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Butterflies rely on color vision extensively to adapt to the natural world. Most species express a broad range of color sensitive Rhodopsins in three stochastically distributed types of ommatidia (unit eyes)1–3. The retinas of Drosophila deploy just two main types, where fate is controlled by the binary stochastic decision to express the transcription factor Spineless (Ss) in R7 photoreceptors4. We investigated how butterflies instead generate three stochastically distributed ommatidial types, resulting in a more diverse retinal mosaic that provides the basis for additional color comparisons and an expanded range of color vision. We show that the Japanese Yellow Swallowtail (Papilio xuthus, Papilionidae) and the Painted Lady (Vanessa cardui, Nymphalidae) have a second R7-like photoreceptor in each ommatidium. Independent stochastic expression of Ss in each R7-like cell results in expression of a Blue (Ss-ON) or a UV (Ss-OFF) Rhodopsin. In Papilio, these choices of Blue/Blue, Blue/UV, or UV/UV in the two R7s are coordinated with expression of additional Rhodopsins in the remaining photoreceptors, and together define the three types of ommatidia. Knocking out ss using CRISPR/Cas95,6 leads to the loss of the Blue fate in R7-like cells and transforms retinas into homogeneous fields of UV/UV-type ommatidia, with all corresponding features. Hence, the three possible outcomes of Ss expression define the three ommatidial types in butterflies. This developmental strategy allowed the deployment of an additional red-sensitive Rhodopsin in Papilio, allowing for the evolution of expanded color vision with a richer variety of receptors7,8. This surprisingly simple mechanism that makes use of two binary stochastic decisions coupled with local coordination may prove to be a general means of generating an increased diversity of developmental outcomes.

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The dissolution rates of olivine have been considered by a plethora of studies in part due to its potential to aid in carbon storage and its ease in obtaining pure samples for laboratory experiments. Due to the relative simplicity of its dissolution mechanism, most of these studies provide mutually consistent results such that a comparison of their rates can provide insight into the reactivity of silicate minerals as a whole. Olivine dissolution is controlled by the breaking of octahedral M 2+-oxygen bonds at or near the surface, liberating adjoining SiO4 4tetrahedra to the aqueous fluid. Aqueous species that adsorb to these bonds apparently accelerate their destruction. For example, the absorption of H + , H2O and, at some conditions, selected aqueous organic species will increase olivine dissolution rates. Nevertheless, other factors can slow olivine dissolution rates. Notably, olivine dissolution rates are slowed by lowering the surface area exposed to the reactive aqueous fluid, by for example the presence and/or growth on these surfaces of either microbes or secondary phases. The degree to which secondary phases decelerate rates

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Investigations of mineral surface reactivity have recently challenged the classical approach of determining dissolution rates from mineral powders as crystals often exhibit heterogeneous and/or anisotropic reactivity. However, face-specific measurements are restricted to small areas at the surface and limited depth and ignore the contribution of the crystal edges to the whole process. Here, we provide a detailed characterization of the dissolution kinetics at pH 4.0 of a single calcite crystal in 3D using X-ray microtomography with a resolution less than 1 μm. The imaging method allows 3D mapping of the crystal surface topography, providing a description of the time-dependent local dissolution fluxes all over the crystal surface, and the calculation of the crystal dissolution rates. The global rate determined at the crystal scale integrates the contribution of all the crystal features, including the faces, edges, and corners, which can be detailed in the local rate distributions. Under acidic conditions, pits develop at the {101̅ 4} surface before dissolution tends to smooth out both the crystal surface asperities and the edges and corners. In addition, a high rate variability is noticed over the crystal surface. The heterogeneous dissolution rates at the crystal surface first led to a local increase of the surface roughness due to pit formation and coalescence, followed by a decrease of the global crystal roughness due to smoothing of the large-scale surface asperities, crystal edges, and corners. Etch pits dominate initially the surface topography, whereas the evolution of the crystal morphology is dominated by the reactivity of edges and corners, whose contribution to dissolution is on average 1.7−3.6 times higher than that of the crystal faces. These results suggest that the dissolution reaction preferentially occurs at the crystal edges and corners, something not considered in most studies of mineral dissolution.