美丽胡枝子核型分析

采用常规制片法,对青岛崂山地区野生美丽胡枝子的染色体数目、核型、染色体体积等进行了研究.结果表明,美丽胡枝子为二倍体,体细胞染色体数目为2n=22,核型公式为2n=2x=12m+8sm+2st;染色体长度组成为2L+6M2+14M1,染色体总长为20.87 μm,核型不对称系数为65.8%,属于2A型.染色体总体积为11.54 μm3,染色体较小,进化上属于较原始状态.     

FUNGAL CONTROL OF POPULATION CHANGES OF THE PLANKTONIC DIATOM ASTERIONELLA FORMOSA IN A SHALLOW EUTROPHIC LAKE1

Parasitic fungi infected an Asterionella formosa Hass. population for 10 months of the year, during which time the population density of the alga was over 100 cells-L^?1. High injection rates of more than 20% of the cells were observed between February and July except on a few occasions such as late April and late June. Mortality of infected diatom cells was temperature dependent in dialysis tube culture experiments; the loss rate of field populations due to fungal injection also was affected by temperature. Loss rates of diatom cells were negligibly small m February even though infection percentages were more than 20%, but increased toward summer with an increase m temperature. High infection percentages and correspondingly high loss rates (e.g. 0.3 d^?1) occurred periodically during the summer. The results suggest that this type of host-parasite interaction is an important control of algal population size in natural waters.     

EVOLUTIONARY ORIGIN OF A PARTHENOFORM,THE AMAZON MOLLY POECILIA FORMOSA,ON THE BASIS OF A MOLECULAR GENEALOGY

The appearance of vertebrate species that reproduce without genetic recombination has been explained by their origin from a rare hybridization event between members of two distantly related species. For the first recognized vertebrate unisexual, the Amazon molly Poecilia formosa, mostly morphological and biochemical genetic information has been available so far with respect to its evolutionary origin. DNA sequence analyses of transcribed portions of the genome (tyrosine kinase proto-oncogenes) demonstrated its hybrid state unequivocally. Both alleles can be traced in a DNA sequence-based phylogenetic tree to extant species that represent the parental species or that are closely related to the corresponding extinct forms, namely P. mexicana limantouri and a so far taxonomically ill-defined north Mexican subspecies of the P. latipinna/P. velifera complex. A rough estimate from the mutation rates dates the hybridization event further back than would have been predicted on the basis of “Muller's ratchet” for an ecologically successful species.     

Host-feeding of three parasitoid species on Bemisia tabaci biotype B and implications for whitefly biological control

Parasitoids in the genera Encarsia and Eretmocerus (Hymenoptera: Aphelinidae) are important biological control agents of whiteflies through their reproductive as well as host‐feeding activities. The feeding capacities of female parasitoids of three species with different reproductive strategies [Encarsia sophia (Girault & Dodd), Encarsia formosa Gahan, and Eretmocerus melanoscutus Zolnerowich & Rose] on their host, sweetpotato whitefly, Bemisia tabaci (Gennadius) biotype B (Homoptera: Aleyrodidae), were evaluated on cabbage in a single‐instar no‐choice experiment in the laboratory and a mixed‐instar choice experiment in the greenhouse. In both single‐ and mixed‐instar experiments, significant differences in host‐feeding capacities were found among the three parasitoid species. Encarsia sophia exhibited superior capacity of host‐feeding compared to E. formosa and E. melanoscutus. In the single‐instar experiment, parasitoids fed more on younger (smaller) hosts than older (larger) hosts. In the mixed‐instar experiments, all three parasitoid species exhibited a clear preference for feeding on older hosts compared to younger hosts. Total number of whitefly nymphs fed on by E. sophia was approximately three times that of the other two parasitoid species. Whitefly mortality accounted for by host‐feeding by E. sophia was up to 59.7%, and, thus, equivalent to parasitization. The significance of host‐feeding of E. sophia for biological control of B. tabaci is discussed.     

Are there lessons from negative results in studies using video playback?

The use of video playback is a growing practice in animal behaviour. Studies that lead to unpredicted negative results are often considered failures and remain unpublished. In this article I review some of the published and unpublished studies that have led to negative results. Analysis of these studies suggests that no particular problem is responsible for more than a small number of negative results. I suggest further investigation of the potential causes of these negative results to assess their importance to the outcome of video playback experiments. Received: 31 January 2000 / Received in revised form: 27 March 2000 / Accepted: 28 March 2000     

Comparison of parasite communities in native and introduced populations of sexual and asexual mollies of the genus Poecilia

The parasite communities of two molly species, the sexual Poecilia latipinna and the clonal Poecilia formosa , were investigated in four populations using a novel method applicable under field conditions. In two native populations from south Texas and two introduced populations from central Texas, four species of microparasites and eight species of macroparasites were recorded. Virtually no differences in the parasite diversity and species composition could be detected between the populations. Mollies from south Texas had a higher individual parasitization index of macroparasites. There was a negative correlation between the relative number of oocytes in gravid females and their individual macroparasite load.     

THE RELATIVE PHOTOSYNTHETIC EFFICIENCY OF ASTERIONELLA FORMOSA HASS. (BACILLARIOPHYTA) IN NATURAL PLANKTON ASSEMBLAGES1

The relative photosynthetic efficiencies of net vs. nanoplankton and diatom vs. non-diatom plankton were estimated weekly in Woodcock Creek Reservoir, Pennsylvania, from April to November 1979, using carbon-14 in conjunction with size fractionation and germanium as a diatom inhibitor. The nanoplankton contributed a proportionally greater share of the community carbon assimilation than their share of the community biomass would indicate. Diatoms, specifically Asterionella formosa Hass. were photosynthetically inefficient and contributed significantly less than expected. The mechanism of periodic diatom dominance in lakes remains unexplained.     

Oogenesis of microlecithal oocytes in the viviparous teleost Heterandria formosa

Viviparous teleosts exhibit two patterns of embryonic nutrition: lecithotrophy (when nutrients are derived from yolk that is deposited in the oocyte during oogenesis) and matrotrophy (when nutrients are derived from the maternal blood stream during gestation). Nutrients contained in oocytes of matrotrophic species are not sufficient to support embryonic development until term. The smallest oocytes formed among the viviparous poeciliid fish occur in the least killifish, Heterandria formosa, these having diameters of only 400 μm. Accordingly, H. formosa presents the highest level of matrotrophy among poeciliids. This study provides histological details occurring during development of its microlecithal oocytes. Five stages occur during oogenesis: oogonial proliferation, chromatin nucleolus, primary growth (previtellogenesis), secondary growth (vitellogenesis), and oocyte maturation. H. formosa, as in all viviparous poeciliids, has intrafollicular fertilization and gestation. Therefore, there is no ovulation stage. The full‐grown oocyte of H. formosa contains a large oil globule, which occupies most of the cell volume. The oocyte periphery contains the germinal vesicle, and ooplasm that includes cortical alveoli, small oil droplets and only a few yolk globules. The follicular cell layer is initially composed of a single layer of squamous cells during early previtellogenesis, but these become columnar during early vitellogenesis. They are pseudostratified during late vitellogenesis and reduce their height becoming almost squamous in full‐grown oocytes. The microlecithal oocytes of H. formosa represent an extreme in fish oogenesis typified by scarce yolk deposition, a characteristic directly related to matrotrophy. J. Morphol., 2011. © 2010 Wiley‐Liss, Inc.