Thermal adaptation in biological membranes: functional significance of changes in phospholipid molecular species composition.

Quantities of 1-palmitoyl 2-docosahexaenoyl phosphatidylcholine (16:0/22:6-PC or PDPC) increase from 24 to 40 weight percent as a consequence of cold acclimation in mitochondrial membranes of rainbow trout liver (J. Comp. Physiol. 156, 665-674, 1986). The present study was undertaken to assess the impact of such a large change in the proportions of a single molecular species on the fluidity, lateral packing (as sensed by phospholipase A2), and permeability of biological membranes. These properties were examined in multilamellar liposomes prepared from binary mixtures of dipalmitoyl phosphatidylcholine (DPPC) and PDPC in proportions increasing from 10 to 40 mole% PDPC. Glucose permeability was positively correlated with both assay temperature and PDPC content. The temperature dependence of Na+ permeability declined steadily as the mole fraction of PDPC increased; consequently, sodium permeability was positively correlated with PDPC content at 5 degrees C, but inversely correlated at 20 degrees C. Phospholipase A2 activity was independent of both assay temperature and vesicle composition. Vesicles of all compositions displayed a single transition in the temperature dependence of 1,6 diphenyl-1,3,5-hexatriene (DPH) fluorescence polarization, which shifted to lower temperature and broadened as proportions of PDPC increased. At temperatures below the transition, fluidity was positively correlated with the mole fraction of PDPC, but interfacial and deeper regions of the bilayer were affected differently by variations in PDPC content. Nonelectrolyte permeability was the only index of membrane structure or function to be significantly correlated with the fluidity of the bilayer interior. The tendencies of PDPC to both fluidize the membrane and to reduce the temperature sensitivity of electrolyte permeation may promote the adaptation of membrane function to low temperature.

Acylation of lysophosphatidylcholine in liver microsomes of thermally acclimated trout.

The specificity of acyl-coenzyme A (CoA): lysophosphatidylcholine acyltransferase (LPCAT; EC 2.3.1.23) was determined for a range for acyl-CoA substrates differing with respect to chain length and degree of unsaturation in liver microsomes of thermally acclimated (5 and 20 degrees C) rainbow trout, Salmo gairdneri. Absolute levels of oleate incorporation into phosphatidylcholine (PC) were determined at substrate concentrations in the physiological range (12 microM) and higher (64 microM). The specificity of LPCAT was determined by the extent to which competing substrates decreased the incorporation of oleate. LPCAT specificity was significantly influenced by both assay and acclimation temperature at total substrate concentrations of both 72 and 256 microM. A clear preference for 14- and 16-carbon monoenes was exhibited by LPCAT from 20 but not 5 degrees C-acclimated trout. Furthermore, LPCAT from 5 degrees C-acclimated trout preferentially incorporated long chain and polyunsaturated fatty acids (eicosadienoyl, arachidonoyl, erucoyl) and excluded 18-carbon unsaturates at an assay temperature of 5 degrees C compared with 20 degrees C; at 20 degrees C, 18-carbon unsaturates were incorporated more readily than 20-carbon species. Linolenic acid (18:3N3) was generally excluded from incorporation, reflecting a possible mechanism by which this precursor of docasahexaenoic acid (22:6N3, n - 3) remains available for modification. These results indicate that trout liver LPCAT preferentially incorporates fatty acids into PC on the basis of both chain length and degree of unsaturation in a manner consistent with the temperature-induced restructuring of membrane phospholipids.

Genetic analysis of a documented population bottleneck: introduced Bennett's wallabies (Macropus rufogriseus rufogriseus) in New Zealand.

Few bottlenecks of wild populations are sufficiently well-documented to constitute models for testing theories about the impact of bottlenecks on genetic variation, and subsequent population persistence. Relevant details of the Bennett's wallaby (Macropus rufogriseus rufogriseus) introduction into New Zealand were recorded (founder number, source and approximate bottleneck duration) and suggest this may provide a rare opportunity to examine the efficacy of tests designed to detect recent bottlenecks in wild populations. We first assessed the accuracy of historic accounts of the introduction using genetic diversity detected in mitochondrial DNA (mtDNA) and at five microsatellite loci. Phylogenetic analyses of mtDNA D-loop sequence haplotypes were consistent with the reported origin of the founders as Tasmania, rather than one of the Bass Strait islands in which Bennett's wallabies are also found. Microsatellite allele frequencies from the Tasmanian source population were then used to seed bottleneck simulations encompassing varying sizes and numbers of generations, in order to assess the severity of bottleneck consistent with diversity observed in the New Zealand population. The results suggested that the founder number was unlikely to have been as small as the three animals suggested by the account of the introduction. Nonetheless, the bottleneck was probably severe; in the range of three to five pairs of wallabies for one to three generations. It resulted in significantly reduced levels of allelic diversity and heterozygosity relative to the source population. This bottleneck is only detectable under the infinite allele model (IAM) and not under the stepwise mutation model (SMM) or the two-phase model (TPM), and possible explanations for this are discussed.