Expression of the Pupal Determinant broad during Metamorphic and Neotenic Development of the Strepsipteran Xenos vesparum Rossi

Derived members of the endoparasitic order Strepsiptera have acquired an extreme form of sexual dimorphism whereby males undergo metamorphosis and exist as free-living adults while females remain larviform, reaching sexual maturity within their hosts. Expression of the transcription factor, broad (br) has been shown to be required for pupal development in insects in which both sexes progress through metamorphosis. A surge of br expression appears in the last larval instar, as the epidermis begins pupal development. Here we ask if br is also up-regulated in the last larval instar of male Xenos vesparum Rossi (Stylopidae), and whether such expression is lost in neotenic larviform females. We clone three isoforms of br from X. vesparum (Xv’br), and show that they share greatest similarity to the Z1, Z3 and Z4 isoforms of other insect species. By monitoring Xv’br expression throughout development, we detect elevated levels of total br expression and the Xv’Z1, Xv’Z3, and Xv’Z4 isoforms in the last larval instar of males, but not females. By focusing on Xv’br expression in individual samples, we show that the levels of Xv’BTB and Xv’Z3 in the last larval instar of males are bimodal, with some males expressing 3X greater levels of Xv’br than fourth instar femlaes. Taken together, these data suggest that neoteny (and endoparasitism) in females of Strepsiptera Stylopidia could be linked to the suppression of pupal determination. Our work identifies a difference in metamorphic gene expression that is associated with neoteny, and thus provides insights into the relationship between metamorphic and neotenic development.

内寄生的捻翅目（Strepsiptera）类群的衍生分支已演化出极端的雌雄二型性（sexual dimorphism）：雄性个体经历完整变态发育（metamorphosis），成为自由生活的成虫；而雌性始终保持幼虫形态（larviform），在宿主（hosts）内完成性成熟（sexual maturity）。已有研究证实，在两性均需经历变态发育的昆虫中，转录因子（transcription factor）Broad（br）的表达是蛹发育（pupal development）的必要条件。当昆虫表皮（epidermis）启动蛹发育程序时，br基因的表达会在末龄幼虫期（last larval instar）出现显著上调峰值。

本研究旨在探讨：雄性维氏胡蜂捻翅虫（Xenos vesparum Rossi，Stylopidae）的末龄幼虫期是否同样存在br基因的上调表达，而幼态持续（neoteny）的幼虫形雌性个体中是否会缺失此类表达。我们从X. vesparum中克隆获得3种br基因同工型（isoforms），并将其命名为Xv’br，序列比对结果显示，它们与其他昆虫的Z1、Z3及Z4型同工型的同源性最高。

通过监测Xv’br在整个发育周期的表达谱，我们发现雄性个体的末龄幼虫期中，总br基因表达量以及Xv’Z1、Xv’Z3和Xv’Z4型同工型的表达水平均显著升高，而雌性个体中未检测到此表达上调现象。通过对单个样本的Xv’br表达进行精准分析，我们发现雄性末龄幼虫期的Xv’BTB型与Xv’Z3型同工型的表达水平呈双峰分布，部分雄性个体的Xv’br表达量较四龄雌性个体高出3倍以上。

综上，本研究数据表明，捻翅目栉首亚目（Stylopidia）雌性个体的幼态持续（neoteny）及内寄生习性可能与蛹决定（pupal determination）过程的抑制密切相关。本研究揭示了与幼态持续相关的变态发育基因表达差异，为阐明变态发育与幼态持续发育之间的内在关联提供了新的研究视角。