Heterotypic interactions regulate cell shape and density during color pattern formation in zebrafish

Abstract

The conspicuous striped coloration of zebrafish is produced by cell-cell interactions among three different types of chromatophores: black melanophores, orange/yellow xanthophores and silvery/blue iridophores. During color pattern formation xanthophores undergo dramatic cell shape transitions and acquire different densities, leading to compact and orange xanthophores at high density in the light stripes, and stellate, faintly pigmented xanthophores at low density in the dark stripes. Here, we investigate the mechanistic basis of these cell behaviors in vivo, and show that local, heterotypic interactions with dense iridophores regulate xanthophore cell shape transition and density. Genetic analysis reveals a cell-autonomous requirement of gap junctions composed of Cx41.8 and Cx39.4 in xanthophores for their iridophore-dependent cell shape transition and increase in density in light-stripe regions. Initial melanophore-xanthophore interactions are independent of these gap junctions; however, subsequently they are also required to induce the acquisition of stellate shapes in xanthophores of the dark stripes. In summary, we conclude that, whereas homotypic interactions regulate xanthophore coverage in the skin, their cell shape transitions and density is regulated by gap junction-mediated, heterotypic interactions with iridophores and melanophores.

Flexibility of pigment cell behavior permits the robustness of skin pattern formation

“…However, mutant fish lacking iridophores cannot form correct stripe patterns in the body trunk, whereas the same fish can develop normal stripes in the fins (Frohnhöfer et al, 2013;Patterson & Parichy, 2013). Temporally, the detailed behavior of pigment cells is provided only for an early stage in the body trunk (Mahalwar, Singh, Fadeev, Nüsslein-Volhard, & Irion, 2016;Mahalwar et al, 2014). As melanophores behave very similarly in both the body trunk and fins, the cause of this difference may be in the behavior of the xanthophores.…”

“…However, most of the parameters are not taken from real data. Temporally, the detailed behavior of pigment cells is provided only for an early stage in the body trunk (Mahalwar, Singh, Fadeev, Nüsslein-Volhard, & Irion, 2016;Mahalwar et al, 2014). In this study, we have investigated the behavior of the melanophores in many different environments and have shown experimentally how the robustness of the skin pattern formation is secured.…”

Topological data analysis of zebrafish patterns

“…Less data is available for zebrafish mutants, and, to our knowledge, global information is in the form of qualitative descriptions of the characteristic features of their patterns. Local measurements, in turn, include cell speeds [40,41] and distances between adjacent cells [33,40,42]. Notably, we are not aware of measurements of pattern variability or stripe straightness.…”

“…Specifically, we use T 0 p = 100 µm and T 0 p = 90 µm as the dimension 0 persistence thresholds for iridophores and melanophores, respectively. We chose these thresholds conservatively, as average xanthophore-xanthophore neighboring distances are 30-60 µm and average melanophore-melanophore distances are roughly 50-60 µm in wild-type [21,33,40,42]. (We are not aware of empirical measurements of iridophore spacing.)…”

“…If Ω long in (1) is too small (e.g., when its inner radius is below 30-80 µm, the average distance between cells [33,40]), it is likely that there are no or very few cells in this annulus region, so that the signal from X d and I d to promote M cell birth is effectively turned off. Intuitively, this should lead to an M shortage in the resulting patterns.…”