白鲟消化道形态学与组织学的初步观察

白鲟消化道具有肉食性鱼类的典型特征,其口咽腔结构既适合捕食又适合吞食与滤食水生动物。咽后消化道可分为食道、胃后行支、胃前行支、小肠、瓣肠、直肠与肛门。幽门盲囊似一致密器官,小肠与瓣肠连接处有一特殊淋巴器官,肛门两侧有腹孔。白鲟口咽腔被覆层扁平上皮,上皮内有味蕾分布。咽后消化道组织分层为粘膜(无粘膜肌层)、粘膜下层(小肠及瓣肠前部无)、肌层与外膜。粘膜上皮为单层柱状上皮,由纤毛柱状细胞、一般柱状细胞和杯状细胞组成,其间还散在有颗粒细胞和游走细胞。食道后部与胃的一般柱状细胞为分泌粘液的细胞,肠内的一般柱状细胞为吸收细胞。胃后行支及部分前行支固有膜内有消化腺,其余各部的固有膜为致密层。小肠前中部粘膜形成蜂窝状粘膜窦,无肠腺。除食道前部肌层中有横纹肌外,其余部的肌层均为平滑肌。外膜内结缔组织有的致密有的疏松,外膜表面细胞柱状或立方形或扁平。     

A possible role for the pcnB gene product of Escherichia coli in modulating RNA: RNA interactions.

Summary The sequence of the PcnB protein of Escherichia coli, a protein required for copy number maintenance of ColE1-related plasmids, was compared with the PIR sequence database. Strong local similarities to the sequence of the E. coli protein tRNA nucleotidyltransferase were found. Since a substrate of the latter protein, tRNA, structurally resembles the RNAs that control ColE1 copy number we believe that we may have identified a region in PcnB that interacts with these RNAs. Consistent with this idea is our observation that PcnB is required for the replication of R1, a plasmid whose replication is also regulated by a small RNA.     

Population dynamics of Gyrodactylus sp. (Monogenea) infecting the sand goby in the Oslo Fjord, Norway

Prevalence and abundance of Gyrodactylus sp. on the 1993 year-class of sand gobies Pomatoschistus minutus in the Oslo Fjord, Norway, decreased slightly during the cold months, increased as temperature increased in spring, reaching a peak in June, then decreased significantly, corresponding with a decline in the number of hosts from August. Abundance and, especially, prevalence increased again in September, and decreased in late autumn. Death of the most heavily infected hosts, and/or host resistance might be responsible for the decline in parasite numbers on post-spawners in summer. The young of the year (1994 year-class) were probably infected shortly after they commenced benthic life in July, and prevalence and abundance increased significantly throughout late summer and autumn. Parasite abundance on males was significantly higher than on females at the height of the breeding season in June, but significantly lower at the end of the breeding season in July. Two-thirds of the parasites infected the gills, oral cavity and pharynx. The proportion of parasites on the body, head and fins increased on both sexes, but significantly more so on males, during the breeding season of the host, when parasite intensity was highest. Possible routes of transmission of the parasite are discussed.     

Changes of the forest-savanna boundary in Brazilian Amazonia during the Holocene revealed by stable isotope ratios of soil organic carbon

The possibility of ecosystem boundary changes in northern Brazilian Amazonia during the Holocene period was investigated using soil organic carbon isotope ratios. Determination of past and present fluctuations of the forest-savanna boundary involved the measurement of natural ¹³C isotope abundance, expressed as ¹³C, in soil organic matter (SOM). SOM ¹³C analyses and radiocarbon dating of charcoal fragments were carried out on samples derived from soil profiles taken along transects perpendicular to the ecotonal boundary. SOM ¹³C values in the upper soil horizons appeared to be in equilibrium with the overlying vegetation types and did not point to a movement of the boundary during the last decades. However, ¹³C values obtained from deeper savanna and forest soil layers indicated that the vegetation type has changed in the past. In current savanna soil profiles, we observed the presence of mid-Holocene charcoals derived from forest species: fire frequency at that time was probably greater, and more extensive savanna may have resulted. Isotope data and the presence of these charcoals thus suggest that the forest-savanna boundary has shifted significantly in the recent Holocene period, forest being more extensive during the early Holocene than today. During the middle Holocene, the forest could have strongly regressed, and fires appeared, with a maximum development of the savanna vegetation. At the beginning of the late Holocene, the forest may have invaded a part of this savanna, and fires occurred again.     

Genetic evidence for functional specificity of the yeast GCN2 kinase

In yeast the GCN2 kinase mediates translational control ofGCN4 by phosphorylating the subunit of eIF-2 in response to extracellular amino acid limitation. Although phosphorylation of eIF-2 has been shown to inhibit global protein synthesis, amino acid starvation results in a specific activation effect onGCN4 mRNA translation. Under the same conditions, translation of other mRNAs appears only slightly affected. The mechanism responsible for the observed selectivity of the GCN2 kinase is not clear. Here, we present genetic evidence that suggests that locally restricted action of the GCN2 kinase facilitatesGCN4-specific translational regulation.     

Fruit set in a deceptive orchid: The effect of flowering phenology,display size,and local floral abundance

We investigated the effect of flowering time, display size, and local floral density on fruit set in Tolumnia variegata, a pollination-limited orchid that offers no reward to its pollinator(s). During 1990, natural variation in flowering time, display size, and fruit set were monitored in 508 plants at one locality in Puerto Rico. The following season, orchid floral abundance per host tree (Randia aculeata) was manipulated to investigate its effect on fruit set. Four floral abundance treatments were established (700, 500, 300, and 100), each replicated four times. Flowering time was the most important trait affecting fruit set. The proportion of plants setting at least one fruit was significantly high early and late in the season, but low during the flowering peak. Thus, strong disruptive selection differential on flowering phenology was found. Display size had little effect on fruit set. A weak, but significant disruptive selection differential on display size was found. Orchid floral abundance per host tree had a significant effect on fruit set. Early in the season, T. variegata flowers with intermediate number of conspecific flowers exhibited a greater probability of setting fruit than those in host trees with fewer or more flowers. Our results show that flowering phenology may be evolutionarily unstable, possibly a consequence of the deception pollination system. Furthermore, a deception strategy would be relatively unsuccessful in populations where plants are found in either very dense or sparse patches.     

Genetic diversity and hierarchical population structure of a rare autotetraploid plant,Aster kantoensis (Asteraceae)

We sampled 17 populations of a rare autotetraploid Aster kantoensis (Asteraceae) from three river systems located in central Japan, and studied them for allelic variation at 22 enzyme loci. There was no significant correlation between the actual population size and three genetic diversity parameters, suggesting that the effective population size was very small even for the large populations, i.e., even large populations may still have a high probability of being of recent origin and remain influenced by the founder effect. Compared to other autotetraploid species, the total genetic variation of A. kantoensis is small. The number of alleles and gene diversity of a population were not significantly different among the river systems, although the percentage of polymorphic loci was different. Genetic differentiation among river systems was larger than between populations within the river systems, thereby indicating that gene flow between river systems is small, especially between the Kinu River system and Tama or Sagami River systems.     

Diversity and conservation of blackwater fishes in Peninsular Malaysia,particularly in the North Selangor peat swamp forest

One of the most extreme freshwater habitats in Peninsular Malaysia is the peat swamp forest, with dark-coloured and highly acidic waters. Surprisingly, little is known about blackwater fishes in Peninsular Malaysia. Until 1968, only 26 fish species were known from blackwaters throughout Peninsular Malaysia, of which only one can be regarded as stenotopic. A recent intensive survey of part of the North Selangor peat swamp forest yielded 47 species, of which 14 are probably stenotopic taxa. These include four undescribed species and several new records for western Peninsular Malaysia. These discoveries are significant in that they include the family Chaudhuriidae which until 1985, was not reported from Sundaic Southeast Asia, and the rare genus Encheloclarias which had not been encountered for over 50 years. The rapid rate of destruction of the peat swamp forest owing to development, forestry and agricultural activities must be halted or slowed significantly to enable the proper zoological surveys and studies to be conducted. Conservation plans and environmental impact assessments based on inadequate sampling and knowledge of species present is acutely dangerous. There are no longer substantial undisturbed blackwater peat swamp forests left in most of Peninsular Malaysia. Conservation of the remaining blackwater biotopes is critically important if extinction of many species, here regarded as economically valuable renewable resources, is to be prevented.     

Comparative limnology,species diversity and biomass relationship of zooplankton and phytoplankton in five freshwater lakes in Kenya

Comparative studies on the limnology, species diversity and standing stock biomass of phytoplankton and zooplankton in five freshwater lakes, Naivasha and Oloidien, Ruiru, Masinga and Nairobi reservoirs, were undertaken. Phytoplankton chlorophyll a, dissolved oxygen and temperature were also measured. Thermocyclops oblongatus (Copepoda) was dominant in all the lakes. Ceriodaphnia cornuta and Diaphanosoma excisum (Cladocera) dominated in lakes Naivasha and Oloiden, whereas in Ruiru, Masinga and Nairobi reservoirs, Brachionus angularis and Hexarthra mira (Rotifera) were the dominant zooplankters. Phytoplankton biomass as chlorophyll a was lowest in Ruiru dam 5.64 ± 4.0 µg l^-1 and highest in the eutrophic Nairobi dam 71.5 ± 12.02 µg l^-1. The endorheic lakes Naivasha and Oloidien showed medium values of 24.5 ± 4.0 µg l^-1.     

Taxonomy, biology and abundance of icefishes, or noodlefishes (Salangidae), in the Yellow River estuary of the Bohai Sea, China

Four species of icefishes were sampled in the Yellow River estuary of the Bohai Sea from April 1984 to April 1985 and from February 1988 to June 1989. Morphological analysis showed that they belonged to three subfamilies of Salangidae: one species from Protosalangicae, Protosalanx chinensis ; two species from Salanginae, Salanx ariakensis and Salanx prognathus ; and one species, Neosalanx andersoni , from Salangichthyinae, a new subfamily. External morphological characteristics, population composition, growth, maturity, food habits, longevity, fecundity, abundance and distribution of these icefishes are described. Based on these analyses, a clear outline of taxonomy, zoogeography aod biology of icefishes in the Yellow River estuary of the Bohai Sea was established.