Genetic Structure of Earthworm Populations at a Regional Scale: Inferences from Mitochondrial and Microsatellite Molecular Markers in Aporrectodea icterica (Savigny 1826)

Despite the fundamental role that soil invertebrates (e.g. earthworms) play in soil ecosystems, the magnitude of their spatial genetic variation is still largely unknown and only a few studies have investigated the population genetic structure of these organisms. Here, we investigated the genetic structure of seven populations of a common endogeic earthworm (Aporrectodea icterica) sampled in northern France to explore how historical species range changes, microevolutionary processes and human activities interact in shaping genetic variation at a regional scale. Because combining markers with distinct modes of inheritance can provide extra, complementary information on gene flow, we compared the patterns of genetic structure revealed using nuclear (7 microsatellite loci) and mitochondrial markers (COI). Both types of markers indicated low genetic polymorphism compared to other earthworm species, a result that can be attributed to ancient bottlenecks, for instance due to species isolation in southern refugia during the ice ages with subsequent expansion toward northern Europe. Historical events can also be responsible for the existence of two divergent, but randomly interbreeding mitochondrial lineages within all study populations. In addition, the comparison of observed heterozygosity among microsatellite loci and heterozygosity expected under mutation-drift equilibrium suggested a recent decrease in effective size in some populations that could be due to contemporary events such as habitat fragmentation. The absence of relationship between geographic and genetic distances estimated from microsatellite allele frequency data also suggested that dispersal is haphazard and that human activities favour passive dispersal among geographically distant populations.     

Molecular determinants of the Arabidopsis AKT1 K+ channel ionic selectivity investigated by expression in yeast of randomly mutated channels

The Arabidopsis thaliana K⁺ channel AKT1 was expressed in a yeast strain defective for K⁺ uptake at low K⁺ concentrations (<3 m M ). Besides restoring K⁺ transport in this strain, AKT1 expression increased its tolerance to salt (NaCl or LiCl), whatever the external K⁺ concentration used (50 μ M , 5 m M , or 50 m M ). We took advantage of the latter phenomenon for screening a library of channels randomly mutated in the region that shares homologies with the pore forming domain (the so-called P domain) of animal K⁺ channels (Shaker family). Cassette mutagenesis was performed using a degenerate oligonucleotide that was designed to ensure, theoretically, a single mutation per P cassette. The mean number of amino acid exchanges per cassette turned out to be 1.4. Mutant channels that conferred on the transformed cells a reduction in salt tolerance (increased Na⁺ content, decreased K⁺ content, and lower growth rate, as compared to control cells expressing the wild-type channel) were selected. By co-expressing them with the wild-type AKT1 cDNA, it was shown that the mutated polypeptides were expressed, stable and correctly targeted to the cell membrane where they formed channels with altered properties. Analysis of the mutation distribution in these channels suggests that the AKT1 P domain has a structure similar to that of animal Shaker channels (a strongly constrained central region lining the tunnel that includes the highly conserved consensus motif TXXTXGYGD, and flanking regions forming the outer mouth of the pore), with an additional selectivity filter located upstream from the tunnel and formed by residues present in the N-terminal flanking region.     

Variation in patriline reproductive success during queen production in orphaned colonies of the thelytokous ant Cataglyphis cursor

In genetically diverse insect societies (polygynous or polyandrous queens), the production of new queens can set the ground for competition among lineages. This competition can be very intense when workers can reproduce using thelytoky as worker lineages that manage to produce new queens gain a huge benefit. Selection at the individual level might then lead to the evolution of cheating genotypes, i.e. genotypes that reproduce more than their fair share. We studied the variation in reproductive success among worker patrilines in the thelytokous and highly polyandrous ant Cataglyphis cursor. Workers produce new queens by thelytoky in orphaned colonies. The reproductive success of each patriline was assessed in 13 orphaned colonies using genetic analysis of 433 workers and 326 worker-produced queens. Our results show that patrilines contributed unequally to queen production in half of the colonies, and the success of patrilines was function of their frequencies in workers. However, over all colonies, we observed a significant difference in the distribution of patrilines between workers and worker-produced queens, and this difference was significant in three of 13 colonies. In addition, six colonies contained a low percentage of foreign workers (drifters), and in one colony, they produced a disproportionably high number of queens. Hence, we found some evidence for the occurrence of rare cheating genotypes. Nevertheless, cheating appears to be less pronounced than in the Cape Honey bee, a species with a similar reproductive system. We argue that worker reproduction by parthenogenesis might not be common in natural populations of C. cursor.     

Root elongation and branching is related to local hexose concentration in Arabidopsis thaliana seedlings

Root architecture can be profoundly affected by the carbon availability in the plant. We hypothesized that this effect could be mediated by the carbon status of root cells involved in elongation and branching processes. Arabidopsis thaliana plants were grown at several photosynthetic photon flux densities (PPFD) and were supplied with various sucrose concentrations in the root medium. Hexose and sucrose concentration was estimated in individual roots in the apical growing region of the primary root and of secondary roots as well as in the zone of primordia development. Local sugar concentration was high in fast‐growing and in highly branched roots and robust relationships between root elongation rate or branching and hexose concentration (but not sucrose) were found that were common to all situations experienced. Moreover, these relationships accounted for the plant‐to‐plant variability within a treatment as well as for the variability among individual secondary roots within a plant. These results support the view that local hexose concentration integrates changes in carbon availability from several sources and acts as a signal to induce at least part of the response of the root architecture to the environment.     

Les collemboles des sables littoraux de Madagascar

Nous avons récolté 32 espèces de Collemboles dans les sables littoraux de Madagascar, dont huit nouvelles pour la science, 23 nouvelles pour l’île. Près de 30% sont endémiques et 70 % psammobiontes. Le nombre total de Collemboles connus de Madagascar est maintenant de 103 espèces, avec 74% d’endémiques.     

Heteromeric K+ channels in plants

Voltage-gated potassium channels of plants are multimeric proteins built of four α-subunits. In the model plant Arabidopsis thaliana , nine genes coding for K⁺ channel α-subunits have been identified. When co-expressed in heterologous expression systems, most of them display the ability to form heteromeric K⁺ channels. Till now it was not clear whether plants use this potential of heteromerization to increase the functional diversity of potassium channels. Here, we designed an experimental approach employing different transgenic plant lines that allowed us to prove the existence of heteromeric K⁺ channels in plants. The chosen strategy might also be useful for investigating the activity and function of other multimeric channel proteins like, for instance, cyclic-nucleotide gated channels, tandem-pore K⁺ channels and glutamate receptor channels.     

H Cotransports in Corn Roots as Related to the Surface pH Shift Induced by Active H Excretion

The surface pH shift induced by active H⁺ excretion in corn (Zea mays L.) roots was estimated using acetic acid influx as a pH probe (H Sentenac, C Grignon 1987 Plant Physiol 84: 1367-1372). At constant bulk pH, buffering the medium strongly reduced the magnitude of the surface pH shift. This was used to study the effect of surface pH shift on H⁺ cotransports. In the absence of buffers, the surface pH shift increased with the bulk pH. Buffers decreased ³²Pi influx and this effect was stronger at pH 7.2 than at pH 5.8, and stronger in the absence than in the presence of an inhibitor of the proton pump (vanadate). Buffers exerted a similar depressive and pH-dependent effect on net NO₃⁻ uptake. They hyperpolarized the cell membrane, and stimulated ⁸⁶Rb⁺ influx, K⁺:H⁺ net exchange, and malate accumulation. These results are consistent with the hypothesis that H⁺ accumulation at the cell surface is effective in driving H⁺ reentry. We concluded that the surface pH shift due to proton pump activity is involved in the energetic coupling of H⁺ cotransports.     

Assembly of plant Shaker-like K(out) channels requires two distinct sites of the channel alpha-subunit

SKOR and GORK are outward-rectifying plant potassium channels from Arabidopsis thaliana. They belong to the Shaker superfamily of voltage-dependent K(+) channels. Channels of this class are composed of four alpha-subunits and subunit assembly is a prerequisite for channel function. In this study the assembly mechanism of SKOR was investigated using the yeast two-hybrid system and functional assays in Xenopus oocytes and in yeast. We demonstrate that SKOR and GORK physically interact and assemble into heteromeric K(out) channels. Deletion mutants and chimeric proteins generated from SKOR and the K(in) channel alpha-subunit KAT1 revealed that the cytoplasmic C-terminus of SKOR determines channel assembly. Two domains that are crucial for channel assembly were identified: i), a proximal interacting region comprising a putative cyclic nucleotide-binding domain together with 33 amino acids just upstream of this domain, and ii), a distal interacting region showing some resemblance to the K(T) domain of KAT1. Both regions contributed differently to channel assembly. Whereas the proximal interacting region was found to be active on its own, the distal interacting region required an intact proximal interacting region to be active. K(out) alpha-subunits did not assemble with K(in) alpha-subunits because of the absence of interaction between their assembly sites.     

Spontaneous use and modification of a feather as a tool in a captive common raven

Here, we report an observation of a zoo‐housed common raven (Corvus corax) modifying and using a raven feather as a tool to access the food cache of her partner. We believe this record is of importance, as it represents one of the first cases of untrained common ravens spontaneously using and modifying a tool, and a rare example of a non‐human animal manufacturing a tool from a body part. This anecdotal observation suggests that tool use and tool modification may be present in the common raven behavioral repertoire in the foraging context; nevertheless, further investigations are needed to assess this possibility in more controlled settings as well as in the wild.     

Calcium-dependent modulation and plasma membrane targeting of the AKT2 potassium channel by the CBL4/CIPK6 calcium sensor/protein kinase complex

Potassium (K(+)) channel function is fundamental to many physiological processes. However, components and mechanisms regulating the activity of plant K(+) channels remain poorly understood. Here, we show that the calcium (Ca(2+)) sensor CBL4 together with the interacting protein kinase CIPK6 modulates the activity and plasma membrane (PM) targeting of the K(+) channel AKT2 from Arabidopsis thaliana by mediating translocation of AKT2 to the PM in plant cells and enhancing AKT2 activity in oocytes. Accordingly, akt2, cbl4 and cipk6 mutants share similar developmental and delayed flowering phenotypes. Moreover, the isolated regulatory C-terminal domain of CIPK6 is sufficient for mediating CBL4- and Ca(2+)-dependent channel translocation from the endoplasmic reticulum membrane to the PM by a novel targeting pathway that is dependent on dual lipid modifications of CBL4 by myristoylation and palmitoylation. Thus, we describe a critical mechanism of ion-channel regulation where a Ca(2+) sensor modulates K(+) channel activity by promoting a kinase interaction-dependent but phosphorylation-independent translocation of the channel to the PM.