Cladocera in space and time: Analysis of lake sediments

Shells of Bosminidae and Chydoridae are quantitatively preserved in lake sediments. The chronological deposition of these remains provides the means for longterm observation of these Cladocera, both in terms of species and communities. Chydorid analysis, as based on subfossil assemblages, is an analysis of community and provides direct observation of community dynamics over extended periods of time. It has proved to be a valuable method to obtain information on the influence of environmental factors and time on community characteristics. Morphological variation inBosmina (Eubosmina) has been followed for some thousand years. This is of special interest for the evaluation of taxonomic rank (species, forms) if closely related taxa have co-existed. Bosmina successions, as well as shifts in the chydorid fauna, are related to environmental change. Thus, cladoceran analysis of lake sediments provides information on the developmental history of lakes and allows observation of the effects of longterm environmental changes, such as climatic changes and eutrophication.     

Ribosomal DNA non-transcribed spacer polymorphism in subterranean mole rats: genetic differentiation,environmental correlates and phylogenetic relationships

Summary An analysis is presented of genetic differentiation in the non-transcribed spacers of ribosomal DNA (NTS rDNA). Diversity, environmental correlates and the phylogenetic relationships are examined within and between species of the actively speciating subterranean mole rat, superspeciesSpalax ehrenbergi (2n=52, 54, 58, 60) in Israel. This analysis is based on a previous study of the geographic distribution of restriction fragment length polymorphisms of NTS rDNA. Here we present results indicating that NTS rDNA diversity exists mostly (66%) within populations, while 20% is between populations within species, and 14% between species. Multivariate discriminant analysis succeeded in separating 10 of the 13 populations (77%) into their correct chromosomal species, on the basis of the combination of three NTS rDNA repetypes. The phylogenetic relationships suggest that the complex involves two pairs of closely related species (2n=52–54 and 2n=58–60). NTS rDNA diversity, as well as the decrease southward in frequency of repetype C, are correlated with climatic factors of humidity and temperature. These data are discussed in terms of the evolutionary forces of migration and selection which may cause NTS rDNA differentiation. Climatic selection appears to be the major differentiating factor of NTS rDNA.     

Sperm aging in the male after sexual rest: Contribution to chromosome anomalies

The chromosome complement was studied in first-cleavage metaphases of mouse zygotes resulting from sperm aged in the male physiologically, after sexual rest. Females were inseminated by control males mating at 3-day intervals while experimentals mated to males that had had a sexual rest of 14 or more days. A total of 1954 eggs were collected 33–35 h post-HCG from 101 superovulated females mated to 42 controls and 43 experimental males. The fertilization rate was similar in both groups, being 84% and 85%, respectively. G-banded or Q-banded chromosomes were analyzed in 301 (68.3%) controls and 392 (49%) experimental first-cleavage metaphases. The overall rate of chromosome anomalies in controls was 4.45% as compared to 10.94% in experimentals, a highly significant difference. In the experimental group compared to controls, the frequency of trisomy, triploidy, structural rearrangements, and tetraploidy increased from 3.9% to 6.9%, 0% to 1.6%, 0.8% to 2.8%, and 0% to 1.3%, respectively. The genomic source of origin of the abnormalities was determined on the basis of differential condensation of the genomes. In the experimentals, grossly unbalanced sperm (diploids, disomics, double disomics, and those with large fragments) fertilized significantly more oocytes compared to controls. Our results implicate an advantage either in numbers or fertilizing capability for chromosomally abnormal sperm in a physiologically aged population.     

Climate and vegetation in China II. Distribution of main vegetation types and thermal climate

The authors examined relationships between Kira's warmth index (WI) and four other important thermal indices: the sums of daily mean temperatures above 5°C and 10°C, Thornthwaite's potential evapotranspiration (PE) and Holdridge's annual biotemperature. The thermal records of 671 meteorological stations evenly located all over China were used to make these comparisons. Close correlations were found within the four relationships, and accordingly WI was used to analyse the thermal distributions of the main vegetation types. Vegetation types around the 671 stations were read from a vegetation map with a scale of 1/4000000. Vegetation types at 269 stations corresponded to the natural or seminatural vegetation, and 29 vegetation types were distinguished by arranging the 269 data into the same or similar types. The geographical distribution of these 29 types and the corresponding main climatic features were described. The relations between WI and distribution of these vegetation types were discussed in detail. As a result, WI values (°C month) corresponding to the vegetation zones could be summarized as follows: (1) arctic or alpine vegetation zone: 0–15; (2) boreal or subalpine vegetation zone: 15-(50–55); (3) cool-temperate vegetation zone: (50–55)–(80–90); (4) warm-temperate vegetation zone: (80–90)–(170–180). These values almost coincided with Kira's values. Chinese postgraduate student in Japan sent by the Chinese Government.     

Mammal diversity and environment evolution during the Plio-Pleistocene in East Africa

Human evolution began in East Africa four million years ago, with a transition from an arboreal state to a more terrestrial one. This evolution seems to be correlated with a large environmental change in East Africa around 2.5 m.y. due to a major climatic change leading to drier and cooler conditions. Cenogram analysis (a graphical representation of community structure) can be used to reconstruct the vegetation cover at a regional scale, and to infer the changing climatic conditions. Using cenogram sequences of different sites along the Rift Valley, we were able to determine the regional ecological context in which mammals and hominids have evolved in East Africa during the last 3 million years. Between 3.5 and 2 m.y., during a general climatic change, successive faunas of South Tanzania reflect the progressive opening of their environment. In contrast around Lake Turkana a mosaic of isolated dry and wet habitats were present throughout this period. At this time, the Rift seems to have been spatially structured in several basins isolated from one other, and isolated faunas experienced separate speciation events (particularly with the appearance ofHomo genus). After 2 m.y., the disappearance of the isolating barriers on one hand, and a regional increase in aridity, on the other hand, led to more homogenous faunas arising throughout the region. Replacements of mammal species occurred (especiallyHomo erectus replacingHomo habilis) and several others mammal species, including australopithecines, disappeared during this same period.