Blastoderm segmentation in Oncopeltus fasciatus and the evolution of insect segmentation mechanisms

Abstract

Segments are formed simultaneously in the blastoderm of the fly Drosophila melanogaster through a hierarchical cascade of interacting transcription factors. Conversely, in many insects and in all non-insect arthropods most segments are formed sequentially from the posterior. We have looked at segmentation in the milkweed bug Oncopeltus fasciatus. Posterior segments are formed sequentially, through what is probably the ancestral arthropod mechanism. Formation of anterior segments bears many similarities to the Drosophila segmentation mode. These segments appear nearly simultaneously in the blastoderm, via a segmentation cascade that involves orthologues of Drosophila gap genes working through a functionally similar mechanism. We suggest that simultaneous blastoderm segmentation evolved at or close to the origin of holometabolous insects, and formed the basis for the evolution of the segmentation mode seen in Drosophila We discuss the changes in segmentation mechanisms throughout insect evolution, and suggest that the appearance of simultaneous segmentation as a novel feature of holometabolous insects may have contributed to the phenomenal success of this group.

Keywords: blastoderm; evo-devo; insects; segmentation; transcription factors.

Figure 1.

Phylogenetic spread of germ types in selected insects. Redrawn and modified from Ten Tusscher [15].

Figure 2.

The development of the expression pattern of four segmentally expressed genes: (i) even-skipped, (ii) Delta, (iii) wingless and (iv) invected. The time axis (in hours after egg lay) is approximate owing to small variability among clutches. Embryos in the same column are of the same age. In all cases, anterior is to the left and dorsal to the top except for b9, which is a dorso-lateral view. Dashed lines connect embryos from the same clutch. The identity of the segments is marked on representative embryos: A1, first abdominal segment; an, antennal; ic, intercalary; lb, labial; md, mandibular; mx, maxillary; oc, ocular; T1–T3, first to third thoracic segment.

Figure 3.

Expression of segmental genes following knock-down of even-skipped. (a) Early expression (32–34 hAEL) of delta is shifted posteriorly. (b) Expression of wingless at 34–36 hAEL is shifted anteriorly. The anterior pre-gnathal patch elongates relative to the normal expression (cf. figure 2c5–c6) and all gnathal and thoracic segmental stripes are lost.

Figure 4.

RNAi phenotypes of delta knock-down pre-hatching larvae, arranged according to severity. (a) Class 1 phenotypes: the anterior (pre-gnathal) head is nearly normal, but the gnathal head and trunk are fused and shrivelled. (b) Class 2 phenotypes: head and thorax are present and nearly normal, but the abdomen is truncated and various ectopic masses of cells are seen. (c) Class 3 phenotypes: all body regions are present but with anomalous morphologies. (d) Wild-type larva for comparison.

Figure 5.

Expression of segmental genes following knock-down of gap genes. All embryos are at 34–36 hAEL, but note that the timing of these embryos is not as precise as those shown in figure 2a–c. Expression of even-skipped, (d–f) expression of Delta, (g–i) expression of wingless and (j–l) expression of invected, following RNAi against giant (left column), Krüppel (middle column) and hunchback (right column). In each embryo, the mandibular segment is marked with an arrowhead and the first thoracic segment with an arrow. Asterisks mark the putative location of missing segmental stripes. Knock-down of giant leads to the loss of the maxillary and labial segments and to disruption of the first thoracic segments. However, in the embryo imaged for invected (j) the labial and first thoracic segmental stripes are missing (this could be related to the position of the deleted stripes relative to the segmental/parasegmental borders). Knock-down of Krüppel leads to the loss of the second and third thoracic segments (occasionally also the first is missing—not shown). Knock-down of hunchback does not lead to the loss segmental expression in any of the genes at the blastoderm stage.

Figure 6.

The evolution of segmentation modes in insects. This analysis looks not only at germ type but also at modes of segment generation (simultaneous versus sequential), and lists a series of hypothetical events that occurred through the evolution of the different modes. (1) Sequential segmentation is the plesiomorphic mode of segment generation in insects and is found in all non-insect arthropods. (2) The appearance of simultaneous segmentation in the blastoderm occurred either at the common ancestor of Holometabola + Paraneoptera (2b) or earlier in the insect lineage (2a). Not enough is known about blastodermal segmentation in Orthoptera, but from the little that is known, we suggest that simultaneous segmentation occurred after the splitting of Orthoptera (option 2b). (3) Heterochronic shifts lead to delayed gastrulation and gastrulation independent segmentation—long germ segmentation. Simultaneous segmentation expands to include more and more segments, but vestiges of a bi-phasic mode of segmentation remain. (4) Pair-rule patterning of segments also appeared at the base of Holometabola. (5) Sequential segmentation is lost completely in several lineages within Diptera and Hymenoptera, leaving extreme long germ simultaneous segmentation of all segments in the blastoderm stage. (6) Simultaneous segmentation is lost completely in some (or all) lineages within Coleoptera (6a), and in some parasitic wasps (6b), leaving sequential segmentation of all segments. Heterochronic shifts in different lineages lead to long, intermediate and short germ development, unrelated to the sequential mechanism of segment generation.