Flight or protection: the genes Ultrabithorax and apterous in the determination of membranous and sclerotized wings in insects

Abstract

Present-day pterygote insects have two pairs of wings, one in the mesothorax (T2), the other in the metathorax (T3), and both have diverged in structure and function in different groups. Studies in endopterygote and paraneopteran species have shown that the gene Ultrabithorax (Ubx) specifies the identity and wing structure in T3, whereas the gene apterous (ap) significantly contributes to forming modified T2 wings. We wondered whether these Ubx and ap mechanisms operate in the lineage of polyneopterans. To explore this possibility, we used the cockroach Blattella germanica (Polyneoptera and Blattodea), in which the T2 wings are sclerotized (tegmina), whereas those of the T3 are membranous. We found that Ubx determines the structure of T3 and the membranous wing, while ap significantly contributes to form the sclerotized T2 tegmina. These results along with the studies carried out on the beetle Tribolium castaneum by Tomoyasu and collaborators suggest that ap plays an important role in the sclerotization and melanization of the T2 wings in neopteran groups that have sclerotized forewings. In turn, the sclerotizing properties of ap demonstrated in beetles and cockroaches suggest that the origin of this function goes back to the emergence of Neoptera, in the mid Devonian.

Keywords: Apterous; Blattella; Ultrabithorax; insect wings; tribolium; wing specialization.

Figure 1.

Effects of Ultrabithorax (Ubx) depletion on wing development in Blattella germanica. (a) Ubx expression in the prothorax (T1), mesothorax (T2), metathorax (T3) and the second and third abdominal segments (A2–3) of 2-day-old sixth nymphal instar (N6D2). (b) Expression pattern of Ubx in T3 during the sixth nymphal instar (N6). (c) Ubx expression in N6D2 in controls and dsUbx-treated (Ubx-i) insects. (d) General view of the adult female, and detail of the T2 tegmina (upper part) and T3 membranous wing (lower part) in control and Ubx-depleted insects (Ubx-i); the dashed line in the T3 wing of control and Ubx-i specimens roughly separates the remigium (above the line) and the anal (below) regions. (e) Adult T3 leg in Ubx-i insects compared with controls; a T2 hind leg from control insects is also shown for comparison. (f) Ubx-i adult female with asymmetric transformation of the T3; notice the difference in size of the right hind leg, typical of the T3 segment and the smaller left leg, such as that of the T2 segment; in the enlarged figure on the right, the two hind legs are highlighted with an oval of dots. In (a,c), expression is represented as mRNA copies per 1000 copies of actin-5c (mean ± s.e.m., n = 3–4); in (c), the asterisk indicates statistically significant differences with respect to controls (p < 0.05), according to REST [20]. In (d,f), scale bars: 2 mm. (Online version in colour.)

Figure 2.

Effects of the depletion of apterous-A (ap-A) and apterous-B (ap-B) on wing development in Blattella germanica. (a) ap-A and ap-B expression in the prothorax (T1), mesothorax (T2), metathorax (T3), and the second and third abdominal segments (A2–3) of 2-day-old sixth nymphal instar (N6D2). (b) Expression pattern of ap-A and ap-B during the sixth nymphal instar. (c) ap-A and ap-B expression in N6D2 in controls and dsAp-A-treated (ap-A-i) insects. (d) General view of the adult female, and detail of the T2 tegmina (upper part) and T3 membranous wing (lower part) in control and ap-A-i insects. (e) ap-B and ap-A expression in N6D2 in controls and dsAp-B-treated (ap-B-i) insects. (f) General view of the adult female, and detail of the T2 tegmina (upper part) and T3 membranous wing (lower part) in control and ap-B-i insects; the tegmina from the ap-B-i insect shown in the panel is one of the 12 (out of 32) that had the posterior edge, at the distal end, slightly depigmented. In (a–c) and (e), expression is represented as mRNA copies per 1000 copies of actin-5c (mean ± s.e.m., n = 3–4); in (c) and (e), the asterisk indicates statistically significant differences with respect to controls (p < 0.05), according to REST [20]. In (d) and (f), scale bars: 2 mm. (Online version in colour.)

Figure 3.

Effects of apterous-A and apterous-B joint depletion on wing development in Blattella germanica. (a) ap-A and ap-B expression in N6D2 in the mesothorax (T2) and metathorax (T3) of control and dsAp-A + dsAp-B-treated (ap-A-i + apB-i) insects. (b) General view of the adult female, and detail of the T2 tegmina (upper part) and T3 membranous wing (lower part) in control and ap-A-i + apB-i insects; in the latter, mild and severe phenotypes have been differentiated. In (a), expression is represented as mRNA copies per 1000 copies of actin-5c (mean ± s.e.m., n = 3–4); the asterisk indicates statistically significant differences with respect to controls (p < 0.05), according to REST [20]. In (b), scale bar: 2 mm. (Online version in colour.)

Figure 4.

Effects of dsUbx and dsAp-A treatment depletion on the expression of other wing-related genes in Blattella germanica. The effects of ap-A and Ubx transcript depletion (Ubx-i and ap-A-i, respectively) on the expression of blistered (bs), cut (ct), delta (Dl), decapentaplegic (dpp), epidermal growth factor receptor (Egfr), engrailed (en), Notch (N), nubbin (nub), rhomboid (rho), spalt major (salm), scalloped (sd), Serrate (Ser), vestigial (vg) and wingless (wg) are shown. Data represent normalized values against control (dsMock-treated) (reference value = 1) and is expressed as mRNA copies per 1000 copies of EIF4A (mean ± s.e.m., n = 3–4); the asterisk indicates statistically significant differences with respect to controls (p < 0.05) according to REST [20]. Reference expression values of these genes in the three thoracic segments of Bl. germanica have been reported previously [22]. (Online version in colour.)