Comptes Rendus de l'Académie des Sciences - Series III - Sciences de la Vie最新文献

The discovery of a second fertilization event that initiates endosperm in flowering plants, just over a century ago, stimulated intense interest in the evolutionary history and homology of endosperm, the genetically biparental embryo-nourishing tissue that is found only in angiosperms. Two alternative hypotheses for the origin of double fertilization and endosperm have been invoked to explain the origin of the angiosperm reproductive syndrome from a typical non-flowering seed plant reproductive syndrome. Endosperm may have arisen from a developmental transformation of a supernumerary embryo derived from a rudimentary second fertilization event that first evolved in the ancestors of angiosperms (endosperm homologous with an embryo). Conversely, endosperm may represent the developmental transformation of the cellular phase of non-flowering seed plant female gametophyte ontogeny that was later sexualized by the addition of a second fertilization event in a strongly progenetic female gametophyte (endosperm homologous with a female gametophyte). For the first time, explicit developmental and evolutionary transitions for both of these hypotheses are examined and compared. In addition, current data that may be congruent with either of these hypotheses are discussed. It is clear that much remains to be accomplished if the evolutionary significance of the process of double fertilization and the formation of endosperm is to be fully understood.

The double fertilization of flowering plants was discovered a century ago. The cytology of the gametes is now well known. However the description of the fertilization steps is still poor and most of the cellular and molecular mechanisms involved are unknown. Recent research using in vitro fertilization demonstrated that the early steps of fertilization share some homology with those in animal species. In particular, gamete fusion is followed by a cytosolic calcium increase in the fertilized egg as well as a calcium influx. Further understanding of fertilization also comes from the analysis of mutants isolated in Arabidopsis thaliana. Important new ideas have already emerged from these studies such as the importance of the female gametophyte in embryo development, and an early silencing of the male genome during the first days following gamete fusion.

Self-incompatibility (SI) is a widespread mechanism in flowering plants that prevents self-fertilization. Self-pollen recognition relies on the products of genes located at the S (self-incompatibility) locus. Significant progress towards understanding molecular interactions allowing stigmatic cells to recognize and reject self-pollen in Brassica has been made during the past two years. Thus, the male and female determinants responsible of the self-incompatibility (SI) response have been identified. The structural features of these molecules strongly suggest that SI response is triggered by a ligand-receptor interaction.

Why mark the centenary of the independant discovery of double fertilization by Sergius Nawashin (1898) and Léon Guignard (1899), when biology has progressed so much since the beginning of the XXth century? This discovery still constitutes one of the key references in plant biology: double fertilization is unique to flowering plants among all living organisms. This meeting is also the occasion to associate angiosperm fertilization with developmental biology because of the localization of this event in the flower. Very important and significant progress has been made in elucidating flower development during the last ten years. And today it is possible to understand the diversity of floral structure present in the angiosperms in the context of a underlying mechanism of flower development inherited from their common ancestor. This special issue also allows a survey of these two broad scientific fields, plant reproduction and plant development (flower and embryo). It might also attract new, talented young scientists.

Most of higher plant species are hermaphroditic and male-sterility is often considered as an accident of development. In fact among the multiple possible causes of male-sterility, the most frequently met in nature is cytoplasmic male-sterility (cms) which is a maternally inherited trait playing an active role in the evolution of gynodioecious species. Recent molecular studies have shown that this trait is determined by additional genes created in plant mitochondrial genomes due to their high recombinogenic activity. The physiological mechanisms by which the products of these genes interfere with the formation of male gametophytes are still the subject of intense research.

Vertition in mites is defined as a meristic variation for a bilateral integumental organ with a separate genetic control for each body side. A prominent hypothesis expressed by Grandjean is the role of vertition in the evolutionary trend towards a reduced number of hair-like organs (mechano- and/or chemo-receptors) known to have occurred in many mite groups. Observations on leg setae in the two-spotted spider mite Tetranychus urticae do not support this hypothesis. Meristic variation for leg setae rather conforms to the notion of fluctuating asymmetry: the difference between the number of leg setae on the right and left sides of the body had a unimodal distribution with a mean of zero. Moreover, lack of heritability for left/right absences of leg setae in an inbred laboratory strain suggests that vertition could be purely environmental. It is therefore argued that meristic variation for hair-like organs in mites is caused by random developmental accidents not corrected by homeostatic mechanisms normally resulting in a perfect bilateral symmetry.

We used knockout mice and receptor antagonist strategies to investigate the contribution of the serotonin (5-hydroxytryptamine, 5-HT) 5-HT_1B receptor subtype in mediating the effects of selective serotonin reuptake inhibitors (SSRIs). Using in vivo intracerebral microdialysis in awake mice, we show that a single systemic administration of paroxetine (1 or 5 mg/kg, i.p.) increased extracellular serotonin levels 〚5-HT〛_ext in the ventral hippocampus and frontal cortex of wild-type and mutant mice. However, in the ventral hippocampus, paroxetine at the two doses studied induced a larger increase in 〚5-HT〛_ext in knockout than in wild-type mice. In the frontal cortex, the effect of paroxetine was larger in mutants than in wild-type mice at the 1 mg/kg dose but not at 5 mg/kg. In addition, either the absence of the 5-HT_1B receptor or its blockade with the mixed 5-HT_1B/1D receptor antagonist, GR 127935, potentiates the effect of a single administration of paroxetine on 〚5–HT〛_ext more in the ventral hippocampus than in the frontal cortex. Furthermore, we demonstrate that SSRIs decrease immobility in the forced swimming test; this effect is absent in 5-HT_1B knockout mice and blocked by GR 127935 in wild-type suggesting therefore that activation of 5-HT_1B receptors mediate the antidepressant-like effects of SSRIs. Taken together these data demonstrate that 5-HT_1B autoreceptors appear to limit the effects of SSRI on dialysate 5-HT levels particularly in the hippocampus while presynaptic 5-HT_1B heteroreceptors are likely to be required for the antidepressant activity of SSRIs.

Insect intracellular symbiotic bacteria (intracellular endosymbionts, or endocytobionts) were positioned within the γ3-Proteobacteria using a non-homogeneous model of DNA evolution, allowing for rate variability among sites, for GC content heterogeneity among sequences, and applied to a maximum likelihood framework. Most of them were found to be closely related within the Enterobacteriaceae family, located between Proteus and Yersinia. These results suggest that such a bacterial group might possess several traits allowing for insect infection and the stable establishment of symbiotic relationships and that this could represent a stem clade for numerous insect endocytobionts. Based on the estimations of the equilibrium GC content and branch lengths in the phylogenetic tree, we have made comparisons of the relative ages of these different symbioses.