Effects of nutritional restriction on nitrogen and carbon stable isotopes in growing seabirds

When using stable isotopes as dietary tracers it is essential to consider effects of nutritional state on isotopic fractionation. While starvation is known to induce enrichment of ¹⁵N in body tissues, effects of moderate food restriction on isotope signatures have rarely been tested. We conducted two experiments to investigate effects of a 50–55% reduction in food intake on δ¹⁵N and δ¹³C values in blood cells and whole blood of tufted puffin chicks, a species that exhibits a variety of adaptive responses to nutritional deficits. We found that blood from puffin chicks fed ad libitum became enriched in ¹⁵N and ¹³C compared to food-restricted chicks. Our results show that ¹⁵N enrichment is not always associated with food deprivation and argue effects of growth on diet–tissue fractionation of nitrogen stable isotopes (Δ¹⁵N) need to be considered in stable isotope studies. The decrease in δ¹³C of whole blood and blood cells in restricted birds is likely due to incorporation of carbon from ¹³C-depleted lipids into proteins. Effects of nutritional restriction on δ¹⁵N and δ¹³C values were relatively small in both experiments (δ¹⁵N: 0.77 and 0.41‰, δ¹³C: 0.20 and 0.25‰) compared to effects of ecological processes, indicating physiological effects do not preclude the use of carbon and nitrogen stable isotopes in studies of seabird ecology. Nevertheless, our results demonstrate that physiological processes affect nitrogen and carbon stable isotopes in growing birds and we caution isotope ecologists to consider these effects to avoid drawing spurious conclusions.     

Chloroplast lipid transfer processes in Chlamydomonas reinhardtii involving a TRIGALACTOSYLDIACYLGLYCEROL 2 (TGD2) orthologue

In plants, lipids of the photosynthetic membrane are synthesized by parallel pathways associated with the endoplasmic reticulum (ER) and the chloroplast envelope membranes. Lipids derived from the two pathways are distinguished by their acyl‐constituents. Following this plant paradigm, the prevalent acyl composition of chloroplast lipids suggests that Chlamydomonas reinhardtii (Chlamydomonas) does not use the ER pathway; however, the Chlamydomonas genome encodes presumed plant orthologues of a chloroplast lipid transporter consisting of TGD (TRIGALACTOSYLDIACYLGLYCEROL) proteins that are required for ER‐to‐chloroplast lipid trafficking in plants. To resolve this conundrum, we identified a mutant of Chlamydomonas deleted in the TGD2 gene and characterized the respective protein, CrTGD2. Notably, the viability of the mutant was reduced, showing the importance of CrTGD2. Galactoglycerolipid metabolism was altered in the tgd2 mutant with monogalactosyldiacylglycerol (MGDG) synthase activity being strongly stimulated. We hypothesize this to be a result of phosphatidic acid accumulation in the chloroplast outer envelope membrane, the location of MGDG synthase in Chlamydomonas. Concomitantly, increased conversion of MGDG into triacylglycerol (TAG) was observed. This TAG accumulated in lipid droplets in the tgd2 mutant under normal growth conditions. Labeling kinetics indicate that Chlamydomonas can import lipid precursors from the ER, a process that is impaired in the tgd2 mutant.     

On the serial homology of the pectoral and pelvic girdles of tetrapods

While fore‐ and hindlimbs are commonly assumed to be serially homologous, the serial homology of the pectoral and pelvic girdles is more ambiguous. We investigate the degree to which a common history, developmental program, and gene network are shared between the girdles relative to the rest of the appendicular skeleton. Paleontological data indicate that pectoral appendages arose millions of years before pelvic appendages. Recent embryological and genetic data suggest that the anatomical similarity between the fore‐ and hindlimbs arose through the sequential, derived deployment of similar developmental programs and gene networks, and is therefore not due to ancestral serial homology. Much less developmental work has however been published about the girdles. Here, we provide the first detailed review of the developmental programs and gene networks of the pectoral and pelvic girdles. Our review shows that, with respect to these programs and networks, there are fewer similarities between pelvic and pectoral girdles than there are between the limbs. The available data therefore support recent hypotheses that the anatomical similarities between the fore‐ and hindlimbs arose during the fin‐to‐limb transition through the derived co‐option of similar developmental mechanisms, while the phylogenetically older pectoral and pelvic girdles have remained more distinct since their evolutionary origin.     

Developmental basis of mammalian digit reduction: a case study in pigs

Digit reduction has occurred in parallel in many mammalian lineages. However, despite this pattern's prevalence, the developmental mechanisms underlying mammalian digit reduction remain controversial. We therefore undertook a study of digit development in the pig (Sus scrofa), a mammal with reduced first, second, and fifth digits. Our results indicate that from its earliest formation, the pig limb bud is significantly narrower than that of the model pentadactyl mammal, mouse. Furthermore, the cartilage condensations of the pig's reduced digits are noticeably smaller than those of their nonreduced counterparts from the time of their formation. In addition, growth rates of pig digits are comparable, as are the patterns of cell death in developing pig and mouse limbs. Taken together, results suggest that pig's first, second, and fifth digits are primarily reduced through evolutionary modifications in the early developmental patterning of their limbs. Results of this study, coupled with those from study of limb development in other mammals, suggest that although major developmental reorganizations (e.g., complete digit or limb loss) during early limb development may be selected against, it may be common for more subtle evolutionary modifications in limb development (e.g., changes in relative digit size) to occur at this time.     

Characterization of the Type III restriction endonuclease PstII from Providencia stuartii

A new Type III restriction endonuclease designated PstII has been purified from Providencia stuartii. PstII recognizes the hexanucleotide sequence 5′-CTGATG(N)_25-26/27-28-3′. Endonuclease activity requires a substrate with two copies of the recognition site in head-to-head repeat and is dependent on a low level of ATP hydrolysis (~40 ATP/site/min). Cleavage occurs at just one of the two sites and results in a staggered cut 25–26 nt downstream of the top strand sequence to generate a two base 5′-protruding end. Methylation of the site occurs on one strand only at the first adenine of 5′-CATCAG-3′. Therefore, PstII has characteristic Type III restriction enzyme activity as exemplified by EcoPI or EcoP15I. Moreover, sequence asymmetry of the PstII recognition site in the T7 genome acts as an historical imprint of Type III restriction activity in vivo. In contrast to other Type I and III enzymes, PstII has a more relaxed nucleotide specificity and can cut DNA with GTP and CTP (but not UTP). We also demonstrate that PstII and EcoP15I cannot interact and cleave a DNA substrate suggesting that Type III enzymes must make specific protein–protein contacts to activate endonuclease activity.