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21Eye absent (Eya) is a protein which has been structurally conserved from hydrozoans to 22 humans which has two functions: it is both a transcription cofactor and a protein 23 tyrosine phosphatase. Eya was first described in the fly Drosophila melanogaster for its 24 role in eye development, and the same functions were also later reported in less derived 25 insects. Studies on the involvement of Eya in insect oogenesis are limited to D. 26 melanogaster, which has meroistic ovaries. In this fly, Eya plays a fundamental role in 27 the first stages of ovarian development because Eya mutations abolish gonad formation. 28 In this present work we studied the function of Eya in the panoistic ovary of the 29 cockroach Blattella germanica. We demonstrated that Eya is essential for correct ovary 30 development also in this ovary type. In B. germanica ovaries, Eya affects both somatic 31 and germinal cells in the germarium and the vitellarium, acting differently in different 32 ovarian regions. Development of the basal ovarian follicles is arrested BgEya-depleted 33 females, while in the germaria, BgEya helps to the maintain the correct number of 34 somatic and germinal stem cells by regulating the expression of ecdysteroidogenic 35 genes in the ovary.36 37 38 39 40 41 42 43 44 45 46 KEYWORDS: panoistic ovary, ecdysone, Halloween genes, cell proliferation, insect 47 oogenesis, Notch 48 49 50 51Maintaining the stability of stem cells is crucial in every organism, and this is especially 52 important in the case of germ stem cells. Oogenesis describes the process of ovary 53 development from the time of germ stem cell differentiation until oocyte maturation. 54During oogenesis, oocytes must synthesize and accumulate all maternal factors needed 55 by embryos to complete their development, thus ensuring reproductive success. 56In insect ovaries, germ stem cells are located in niches in the germarium of each 57 ovariole. The control of their proliferation and differentiation has been thoroughly 58 studied, mainly in species with meroistic polytrophic ovaries such as the fruit fly 59 Drosophila melanogaster (see Ameku et al., 2017; Belles and Piulachs, 2015; Dai et al., 60 2017). In contrast, the number of genes involved in regulating oogenesis so far 61 identified in less modified panoistic ovaries such as those of the cockroaches including 62 Blattella germanica is very limited. This prevents study of the mechanisms that control 63 oocyte growth and maturation in these basal insects. B. germanica is emerging as a 64 choice model in which to study panoistic ovaries. In this cockroach, each ovary has 65 around 20 ovarioles and only the most basal ovarian follicle of each ovariole matures 66 during a given gonadotrophic cycle, a process that starts early in the last nymphal instar. 67The basal ovarian follicles are almost ready to mature in freshly ecdysed females, 68 whereas the development of the remaining ovarian follicles of each ovariole is arrested 69 until these basal ones are oviposited. 70In previous co...