Expression of homeobox genes shows chelicerate arthropods retain their deutocerebral segment

Abstract

Expression patterns of six homeobox containing genes in a model chelicerate, the oribatid mite Archegozetes longisetosus, were examined to establish homology of chelicerate and insect head segments and to investigate claims that the chelicerate deutocerebral segment has been reduced or lost. engrailed (en) expression, which has been used to demonstrate the presence of segments in insects, fails to demonstrate a reduced deutocerebral segment. Expression patterns of the chelicerate homologs of the Drosophila genes Antennapedia (Antp), Sex combs reduced (Scr), Deformed (Dfd), proboscipedia (pb), and orthodenticle (otd) confirm direct correspondence of head segments. The chelicerate deutocerebral segment has not been reduced or lost. We make further inferences concerning the evolution of heads and Hox genes in arthropods.

Figure 1

Two hypotheses of segmental homologies between chelicerate and crustacean/insect head regions. (A) Deutocerebral segment missing. The cross indicates the antennal segment thought to have been reduced or lost in the chelicerates relative to the insects and crustaceans. This theory supposes that the chelicerae (Ch) are homologous to the second antennal segment of crustaceans (An2) and the intercalary segment of insects (Ic). (B) Deutocerebral segment present. Diagonal lines between A and B emphasize the shift in segmental register. This theory supposes that the chelicerae are homologous to the first antennal segment of insects and crustaceans. Lr, labrum; Oc, ocular; Md, mandible; Pp, pedipalp; Mx1 and 2, maxilla 1 and 2; Lb, labium; L1–4, legs 1–4; T1–3, thorax 1–3; Op1–2, opisthosomal 1–2. More posterior segments are not shown.

Figure 2

Conceptual translations of homeodomains of Archegozetes (Al) genes compared with homologs from other arthropods and an onychophoran. (A) Engrailed (en) and invected (inv). (B) proboscipedia (pb). (C) Deformed (Dfd). (D) Sex combs reduced (Scr). (E) Antennapedia (Antp) (F) orthodenticle (otd). Insect sequences obtained from the GenBank database: Dm, Drosophila melanogaster; Af, Artemia franciscana; Sg, Schistocerca gregaria; Tc, Tribolium castaneum. Noninsect sequences from ref. : Cen, centipede (Ethmostigmus rubripes); Ony, onychophoran (Acanthokara kaputensis). Matches are indicated with a period and missing data by a dashed line.

Figure 3

Expression patterns of Homeobox-containing genes in Archegozetes demonstrating homology of anterior segments. The embryo is ≈170 μm long in each case. Abbreviations as in Fig. 1. (A) Expression of the Archegozetes engrailed (Alen) homolog in an embryo after the formation of the labrum. Alen is expressed in the posterior portion of each segment with visible limbs. Despite strong staining in these segments, no engrailed expression is evident anterior to the chelicerae. (B) Alpb is expressed in all of the visible walking legs with an anterior boundary at the front of the pedipalp. (C) AlDfd is expressed throughout the posterior of the embryo, excepting the terminal segments, with an anterior boundary at the front of the first walking leg. (D) AlScr is expressed strongly in the third leg and weakly in the second. It appears to be expressed only in the more distal regions of the second leg though the staining is diffuse. (E) Close up of opisthosoma and fourth leg bud. AlAntp expression is strong in the opisthosoma and continues into the rear portion of the fourth leg bud. Because of the small size of the fourth limb bud and its position, tucked in next to the opisthosoma, it has proved very hard to photograph; this expression therefore is schematized in a camera lucida drawing in F. (F) Camera lucida drawing highlighting expression of AlAntp in the opisthosoma and rear of fourth leg bud. (G) Optical mid-sagittal expression showing AlAntp expressed strongly in the opisthosoma with no expression in the chelicerae, pedipalps, or legs 1–3. Leg 4, a small bud, is not in focus in this picture. (H) Alotd is expressed in the ocular lobes and in the ventral midline. The photograph is a sagittal optical section focusing on the expression in the ventral midline. No segmental expression is seen posterior to the ocular segment. (I) Alotd expression is seen in the ocular lobes but not in the chelicerae. View is anterior-ventral with the anterior to the left.

Figure 4

Homologies between anterior segments of chelicerates and insects as determined by overlapping patterns of expression of homologous genes. Posterior boundary of otd and anterior boundaries of Dfd, Scr, and Antp are identical in insects and chelicerates. The anterior boundary of Archegozetes pb is as seen in some but not all insect groups. Appendages with Dll expression are colored black (results not shown). The inferred chelicerate homolog of the insect mandible (leg 1) does express Dll. Abbreviations as in Fig. 1.