruitfly

Abstract We have studied the binding pattern of antibody mAB 2B8 directed against even-skipped orthologous proteins (EVE) in honeybee embryos. Primary and secondary EVE stripes form in roughly anterior-to-posterior succession; there are 8 primary and 16 secondary stripes. The most posterior primary stripes appear only after the onset of gastrulation. The secondary stripes form by a splitting of primary stripes; they demarcate the parasegmental pattern. While these findings resemble EVE expression in long-germ beetles, the honeybee differs from both beetles and dipterans by two transient pair-rule traits in the parasegmental EVE pattern: the secondary stripes in head and thorax alternate in strength, yet out of register with the Drosophila pattern, and over the whole pattern the odd-numbered stripes vanish earlier than their even-numbered counterparts. As in Drosophila, however, the strong EVE stripes coincide with the weak engrailed (EN) stripes. These findings are taken to indicate that (1) honeybee and beetles share a conserved mode of EVE stripe formation whilst Drosophila has diverged in this respect, (2) honeybee and Drosophila have diverged from the beetles in specific pair-rule traits during the parasegmental expression of both EVE and EN, and (3) some of these traits differ in the register of segment pairing and thus may reflect regulatory divergences at the pair-rule level between dipterans and the honeybee.

The spatial arrangement of germ line cells in ovarian follicles of the mutantdicephalic inDrosophila melanogaster

SummaryThe pattern of intercellular connections between germ line cells has been studied in follicles of the mutantdicephalic (dic), which possess nurse cell clusters at both poles. Staining of follicles with a fluorescent rhodamine conjugate of phalloidin reveals ring canals and cell membranes and thus allows us to reconstruct the spatial organization of the follicle. Each germ line cell can be identified by the pattern of cell-cell connections which reflect the mitotic history of individual cells in the 16-cell cluster. The results indicate that in both wild-type anddicephalic cystocyte clusters one of the two cells with four ring canals normally becomes the pro-oocyte. However, in some follicles (dicephalic and wild-type) oocytes were found with fewer or more than four ring canals. Indic follicles, one or several nurse cells may become disconnected from the other cells during oocyte growth at stage 9–10. Such disconnected cells cannot later on empty their cytoplasm into the oocyte. This, in turn, might be of consequence for the determination of axial polarity of the embryo.

Entwicklungsphysiologische Untersuchungen am embryonalen Mycetom von Euscelis plebejus F. (Homoptera, Cicadina): I. Ausschaltung und abnorme Kombination einzelner Komponenten des symbiontischen Systems

Zusammenfassung 1. 1. Das Ei der Kleinzikade Euscelis plebejus enthalt im Hinterpol eine Symbiontenmasse aus Bakterien der Typen a und t. Die a-Symbionten gelangen bei der Einrollung des Keimstreifs in primare a-Mycetocyten (a1-Mycetocyten). Diese haben nur transitorische Funktion und werden vor der Ausrollung des Keimstreifs durch die sekundaren a-Mycetocyten (a2-Mycetocyten) ersetzt. Die t-Symbionten werden kurz vorher von t-Mycetocyten aufgenommen, nachdem sie bis dahin als extrazellulare Masse zwischen den a1-Mycetocyten gelegen hatten. 2. 2. Das Ei kann durch Schnurung in 2 Teile von verschiedener Grosse zerlegt werden. Die Symbiotenmasse lasst sich innerhalb der hinteren 2 3 des Eies beliebig verlagern. Dadurch ist die Moglichkeit zur Ausschaltung der Symbionten oder des embryonalen Gewebes gegeben. Ausserdem lassen sich die Symbionten mit verschiedenen Keimstreifregionen atypisch kombinieren. 3. 3. In Eiteilen ohne Symbiontenmasse entstehen die a1-Mycetocyten nur bei Anwesenheit der hinteren Abdominalsegmente, und auch dann nicht immer. Die t- und a2-Mycetocyten werden nach Ausschaltung der Symbionten ebenso wie im Normalfall im Mesoderm bestimmter Abdominalsegmente gebildet. Bei entsprechendem Entwicklungszustand dieser Segmente sind sie immer zu finden. 4. 4. Verlagerte Symbionten konnen in ihrer neuen Lage weder t- noch a2-Mycetocyten induzieren. Dagegen erscheint eine Umstimmung von Zellen anderer prospektiver Bedeutung zu a1-Mycetocyten durch die Symbionten moglich. 5. 5. Im Dottersystem isolierte Symbiontenballe zerfallen trotz Besiedelung durch a1-mycetocyten in der Regel spatestens vom 8. Entwicklungstag an. Der Zerfall beginnt auch dann, wenn der Keimstreif im gleichen Eiteil noch nicht das Entwicklungsstadium erreicht hat, auf dem normalerweise die a1-Mycetocyten degenerieren. Kontakt des Symbiontenballs mit einer beliebigen Keimstreifregion kann den Zerfall hinauszogern oder verhindern; die a1-Mycetocyten bleiben in dieser Lage bisweilen sogar uber das normale Zerfallsstadium hinaus intakt.

A dicephalic monster embryo ofDrosophila melanogaster

SummaryA monster embryo with a head on each end developed from an abnormalDrosophila egg produced by a M(1)oSp/Binsn female. The embryo probably represents a longitudinal mirror duplication of head and all 3 thoracic segments. The significance of this type of pattern anomaly for embryonic pattern specification inDrosophila is discussed.

Switch in pattern formation after puncturing the anterior pole of Smittia eggs (Chironomidae, Diptera)☆

Abstract The aberrant pattern, “double abdomen,” previously induced in the egg of Smittia by uv irradiation of anterior pole regions was also produced by puncturing of the egg at the anterior pole. Double abdomens and embryos with anterior defects developed in eggs in which puncturing had locally prevented the regular arrangement of cleavage nuclei in the periplasm. The resulting gap in the blastoderm at the anterior pole was subsequently closed under exclusion of a small amount of egg material. Double abdomens did not develop in eggs where exclusion of anterior egg material was not observed. Thus a basic switch in the developmental program of the egg appears to depend upon the functional elimination of some crucial components in the anterior egg region.

Inverse correlation between mean nuclear DNA content and cell number in nurse cell clusters of Drosophila

In the Drosophila ovarian follicle, 15 polyploid nurse cells (NC) are linked by cytoplasmic bridges to the anterior pole of the oocyte. The mutant dicephalic occasionally produces aberrant follicles whose 15 nurse cells are separated into two clusters, each attached to one oocyte pole [split nurse cell (SNC) follicles]. Feulgen cytophotometry of isolated nuclei from such clusters and from normal follicles of the same females revealed an inverse correlation between NC number per cluster and mean DNA content per nucleus. The maximum DNA content of nuclei from split NC clusters falls into the 4,096 C class as opposed to the 2,048 C class in 15 NC clusters; the mean DNA content per NC nucleus is 1,940 C in the former and 1,340 C in the latter. These correlations are confirmed by measurements on follicles containing fewer or more than 15 NC, and on SNC follicles from another mutant (spindel C). These findings indicate that replication in NC nuclei is controlled with reference to the transcriptional capacity of the individual NC cluster rather than the total capacity available per oocyte or follicle. We discuss this finding with respect to embryonic patterning as influenced by nurse cell location and list and discuss conflicting published reports on over- and underreplication in Drosophila follicles.

Of gradients and genes: Developmental concepts of Theodor Boveri and his students

Gradients, for decades a “dirty word” in developmental biology (Lawrence 1992), have become respectable of late and are likely to stay with us after their role in pattern formation has been demonstrated at the level of gene regulation (Driever and Nusslein-Volhard 1988). The present essay is to illuminate the often misunderstood “relativistic” gradient concepts of Theodor Boveri (who actually introduced the term Gefalle = gradient), and their influences on his students. Boveri also paved the way, by his studies on chromosomes and development (see Essays 15 and 16), for a chromosomal location of the Mendelian hereditary factors (since 1909 called genes). His most influential students reacted quite differently to his insights on both gradients and genes. Hans Spemann (1869–1941) was remarkably reluctant in linking either con-. cept with his organizing center (see Sander 1986; Wolpert 1986). Waldemar Schleip (1879–1948), however, extended Boveri’s idea of interplay between cytoplasmic factors and genes in development, yet maintained a specific “Boverian” definition of gradient. Leopold von Ubisch (1885–1965) finally achieved a synthesis linking the emerging modern concept of morphogenetic gradients with differential gene activity in order to generate spatial patterns.