In the model plant Arabidopsis thaliana, a core eudicot, the floral homeotic C-class gene AGAMOUS (AG) has a dual role specifying reproductive organ identity and floral meristem determinacy. We conduct a functional analysis of the putative AG ortholog ThtAG1 from the ranunculid Thalictrum thalictroides, a representative of the sister lineage to all other eudicots. Down-regulation of ThtAG1 by virus-induced gene silencing resulted in homeotic conversion of stamens and carpels into sepaloid organs and loss of flower determinacy. Moreover, flowers exhibiting strong silencing of ThtAG1 phenocopied the double-flower ornamental cultivar T. thalictroides 'Double White.' Molecular analysis of 'Double White' ThtAG1 alleles revealed the insertion of a retrotransposon causing either nonsense-mediated decay of transcripts or alternative splicing that results in mutant proteins with K-domain deletions. Biochemical analysis demonstrated that the mutation abolishes protein-protein interactions with the putative E-class protein ThtSEP3. C-and E-class protein heterodimerization is predicted by the floral quartet model, but evidence for the functional importance of this interaction is scarce outside the core eudicots. Our findings therefore corroborate the importance and conservation of the interactions between C-and E-class proteins. This study provides a functional description of a full C-class mutant in a noncore ("basal") eudicot, an ornamental double flower, affecting both organ identity and meristem determinacy. Using complementary forward and reverse genetic approaches, this study demonstrates deep conservation of the dual C-class gene function and of the interactions between C-and E-class proteins predicted by the floral quartet model. floral organ identity genes | MADS-box genes | solo long terminal repeats | RNA silencing C urrent understanding of floral patterning has emerged primarily from studies in the core eudicot model plants Arabidopsis thaliana and Antirrhinum majus. In these species, the genetic ABCE model predicts how combinatorial expression of four classes of transcription factors specifies organ identity in the floral meristem (1-4). According to the latest Arabidopsis model, which incorporates the role of the E-class proteins, once flowering has initiated, A-and E-class proteins specify sepals; A-, B-, and E-class proteins specify petals; B-, C-, and E-class proteins specify stamens; and C-and E-class proteins specify carpels and terminate floral meristem development (2, 5, 6). The underlying biochemical mechanism for specifying organ identity has been described by the floral quartet model, which predicts that correct transcription of organ-specific genetic programs requires the formation of hetero-multimeric complexes between these four interacting classes of transcription factors (5,(7)(8)(9). Mutations affecting the class-A, -B, -C, and -E functions are homeotic, resulting in the replacement of one organ type by another. Loss of expression of the Arabidopsis C-class gene AGAMOUS (AG) results in conversi...

Perennial woodland herbs in the genus Thalictrum exhibit high diversity of floral morphology, including four breeding and two pollination systems. Their phylogenetic position, in the early-diverging eudicots, makes them especially suitable for exploring the evolution of floral traits and the fate of gene paralogs that may have shaped the radiation of the eudicots. A current limitation in evolution of plant development studies is the lack of genetic tools for conducting functional assays in key taxa spanning the angiosperm phylogeny. We first show that virus-induced gene silencing (VIGS) of a PHYTOENE DESATURASE ortholog (TdPDS) can be achieved in Thalictrum dioicum with an efficiency of 42% and a survival rate of 97%, using tobacco rattle virus (TRV) vectors. The photobleached leaf phenotype of silenced plants significantly correlates with the down-regulation of endogenous TdPDS (P<0.05), as compared to controls. Floral silencing of PDS was achieved in the faster flowering spring ephemeral T. thalictroides. In its close relative, T. clavatum, silencing of the floral MADS box gene AGAMOUS (AG) resulted in strong homeotic conversions of floral organs. In conclusion, we set forth our optimized protocol for VIGS by vacuum-infiltration of Thalictrum seedlings or dormant tubers as a reference for the research community. The three species reported here span the range of floral morphologies and pollination syndromes present in Thalictrum. The evidence presented on floral silencing of orthologs of the marker gene PDS and the floral homeotic gene AG will enable a comparative approach to the study of the evolution of flower development in this group.

Skeletal muscles arise from diverse embryonic origins in vertebrates, yet converge on extensively shared regulatory programs that require muscle regulatory factor (MRF)-family genes. Myogenesis in the tail of the simple chordate Ciona exhibits a similar reliance on its single MRF-family gene, and diverse mechanisms activate Ci-Mrf. Here, we show that myogenesis in the atrial siphon muscles (ASMs) and oral siphon muscles (OSMs), which control the exhalant and inhalant siphons, respectively, also requires Mrf. We characterize the ontogeny of OSM progenitors and compare the molecular basis of Mrf activation in OSM versus ASM. In both muscle types, Ebf and Tbx1/10 are expressed and function upstream of Mrf. However, we demonstrate that regulatory relationships between Tbx1/10, Ebf and Mrf differ between the OSM and ASM lineages. We propose that Tbx1, Ebf and Mrf homologs form an ancient conserved regulatory state for pharyngeal muscle specification, whereas their regulatory relationships might be more evolutionarily variable.

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Gap-junctional signaling mediates myriad cellular interactions in metazoans. Yet, how gap junctions control the positioning of cells in organs is not well understood. Innexins compose gap junctions in invertebrates and affect organ architecture. Here, we investigate the roles of gap-junctions in controlling distal somatic gonad architecture and its relationship to underlying germline stem cells in the nematode Caenorhabditis elegans. We show that a reduction of soma-germline gap-junctional signaling causes displacement of distal sheath cells (Sh1) towards the distal end of the gonad. We show that a somatically expressed innexin fusion protein, which was used as marker in a prior study asserting that the wild type lacked a bare region between the distal tip cell (DTC) and Sh1, encodes a poisonous gap junction subunit. We determine that, contrary to the model put forth in the prior study based on this marker, Sh1 mispositioning does not markedly alter the position of the borders of the stem cell pool or of the progenitor cell pool. Together, these results demonstrate that gap junctions can control the position of Sh1, but that Sh1 position is neither relevant for GLP-1/Notch signaling nor for the exit of germ cells from the stem cell pool.

Gap-junctional signaling mediates myriad cellular interactions in metazoans. Yet, how gap junctions control the positioning of cells in organs is not well understood. Innexins compose gap junctions in invertebrates and affect organ architecture. Here, we investigate the roles of gap-junctions in controlling distal somatic gonad architecture and its relationship to underlying germline stem cells in Caenorhabditis elegans. We show that a reduction of soma–germline gap-junctional activity causes displacement of distal sheath cells (Sh1) towards the distal end of the gonad. We confirm, by live imaging, transmission electron microscopy, and antibody staining, that bare regions—lacking somatic gonadal cell coverage of germ cells—are present between the distal tip cell (DTC) and Sh1, and we show that an innexin fusion protein used in a prior study encodes an antimorphic gap junction subunit that mispositions Sh1. We determine that, contrary to the model put forth in the prior study based on this fusion protein, Sh1 mispositioning does not markedly alter the position of the borders of the stem cell pool nor of the progenitor cell pool. Together, these results demonstrate that gap junctions can control the position of Sh1, but that Sh1 position is neither relevant for GLP-1/Notch signaling nor for the exit of germ cells from the stem cell pool.

Like many animals and humans, reproduction in the nematode C. elegans declines with age. This decline is the cumulative result of age-related changes in several steps of germline function, many of which are highly accessible for experimental investigation in this short-lived model organism. Here we review recent work showing that a very early and major contributing step to reproductive decline is the depletion of the germline stem and progenitor cell pool. Since many cellular and molecular aspects of stem cell biology and aging are conserved across animals, understanding mechanisms of age-related decline of germline stem and progenitor cells in C. elegans has broad implications for aging stem cells, germline stem cells, and reproductive aging.

Skeletal muscles arise from diverse embryonic origins, yet converge on common regulatory programs involving muscle regulatory factor (MRF)-family genes. Here, we compare the molecular basis of myogenesis in two separate muscle groups in the simple chordate Ciona: the atrial and oral siphon muscles. Here, we describe the ontogeny of OSM progenitors and characterize the clonal origins of OSM founders to compare mechanisms of OSM specification to what has been established for ASM. We determined that, as is the case in the ASM, Ebf and Tbx1/10 are both expressed and function upstream of Mrf in the OSM founder cells. However, regulatory relationships between Tbx1/10, Ebf and Mrf differ between the OSM and ASM lineages: while Tbx1/10, Ebf and Mrf form a linear cascade in the ASM, Ebf and Tbx1/10 are expressed in the inverse temporal order and are required together in order to activate Mrf in the OSM founder cells.