Left-right axis formation in the dorsalized or ventralized Xenopus laevis embryos : Does dorsal-midline development affect the left-right orientation of visceral organs?

Yostら(1995)は、ツメガエル受精卵の表層回転を阻害することで頭部組織を欠損させた胚や、卵割期の背側割球にXwnt-8 mRNAを微量注射することで頭部組織を欠損させた胚において、心臓逆位が高率に生じることを発見した。彼らはその理由について「頭部中軸組織は内臓の左右軸の決定に関与する因子を放出するので、頭部が欠けると内臓が左右軸の手がかりを失い内臓逆位が生じる」とした。筆者らは、彼らとは異なる以下の3つの処理方法を用いて頭部欠損症状を持った胚を人工的に作出し、それらの心臓や腸管の左右非対称性の正位/逆位を調べた。頭部欠損の程度と内臓逆位の出現率とが相関するか否かを解析することで、Yostらの仮説の検証を試みた。胚の頭部形成を阻害することが知られるレチノイン酸で短時間暴露処理(パルス処理)したツメガエル胞胚∿神経胚の内臓逆位の出現率について調べた。1-10μMレチノイン酸処理胚は著しい頭部欠損症状を示したにもかかわらず、それらの内臓逆位出現率は1%(3/263)と、無処理胚における逆位の自然発生率と同レベルであった。カルシウムイオノフォアA23187のパルス処理によって、発生段階依存的に頭部欠損胚または尾部欠損胚が得られる(後藤ら, 1994)が、その場合の内臓逆位出現率について調べた。後藤らの報告では、初期卵割期の処理では胴尾部欠損胚が得られ、卵割後期∿胞胚期の処理では頭部欠損胚が得られるとあるが、我々の実験結果はそれと正反対のものであった。初期卵割期の処理では頭部欠損が見られ、後期胞胚∿原腸胚期の処理では胴尾部欠損(特に尾部欠損)が観察された。原腸胚期にA23187処理を行った胚の63%(15/24)が内臓逆位を示した。処理によって尾部を欠損した胚の多くは内臓逆位胚であった。塩化リチウム処理で内臓逆位が生じることが、高谷(1949)によって明らかにされており、一方、品川ら(1989)により胞胚期∿原腸胚期のリチウム処理で頭部欠損胚が得られることが報告されている。そこで、リチウム処理による頭部欠損の程度と内臓逆位の出現との相関について調べた。原腸胚期のリチウム処理で、11%(8/72)の個体に内臓逆位が生じた。同じ処理群の中に頭部が欠損した胚も多数得られ、頭尾軸欠損指標Dorso-Anterior Index (Kao & Elinson, 1988)の平均は4.35であった(n=72)。しかしながら、個々の胚について頭部欠損の程度と逆位発生との相関を調べると、逆位胚の75%はDAI 5の頭尾軸は正常な胚であり、頭部欠損の度合いと内臓逆位出現率とは相関がなかった。以上の結果から、「胞胚期以降の胚においては、内臓の左右性に関連する位置情報はすでに心臓や腸管の予定細胞群自身が獲得していて、それらの左右軸は、頭部組織の有無に影響を受けない程度には決定されている。」と考えられる

Interlaboratory study for coral Sr/Ca and other element/Ca ratio measurements

The Sr/Ca ratio of coral aragonite is used to reconstruct past sea surface temperature (SST). Twentyone laboratories took part in an interlaboratory study of coral Sr/Ca measurements. Results show interlaboratory bias can be significant, and in the extreme case could result in a range in SST estimates of 7°C. However, most of the data fall within a narrower range and the Porites coral reference material JCp- 1 is now characterized well enough to have a certified Sr/Ca value of 8.838 mmol/mol with an expanded uncertainty of 0.089 mmol/mol following International Association of Geoanalysts (IAG) guidelines. This uncertainty, at the 95% confidence level, equates to 1.5°C for SST estimates using Porites, so is approaching fitness for purpose. The comparable median within laboratory error is <0.5°C. This difference in uncertainties illustrates the interlaboratory bias component that should be reduced through the use of reference materials like the JCp-1. There are many potential sources contributing to biases in comparative methods but traces of Sr in Ca standards and uncertainties in reference solution composition can account for half of the combined uncertainty. Consensus values that fulfil the requirements to be certified values were also obtained for Mg/Ca in JCp-1 and for Sr/Ca and Mg/Ca ratios in the JCt-1 giant clam reference material. Reference values with variable fitness for purpose have also been obtained for Li/Ca, B/Ca, Ba/Ca, and U/Ca in both reference materials. In future, studies reporting coral element/Ca data should also report the average value obtained for a reference material such as the JCp-1

Carbon and Oxygen Isotope Records from Tridacna derasa Shells: Toward Establishing a Reliable Proxy for Sea Surface Environments.

We report the carbon (δ13C) and oxygen (δ18O) isotope records of three modern Tridacna derasa shells from Ishigaki-jima, southwestern Japan. The high-resolution δ13C profiles of samples from the inner shell layer on cross-sections fall within similar narrow ranges and display no regular variations or trends, such as an ontogenetic trend or abrupt short-term drops likely to be related to reproductive activity. This suggests that the calcification site of this species is not likely affected by photosynthetic CO2 uptake or CO2 incorporation during respiration. The δ18O profiles show distinct seasonal cycles. The intraspecific variability in the δ18O values is small in parts of the shell precipitated in the adult stage, but is not negligible in the juvenile and senescent stages. The differences in the monthly and seasonally resolved δ18O values among shells are less than 0.51‰ and 0.76‰, respectively. The shell δ18O values are nearly identical or close to the δ18O values for aragonite precipitated in oxygen isotope equilibrium with ambient seawater (δ18OEA). The largest differences between the shell δ18O and δ18OEA values calculated from the monthly and seasonally resolved data correspond to an overestimate of the seawater temperature by as much as 1.7°C and 2.3°C, respectively. However, these differences are smaller in the adult stage (<0.25‰) than in the other stages. This small difference allows an accurate reconstruction of the seawater temperature with an error of <1.1°C. Consequently, we recommend that multiple shell records be obtained because of the non-negligible intraspecific variations in their δ18O values. Growth banding, composed of alternating narrow white bands and wide light-grey bands, is discernible on cross-sections of the inner shell layer. The δ18Oshell data indicate that they were formed in winter and the other seasons, respectively