It’s no secret that minnows, suckers (as I found out tonight), and loaches are extremely popular in the aquarium trade. Part of this is due to the hardiness of (some of) these species, but more so due to their extremely amazing color patterns. Some species have horizontal stripes running the length of their bodies (e.g., North American minnows) whereas others have stripes running from the dorsal to ventral side of the body (e.g. clown loach; Chromobotia maracanthus). Some species have similar spots all over their body while others have spots of various sizes (e.g., imperial flower loach, Leptobotia elongata). Some have red, yellow, blue/purple, and black pigments all the time while only males develop these colors during the breeding season in other species. This amazing variation in pigmentation patterns is what drives a lot of the interest in this group in the aquarium trade.
A new article entitled “Local reorganization of xanthophores fine-tunes and colors the striped pattern of zebrafish” is hot off the press in Science. The authors of this paper developed a protocol whereby they could track in developing zebrafish (Danio rerio) the development of three types of cells associated with color production in fishes: xanthophores (yellow/red pigment), iridiophores (reflective), and melanophores (black pigment). It has long been known that these pigments contribute to color patterns in fishes, but how they do from embryo to adult has long remained a mystery. If you think about how small each cell is and the fact that it has be tracked throughout development to determine their embryonic origin, you can figure out why studying these cells have been difficult for so many years. Although difficult, if we knew how these cells formed, we could then understand the striking patterns of color patterns in fishes.
So what did they find? Well, read the paper for the exact details, but the xanthophores first arrive on these scene and form the layer of xanthophores as observed in adults. The iridiophores and melanophores ultimately arrive on the scene taking different cellular routes to arrive. Once the iridiophores arrive, the yellow xanthophores form a dense layer over the iridiophores producing the yellow inter-stripe. The black “zebra” stripes are formed by melanophores with a loose, scatter of xanthophores over the top of it. Why is it necessary for the xanthophores to densely cover the iridiophores and only loosely cover the melanophores? According to the authors, it produces and sharpens the characteristic yellow/black zebra pattern.
If you do a thought experiment, you can see how lacking xanthophores over the melanophores would probably produce a more “monotonous” color pattern. Think about how something changes when you put (a little) glitter on it. It adds some “flare” to whatever you put the glitter on. …I know this analogy isn’t perfect, but hopefully it gets the point across.
So, where do we go from here? Well, this is very exciting. If we understand how such color patterns are formed in one species, we can then ask questions about more complex color patterns as well as distribution of underlying mechanisms — i.e., does the same underlying mechanism produce color patterns in all fishes or are there multiple development mechanisms (convergent evolution). Although this is HUGE step, I still think we have other questions to address. What signals are these cells using? What genes are involved in the recruitment of these cells and how does their expression influence the production of color patterns? There has been a lot of interest in the evolution of genes associated with color production (Kelsh et al. 2004; Braasch et al. 2007; Braasch et al. 2008; Braasch et al. 2009) and even in the expression of these genes (Salzburger et al. 2007; Henning et al. 2010; Gunter et al. 2011). Now, the role of these genes can be much more thoroughly assessed using the various pigmentation mutants of zebrafish (e.g., Watanbe et al. 2006). So, stay tuned. I’m sure there will be more studies in the not too distant future that shed more light on the molecular development of pigmentation patterns in fishes!
Braasch I, Schartl M, Volff J-N (2007) Evolution of pigment synthesis pathways by gene and genome duplication in fish. BMC Evolutionary Biology 7(74) doi: 10.1186/1471-2148-7-74.
Braasch I, Volff J-N, Schartl M (2008) The evolution of teleost pigmentation and the fish-specific genome duplication. Journal of Fish Biology 73(8):1891-1918.
Braasch I, Brunet F, Volff J-N, Schartl M (2009) Pigmentation pathway evolution after whole-genome duplication in fish. Genome Biology and Evolution 1:479-493.
Gunter HM, Clabaut C, Salzburger W, Meyer A (2011) Identification and characterization of gene expression involved in the coloration of cichlid fish using microarray and qRT-PCR approaches. Journal of Molecular Evolution 72:127-137.
Henning F, Renz AJ, Fukamachi S, Meyer A (2010) Genetic, comparative genomic, and expression analyses of the Mc1r locus in the polychromatic Midas cichlid fish (Teleostei, Cichlidae, Amphilophus sp.) species group. Journal of Molecular Evolution 70:405-412.
Kelsh, K (2004) Genetics and evolution of pigment patterns in fish. Pigment Cell Research 17:326-336.
Mahalwar P, Walderich B, Singh AP, Nusslein-Volhard C (2014) Local reorganization of xanthophores fine-tunes and colors the striped pattern of zebrafish. Science doi: 10.1126/science.1254837.
Salzburger W, Braasch I, Meyer A (2007) Adaptive sequence evolution in a color gene involved in the formation of the characteristic egg-dummies of male haplochromine cichlid fishes. BMC Biology 5(51), doi: 10.1186/1741-7007-5-51.
Watanbe M, Iwashita M, Ishii M, Kurachi Y, Kawakami A, Kondo S, Okada K (2006) Spot pattern of leopard Danio is caused by mutation in zebrafish connexin41.8 gene. EMBO Reports 7(9):893-897.