Plant viruses and new perspectives in cross-protection

Cross-protection in plants is the phenomenon whereby a plant preinoculated with a mild virus strain becomes resistant to subsequent inoculation by a related severe strain. It has been used on a large scale in cases where no resistant plants are available. Although several hypotheses have been proposed to explain the molecular mechanism underlying cross-protection, no single hypothesis can account for all the data obtained. Recently, a phenomenon akin to cross-protection has been achieved in transformed plants harboring the cDNA of a part of a viral RNA genome. These results obtained by genetic engineering raise new hopes for obtaining plants resistant to virus infection.     

From limb to fin: an Eocene protocetid forelimb from Senegal sheds new light on the early locomotor evolution of cetaceans

Cetaceans constitute a textbook example of the secondary adaptation of tetrapods to aquatic life. This major event in the evolutionary history of mammals is often linked in the literature to the limb-to-fin transition. Paradoxically, limb bones are scarce in the fossil record of early cetaceans, and the transition from a limb-adapted morphology for an amphibious life in shallow water to a fin-adapted morphology for a pelagic lifestyle remains poorly documented. Here, we describe new protocetid remains from the upper Lutetian of Senegal, including a nearly complete articulated forelimb. A cladistic analysis including 24 taxa and 137 morphological characters recovers the new African specimen close to Carolinacetus. It also confirms that cetacean dispersal to the New World was not the result of a single colonization event. A 3D model of the forelimb was reconstructed. Anatomical comparisons suggest that it is unlikely that the Senegalese forelimb was used as a rigid pectoral flipper for steering as in basilosaurids and modern cetaceans. Instead, we suggest that the hand was actively used during swimming. This challenges previous reconstructions of protocetids as mainly foot-powered swimmers, and suggests that swimming specializations of early cetaceans were probably more diverse than previously considered.     

Heterogeneity of system II secondary electron acceptors in Tris-washed chloroplasts

Tris-washed chloroplasts were submitted to saturating short flashes, and then rapidly mixed with dichlorophenyldimethylurea (DCMU). The amount of singly reduced secondary acceptor was estimated from the DCMU-induced increase in fluorescence, caused by the reverse electron flow from secondary to primary acceptor. The back-transfer from the singly reduced secondary acceptor to the primary acceptor Q induced by DCMU addition affects only a part (60%) of the variable fluorescence (ΔF_max). As previously shown, the quenchers involved in this phenomenon, ‘B-type’ quenchers, are different from those controlling the complementary part of the fluorescence, the non-B-type. In this report, we show that at pH 8.5 in the B-type systems, there exist two kinds of secondary electron acceptors: B, a two-electron acceptor, the corresponding Q accounting for 40% of the variable fluorescence; B′, a one-electron acceptor, the corresponding Q accounting for 20% of the variable fluorescence. The lifetimes of B^? and B′^? in the absence of DCMU are 40 and 1 s, respectively. The primary acceptors of the B and B′ systems can be considered as corresponding to the Q₁s defined previously (Joliot, P. and Joliot, A. (1981) in Proceedings of the 5th International Congress on Photosynthesis (Akoynoglou, G., ed.), pp. 885–899, Balaban International Science Services, Philadelphia). The B′ centers seems to be equivalent to the Q_β centers as defined by other workers (Van Gorkom, H.J., Thielen, A.P.G.M. and Gorren, A.C.F. (1982) in The Function of Quinones in Energy Conserving Systems (Trumpower, B.L., ed.), Academic Press, New York, in the press).