Short and Intermediate Germ Band Insects
Drosophila is an example of a long germ band insect. That is to say, the embryonic primordium of the embryo, the germ band (keimanlage) extends through the entirety of the egg. Indeed, in Drosophila, the germ band has to wrap itself around the egg. This long germ band contains the primordia of all the segments that will form in the embryo (and adult). Long germ band insects include the Diptera (flies), Lepidoptera (butterflies and moths), Hymenoptera (bees, wasps, ants), and some Coleoptera (beetles).
In addition, there are those insects with short or intermediate germ bands. In species with short germ bands (such as the short-horned Orthopterans--the grasshoppers), the germ band is a relatively short anterior structure that will eventually form the anterior parts of the head. However, in the caudal portion of that short germ band is a posterior proliferation zone that will bud off new cells to form the additional segments. This posterior proliferation zone remains in the posterior portion of the insect, continually producing cells immediately anterior to it (Anderson, 1973; Figure 1A). This type of segmentation can be seen in Tribolium, a short germ band beetle. In situ hybridization in Drosophila shows that the engrailed mRNA is localized in the posterior cells of each segment. The expression of engrailed mRNA in Drosophila is seen to occur in each segment at the same time. In Tribolium, there is still one stripe of engrailed mRNA per segment, but each segment is added sequentially (Sulston and Anderson, 1996; Figure 1B).
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| Figure 1 Short germ band insects. (A) The posterior proliferation zone of the Petrobius, a wingless thysanuran. The germ band gives rise to the anterior head, while the posterior proliferation zone (dark stipples) continues to divide to provide cells for the posterior head, thorax, and abdominal segments. (B) In situ hybridization of engrailed shows the segmentation of the short germ band beetle, Tribolium. The left shows an early stage when only the gnathal (jaw, G1) segment has been laid down by the posterior zone of proliferation. The center figure shows a later stage where thoracic (T) and abdominal (A) segments have also been made. The right figure is at the end of segmentation, and the brain (br) antennal segment (ant) and intercalary (int) segment are differentiating. (A after Anderson, 1973 and Gerhart and Kirschner, 1997; B after Sulston and Anderson, 1996). |
In intermediate germ band insects such as damsel flies (Odonata) and long-horned Orthoptera (i.e., crickets), germ bands form from two ventrolateral aggregations that cover about 50% of the surface of the egg. After these two rudiments have fused ventrally, they become the primordia of the head and thoracic segments, while a posterior proliferation zone buds off the abdominal segments (Schwalm, 1988; 1997).
In intermediate and short germ band insects, the mesoderm forms by cell invagination and the delamination of newly divided cells along a primitive groove that begins centrally behind the presumptive head. The posterior proliferative zone forms the precursors of ectoderm and mesoderm simultaneously. These embryos develop much more like the other arthropods (spiders, crustaceans) than Drosophila does, and like these other arthropods, the embryo immerses itself into the yolk and then resumes its place on the surface (Figure 2A).
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| Figure 2 Development of the cricket Acheta domesticus, an intermediate germ band insect. (A) Major features of development. (1) The ventral germ band becomes immersed inside the yolk and migrates posteriorly. After establishing segmentation, the embryo revolves around the posterior end of the egg and assumes its final position. (2-4) The head emerges from the yolk and moves anteriorly. The embryo begins to progressively cover the yolk sac as tissues differentiate. (5, 6) The embryo closes its dorsal flanks. The leading edge contains the mesodermal cardioblasts. (7) Frontal view of the cricket embryo during the completion of organogenesis. (B) Schematic diagram of the development of the amniotic cavity in intermediate germ band insects. (1) Lateral folds starting at the edges of the germ band grow over the germ band. (2) The lateral folds meet ventrally beneath the germ band. (3) The membranes fuse, causing the separation of the amnion and serosa, thereby surrounding the embryo with yolk. (After Schwalm, 1997). |
As the embryos thicken, they produce amniotic folds from the surface, extending toward the center of the embryo. This results in a cellular covering over the ventral portion of the embryo (Figure 2B). These folds will pull the serosa downward so that the embryo will eventually be covered by two layers of cells. The head, which was originally formed over the ventral surface, moves around the posterior end of the egg such that it is now in the anterior. The dorsal part is last to differentiate, and eventually the flanks of the folds extend dorsally and fuse. From then on development appears similar to that of a long germ band embryo.
Literature Cited
Anderson, D. T. 1973. Embryology and Phylogeny in Annelids and Arthropods. Pergamon Press, NY.
Gerhart, J. and Kirschner, M. 1997. Cells, Embryos, and Evolution. Blackwell Science, Malden, MA.
Sulston, I.A. and Anderson, K. V. 1996. Embryonic patterning mutants in Tribolium castaneum. Development 122: 805-814.
Schwalm, F. E. 1988. Insect Morphogenesis. Monographs in Developmental Biology. Vol. 20. H. W. Sauer, Basel.
Schwalm, F. E. 1997. Arthropods: The Insects. In Embryology: Constructing the Organism. (S. F. Gilbert and A. M. Raunio, eds.) Sinauer Associates, Inc., Sunderland, MA p. 259-278.