Similarities in the chromosomal distribution of AG and AC repeats within and between Drosophila, human and barley chromosomes

Two simple sequence repeats (SSRs), AG and AC, were mapped directly in the metaphase chromosomes of man and barley (Hordeum vulgare L.), and in the metaphase and polytene chromosomes of Drosophila melanogaster. To this end, synthetic oligonucleotides corresponding to (AG)(12) and (AC)(8) were labelled by the random primer technique and used as probes in fluorescent in situ hybridisation (FISH) under high stringency and strict washing conditions. The distribution and intensity of the signals for the repeat sequences were found to be characteristic of the chromosomes and genomes of the three species analysed. The AC repeat sites were uniformly dispersed along the euchromatic segments of all three genomes; in fact, they were largely excluded from the heterochromatin. The Drosophila genome showed a high density of AC sequences on the X chromosome in both mitotic and polytene nuclei. In contrast, the AG repeats were associated with the euchromatic regions of the polytene chromosomes (and in high density on the X chromosome), but were only seen in specific heterochromatic regions in the mitotic chromosomes of all three species. In Drosophila, the AG repeats were exclusively distributed on the tips of the Y chromosome and near the centromere on both arms of chromosome 2. In barley and man, AG repeats were associated with the centromeres (of all chromosomes) and nucleolar organizer regions, respectively. The conserved chromosome distribution of AC within and between these three phylogenetically distant species, and the association of AG in specific chromosome regions with structural or functional properties, suggests that long clusters of these repeats may have some, as yet unknown, role.     

External sense organs in freshwater oligochaetes (Annelida, Clitellata) revealed by scanning electron microscopy

Freshwater oligochaetes have at least two kinds of external sense organs: multiciliate organs of short cilia (also present in earthworms) and sense organs with one to three long cilia (unknown in earthworms and possibly acting as rheoreceptors). Ciliate sense organs of freshwater oligochaetes are distributed over their entire body surface, including the clitellum. They are scattered on the prostomium and pigidium and are arranged into a transversal chaetal row and dispersed or forming a few other discrete transversal rows on chaetal segments. Three species display very prominent sense organs (sensory buds in Protuberodrilus tourenqui and papillae in Ophidonais serpentina and Spirosperma velutinus). The number of cilia per organ at the prostomium of freshwater families appears to be fewer than that of terrestrial ones. It is suggested that the total number of cilia at the prostomium of the freshwater species could be related to their habitat, evolving from an epibenthic to an endobenthic way of life.     

Haploinsufficiency of RanBP2 is neuroprotective against light-elicited and age-dependent degeneration of photoreceptor neurons

Prolonged light exposure is a determinant factor in inducing neurodegeneration of photoreceptors by apoptosis. Yet, the molecular bases of the pathways and components triggering this cell death event are elusive. Here, we reveal a prominent age-dependent increase in the susceptibility of photoreceptor neurons to undergo apoptosis under light in a mouse model. This is accompanied by light-induced subcellular changes of photoreceptors, such as dilation of the disks at the tip of the outer segments, prominent vesiculation of nascent disks, and autophagy of mitochondria into large multilamellar bodies. Notably, haploinsufficiency of Ran-binding protein-2 (RanBP2) suppresses apoptosis and most facets of membrane dysgenesis observed with age upon light-elicited stress. RanBP2 haploinsufficiency promotes decreased levels of free fatty acids in the retina independent of light exposure and turns the mice refractory to weight gain on a high-fat diet, whereas light promotes an increase in hydrogen peroxide regardless of the genotype. These studies demonstrate the presence of age-dependent and RanBP2-mediated pathways modulating membrane biogenesis of the outer segments and light-elicited neurodegeneration of photoreceptors. Furthermore, the findings support a mechanism whereby the RanBP2-dependent production of free fatty acids, metabolites thereof or the modulation of a cofactor dependent on any of these, promote apoptosis of photoreceptors in concert with the light-stimulated production of reactive oxygen species.     

Evolution of Iris subgenus Xiphium based on chromosome numbers, FISH of nrDNA (5S, 45S) and trnL–trnF sequence analysis

The subgenus Xiphium is one of the six infrageneric divisions of the genus Iris. Chromosome numbers of six of the seven Xiphium species are known. Here the aim was to infer genetic and phylogenetic relationships based on chromosome numbers, chromosome markers and plastid sequences. Chromosomal locations of 5S and 45S rDNA loci were determined in 19 populations of the 7 species by fluorescence in situ hybridization (FISH). Additionally, the trnL–trnF plastid spacer was sequenced and a phylogenetic analysis performed. Based on chromosome markers, subgenus Xiphium species were classified into four groups that differed in the number and locations of both types of nrDNA: (1) I. tingitana (2n = 28), I. filifolia (2n = 30, 34) and I. xiphium (2n = 34), (2) I. juncea (2n = 32) and I. boissieri (2n = 36), (3) I. serotina (2n = 34) and (4) I. latifolia (2n = 42). Although the trnL–trnF phylogeny was not fully resolved, the sequence analysis showed a well-supported subgroup of I. filifolia, I. tingitana and I. xiphium, as well as I. juncea. FISH physical maps of the Iris subgenus Xiphium taxa are species dependent. I. filifolia, I. tingitana and I. xiphium are very closely related species and share cytogenetic characteristics. Disploidy appears to have been central in the evolution of this subgenus, given a series of chromosome numbers (2n = 28, 30, 32, 34, 36, 42) and our phylogenetic results. Clear differences were found among European and African populations of I. filifolia. A different taxonomic treatment of I. filifolia is supported for populations on both sides of the Strait of Gibraltar.     

A new cardiid bivalve from the Pliocene Baklan Basin (Turkey) and the origin of modern Ponto‐Caspian taxa

Abstract: We present the first record of the cardiid genus Monodacna from the Pliocene of Anatolia, Turkey. Monodacna imrei sp. nov. was found in the Pliocene Killik Formation from the western margin of the Baklan Basin, in very marginal brackish to freshwater lacustrine deposits. The new record extends the stratigraphic range of the modern Ponto‐Caspian genus back into the Pliocene and adds to earlier evidence that modern Ponto‐Caspian taxa originated in the Pliocene of south‐western Turkey.     

Climatic niche and neutral genetic diversity of the six Iberian pine species: a retrospective and prospective view

Quaternary climatic fluctuations have left contrasting historical footprints on the neutral genetic diversity patterns of existing populations of different tree species. We should expect the demography, and consequently the neutral genetic structure, of taxa less tolerant to particular climatic extremes to be more sensitive to long‐term climate fluctuations. We explore this hypothesis here by sampling all six pine species found in the Iberian Peninsula (2464 individuals, 105 populations), using a common set of chloroplast microsatellite markers, and by looking at the association between neutral genetic diversity and species‐specific climatic requirements. We found large variation in neutral genetic diversity and structure among Iberian pines, with cold‐enduring mountain species (Pinus uncinata, P. sylvestris and P. nigra) showing substantially greater diversity than thermophilous taxa (P. pinea and P. halepensis). Within species, we observed a significant positive correlation between population genetic diversity and summer precipitation for some of the mountain pines. The observed pattern is consistent with the hypotheses that: (i) more thermophilous species have been subjected to stronger demographic fluctuations in the past, as a consequence of their maladaptation to recurrent glacial cold stages; and (ii) altitudinal migrations have allowed the maintenance of large effective population sizes and genetic variation in cold‐tolerant species, especially in more humid regions. In the light of these results and hypotheses, we discuss some potential genetic consequences of impending climate change.