Post-translational hydroxylation at the N-terminus of the prion protein reveals presence of PPII structure in vivo

The transmissible spongiform encephalopathies are characterized by conversion of a host protein, PrP(C) (cellular prion protein), to a protease-resistant isoform, PrP(Sc) (prion protein scrapie isoform). The importance of the highly flexible, N-terminal region of PrP has recently become more widely appreciated, particularly the biological activities associated with its metal ion-binding domain and its potential to form a poly(L-proline) II (PPII) helix. Circular dichroism spectroscopy of an N-terminal peptide, PrP(37-53), showed that the PPII helix is formed in aqueous buffer; as it also contains an Xaa-Pro-Gly consensus sequence, it may act as a substrate for the collagen-modifying enzyme prolyl 4-hydroxylase. Direct evidence for this modification was obtained by mass spectrometry and Edman sequencing in recombinant mouse PrP secreted from stably transfected Chinese hamster ovary cells. Almost complete conversion of proline to 4-hydroxyproline occurs specifically at residue Pro44 of this murine protein; the same hydroxylated residue was detected, at lower levels, in PrP(Sc) from the brains of scrapie-infected mice. Cation binding and/or post-translational hydroxylation of this region of PrP may regulate its role in the physiology and pathobiology of the cell.     

Radiation of the Australian flora: what can comparisons of molecular phylogenies across multiple taxa tell us about the evolution of diversity in present-day communities?

The Australian fossil record shows that from ca. 25 Myr ago, the aseasonal-wet biome (rainforest and wet heath) gave way to the unique Australian sclerophyll biomes dominated by eucalypts, acacias and casuarinas. This transition coincided with tectonic isolation of Australia, leading to cooler, drier, more seasonal climates. From 3 Myr ago, aridification caused rapid opening of the central Australian arid zone. Molecular phylogenies with dated nodes have provided new perspectives on how these events could have affected the evolution of the Australian flora. During the Mid-Cenozoic (25-10 Myr ago) period of climatic change, there were rapid radiations in sclerophyll taxa, such as Banksia, eucalypts, pea-flowered legumes and Allocasuarina. At the same time, taxa restricted to the aseasonal-wet biome (Nothofagus, Podocarpaceae and Araucariaceae) did not radiate or were depleted by extinction. During the Pliocene aridification, two Eremean biome taxa (Lepidium and Chenopodiaceae) radiated rapidly after dispersing into Australia from overseas. It is clear that the biomes have different histories. Lineages in the aseasonal-wet biome are species poor, with sister taxa that are species rich, either outside Australia or in the sclerophyll biomes. In conjunction with the fossil record, this indicates depletion of the Australian aseasonal-wet biome from the Mid-Cenozoic. In the sclerophyll biomes, there have been multiple exchanges between the southwest and southeast, rather than single large endemic radiations after a vicariance event. There is need for rigorous molecular phylogenetic studies so that additional questions can be addressed, such as how interactions between biomes may have driven the speciation process during radiations. New studies should include the hitherto neglected monsoonal tropics.     

Genomic divergence in sympatry indicates strong reproductive barriers and cryptic species within Eucalyptus salubris

Genetic studies are increasingly detecting cryptic taxa that likely represent a significant component of global biodiversity. However, cryptic taxa are often criticized because they are typically detected serendipitously and may not receive the follow‐up study required to verify their geographic or evolutionary limits. Here, we follow‐up a study of Eucalyptus salubris that unexpectedly detected two divergent lineages but was not sampled sufficiently to make clear interpretations. We undertook comprehensive sampling for an independent genomic analysis (3,605 SNPs) to investigate whether the two purported lineages remain discrete genetic entities or if they intergrade throughout the species’ range. We also assessed morphological and ecological traits, and sequenced chloroplast DNA. SNP results showed strong genome‐wide divergence (F _ST = 0.252) between two discrete lineages: one dominated the north and one the southern regions of the species’ range. Within lineages, gene flow was high, with low differentiation (mean F _ST = 0.056) spanning hundreds of kilometers. In the central region, the lineages were interspersed but maintained their genomic distinctiveness: an indirect demonstration of reproductive isolation. Populations of the southern lineage exhibited significantly lower specific leaf area and occurred on soils with lower phosphorus relative to the northern lineage. Finally, two major chloroplast haplotypes were associated with each lineage but were shared between lineages in the central distribution. Together, these results suggest that these lineages have non‐contemporary origins and that ecotypic adaptive processes strengthened their divergence more recently. We conclude that these lineages warrant taxonomic recognition as separate species and provide fascinating insight into eucalypt speciation.     

AMPLIFICATION OF THE POLYMORPHIC 5.8S rRNA GENE FROM SELECTED AUSTRALIAN GIGARTINALEAN SPECIES (RHODOPHYTA) BY POLYMERASE CHAIN REACTION1

We have initiated comparative studies of ribosomal RNA (rRNA) gene structure to explore its potential to provide taxonomically useful data within the large red algal order Gigartinales. In southern Australia, this group is extremely diverse and includes large numbers of endemic taxa, many of potential economic importance. The 5.8S rRNA gene occurs in the middle region of the ribosomal DNA (rDNA) cistron and is flanked by two internal transcribed spacers (ITSs). These spacers contain regions of DNA, which are highly consented at the generic level and above, interspersed with highly divergent sequences. The 5.8S and associated ITS s of 11 species of Gigartinales (including five species of the largest Australian endemic marine algal genus, Mychodea), plus five taxa belonging to other orders, were amplified by the polymerase chain reaction. The size of the 5.8S rDNA and its flanking ITSs varied not only within and between genera, but also at the species level. However, this rDNA sequence appears to be relatively constant within populations find may be useful as a populational marker.     

Differential mRNA Expression and Glucocorticoid-Mediated Regulation of TRPM6 and TRPM7 in the Heart and Kidney throughout Murine Pregnancy and Development

The transient receptor potential (TRP) channels TRPM6 and TRPM7 are critically involved in maintaining whole body and cellular Mg²⁺ homeostasis and ensuring the normal function of organs such as the heart and kidney. However, we do not know how the expression of TRPM6 and TPRM7 in these organs changes throughout fetal development and adult life, and whether this expression can be hormonally regulated. This study determined the ontogeny of TRPM6 and TRPM7 mRNA expression from mid-gestation through to adulthood in the mouse. In a second series of experiments, we examined how maternal administration of the glucocorticoids corticosterone and dexamethasone between embryonic days 12.5–15 affected TRPM6 and TRPM7 channel mRNA expression in the mother and fetus. Whilst renal TRPM7 expression was relatively constant throughout development, renal TRPM6 expression was markedly upregulated after birth. In contrast, cardiac TRPM7 expression was 2–4 fold higher in the fetus than in the adult. Surprisingly, TRPM6 expression was detected in the fetal heart (qPCR and in situ hybridization). Glucocorticoid administration during gestation increased fetal cardiac expression of both channels without affecting renal expression. In contrast, in the dam renal TRPM6 and TRPM7 expression was increased by glucocorticoids with no change in the cardiac channel expression. These data suggest that TRPM6 and TRPM7 channels are important in organogenesis, and that elevated maternal glucocorticoid levels can alter the expression of these channels. This suggests that perturbations in hormonal regulatory systems during pregnancy may adversely impact upon normal fetal development, at least in part by altering expression of TRPM channels.     

Incongruence between chloroplast and species phylogenies in Eucalyptus subgenus Monocalyptus (Myrtaceae)

Seventy-eight polymorphic cpDNA (chloroplast DNA) characters were found in 13 closely related taxa from Eucalyptus series Amygdalinae (subgenus Monocalyptus) and seven potential outgroup taxa. The strict consensus of six cladograms generated from cpDNA data confirmed monophyly of Monocalyptus. However, cpDNA phylogeny within Monocalyptus was incongruent with taxonomic classification, being more related to geography, even when accessions were from divergent series. Monocalyptus cpDNA formed two major clades. On the island of Tasmania cpDNA was restricted to a single clade, exhibited very little variation, and was phylogenetically related to cpDNA found in central and western Victoria. In contrast, cpDNA of mainland monocalypt taxa was more variable, even within the Amygdalinae. Four out of six Tasmanian Amygdalinae species were polymorphic. The difference between cpDNA of replicates was often greater than differences between species from different series. The low level of cpDNA variation and extensive morphological intergradation between the Tasmanian endemics suggest recent speciation. However, the transfer of cpDNA through hybridization between lineages is the most likely explanation for the observed sharing of cpDNA across series. This study highlights that the geographical pattern to cpDNA variation in Eucalyptus may be an important source of information on past plant distributions in Australia.     

Phylogenetic Responses of Forest Trees to Global Change

In a rapidly changing biosphere, approaches to understanding the ecology and evolution of forest species will be critical to predict and mitigate the effects of anthropogenic global change on forest ecosystems. Utilizing 26 forest species in a factorial experiment with two levels each of atmospheric CO₂ and soil nitrogen, we examined the hypothesis that phylogeny would influence plant performance in response to elevated CO₂ and nitrogen fertilization. We found highly idiosyncratic responses at the species level. However, significant, among-genetic lineage responses were present across a molecularly determined phylogeny, indicating that past evolutionary history may have an important role in the response of whole genetic lineages to future global change. These data imply that some genetic lineages will perform well and that others will not, depending upon the environmental context.     

The rare silver gum, Eucalyptus cordata, is leaving its trace in the organellar gene pool of Eucalyptus globulus

The process of genetic assimilation of rare species by hybridizing congeners has been documented in a number of plant genera. This raises the possibility that some of the genetic diversity found in phylogeographical studies of widespread species has been acquired through hybridization with species that are now rare or extinct. In this fine-scale phylogeographical analysis, we show that a rare eucalypt species is leaving its trace in the chloroplast genome of a more abundant congener. The heart-leafed silver gum, Eucalyptus cordata, is a rare endemic of south-eastern Tasmania. Its populations are scattered amidst populations of more abundant related species, including the Tasmanian blue gum, Eucalyptus globulus. Using 339 samples from across the full range of both species, we compared chloroplast (cp) DNA haplotype phylogeography in E. globulus and E. cordata. The genealogy and distribution of chloroplast haplotypes suggest that E. globulus has acquired cpDNA from E. cordata in at least four different mixed populations. Shared haplotypes are highest in E. globulus sampled within 2 km of known E. cordata populations and drop to zero at a distance of 25 km from the nearest known E. cordata population. Localized haplotype sharing occurs in the absence of obvious hybrid zones or locally shared nuclear ribosomal DNA sequences. Given that the future loss of E. cordata from some mixed populations is likely, these findings indicate that phylogeographical analyses of organellar DNA should consider the possibility of introgression, even from species that have been eliminated from the sites of interest.     

ITS sequence data resolve higher level relationships among the eucalypts.

Sequences of the internal transcribed spacer (ITS) region of the nuclear ribosomal DNA were obtained for 35 species of Eucalyptus s.s. and seven taxa representing five outgroup genera (Allosyncarpia, Angophora, Arillastrum, Corymbia, and Stockwellia). The sequences were analyzed cladistically. The data distinguished clearly between the two major subgenera of Eucalyptus s.s. (Symphyomyrtus and Monocalyptus) but indicated that subgenus Eudesmia may be paraphyletic. ITS sequence data demonstrated the potential to resolve relationships between sections within subgenus Symphyomyrtus. Within sections, however, taxa were poorly differentiated. At the generic level, Corymbia appeared to be paraphyletic due to the exclusion of Angophora. The positions of Allosyncarpia and Arillastrum relative to the ingroup remain unresolved. ITS sequence data may prove valuable for resolving other phylogenetic relationships at higher taxonomic levels within Eucalyptus.