The chordate amphioxus: an emerging model organism for developmental biology

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Retinoic acid signaling acts via Hox1 to establish the posterior limit of the pharynx in the chordate amphioxus

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Amphi-COUP-TF, a nuclear orphan receptor of the lancelet Branchiostoma floridae, is implicated in retinoic acid signaling pathways.

International audienceIn vertebrates, the orphan nuclear receptors of the COUP-TF group function as negative transcriptionalregulators that inhibit the hormonal induction of targetgenes mediated by classical members of the nuclear hor-mone superfamily, such as the retinoic acid receptors(RARs) or the thyroid hormone receptors (TRs). To in-vestigate the evolutionary conservation of the roles of COUP-TF receptors as negative regulators in the retinoidand thyroid hormone pathways, we have characterizedAmphiCOUP-TF, the homologue of COUP-TFI andCOUP-TFII, in the chordate amphioxus ( Branchiostoma floridae ), the closest living invertebrate relative of thevertebrates. Electrophoretic mobility shift assays(EMSA) showed that AmphiCOUP-TF binds to a widevariety of response elements, as do its vertebrate homo-logues. Furthermore, AmphiCOUP-TF is a transcription-al repressor that strongly inhibits retinoic acid-mediatedtransactivation. In situ hybridizations revealed expres-sion of AmphiCOUP-TF in the nerve cord of late larvae,in a region corresponding to hindbrain and probably an-terior spinal cord. Although the amphioxus nerve cordappears unsegmented at the gross anatomical level, thispattern reflects segmentation at the cellular level withstripes of expressing cells occurring adjacent to the endsand the centers of each myotomal segment, which mayinclude visceral motor neurons and somatic motor neu-rons respectively, among other cells. A comparison of the expression pattern of AmphiCOUP-TF with those of its vertebrate homologues, suggests that the roles of COUP-TF in patterning of the nerve cord evolved priorto the split between the amphioxus and vertebrate lineag-es. Furthermore, in vitro data also suggest that Amphi-COUP-TF acts as a negative regulator of signalling byother nuclear receptors such as RAR, TR or ER

Can Genetic Algorithms Explain Experimental Anomalies?

In experimental data, it is common to find persistent oscillations in the aggregate outcomes and high levels of heterogeneity in individual behavior. Furthermore, it is not unusual to find significant deviations from aggregate Nash equilibrium predictions. In this paper, we employ an evolutionary model with boundedly rational agents to explain these findings. We use data from common property resource experiments (Casari and Plott, 2003). Instead of positing individual-specific utility functions, we model decision makers as selfish and identical. Agent interaction is simulated using an individual learning genetic algorithm (GA), where agents have constraints in their working memory, a limited ability to maximize, and experiment with new strategies. We show that the model replicates most of the patterns that can be found in common property resource experiments. Copyright Kluwer Academic Publishers 2004bounded rationality, common-pool resources, experiments, genetic algorithms,

Phylogenetic analysis of phenotypic characters of Tunicata supports basal Appendicularia and monophyletic Ascidiacea

With approximately 3000 marine species, Tunicata represents the most disparate subtaxon of Chordata. Molecular phylogenetic studies support Tunicata as sister taxon to Craniota, rendering it pivotal to understanding craniate evolution. Although successively more molecular data have become available to resolve internal tunicate phylogenetic relationships, phenotypic data have not been utilized consistently. Herein these shortcomings are addressed by cladistically analyzing 117 phenotypic characters for 49 tunicate species comprising all higher tunicate taxa, and five craniate and cephalochordate outgroup species. In addition, a combined analysis of the phenotypic characters with 18S rDNA ‐sequence data is performed in 32 OTU s. The analysis of the combined data is congruent with published molecular analyses. Successively up‐weighting phenotypic characters indicates that phenotypic data contribute disproportionally more to the resulting phylogenetic hypothesis. The strict consensus tree from the analysis of the phenotypic characters as well as the single most parsimonious tree found in the analysis of the combined dataset recover monophyletic Appendicularia as sister taxon to the remaining tunicate taxa. Thus, both datasets support the hypothesis that the last common ancestor of Tunicata was free‐living and that ascidian sessility is a derived trait within Tunicata. “Thaliacea” is found to be paraphyletic with Pyrosomatida as sister taxon to monophyletic Ascidiacea and the relationship between Doliolida and Salpida is unresolved in the analysis of morphological characters; however, the analysis of the combined data reconstructs Thaliacea as monophyletic nested within paraphyletic “Ascidiacea”. Therefore, both datasets differ in the interpretation of the evolution of the complex holoplanktonic life history of thaliacean taxa. According to the phenotypic data, this evolution occurred in the plankton, whereas from the combined dataset a secondary transition into the plankton from a sessile ascidian is inferred. Besides these major differences, both analyses are in accord on many phylogenetic groupings, although both phylogenetic reconstructions invoke a high degree of homoplasy. In conclusion, this study represents the first serious attempt to utilize the potential phylogenetic information present in phenotypic characters to elucidate the inter‐relationships of this diverse marine taxon in a consistent cladistic framework.Peer Reviewe

Rapid temporal change in the expression and age-related information content of a sexually selected trait

The expression of sexual signals is often phenotypically plastic and also evolves rapidly. Few studies have considered the possibility that proximate determination - the pathway between genes and trait expression - may also be subject to both phenotypic plasticity and evolutionary change. We examined long-term patterns in size, condition- and age-dependence, repeatability and heritability of forehead patch size, a sexually selected plumage trait in male collared flycatchers. We also estimated survival and sexual selection on the phenotypic value of the trait. Forehead patch size linearly declined during the 15 years, probably due to the significantly negative survival selection. In addition, the expression of genetic variation for the ornament apparently underwent an age-limited change, which implies a change in the information content of the signal to receivers. The persistent lack of condition-dependence makes phenotypic plasticity an unlikely explanation to our results. This raises the possibility of a microevolutionary change of both expression and proximate determination during the study period