EVOLUTIONARY INTERACTIONS BETWEEN SEXUAL AND ALL-FEMALE TAXA OF CYPRINOTUS (OSTRACODA: CYPRIDIDAE)

Freshwater ostracodes show both an exceptionally high incidence of transitions to unisexuality and, in some cases, an extraordinary level of clonal diversity. There is no understanding of the agents promoting these transitions to thelytoky, although it has been suggested that their frequency may set the stage for sexual taxa to infuse clonal diversity into unisexuals. This study examines the nature and origins of clonal diversity in the unisexual ostracode Cyprinotus incongruens. A combination of allozyme and cytogenetic studies revealed the presence of two diploid clones of this species at three temperate sites and ten clones at one arctic site including three diploids, five triploids, and two tetraploids. The low heterozygosity (0%–20%) of its diploid clones suggests that parthenogenesis has arisen spontaneously in C. incongruens rather than through hybridization, as in vertebrate asexuals. Polyploid clones appear to owe their origin to genome additions from sexual taxa, although subsequent mutational divergence has played a role in further enhancing diversity. Two triploid clones have apparently originated from the incorporation of a haploid genome from the sexually reproducing C. glaucus, as evidenced by their high heterozygosity and possession of alleles otherwise found only in that species. Other polyploid clones have likely arisen as a result of interbreeding between bisexual and unisexual C. incongruens. These results suggest that both the incidence of spontaneous transitions to clonality and the frequency of interbreeding with relatives may be the key processes that govern clonal diversity in unisexual ostracodes.     

La limite Oligocène-Miocène en Limagne : changements fauniques chez les mammifères, oiseaux et ostracodes des différents niveaux de Billy-Créchy (Allier, France)

The lacustrine sequence of Billy-Créchy (Allier) yielded four superposed fossiliferous levels that can be precisely dated for the first time and represent the upper part of the Oligocene and the basis of the Miocene. The age of the localities is given by mammals and particularly by the Rodent and Lagomorph faunas: three are ascribed to the Paleogene mammalian levels MP 29-MP30 and the upper one to the Neogene MN 1. The Paleogene faunas evidence a high degree of similarity with those of the Oligo-Miocene Lower Freshwater Molasse of Switzerland and the localities Créchy 1 and 2 correspond particularly well to the Swiss locality Brochene-Fluh 53, to which a magnetostratigraphic calibration ascribed an age between 24.1 and 23.6 Ma, the date of the Oligo-Miocene boundary being fixed at 23.8 Ma. Changes in the Rodent composition and abundance in these faunas document the impact of the Oligo-Miocene crisis, the uppermost fauna being very impoverished. The avifaunas differ from those of the Aquitanian localities of the Saint-Gérand-le-Puy area by the presence of two forms which are no longer found in the more recent deposits. These birds are mainly aquatic and indicate the proximity of shallow to moderately deep lakes, along the edge of which the Flamingo-related Palaelodidae were nesting. In Ostracod assemblages - numerous along the sequence but in most cases crushed or completely encrusted -, small-sized specimens are abundant, well-preserved, and among them Cypridopsis sp. is dominant; they are represented by adults and juveniles: they indicate shallow-water environmental conditions and unstable environment. Among large specimens, freshwater and brackish species occur; all of them are broken, coated by CaCO3: they belong to different biocenoses and attest some transport.     

Salinity tolerance of three ostracode species (Crustacea:Ostracoda) of Iberian saline lakes

Laboratory experiments and field data were used to determine salinity tolerance limits of three ostracode species (Prionocypris aragonica, Eucypris mareotica and Heterocypris barbara) from Iberian saline lakes. Salinity tolerance appeared related to ionic composition and temperature. Implications for ostracode ecology and geographical distribution are evaluated.     

Late miocene ostracodes (Ostracoda,Crustacea) from the Enikal Strait (Eastern Paratethys) as indicators of salinity changes

The species composition of ostracodes from the upper Maeotian and lower Pontian deposits of the Kerch-Taman Depression (Enikal Strait, Eastern Paratethys) is reported. The sedimentological and paleoenvironmental study shows that mass ostracode occurrences were controlled by hydrological changes in the Late Miocene brackish-water basins under consideration.     

Paleolimnology of mid-elevation lakes in the Kenya Rift Valley

Three small lakes of the Kenya Highlands have been subject to intermittent paleoenvironmental investigation for more than 50 years. In this report, the stratigraphic, core-based history of Lake Naivasha (elev. 1 890 m) is extended backward to 20 000 B.P. and is related to less complete 30 000-year core stratigraphies from nearby lakes Nakuru and Elmenteita. The paleolimnological reconstructions, primarily diatom-based, include geochemical parameters and the first ostracode stratigraphies from tropical Africa. The analyses from all three lakes converge on a history of low and fluctuating water levels over approximately the past 4 500 years, preceded by a major Holocene high stand (ca. 10 000-6 000 B.P.) and a brief earlier rise centered on 12 500 B.P. The lakes were mostly very low during the late Pleistocene (13 000–30 000 B.P.), though modestly elevated levels may have occurred early in this time period. The ostracode record, comprising at least 15 species, was relatively continuous in the Nakuru core but was restricted to sharply demarcated regions in cores from the other lakes.     

松辽盆地泉头组三、四段介形类化石的分布特征与扶、杨油层细分

本文简要叙述了松辽盆地泉头组三、四段的沉积特征,详细讨论了泉头组三、四段及其所属扶余、杨大城子油层介形类化石的分布,总结出划分油层和油层组的介形类标志化石。利用这些介形类标志化石可进一步将扶余油层细分为三个油层组,杨大城子油层细分为五个油层组,为油层细分层对比提供了古生物依据。     

Paleogene ostracodes from the Dawenkou Basin,East China and their biostratigraphic significance for the age of mineral resources

The Dawenkou Basin is a Cenozoic terrestrial fault basin in Shandong Province, East China, containing abundant mineral resources, especially gypsum. Recently a borehole was drilled in this basin unearthing the upper and middle members of the Dawenkou Formation. From the upper member, well-preserved ostracodes were discovered, which are entirely non-marine taxa including species of Eucypris, Caspiolla, Candona and Candoniella. Based on the new material, a new species Candona dawenkouensis n. sp. was erected. The ostracode assemblage biostratigraphically indicates an age of middle Eocene to Oligocene of the upper member of the Dawenkou Formation and the strata bearing mineral resources (mainly gypsum) of the middle member of the Dawenkou Formation is likely early Eocene.     

The population genetics of the ostracode Cyprinotus incongruens on the basis of esterase markers

Different digenic esterase clones of Cyprinotus incongruens reach maturity and die in their own time. Thus, when comparing populations, the presence or absence of clones rather than their frequency should be taken into account. There are no clear geographical relationships concerning the clonal composition of populations. The migration rate is in general quite high, especially to longer distances of about 200 km; the beginning of the isolation of populations by distance can be detectable in Central Europe over distances of at least 400 km. This clonal composition of populations is determined firstly by the founder effect, amplified by the “priority effect” when the founder population increases in numbers, and secondly by mutations. With regards to loci studied here (those coding for esterases), founder individuals are more likely to be doubly homozygous than singly or doubly heterozygous. Thus, the heterozygosity which arises by mutation is severely reduced at the suprapopulation level.     

Quantitative analysis of freshwater ostracode assemblages in southwestern Yukon Territory,Canada

Relations between modern ostracode assemblages and environmental variables from lakes in the southwest Yukon and northern British Columbia were explored. A total of 29 freshwater species representing 8 genera were identified from the sediments of 36 lakes, with the number of species ranging between 3 and 8 per lake. Species widespread throughout the study area include Cyclocypris ampla, Candona candida, Cypria turneri, Cypria ophtalmica, and Candona protzi. The Mg/Ca ratio is an important factor determining the ostracode species composition of a lake. Species richness is at a maximum when the lake water has intermediate values of conductivity. Lakes in which one species clearly dominates the assemblage (‰>‰70% relative abundance) have water saturated with respect to CaCO₃. Mg/Ca ratio, depth and Sr are the environmental factors that are most highly correlated with species distributions in this region.     

柴达木盆地北缘昆特依凹陷始新统下干柴沟组微体古生物研究

昆2井是柴达木盆地北部昆特依凹陷区潜伏Ⅰ号构造上的第一口勘探井,为了建立该地区地层层序,对该井的岩屑及岩芯样品进行了系统的微体古生物学研究,仅在下干柴沟组发现少量轮藻化石和零星的介形类化石。下干柴沟组下段出现的轮藻类Gyrogona qianjiangica和Lamprothamnium? brevis是我国中始新世最常见的化石。该层段的轮藻类可命名为Gyrogona qianjiangica-Lamprothamniumcurta-L. brevis组合,时代为中、晚始新世。本文以Gyrogona qianjiangica分布的顶、底界作为昆2井下干柴沟组下段的标志,划分结果与其它轮藻及介形类化石的对比结果基本一致。氯离子含量分布显示,轮藻类化石产出的下干柴沟组是整个地层剖面中含盐度最低的沉积阶段,表明下干柴沟组沉积时期,该地区处于水域比较广阔的淡水湖泊环境,但在个别层段出现咸化环境,产出以喜盐的Austrocypris为代表的介形类化石。