Jun-Yuan Chen

Fitness costs of R-gene-mediated resistance in Arabidopsis thaliana.

Resistance genes (R-genes) act as an immune system in plants by recognizing pathogens and inducing defensive pathways. Many R-gene loci are present in plant genomes, presumably reflecting the need to maintain a large repertoire of resistance alleles. These loci also often segregate for resistance and susceptibility alleles that natural selection has maintained as polymorphisms within a species for millions of years. Given the obvious advantage to an individual of being disease resistant, what prevents these resistance alleles from being driven to fixation by natural selection? A cost of resistance is one potential explanation; most models require a lower fitness of resistant individuals in the absence of pathogens for long-term persistence of susceptibility alleles. Here we test for the presence of a cost of resistance at the RPM1 locus of Arabidopsis thaliana. Results of a field experiment comparing the fitness of isogenic strains that differ in the presence or absence of RPM1 and its natural promoter reveal a large cost of RPM1, providing the first evidence that costs contribute to the maintenance of an ancient R-gene polymorphism.

An Early Cambrian craniate-like chordate

Since the identification of the Lower Cambrian Yunnanozoon as a chordate in 1995 (ref. 1), large numbers of complete specimens of soft-bodied chordates from the Lower Cambrian Maotianshan Shale in central Yunnan (southern China) have been recovered. Here we describe a recently discovered craniate-like chordate, Haikouella lanceolata, from 305 fossil specimens in Haikou near Kunming. This 530 million-year-old (Myr) fish-like animal resembles the contemporaneous Yunnanozoon from the Chengjiang fauna (about 35 km southeast of Haikou) in several anatomic features. But Haikouella also has several additional anatomic features: a heart, ventral and dorsal aorta, an anterior branchial arterial, gill filaments, a caudal projection, a neural cord with a relatively large brain, a head with possible lateral eyes, and a ventrally situated buccal cavity with short tentacles. These findings indicate that Haikouella probably represents a very early craniate-like chordate that lived near the beginning of the Cambrian period during the main burst of the Cambrian explosion. These findings will add to the debate on the evolutionary transition from invertebrate to vertebrate.

A new ‘great‐appendage’ arthropod from the Lower Cambrian of China and homology of chelicerate chelicerae and raptorial antero‐ventral appendages

The uniramous ‘great appendages’ of several arthropods from the Early to Middle Cambrian are a characteristic pair of pre-oral limbs, which served for prey capture. It has been assumed that the morphological differences between the ‘great-appendage’ arthropods indicate that raptorial antero-ventral and anteriorly pointing appendages evolved more than once in arthropod phylogeny. One set of Cambrian ‘great-appendage’ arthropods has, however, very similar short antero-ventral appendages with a peduncle of two segments angled against each other (elbowed) and with stout distally or medio-distally directed spines or long flexible flagellate spines on each of the four distal segments. Moreover, the head appendages of all these forms comprise the ‘great appendages’ and three pairs of biramous limbs. To this set of taxa we can add a new form from the Lower Cambrian Maotianshan Shale of southern China, Haikoucaris ercaiensis n. gen. and n. sp. It is known from three specimens, possibly being little abundant in the faunal community. It can be distinguished from all other taxa by the prominence of the proximal claw segment of its ‘great appendages’ and by only three distal spines (one on each of the distal segments). The similarity of the short, spiky ‘great appendages’ of Haikoucaris with the chelicera of the Chelicerata leads us to hypothesize that this particular type of ‘great appendages’ was the actual precursor of the chelicera. Homeobox gene and developmental data recently demonstrated the homology between the antenna of ateloceratans and the antennula of crustaceans on one side and the chelicera of chelicerates on the other. To this we add palaeontological evidence for the homology between the chelicerae of chelicerates and the ‘short great appendages’ of certain Cambrian arthropods, which leads us to hypothesize that the evolutionary path went from the ‘short great appendages’, by progressive compaction, toward the chelicera with only a two-spined chela. The new form from China is regarded as the possible latest offshoot, whereas the other ‘great appendages’ arthropods with similar short grasping limbs were derivatives of the stem lineage of the crown-group Chelicerata. Consequently, the chelicera with a chela with one fixed and one mobile finger is an autapomorphy of the crown group of Chelicerata, whereas a raptorial, but more limb-like antenna, with more distal spine-bearing segments, characterized the ground pattern of Chelicerata. Further taxa having ‘great appendages’, including the large Anomalocarididae, are also discussed in the light of their possible affinities to the Chelicerata and possible monophyly of all of these arthropods with raptorial anterior appendages.

The first tunicate from the Early Cambrian of South China

Here we report the discovery of eight specimens of an Early Cambrian fossil tunicate Shankouclava near Kunming (South China). The tunicate identity of this organism is supported by the presence of a large and perforated branchial basket, a sac-like peri-pharyngeal atrium, an oral siphon with apparent oral tentacles at the basal end of the siphonal chamber, perhaps a dorsal atrial pore, and an elongated endostyle on the mid-ventral floor of the pharynx. As in most modern tunicates, the gut is simple and U-shaped, and is connected with posterior end of the pharynx at one end and with an atrial siphon at the other, anal end. Shankouclava differs from Cheungkongella, which was previously called a tunicate. Based on new, more complete “Cheungkongella” specimens that show branching tentacles, this form may be a lophophorate, and in any case is not a tunicate.

Raman spectra of a Lower Cambrian ctenophore embryo from southwestern Shaanxi, China

The Early Cambrian (≈540 million years old) Meishucun fossil assemblage of Ningqiang County (Shaanxi Province), China, contains the oldest complex skeletonized organisms known in the geological record. We here report the finding in this assemblage of an exquisitely preserved late-stage embryo of a ctenophore (“comb jelly”), its fine structure documented by confocal laser scanning microscopy and shown by Raman spectroscopy to be composed of carbonaceous kerogen permineralized in apatite. In its spheroidal morphology, the presence of eight comb rows and the absence of tentacles, this embryo resembles an adult ctenophore (Maotianoascus octonarius) known from the immediately younger Chengjiang fauna of Yunnan, China. The oldest ctenophore and the only embryonic comb jelly known from the fossil record, this exceptionally well preserved specimen provides important clues about the early evolution of the phylum Ctenophora and of metazoans in general.

Phylogeny and life habits of Early Arthropods——Predation in the Early Cambrian Sea

Abstract We investigated two new arthropods from the Maotianshan-Shale fauna of southern China in the course of our research on life strategies, particularly predation, in Early Cambrian marine macrofaunal biota. One form clearly belongs to the so-called “great-appendage” arthropods, animals that were, most likely, active predators catching prey with their first pair of large, specialized frontoventral appendages. Based on this, we hypothesize that the new species and many others, if not all of the “great-appendage” arthropods were derivatives of the chelicerate stem lineage and not forms having branched off at different nodes along the evolutionary lineage of the Arthropoda. Rather, we consider the “great-appendage” arthropods as belonging to a monophyletic clade, which modified autapomorphically their first pair of appendages (antennae in general arthropod terminology) into raptorial organs for food capture. The second new form resembles another Maotianshan-Shale arthropod. Fuxianhuia protensa, in shari...

Spatial and temporal variations in carbon and sulfur isotopic compositions of Sinian sedimentary rocks in the Yangtze platform, South China

Abstract Eight stratigraphic sections from the Yangtze Platform, South China were selected for a study of spatial and temporal variations in carbon and sulfur isotope compositions of Sinian sedimentary rocks. Carbon isotope compositions of carbonate from lower Sinian strata deposited between pulses of glacial diamictite range from −3.9 to −9.9‰; the more negative values recorded from Mn carbonates suggest that, in part, the carbon was derived from the oxidation of organic matter during early diagenesis. In the upper Sinian strata δ13C varies with the facies. They are positive for the platform carbonates, whereas they are negative from about −3 to −12‰ for the carbonates formed in or near basin environments. It is evident that post-depositional alteration accounts for the strongly negative δ13C values. However, some information on primary isotopic compositions can be obtained by evaluating sample quality. The primary δ13C values of carbonates from the Doushantuo Formation are ca −3 to −4‰, and from the Dengying Formation as low as −6‰. δ34S values for pyrite from the lower Sinian rocks are highly positive, and range from values near the coeval seawater sulfate up to ca +60‰. In contrast, δ34S values for pyrite from the upper Sinina rocks are negative down to −27 to −30‰. The feature of sulfur isotopic composition for the lower Sinian rocks is consistent with the late Proterozoic sulfur isotopic record. It might be explained by the ‘supercontinent model’: many basins on the Yangtze block in a supercontinent tectonic background had limited or no access to the open ocean and the basin waters contained sulfate with considerably higher δ34S values than the coeval seawater for a geologically long period. An alternative explanation is based on the sulfate-minimum zone (SMZ) modal. Because of the slow sinking of organic matter bacterial sulfate reduction would be important in the lower Sinian shallow water environment. Particularly in the SMZ, the residual sulfate might have been significantly enriched in 34S, so that pyrite produced in closed diagenetic system would yield very positive δ34S values.

Information landscapes and sensory ecology of the Cambrian Radiation

Abstract Organisms emit, detect, and respond to a huge array of environmental signals. The distribution of a given signal is dependent, first of all, upon the original spatial distribution of signal sources, the source landscape. The signal sources can be fixed or moving and their output can be stable or ephemeral. Different sources can also occupy the same general spatial location, such as insects living on a host plant. The emitted signals are modified by relevant transport processes, which are often strongly scale and environment dependent. Chemical signals, for example, are propagated by diffusion and turbulence. The resulting complex, three-dimensional, and dynamic distribution of signals in the environment is the signal landscape; it is the environment of potentially available information in which sensory systems function and have evolved. Organisms also differ widely in what signals they can actually detect; the distribution of signals that an organism can potentially respond to is its information landscape. Although increasing the kinds and specificity of signals that can be detected and processed can lead to improved decision making, it almost always comes at an increased cost. The greater the spatial and temporal complexity of the environment, the greater are the costs of incomplete information and the more advantageous is the development of improved information-gathering capabilities. Studies with simulation models suggest how variability in the spatial structure of source and signal landscapes may control patterns of animal movement that could be represented in the trace fossil record. Information landscapes and the corresponding sensory systems should have evolved in concert with major transitions in the history of life. The Ediacaran to Cambrian interval is one of the most intensively studied periods in the history of life, characterized by the profound environmental and biological changes associated with the bilaterian radiation. These include the advent of macroscopic predation, an increase in the size and energy content of organisms, and the transition in seafloors from laminated matgrounds to mixgrounds produced by the development of macroscopic infaunal bioturbation. The overall effect of these transitions was to markedly increase the spatial complexity of the marine environment. We suggest that this increased spatial complexity, in turn, drove the evolution of macroscopic sense organs in mobile bilaterians, leading to their first appearance during the Cambrian. The morphology and distribution of these sense organs should reflect the life habits of the animals that possessed them. Our overall hypothesis was that there was a “Cambrian Information Revolution,” a coevolutionary increase in the information content of the marine environment and in the ability of and necessity for organisms to obtain and process this information. A preliminary analysis of the Maotianshan Shale (Chengjiang) biota indicates that the distribution of eyes and antennae in these animals is consistent with predictions based on their life habit.

Lower Cambrian yolk-pyramid embryos from Southern Shaanxi, China

Abstract Phosphatized globules with radially arranged pillars were recently recorded from the Lower Cambrian phosphate deposit. Ningqiang, Shaanxi by Yue and Bengtson. These authors interpreted the globules as blastula stage of embryos and the pillars as blas- tomeres. On the basis of new additional material, we reinterpret thesestructures as yolk-pyramid stages of possible arthropod eggs. The 20 embryos under present study range from 380 μm to 600 μm in diameter and contain about 120 pyramids. Some embryos having a higher number of pyramids are tentatively interpreted as slightly later developmental stages of the same animal. These 543- million-year-old embryos may push back the evolutionary history of the arthropods to a deeper time and also suggest that one important pattern of arthropod development was already present at the beginning of the Cambrian.

An Early Cambrian problematic fossil: Vetustovermis and its possible affinities

The Early Cambrian problematic fossil Vetustovermis (Glaessner 1979 Alcheringa 3, 21–31) was described as an annelid or arthropod. Anatomical analysis of 17 new specimens from the Lower Cambrian Maotianshan Shale at Anning, Kunming (South China) does not support its affinities with annelids or arthropods. Anatomical features instead resemble other animal groups including modern flatworms, nemertines and molluscs. The presence of a pelagic slug-like form and ventral foot, as well as a head with eyes and tentacles indicates a possible affinity with molluscs, but these characters are not present only in molluscs; some of them are shared with other animal groups, including flatworms and nemertines. For example, a ventral foot-like structure is found in nemertines, ‘turbellarians’, and some polychaete groups. The well differentiated head is seen in separate bilaterian groups, but among molluscs it did not occur before the evolutionary level of the Conchifera. Unlike the ctenia-gills in molluscs, the gills in Vetustovermis are bar-like. All the characters displayed in this 525 million-year old soft-bodied animal fail to demonstrate clear affinity with molluscs or any other known extant or extinct animal groups, but argue for representing an independently evolved animal group, which flourished in Early Cambrian and possibly in Middle Cambrian time.