Development of segments and appendages in embryos of the desert scorpion Paruroctonus mesaensis (Scorpiones: Vaejovidae)

Abstract

The scanning electron microscope was used to study the changing features of scorpion embryos from the blastula through early stages in the development of appendages. The earliest scorpion fossils (Silurian period) have structures more advanced than the embryos herein, so the possibility is considered that these embryos still retain and display some features indicative of evolutionary patterns in adult pre-Silurian ancestors. The blastodisc stage is followed by a knob-like germinal center that gives rise to most of the embryo body. The germinal center elongates on the ventral surface of the spherical yolk mass. The broad cephalic lobe is first delineated from the following pedipalpal segment. The limbbuds for the pedipalps and anterior walking legs appear, as additional segments are added at a growth zone at the rear of the embryo body. Initially, in the cephalic lobe there are no limbbuds; then the cheliceral buds emerge from the posterior part of the lobe. The stomodeum appears first in the anterior half of the cephalic lobe, but an oral groove forms and the mouth is displaced posteriorly within the groove. This repositioning allows space anteriorly for invagination (semilunar grooves) of epithelium for the brain and medial eyes. The mouth is directed ventrally in all stages of this study. The widespread chelicerae are initially posterior to the mouth, but later move anterior and dorsal to it. Small limbbud bulges on mesosomal segments disappear later and never become protruding appendages. Metasomal segments are produced free from the yolk surface in a ventral flexure beneath the embryo body. The telson starts as two spherical lobes, but later elongates and tapers distally, not yet developing the sharp sting (aculeus) seen in Silurian and all subsequent scorpions. The walking legs are digitigrade, as in most fossil aquatic scorpions. Segments are delineated in the appendages; the chelicerae and pedipalps are divided distally for chela (claw) formation. Bilateral swellings (limbbuds) on the third abdominal segment become larger than the others, indicating the site of pectine formation. The early fin-like pectines are somewhat posterior in the mesosoma, suggesting ancestral swimming, maneuvering, and balancing for the elongate abdomen. The pectinal surface is initially smooth but later transverse striations increase the surface area as a possible respiratory adaptation. Pectinal teeth (present in Silurian and all subsequent scorpions) and forward movement and merging of anterior abdominal segments are not yet evident in embryos of this study.

Development of respiratory structures in embryos and first and second instars of the bark scorpion, Centruroides gracilis (Scorpiones: Buthidae)

“…Small swellings on the ventral surface of the teeth develop a pore in the center (arrow) and lengthen to become peg sensilla (Fig. 18;Farley, 2001c). The pectines have separated from the ventral surface of the fourth opisthosomal segment but remain attached at the midline of segment 3.…”

“…4B; Farley, 2001cFarley, ,d, 2005. Transverse striations in the pectine gradually become teeth at the posterior margin, and it is at this stage that the spiracles can be discerned on the ventral surface of the mesosoma (Figs.…”

“…1; Farley, 2001bFarley, ,d, 2005. The posterior edges of the primordial sternites become evident at about the same time as the spiracles appear on the ventral surface of the mesosoma.…”

Hox gene duplications correlate with posterior heteronomy in scorpions

“…Females were anaesthetized with CO 2 and embryos dissected from the ovary following a published protocol [31]. RNA was extracted from one clutch of limb bud stage embryos using Trizol (Invitrogen) and first strand cDNA synthesis was performed using SuperScriptIII (Invitrogen).…”

“…Only the collective protein expression of Ubx and abd-A (termed 'UbdA') has been surveyed in the scorpion Smeringurus mesaensis, which shows an anterior expression boundary in part of the O2 segment, comparable to harvestmen [28,30]. Scorpion life-history characteristics, including live birth in all extant species and gestation periods lasting several months have heretofore precluded extensive developmental genetic study in this lineage [30][31][32][33] …”