Soldier Caste-Specific Protein 1 Is Involved in Soldier Differentiation in Termite Reticulitermes aculabialis

Abstract

Termite soldiers are a unique caste among social insects, and their differentiation can be induced by Juvenile hormone (JH) from workers through two molts (worker–presoldier–soldier). However, the molecular mechanism underlying the worker-to-soldier transformation in termites is poorly understood. To explore the mechanism of soldier differentiation induced by JH, the gene soldier caste-specific protein 1 (RaSsp1, NCBI accession no: MT861054.1) in R. aculabialis was cloned, and its function was studied. This gene was highly expressed in the soldier caste, and the protein RsSsp1 was similar to the JHBP (JH-binding protein) domain-containing protein by Predict Protein online. In addition, JHIII could be anchored in the hydrophobic cage of RaSsp1 as the epoxide of the JHBP-bound JH according to the protein ligand molecular docking online tool AutoDock. The functional studies indicated that knocking down of the RaSsp1 shorted the presoldier’s head capsule, reduced mandible size, delayed molting time and decreased molting rate (from worker to presoldier) at the beginning of worker gut-purging. Furthermore, knocking down of the RaSsp1 had a more pronounced effect on soldier differentiation (from presoldier to soldier), and manifested in significantly shorter mandibles, rounder head capsules, and lower molting rate (from worker to presoldier) at the beginning of presoldier gut-purging. Correspondingly, the expressions of JH receptor Methoprene-tolerant (Met), the JH-inducible transcription factor Krüppel homolog1 (Kr-h1) and ecdysone signal genes Broad-complex (Br-C) were downregulated when knocking down the RaSsp1 at the above two stages. All these results that RaSsp1 may be involved in soldier differentiation from workers by binding and transporting JH.

Caste-specific expressions and diverse roles of takeout genes in the termite Reticulitermes speratus

“…The takeout gene reported in R. aculabialis ( RaSsp1 ), which is most closely related to RsTO2 (Supplementary Figure S2 ), may bind to JH III and is involved in the regulation of soldier differentiation 20 . To clarify whether RsTO2 interacts with JH III before molts and is involved in soldier differentiation, functional analysis should be performed during presoldier and soldier differentiation under natural conditions without hormone treatment.…”

“…For example, it was shown that takeout (named Deviate ) elicited trail-following behavior in R. flavipes 19 . In R. aculabialis , takeout (named RaSsp1 ) was highly expressed in soldiers and was suggested to interact with JH III and participate in soldier differentiation 20 . In the termitid termite Nasutitermes takasagoensis , takeout (named Ntsp1 ) is highly expressed in the soldier frontal gland and probably combines with gland secretions 21 .…”

Juvenile Hormone Involved in the Defensive Behaviors of Soldiers in Termite Reticulitermes aculabialis

“…To analysis the effect of JH on the soldier's defensive behaviors, there was minor treatment changes in JHA (Pyriproxyfen, Sigma, Steinheim, Germany) fed to the termites. According to the previous reports, the purity of pyriproxyfen is 99% [17]. The induction rate of 120 µg pyriproxyfen-treated soldiers is like that of 200 ug, but the mortality rate is quite different [27].…”

“…As one of the most phenotypically plastic insects, the soldier caste fate in termites could be changed during postembryonic development. That is, a constant high juvenile hormone (JH), or the juvenile hormone analogue pyriproxyfen (JHA), titer triggers soldier differentiation from workers through two molting periods via a pre-soldier stage (that is, worker-pre-soldier-soldier) [4,17]. During this differentiation process, the allometry of soldier's head occurs, especially regarding the exaggeratedly enlarged mandibles, which are special weapons of soldiers' defense [18][19][20][21][22].…”

“…Previous studies have shown that RaSsp1, a soldier caste-specific protein in Reticulitermes aculabialis, is homologous with the JH-binding protein and is extremely highly expressed in the soldier caste [17]. Silencing RaSsp1 leads to a shortened head capsule, reduced mandible size, delayed molting time, and decreased molting rate at the beginning of worker or pre-soldier gut-purging [17].…”