Asymmetry in structure of the eggshell in Osmylus fulvicephalus (Neuroptera: Osmylidae): an exceptional case of breaking symmetry during neuropteran oogenesis

Ovaries of neuropterans are of meroistic-polytrophic type. The ovarian tubes, the ovarioles, are divided into two major parts: a germarium, comprised of newly formed germ cell clusters; and a vitellarium, housing linearly arranged ovarian follicles. Each ovarian follicle consists of the germ cell cluster diversified into different number of nurse cells, and the oocyte enclosed by follicular epithelium. In Osmylus fulvicephalus, a representative of Neuroptera, during consecutive stages of oogenesis, the follicular cells undergo a multistep process of diversification which leads to the appearance of several follicular cell subpopulations i.e., the main-body follicular cells, the stretched cells, the anterior centripetal cells, and posterior centripetal cells. The anterior centripetal cells occupy the anterior pole of the oocyte and in advanced oogenesis due to hypertrophy that transform into anterior fold cells. Initially, the anterior fold cells form a symmetric fold, but in advanced oogenesis, quite different from other neuropterans studied so far, they undergo uneven hypertrophic growth which results in breaking symmetry of the anterior fold that becomes shifted to the ventral side of the oocyte. Since the anterior fold cells participate in the production of the specialized chorion structure, the micropyle, asymmetric structure of the anterior fold, is reflected both in its asymmetric position and in the asymmetric construction of the micropyle. As a consequence of breaking symmetry of the anterior fold, Osmylus eggshell gains dorso-ventral polarity, which is unusual for neuropterans.