Holocene coquina beachrock from Haishan Island, east coast of Guangdong Province, China

International audienceCoquina beachrock, with a thickness of 10 m and an exposed surface area covering 3.4 km2, is well developed along the southern coast of Haishan Island on the east coast of Guangdong Province, South China. Part of the deposit not treated in this study also extends under the surface of the sea. The coquina beachrock was deposited in a large embayment semi-enclosed by palaeocoastal hills and islands, but with good marine circulation. Holocene strata on Haishan Island demonstrate a mixed carbonate/siliciclastic regime in a series of fining- and deepening-upward shelly limestone successions. Mass accumulations of whole and fragmented mollusc shells are the most remarkable features expressed throughout the series. Coralline red algae are commonly found overgrowing the bivalve bioclasts. Other bioclasts include gastropods, coralline red algae, bryozoans, foraminifera, and scleractinian coral. Cement is well developed within the interstices formed by the bioclasts, showing two or three generations of development. On both a qualitative and quantitative basis, the consecutive coquina series document an overall shallowing-upward pattern of beachrock deposition, which was strongly influenced by marine transgression on Haishan Island. Dating by AMS 14C indicates an age in the Middle to early Late Holocene for these coquina beachrock deposits. Siliciclasts incorporated within the beachrock deposits mainly consist of quartz grains. Not fully hardened, the binding cement is high in calcium carbonate, including aragonite mud, fibrous aragonite and granular calcite. Such carbonate deposits dominated by mollusc bioclasts with quartz grains indicate a pattern of littoral sedimentation. The massive coquina beachrock on the southern coast of Haishan Island formed during Middle to early Late Holocene time, and represents a highstand of sea level from 4000 BP to 2800 BP with short-lived and rapid sea level fall

Genome-wide association analysis identifies resistance loci for bacterial blight in a diverse collection of indica rice germplasm

Bacterial blight, which is caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most devastating rice diseases worldwide. The development and use of disease-resistant cultivars have been the most effective strategy to control bacterial blight. Identifying the genes mediating bacterial blight resistance is a prerequisite for breeding cultivars with broad-spectrum and durable resistance. We herein describe a genome-wide association study involving 172 diverse Oryza sativa ssp. indica accessions to identify loci influencing the resistance to representative strains of six Xoo races. Twelve resistance loci containing 121 significantly associated signals were identified using 317,894 single nucleotide polymorphisms, which explained 13.3–59.9% of the variability in lesion length caused by Xoo races P1, P6, and P9a. Two hotspot regions (L11 and L12) were located within or nearby two cloned R genes (xa25 and Xa26) and one fine-mapped R gene (Xa4). Our results confirmed the relatively high resolution of genome-wide association studies. Moreover, we detected novel significant associations on chromosomes 2, 3, and 6–10. Haplotype analyses of xa25, the Xa26 paralog (MRKc; LOC_Os11g47290), and a Xa4 candidate gene (LOC_11g46870) revealed differences in bacterial blight resistance among indica subgroups. These differences were responsible for the observed variations in lesion lengths resulting from infections by Xoo races P1 and P9a. Our findings may be relevant for future studies involving bacterial blight resistance gene cloning, and provide insights into the genetic basis for bacterial blight resistance in indica rice, which may be useful for knowledge-based crop improvement. (Résumé d'auteur