Patterns of seedling recruitment in West-Mediterranean Quercus ilex forest influenced by canopy development

Abstract. Quercus ilex (holm oak) coppice forests belong to the most representative communities in the western part of the Iberian Peninsula. Due to the high sprouting potential of holm oak after coppicing, the dynamics of recruitment of new individuals in existing populations has been largely overlooked. In these forests, the density of seedlings increases in old stands, as a result of the remarkable increase in the recruitment of 1-yr and 2-yr old seedlings, probably due to better environmental conditions for germination. Older seedlings show a tendency towards increasing survival under some degree of canopy closure, since their density appears to be higher just before complete canopy closure takes place. Nevertheless, the reduced growth of seedlings in older stands due to low light levels causes age distributions of holm oak seedlings to change drastically, with saplings older than 15 yr becoming very unusual in these stands. Therefore, a bottleneck of recruitment appears at this sapling stage and prevents vigorous regeneration just before thinning. Present conditions of genet density and management tend to prevent population renewal. Dynamics of the seedling bank reinforce the role of gap formation through small or large scale perturbations in population turnover.     

A molecular phylogeny for the Drosophila melanogaster subgroup and the problem of polymorphism data

Drosophila melanogaster belongs to a closely related group of eight species collectively known as the melanogaster subgroup; all are native to sub-Saharan Africa and islands off the east coast of Africa. The phylogenetic relationships of most species in this subgroup have been well documented; however, the three most closely related species, D. simulans, D. sechellia, and D. mauritiana, have remained problematic from a phylogenetic standpoint as no data set has unambiguously resolved them. We present new DNA sequence data on the nullo and Serendipity-alpha genes and combine them with all available nuclear DNA sequence data; the total data encompass 12 genes and the ITS of rDNA. A methodological problem arose because nine of the genes had information on intraspecific polymorphisms in at least one species. We explored the effect of inclusion/exclusion of polymorphic sites and found that it had very little effect on phylogenetic inferences, due largely to the fact that 82% of polymorphisms are autapomorphies (unique to one species). We have also reanalyzed our previous DNA-DNA hybridization data with a bootstrap procedure. The combined sequence data set and the DNA-DNA hybridization data strongly support the sister status of the two island species, D. sechellia and D. mauritiana. This at least partially resolves what had been a paradox of parallel evolution in these two species.      

Chromosomal localization of a tandemly repeated DNA sequence in Trifolium repens L

INTRoDUCTIoNlYho1iumrePensL,whiteclover,isaneconomicallyimportantplantspeciesintemperatepastures.Asbrieflyreportedby[1],ithas16pairsofchromosomes(2n=32).Asyet,nodetailedcytologicalexaminationofthisspecies,suchasC-banding,hasbeenrep0rted.Inthelastdecade,thetechnique0fC-bandinghasbeenusedt0examinehighlyrepeatedsequencesinplantchrom0s0mesandhasprovidedausefultoolf0rtheanalysis0fcyt0geneticstructureincr0pplants[2-71.Inplants,thechr0m0s0mall0calizationofhighlyrepeatedDNAsequencesbyinsituhybr…     

Interspecific and intraspecific comparisons of the period locus in the Drosophila willistoni sibling species

The period (per) locus has received much attention in molecular evolution studies because it is one of the best studied "behavioral genes" and because it offers insight into the evolution of repetitive sequences. We studied most of the coding region of per in Drosophila willistoni and confirmed previously observed patterns of conservation and divergence among distantly related species. Five regions are so highly diverged that they cannot be aligned, whereas a region encompassing the PAS domain is very conserved. Structural and nucleotide polymorphism patterns in the willistoni group are not the same as those observed in previously studied species. We sequenced the region homologous to the highly polymorphic threonine-glycine repeat of D. melanogaster in multiple strains of D. willistoni, as well as in other members of willistoni group, and found an unusual amount of conservation in this region. However, the next nonconserved region downstream in the sequence is quite variable and polymorphic for the number of repeated glycines. The glycine codon usage is significantly different in this glycine repeat as compared to other parts of the gene. We were able to plot the directionality of change in the glycine repeat region onto a phylogeny and find that the addition of glycines is the general trend with the diversification of the willistoni group.      

L’electromyographie du penis: Une nouvelle exploration du muscle lisse caverneux et de son controle neurologique?

The authors have investigated the electrical activity of corpus cavernous, first according to Wagner and Gerstenberg’ method, then, since one year, according to Stief method, in the basal state and following intracavernosal injection of vasoactive agents, or following various stimulation tests. They have found again the electrical activity described by these authors, but have been confronted with the difficulty in quantifying the data. Specially single potential analysis seems to them little reliable and reproducible for an objective interpretation. At this stage of their experience, they test two simpler criteria interpretation: the richness of the electrical activity, and the reactivity of the recording to various stimulations. Their preminary results suggest a correlation between those criteria and the state of the autonomic nervous system of the penis.     

Complete sequences of the rRNA genes of Drosophila melanogaster

In this, the first of three papers, we present the sequence of the ribosomal RNA (rRNA) genes of Drosophila melanogaster. The gene regions of D. melanogaster rDNA encode four individual rRNAs: 18S (1,995 nt), 5.8S (123 nt), 2S (30 nt), and 28S (3,945 nt). The ribosomal DNA (rDNA) repeat of D. melanogaster is AT rich (65.9% overall), with the spacers being particularly AT rich. Analysis of DNA simplicity reveals that, in contrast to the intergenic spacer (IGS) and the external transcribed spacer (ETS), most of the rRNA gene regions have been refractory to the action of slippage-like events, with the exception of the 28S rRNA gene expansion segments. It would seem that the 28S rRNA can accommodate the products of slippage-like events without loss of activity. In the following two papers we analyze the effects of sequence divergence on the evolution of (1) the 28S gene "expansion segments" and (2) the 28S and 18S rRNA secondary structures among eukaryotic species, respectively. Our detailed analyses reveal, in addition to unequal crossing-over, (1) the involvement of slippage and biased mutation in the evolution of the rDNA multigene family and (2) the molecular coevolution of both expansion segments and the nucleotides involved with compensatory changes required to maintain secondary structures of RNA.