Perception et réaction géotropiques de l'épicotyle d'Asparagus officinalis

Geoperception and georeaction in Asparagus officinalis epicotyls The location and the role of the statenchyma were investigated in Asparagus officinalis epicotyls 1, 2 or 3 cm in length. The statocytes represent a monocellular sheath which comprises the outer layer of the pericycle. Three phases of differentiation (S₁, S₂, S₃) of the statocytes may be distinguished by criteria of cell length, the state of vacuolisation and the sedimentation of amyloplasts and nuclei with respect to gravity. The statenchyma was found to be located only in the first 10 mm of the epicotyl 1, 2 or 3 cm in length; in effect the oldest statocytes (S₃) differentiate into sclerenchyma beneath this level. The geotropic curvature of the epicotyls begins with a positive (downward) curvature and the negative (upward) response is only realized after 15–30 min of geotropic stimulation. The upper limit of the bending is located in the zone containing the mature statocytes (S₃), while the lower limit is located much closer to the base of the epicotyl. Experiments on decapitation of the epicotyls have indicated that the S₁ and S₂ statocytes do not play an important role in the perception of gravity. However, with S₃ statocytes, it has been demonstrated that there is a statistical correlation between the initial rate of curvature of the epicotyls and the length of their sheaths. Thus, the longer the sheath, the more the epicotyls curve. From the results obtained it is proposed that the statocytes are responsible for the geotropic perception of Asparagus officinalis epicotyls.     

The vanishing clone: karyotypic evidence for extensive intraclonal genetic variation in the peach potato aphid,Myzus persicae (Hemiptera: Aphididae)

Analysis of holocentric mitotic metaphase chromosomes of the peach‐potato aphid Myzus persicae (Sulzer) clone 33H revealed different chromosome numbers, ranging from 12 to 17 within each embryo, in contrast to the standard karyotype of this species (2n = 12). Chromosome length measurements revealed that the observed chromosomal mosaicism is the result of recurrent fragmentations of chromosomes X, 1 and 3 because of fragile sites or hot spots of recombination. Fluorescent in situ hybridization experiments showed that X chromosomes were frequently involved in recurrent fragmentations, in particular their telomeric end opposite to the nucleolar organizer region. Experiments to induce males showed that M. persicae clone 33H is obligately parthenogenetic. The reproduction by apomictic parthenogenesis, together with a high telomerase expression that stabilized the chromosomes involved in the fragmentations observed in the M. persicae clone 33H, appears to favour the stabilization of the observed chromosome instability. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 350–358.     

Relation entre réaction géotropique et évolution du statenchyme dans la racine d'Asperge

Relationship between the Geotropic Response and the Evolution of the Statenchyma in Roots of Asparagus officinalis. The evolution of the statenchyma in roots of Asparagus of ficinalis seedlings, grown in obscurity, was followed during the first 17 days. After 7 days of etiolation, a decrease of both the average diameter of the amyloplasts and the average number of these organelles was observed in the central root cap cells. If the seedlings were illuminated (with white light) from the 7th day, the average number of statoliths increased rapidly in the statocytes. The volume of these organelles undergoes the same variation in etiolated and in illuminated plants. The initial rate of curvature (V_i) of the roots (stimulated in a horizontal position) and the volume of amyloplasts (V_ac) in their caps were analysed as a function of the time of germination in obscurity (from the 8th to the 17th day). It was found that V_i increased as a linear function of the logarithm of V_ac, which confirms that the weight of the amyloplasts of the statocytes may play a role in the geotropic stimulation of the roots.