Skeletal elements and pigment patterns have their cellular origin in the vertebrate cell type called the neural crest ( Gans & Northcutt 1983, Hall 1999).

These traits have received considerable attention from researchers and fit together conceptually. We focus on three types of traits: skeletons (including craniofacial and postcranial elements), pigmentation, and sex (gender) determination. Here, we review recent advances in the developmental genetics of adaptation in teleost fishes. Understanding the genetics of development in natural lineages would provide theoretically novel insights into gene function because ( a) new genes, not identified in mutant screens, might be involved and ( b) new mutations, compatible with adult viability, would likely play a role. Numerous natural lineages are amenable to genetic and developmental analysis because barriers to hybridization are minimal or absent and embryos are easy to manipulate (e.g., danios, sticklebacks, cichlids). Closely related species differ in a wide range of traits, many of which we explore below. Second, the species richness and diversity of fishes are unrivaled among vertebrates. First, the group contains bona fide model organisms ( Danio, Takifugu, Tetraodon, Oryzias), with research programs in forward and reverse genetics, molecular biology, and genomics providing information, hypotheses, and technical insight. Teleost fishes represent a unique assemblage in which to study the genetics of adaptation and evolutionary novelty, or how development works. Notably, understanding the origin of biological diversity was named one of the вЂњ25 Hard QuestionsвЂќ by Science magazine in July 2005, and вЂњEvolution in ActionвЂќ was Science's 2005 Breakthrough of the Year. As such, a major and complementary objective of current research is to understand the molecular basis of natural diversity. This approach has advanced our knowledge of gene function, but has also underscored the notion that genes do not operate in a vacuum, that environmental and genomic context matters. Subsequently, biologists have inferred how development works by studying how development fails. Second, most zebrafish mutants were embryonic lethals they never developed to function as adults. First, the classical experimental paradigm of forward genetics has sought to minimize complexity by isolating the effects of single mutations. Conceptually, mutant screens are an imperfect metaphor for the identification of genotype-phenotype associations in nature. This mindset was accompanied by major challenges. Comparative biologists recognized that understanding the key to complex phenotypes and evolutionary novelty, encoded in the genome and unveiled through the developing embryo, was a tractable research objective.

The zebrafish mutants, first the domain of biomedicine, contributed to an undercurrent of discovery that adaptation (when development works) was just the flip side of disease (when development fails). The landmark issue of Development was particularly inspirational to those interested in evolution. Students with favorite traits now had favorite mutants.

The simple figures used to document phenotypes (e.g., cleared and stained embryos lacking jaw bones or with duplicated cartilages, fishes without melanophores) provided visual compendia of developmental diversity. The description of mutants in pathways affecting most aspects of vertebrate morphology (brains, eyes, jaws, fins, pigment) provided resounding evidence of the interplay between genes and development on a comprehensive scale. The publication in December 1996 of an entire issue of Development dedicated to the zebrafish embryo and its embryogenesis changed the way evolutionary biologists think about fishes. INTRODUCTION Fishes, Novelty, and How Development Works General rules of evolutionary change have not materialized however, the field is wide open, and fishes will likely continue to contribute insights to this central biological question. We synthesize the current state of knowledge to evaluate theories about the interplay between development and evolution. Thus far, the genes involved in adaptation among fishes belong to well-characterized molecular pathways. We review the highlights of this research, concentrating our attention on skeletal morphology (cranial and postcranial), pigmentation patterning, and sex determination. A number of fish models have emerged at the forefront of this effort to dissect the developmental genetic and molecular basis of evolutionary novelty and adaptation. During the past decade of study in evolutionary developmental biology, we have seen the focus shift away from the stunning conservation of form and function between distantly related taxa toward the causal explanation of differences between closely related species.

Ob red zebra cichlid genetics