Archiv für Entwicklungsmechanik der Organismen

Die Melanisierung der Kohlweißlingspuppen variiert in Abhängigkeit von den Lichtbedingungen. Die Steuerung arbeitet offensichtlich im Sinne einer abgestuften Hemmung; denn durch eine Ligatur isolierte Körperstücke werden stark melanisiert. Bei den hellen Puppen sind die Melaninflecken nicht blasser als bei den dunklen, sondern kleiner. Demnach besteht ein Gefälle der Hemmbarkeit vom Fleckenrand zum Zentrum. Die Flecken unterscheiden sich auch untereinander in dem Ausmaß der Hemmung. Die Flecken entstehen während der ersten 6 Std nach der Puppenhäutung in einer bestimmten Reihenfolge. Diejenigen, welche als erste und als letzte erscheinen, reagieren am empfindlichsten auf die Hemmung. Auf hellen Tieren sind sie völlig unterdrückt. Eine Klassifizierung des Melanisierungsgrades aufgrund leicht erkennbarer Formunterschiede der abdominalen Mittelflecken läßt sich auch bei operierten Tieren und isolierten Körperstücken anwenden. Die gleiche starke Melanisierung wie durch eine Ligatur wird auch von einer Durchtrennung der Schlundkonnektive hervorgerufen. Etwas schwächer wirkt die Durchtrennung des Bauchmarks hinter dem Unterschlundganglion. Die melanisierungsfördernde Wirkung von Ligatur und Nervendurchschneidung erlischt in einer kritischen Periode, welche 12 Stunden nach dem Spinnen des Gürtelfadens abgeschlossen ist. Es wird angenommen, daß während dieser kritischen Periode ein melanisierungshemmender Faktor von einem thorakalen Zentrum abgegeben wird, welches dazu seinerseits über die Nervenverbindungen vom Gehirn her veranlaßt wird.

Antibodies were used to examine the expression patterns of Antennapedia (Antp), Ultrabithorax (Ubx), Ubx and abdominal-A combined(Ubx/abd-A),and Distalless (Dll) in the embryos of the moth Manduca sexta. We found that the spatial and temporal pattern of Antp expression in Manduca was correlated with the anterior migration of two patches of epithelium that include the anterior-most tracheal pits, and with the development of functional spiracles. Ubx expression showed an intricate pattern which suggests complex regulation during development. Throughout Manduca embryogenesis the expression of Ubx/Abd-A and Dll was similar to that reported for other insects. However, there was no apparent reduction in Ubx/Abd-A expression in the Manduca abdominal proleg primordia that expressed Dll. The expression of these four proteins was also examined in embryosof the Manduca homozygous homeotic mutant Octopod (Octo). The Octo mutation results in the transformation of A1 and A2 in the anterior direction, with homeotic legs appearing on A1 and occasionally A2. Our results suggest that in Octo animals there is a reduction in the level of Ubx protein expression throughout its domain. Based on homeotic gene expression in wild-type and mutant Manduca and in other insects, we discuss potential roles of homeotic genes in insect morphological evolution.

The significance of the first quartet of micromeres for the morphogenesis ofBithynia — a polar lobe-forming gastropod-has been studied by deletion experiments. After removal of the whole first quartet at the 8-cell stage a dorsoventrally organized veliger larva is formed. Apparently, an interaction between the animal micromeres and a vegetal macromere, which is essential for the origin of a dorsoventral organization in equally cleaving gastropods, is not required in polar lobe forming eggs. It is concluded that in these eggs dorsoventrality is determined by segregation of the polar lobe. The embryos, in which the first quartet has been removed, never develop head structures. This indicates that the capacity to form head structures is restricted to the first quartet of micromeres. Deletion of a specific first quartet micromere (1a, 1b, 1c or 1d) showed, however, that the individual cells of this quartet are not strictly determined right from their origin. Frequently regulative development was observed after removal of individual first quartet cells.

The origin, diversification, and secondary loss of sexually dimorphic characters are common in animal evolution. In some cases, structurally and functionally similar traits have evolved independently in multiple lineages. Prominent examples of such traits include the male-specific grasping structures that develop on the front legs of many dipteran insects. In this report, we describe the evolution and development of one of these structures, the male-specific “sex brush.” The sex brush is composed of densely packed, irregularly arranged modified bristles and is found in several distantly related lineages in the family Drosophilidae. Phylogenetic analysis using 250 genes from over 200 species provides modest support for a single origin of the sex brush followed by many secondary losses; however, independent origins of the sex brush cannot be ruled out completely. We show that sex brushes develop in very similar ways in all brush-bearing lineages. The dense packing of brush hairs is explained by the specification of bristle precursor cells at a near-maximum density permitted by the lateral inhibition mechanism, as well as by the reduced size of the surrounding epithelial cells. In contrast to the female and the ancestral male condition, where bristles are arranged in stereotypical, precisely spaced rows, cell migration does not contribute appreciably to the formation of the sex brush. The complex phylogenetic history of the sex brush can make it a valuable model for investigating coevolution of sex-specific morphology and mating behavior.