Background: ACh-induced phosphorylation drives assembly of reflectins, dynamically tuning iridescence from subcellular Bragg reflectors in squid iridocytes. Results: Reflectin sequences and phosphorylation sites are characterized from iridocytes with different photonic behaviors. Conclusion: Differences in reflectin structures and phosphorylation determine the emergent photonic behavior of reflective squid tissues. Significance: Biomolecular mechanisms of adaptive iridescence provide new insights into protein-dependent energy transduction and approaches to tunable optical materials.

SUMMARYLoliginid squid use tunable multilayer reflectors to modulate the optical properties of their skin for camouflage and communication. Contained inside specialized cells called iridocytes, these photonic structures have been a model for investigations into bio-inspired adaptive optics. Here, we describe two distinct sexually dimorphic tunable biophotonic features in the commercially important species Doryteuthis opalescens: bright stripes of rainbow iridescence on the mantle just beneath each fin attachment and a bright white stripe centered on the dorsal surface of the mantle between the fins. Both of these cellular features are unique to the female; positioned in the same location as the conspicuously bright white testis in the male, they are completely switchable, transitioning between transparency and high reflectivity. The sexual dimorphism, location and tunability of these features suggest that they may function in mating or reproduction. These features provide advantageous new models for investigation of adaptive biophotonics. The intensely reflective cells of the iridescent stripes provide a greater signal-to-noise ratio than the adaptive iridocytes studied thus far, while the cells constituting the white stripe are adaptive leucophores -unique biological tunable broadband scatterers containing Mie-scattering organelles activated by acetylcholine, and a unique complement of reflectin proteins. Supplementary material available online at

X-chromosome dosage compensation in female placental mammals is achieved by X-chromosome inactivation (XCI). An exception are human pre-implantation embryos, where dosage compensation occurs by X-chromosome dampening (XCD). Here, we examined whether XCD extends to human prenatal germ cells given their similarities with naïve pluripotent cells. We found that female human primordial germ cells (hPGCs) display reduced X-linked gene expression before entering meiosis. Moreover, in hPGCs, both X-chromosome are active and express the long non-coding RNAs XACT and XIST , the master regulator of XCI, which are silenced upon entry into meiosis. These findings uncover XACT as hPGC-marker, describe XCD associated with XIST -expression in hPGCs, and suggest that XCD evolved in humans to regulate X-linked genes in pre-implantation embryos and PGCs. Additionally, we found a unique X-chromosome regulation in human primordial oocytes. Therefore, future studies of human germline development must consider the sexually dimorphic X-chromosome dosage compensation mechanisms in the prenatal germline.

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons are at the apex of the hypothalamic-pituitary-gonadal axis that regulates mammalian fertility. Herein we demonstrate a critical role for the homeodomain transcription factor ventral anterior homeobox 1 (VAX1) in GnRH neuron maturation and show that Vax1 deletion from GnRH neurons leads to complete infertility in males and females. Specifically, global Vax1 knock-out embryos had normal numbers of GnRH neurons at 13 d of gestation, but no GnRH staining was detected by embryonic day 17. To identify the role of VAX1 specifically in GnRH neuron development, Vax1 flox mice were generated and lineage tracing performed in Vax1 flox/flox :GnRH cre :RosaLacZ mice. This identified VAX1 as essential for maintaining expression of Gnrh1. The absence of GnRH staining in adult Vax1flox/flox :GnRH cre mice led to delayed puberty, hypogonadism, and infertility. To address the mechanism by which VAX1 maintains Gnrh1 transcription, the capacity of VAX1 to regulate Gnrh1 transcription was evaluated in the GnRH cell lines GN11 and GT1-7. As determined by luciferase and electrophoretic mobility shift assays, we found VAX1 to be a direct activator of the GnRH promoter through binding to four ATTA sites in the GnRH enhancer (E1) and proximal promoter (P), and able to compete with the homeoprotein SIX6 for occupation of the identified ATTA sites in the GnRH promoter. We conclude that VAX1 is expressed in GnRH neurons where it is required for GnRH neuron expression of GnRH and maintenance of fertility in mice.

Mating behavior in males and females is dependent on olfactory cues processed through both the main olfactory epithelium (MOE) and the vomeronasal organ (VNO). Signaling through the MOE is critical for the initiation of male mating behavior, and the loss of MOE signaling severely compromises this comportment. Here, we demonstrate that dosage of the homeodomain gene Six3 affects the degree of development of MOE but not the VNO. Anomalous MOE development in Six3 heterozygote mice leads to hyposmia, specifically disrupting male mounting behavior by impairing detection of volatile female estrus pheromones. Six3 is highly expressed in the MOE, main olfactory bulb (MOB), and hypothalamus; all regions essential in the proper migration of the gonadotropin-releasing hormone (GnRH) neurons, a key reproductive neuronal population that migrates along olfactory axons from the developing nose into the brain. Interestingly, we find that the reduction in Six3 expression in Six3 heterozygote mice compromises development of the MOE and MOB, resulting in mis-migration of GnRH neurons due to improper olfactory axon targeting. This reduction in the hypothalamic GnRH neuron population, by 45% in adulthood, leads to female subfertility, but does not impact male hormone levels, suggesting that male infertility is not related to GnRH neuron numbers, but exclusively linked to abnormal olfaction. We here determine that Six3 is haploinsufficient for MOE development, GnRH neuron migration, and fertility, and represents a novel candidate gene for Kallmann syndrome, a form of inherited infertility.