Latent developmental potential to form limb-like skeletal structures in zebrafish, bioRxiv, 2018-10-23

AbstractThe evolution of fins into limbs was a key transition in vertebrate history. A hallmark of this transition is the addition of multiple long bones to the proximal-distal axis of paired appendages. Whereas limb skeletons are often elaborate and diverse, teleost pectoral fins retain a simple endoskeleton. Fins and limbs share many core developmental processes, but how these programs were reshaped to produce limbs from fins during evolution remains enigmatic. Here we identify zebrafish mutants that form supernumerary long bones along the proximal-distal axis of pectoral fins with limb-like patterning. These new skeletal elements are integrated into the fin, as they are connected to the musculature, form joints, and articulate with neighboring bones. This phenotype is caused by activating mutations in previously unrecognized regulators of appendage development, vav2 and waslb, which we show function in a common pathway. We find that this pathway functions in appendage development across vertebrates, and loss of Wasl in developing limbs results in patterning defects identical to those seen in Hoxall knockout mice. Concordantly, formation of supernumerary fin long bones requires the function of hoxall paralogs, indicating developmental homology with the forearm and the existence of a latent functional Hox code patterning the fin endoskeleton. Our findings reveal an inherent limb-like patterning ability in fins that can be activated by simple genetic perturbation, resulting in the elaboration of the endoskeleton.

biorxiv evolutionary-biology 0-100-users 2018

Polygenic Adaptation From sweeps to subtle frequency shifts, bioRxiv, 2018-10-23

Evolutionary theory has produced two conflicting paradigms for the adaptation of a polygenic trait. While population genetics views adaptation as a sequence of selective sweeps at single loci underlying the trait, quantitative genetics posits a collective response, where phenotypic adaptation results from subtle allele frequency shifts at many loci. Yet, a synthesis of these views is largely missing and the population genetic factors that favor each scenario are not well understood. Here, we study the architecture of adaptation of a binary polygenic trait (such as resistance) with negative epistasis among the loci of its basis. The genetic structure of this trait allows for a full range of potential architectures of adaptation, ranging from sweeps to small frequency shifts. By combining computer simulations and a newly devised analytical framework based on Yule branching processes, we gain a detailed understanding of the adaptation dynamics for this trait. Our key analytical result is an expression for the joint distribution of mutant alleles at the end of the adaptive phase. This distribution characterizes the polygenic pattern of adaptation at the underlying genotype when phenotypic adaptation has been accomplished. We find that a single compound parameter, the population-scaled background mutation rate Θbg, explains the main differences among these patterns. For a focal locus, Θbg measures the mutation rate at all redundant loci in its genetic background that offer alternative ways for adaptation. For adaptation starting from mutation-selection-drift balance, we observe different patterns in three parameter regions. Adaptation proceeds by sweeps for small Θbg ≲0.1, while small polygenic allele frequency shifts require large Θbg ≳100. In the large intermediate regime, we observe a heterogeneous pattern of partial sweeps at several interacting loci.

biorxiv evolutionary-biology 0-100-users 2018

 

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