Evo-devo

Spatial expression of Hox cluster genes in the ontogeny of a sea urchin. Arenas-Mena, C. et al. Development 127 , 4631–4643 (2000). [ Contents page] [ Pubmed]

The patterning function of the Hox gene cluster has mostly been studied in animals, such as chordates and arthropods, which have two-fold symmetry around the adult anterior–posterior axis. The embryo and larva of the sea urchin are bilaterally symmetical, but develop into five-fold symmetrical adults. Phylogenetic studies suggest that the five-fold axis of symmetry evolved from a bilateral ancestor. The theory is now clearly supported by the expression of five sea urchin Hox genes during the development of the adult body plan.

Gene expression

A calmodulin-related protein that suppresses posttranscriptional gene silencing in plants. Anandalakshmi, R. et al. Science 290 , 142–144 (2000). [ PubMed]

A viral movement protein prevents spread of the gene silencing signal in Nicotiana benthamiana. Voinnert, O. et al. Cell 103 , 157–167 (2000). [ Contents page]

Post-transcriptional gene silencing (PTGS) in plants is an anti-viral defence system that directs the sequence-specific degradation of target RNAs. Using a yeast two-hybrid assay, with a viral suppressor of PTGS (HC-Pro) as a bait, Anandalakshmi and colleagues have discovered the first cellular suppressor of silencing. This protein, called rgs-CaM, might point to a role for calcium in regulating the PTGS pathway. PTGS is non-cell-autonomous, as the signal of gene silencing (probably a nucleic acid) can move long distances through the plant vasculature, to potentiate RNA sequence-specific virus resistance in as yet uninfected tissues. Voinnert and colleagues report the finding of a 25 kDa viral protein (p25) encoded by potato virus X, which can suppress the anti-viral effect of local and systemic PTGS.

Development

Combinatorial signaling in the specification of unique cell fates.  Flores, G. V. et al. Cell 103 , 75–85 (2000). [Contents page]

Overlapping activators and repressors delimit transcriptional response to receptor tyrosine kinase signals in the Drosophila eye.  Xu, C. et al. Cell 103 , 87–97 (2000). [ Contents page]

How do multiple signals specify unique cell fates during development? These papers address this issue, in the Drosophila eye. In this system, cell-type specific transcription factors have been shown to be important in cell fate specification. The two papers demonstrate how signals (such as Notch and EGF) are interpreted in a combinatorial fashion and lead to the expression of the appropriate transcription factor. The implication is that a small number of signals can be integrated to generate a variety of cell-specific outcomes.