Phylogenomics of C(4) photosynthesis in sedges (Cyperaceae): multiple appearances and genetic convergence.

C(4) photosynthesis is an adaptive trait conferring an advantage in warm and open habitats. It originated multiple times and is currently reported in 18 plant families. It has been recently shown that phosphoenolpyruvate carboxylase (PEPC), a key enzyme of the C(4) pathway, evolved through numerous independent but convergent genetic changes in grasses (Poaceae). To compare the genetics of multiple C(4) origins on a broader scale, we reconstructed the evolutionary history of the C(4) pathway in sedges (Cyperaceae), the second most species-rich C(4) family. A sedge phylogeny based on two plastome genes (rbcL and ndhF) has previously identified six fully C(4) clades. Here, a relaxed molecular clock was used to calibrate this tree and showed that the first C(4) acquisition occurred in this family between 19.6 and 10.1 Ma. According to analyses of PEPC-encoding genes (ppc), at least five distinct C(4) origins are present in sedges. Two C(4) Eleocharis species, which were unrelated in the plastid phylogeny, acquired their C(4)-specific PEPC genes from a single source, probably through reticulate evolution or a horizontal transfer event. Acquisitions of C(4) PEPC in sedges have been driven by positive selection on at least 16 codons (3.5% of the studied gene segment). These sites underwent parallel genetic changes across the five sedge C(4) origins. Five of these sites underwent identical changes also in grass and eudicot C(4) lineages, indicating that genetic convergence is most important within families but that identical genetic changes occurred even among distantly related taxa. These lines of evidence give new insights into the constraints that govern molecular evolution.

Prograde mica 40Ar/39Ar growth ages recorded in high pressure rocks (Syros, Cyclades, Greece)

Phengites from the eclogite and blueschist-facies sequences of the Cycladic island of Syros (Greece) have been dated by the in situ UV-laser ablation Ar-40/Ar-39 method. A massive, phengite-rich eclogite and an omphacite-rich metagabbro were investigated. The phengites are eubedral and coarse-grained (several 100 mum), strain-free and exhibit no evidence for late brittle deformation or recrystallization. Apparent ages in these samples range from 43 to 50 Ma for the phengite-rich eclogite and 42 to 52 Ma for the ompbacitic metagabbro. This large spread of ages is visible at all scales-within individual grains as well as in domains of several 100 mum and across the entire sample (ca. 2 cm). Such variations have been traditionally attributed to metamorphic cooling or the incorporation of excess argon. However, the textural equilibrium between the phengites and other high pressure phases and the subtle compositional variations within the phengites, especially the preservation of growth textures, alternatively suggest that the observed range in ages may reflect variations of radiogenic argon acquired during phengite formation and subsequent growth, thus dating a discrete event on the prograde path. This implies that the oldest phengite 40Ar/39Ar ages provide the best estimate of a minimum crystallization age, which is in agreement with recently reported U-Pb and Lu-Hf geochronological data. Our results are consistent with available stable isotope data and further suggest that, under fluid-restricted conditions, both stable and radiogenic isotopic systems can survive without significant isotopic exchange during subduction and exhumation from eclogite-facies P-T conditions. (C) 2004 Elsevier B.V. All rights reserved.

The sandstone-hosted Beverley uranium Deposit, Lake Frome Basin, South Australia: Mineralogy, geochemistry, and a time-constrained model for its genesis

The sandstone-hosted Beverley uranium deposit is located in terrestrial sediments in the Lake Frome basin in the North Flinders Ranges, South Australia. The deposit is 13 km from the U-rich Mesoproterozoic basement of the Mount Painter inlier, which is being uplifted 100 to 200 m above the basin by neotectonic activity that probably initiated in the early Pliocene. The mineralization was deposited mainly in organic matter-poor Miocene lacustrine sands and partly in the underlying reductive strata comprising organic matter-rich clays and silts. The bulk of the mineralization consists of coffinite and/or uraninite nodules, growing around Co-rich pyrite with an S isotope composition (delta S-34 = 1.0 +/- 0.3 parts per thousand), suggestive of an early diagenetic lacustrine origin. In contrast, authigenic sulfides in the bulk of the sediments have a negative S isotope signature (delta S-34 ranges from -26.2 to -35.5 parts per thousand), indicative of an origin via bacterially mediated sulfate reduction. Minor amounts of Zn-bearing native copper and native lead also support the presence of specific, reducing microenvironments in the ore zone. Small amounts of carnotite are associated with the coffinite ore and also occur beneath a paleosoil horizon overlying the uranium deposit. Provenance studies suggest that the host Miocene sediments were derived from the reworking of Early Cretaceous glacial or glaciolacustrine sediments ultimately derived from Paleozoic terranes in eastern Australia. In contrast, the overlying Pliocene strata were in part derived from the Mesoproterozoic basement inlier. Mass-balance and geochemical data confirm that granites of the Mount Painter domain were the ultimate source of U and BEE at Beverley. U-Pb dating of coffinite and carnotite suggest that the U mineralization is Pliocene (6.7-3.4 Ma). The suitability of the Beverley deposit for efficient mining via in situ leaching, and hence its economic value, are determined by the nature of the hosting sand unit, which provides the permeability and low reactivity required for high fluid flow and low chemical consumption. These favorable sedimentologic and geometrical features result from a complex conjunction of factors, including deposition in lacustrine shore environment, reworking of angular sands of glacial origin, deep Pliocene weathering, and proximity to an active fault exposing extremely U rich rocks.