家蝇（Musca domestica）对三氟氯氰菊酯的抗药性遗传

研究了家蝇对三氟氯氰菊酯抗性的遗传,通过对抗性品系和敏感品系杂交后代的抗性遗传分析发现家蝇对三氟氯氰菊酯的抗性受多因子控制,抗性显性率为－0．102,为不完全隐性。其对三氟氯氰菊酯的抗性现实遗传力为0．120。     

甜菜夜蛾对三种拟除虫菊酯类杀虫剂的抗性遗传力及风险评估

在室内抗性选育的基础上,应用数量遗传学中的域性状分析法,分别研究了甜菜夜蛾(Spodoptera exigua)对三氟氯氰菊酯、氰戊菊酯和顺式氯氰菊酯3种拟除虫菊酯类杀虫剂的抗性现实遗传力(h²),并对3种药剂在不同杀死率下的抗性发展速率进行了预测.结果表明,用三氟氯氰菊酯、氰戊菊酯、顺式氯氰菊酯分别连续汰选12、10和10代后,甜菜夜蛾对3种药剂的抗性分别提高了18.1、27.6和45.4倍,抗性现实遗传力分别为0.2567、0.3571和0.4239.假设遗传力为实验室汰选估算值的一半、药剂杀死率为50%～90%、预计抗性增长10倍时,三氟氯氰菊酯需要约9～20代,氰戊菊酯需要约6～14代,顺式氯氰菊酯需要约5～12代.甜菜夜蛾对以上3种拟除虫菊酯类杀虫剂均存在抗性风险.相比之下,顺式氯氰菊酯和氰戊菊酯的抗性风险大于三氟氯氰菊酯.     

不同抗性水平棉铃虫地理种群的基因交流对抗性演化的影响

采自我国长江流域棉区的棉铃虫Helicoverpa armigera(Hubner)四川简阳种群、湖北武穴种群、湖南衡阳种群、江苏南京种群和浙江杭州种群对辛硫磷、三氟氯氰菊酯和硫丹的抗性水平分别为8.05～38.80、2.86～290.00和5.87～19.83倍。抗性较高的简阳种群和较低的杭州种群正、反交F₁代对上述3种农药的抗性为13.16和16.69倍、10.43和19.00倍及12.42和10.21倍,抗性遗传的显性系数为-0.2220、-0.2147和0.0700,表明对辛硫磷、三氟氯氰菊酯抗性呈不完全隐性方式遗传,而硫丹抗性则为半隐性遗传。P1自交F2代对辛硫磷、三氟氯氰菊酯和硫丹的抗性水平分别为17.66、7.57和9.03倍,F₁代对杭州种群回交F₂代的抗性倍数分别为8.79、3.Oo和6.61。其结果,高抗区和低抗区棉铃虫种群的基因流动将导致对三种农药抗性水平明显下降。     

霍乱毒素对三氟氯氰菊酯抗性及敏感棉铃虫神经细胞L型钙通道的调节作用

霍乱毒素（CTX）可激活兴奋性异三聚体G蛋白（Gα_s）的α-亚基和刺激电压门控L-型钙通道,而昆虫的L-型钙通道可能是拟除虫菊酯类杀虫剂的作用靶点。为进一步探讨农业害虫对拟除虫菊酯类杀虫剂产生抗药性的作用机理,我们检测了CTX对三氟氯氰菊酯抗性及敏感棉铃虫Helicoverpa armigera中枢神经细胞电压门控L-型钙通道的调节作用。分别急性分离三氟氯氰菊酯抗性及敏感的3～4龄棉铃虫幼虫胸腹神经节细胞,并在改良的L15培养基（加入或未加入700 ng/mL的CTX）中培养12～16 h。钡离子为载流子,应用全细胞膜片钳技术记录电压门控L_型钙通道电流。结果显示,CTX可使敏感组棉铃虫神经细胞L-型钙通道的峰值电流密度增大36％、峰值电压左移5 mV,但对抗性组棉铃虫神经细胞L-型钙通道无上述作用。并且,CTX对敏感组及抗性组棉铃虫神经细胞L_型钙通道的激活电位、翻转电位、激活曲线和失活曲线等其他一些参数的影响也不明显。在无CTX作用时,所检测到的抗性组与敏感组棉铃虫神经细胞L_型钙通道的上述参数值间差异不显著。结果提示,棉铃虫神经细胞内存在Gs腺苷酸环化酶(AC)-cAMP-蛋白激酶A (PKA)-L-型钙通道信号调节系统;与敏感棉铃虫神经细胞L-型钙通道相比,三氟氯氰菊酯抗性棉铃虫神经细胞L-型钙通道的活性相对不易受到CTX调节,这可能与昆虫对拟除虫菊酯产生抗药性的机理有关。     

棉铃虫对氰戊菊酯-辛硫磷混剂的抗性演化及解毒酶活性变化

用氰戊菊酯-辛硫磷混剂（有效成分1∶10,简称氰-辛混剂）对棉铃虫Helicoverpa armigera室内品系(YS)进行16代的抗性选育,获得棉铃虫对氰-辛混剂的抗性品系（YS-FP）。YS-FP品系与YS品系相比,对氰-辛混剂的抗性为14.7倍,对其中的单剂氰戊菊酯和辛硫磷的抗性分别为2 170倍和3.1倍。随着筛选的进行,氰戊菊酯和辛硫磷之间的共毒系数在F₂代出现短暂的增加,然后逐渐降低,它们之间的互作由增效变为拮抗。交互抗性测定结果表明,YS-FP品系对氯氰菊酯、溴氰菊酯、三氟氯氰菊酯、三唑磷和灭多威产生了明显的交互抗性,对硫丹、多杀菌素和爱玛菌素没有产生交互抗性。YS-FP品系6龄幼虫中肠细胞色素P450氧化酶甲氧基香豆素O-脱甲基活性为YS品系的10倍,3龄幼虫谷胱甘肽S-转移酶和酯酶活性分别是YS品系的1.7倍（CDNB结合作用）和2.4倍（α-NA 酯酶水解作用）。氰-辛混剂的筛选导致了棉铃虫多种解毒酶活性的增加,特别是细胞色素P450氧化酶活性增强最为明显。本研究结果表明氰-辛混剂对棉铃虫的筛选导致了广谱的交互抗性和多种代谢抗性机理,并且两个单剂之间的互作由增效变为拮抗,因此氰 辛混剂在棉铃虫抗性治理中的作用是有限的和暂时的。     

棉铃虫对氰戊菊酯等杀虫剂抗性的选育及其生化机理

从用药水平低的棉田采集棉铃虫Helicover pa armigera(Hubner),在室内恒温条件下以人工饲料饲养,用氰戊菊酯等4种杀虫剂和1种混合剂经点滴法分别逐代处理棉铃虫幼虫,以选育其抗药性。用氰戊菊酯选择15代,抗性达311倍;而用灭多威、甲基对硫磷和辛硫磷分别选择13代、14代及13代,抗性仅分别达10.8倍、3.5倍及5.2倍,抗性发展较慢;甲基对硫磷与辛硫磷的混合剂选择12代,抗性也只有4.8倍。氰戊菊酯和溴氰菊酯、三氟氯氰菊酯间存在明显的交互抗性。用生测法测定酶抑制剂和生化法测定酶活性的结果表明,棉铃虫对氰戊菊酯的抗性主要与幼虫体内多功能氧化酶和羧酸酯酶的活性提高有关。     

泉州地区小菜蛾对三氟氯氰菊酯和阿维菌素的抗性监测

用浸叶法对泉州地区小菜蛾对三氟氯氰菊酯和阿维菌素的抗性进行了监测。结果表明,小菜蛾对三氟氯氰菊酯和阿维菌素的抗性呈逐年上升趋势,2003年的分别为2000年的7．4和4．3倍。     

菜缢管蚜、棉铃虫对杀虫混剂及其单剂的抗性遗传力分析

采用阈性状分析法,估算了菜缢管蚜Lipaphis erysimi(Kaltenbach)和棉铃虫Helicoverpa armigera(Hubuer)对菊马混剂及其单剂、棉铃虫对灭铃威混剂及其单剂的抗性现实遗传力,并对抗性风险和混剂延缓抗性的作用进行了评估。结果表明,菜缢管蚜和棉铃虫对混剂的现实抗性遗传力均明显小于各单剂,依次为：拟除虫菊酯>氨基甲酸酯>有机磷>混剂。混剂的使用寿命大于组成其各个单剂的使用寿命之和,抗性风险依次为：拟除虫菊脂>氨基甲酸酯>有机磷>混剂。菊马混剂对菜缢管蚜和棉铃虫,灭铃威混剂对棉铃虫均有延缓抗性发展的作用。灭铃威对棉铃虫抗性发展的延缓作用最显著。     

家蝇（Musca domestica L.）成虫和幼虫对杀虫剂敏感性差异研究

报导了家蝇成虫和幼虫对杀虫剂敏感性的差异及其机理,家蝇的幼虫对杀虫剂的耐药性比成虫高,敏感性家蝇幼虫对三氟氯氰菊酯的耐药性是成虫的205．5倍,机理研究表明幼虫高GST活性与其较高的耐药性有关,三氟氯氰菊酯和二硫氰基甲烷混剂（4：1）对菊酯抗性家蝇的共毒系数达188。     

瓢虫对杀虫剂的敏感性研究

用4种不同类别杀虫剂对3种不同用药水平地区的七星瓢虫Coccinella septempunctataLinnaeus、龟纹瓢虫Propylea japonica (Thunberg)的毒力进行了测定,同一种用药水平地区的瓢虫对不同药剂的敏感性差异均极显著,三氟氯氰菊酯>灭多威>甲胺磷>硫丹;三氟氯氰菊酯对用药水平较高地区的七星瓢虫幼虫、成虫的毒力分别为硫丹的8547和617倍。同种药剂对不同用药水平地区的七星瓢虫的毒力差异不显著,而用药水平较高地区的龟纹瓢虫较用药水平较低地区的龟纹瓢虫对三氟氯氰菊酯产生了30.6倍的抗药性,这与羧酸酯酶活性提高有关。不同种类杀虫剂对七星瓢虫和抗性棉蚜Aphis gossypii Glover的选择指数差异极为显著,硫丹>灭多威>甲胺磷>三氟氯氰菊酯,如硫丹为4.9,而三氟氯氰菊酯却仅为7.3X10^-6。硫丹是防治抗性棉蚜值得重视的一种杀虫剂。     

不同杀虫剂选育对家蝇抗药性水平及kdr基因频率的影响

采用杀虫剂（溴氰菊酯和甲基嘧啶磷）筛选及不接触药物自然衰退的方法,研究了家蝇Musca domestica氯氟氰菊酯高抗品系(Cyh-R)对杀虫剂的抗性变化,探讨蝇类抗药性治理的方法。用点滴法测定氯氟氰菊酯对不同家蝇品系的毒力,比较抗药性的变化,结合特异性等位基因PCR扩增（PASA）技术检测了不同家蝇品系的kdr基因频率,探讨kdr基因频率与抗性水平之间的关系。结果表明:甲基嘧啶磷筛选后,氯氟氰菊酯对第2~8代Cyh-R品系的LD₅₀呈递减趋势,从F₀的2.8434 μg/蝇降为F₈的0.4404 μg/蝇,但第8~18代Cyh-R品系的LD₅₀呈逐代递增趋势;溴氰菊酯筛选后,氯氟氰菊酯对Cyh-R品系第2~16代的LD₅₀呈上升趋势,从F₀的2.8434 μg/蝇升至F₁₆的24.3249 μg/蝇;表明了施用有机磷杀虫剂可降低其对氯氟氰菊酯的抗药性,而施用拟除虫菊酯药剂则有助于其对氯氟氰菊酯抗药性的增长;不使用任何杀虫剂也能降低其对氯氟氰菊酯的抗药性,但下降速率低于甲基嘧啶磷。PASA技术检测表明Cyh-R品系的kdr抗性基因频率为88.8%,不经过任何药剂筛选其kdr抗性基因频率下降程度最大,达到69.7%;甲基嘧啶磷筛选后其结果降为78.8%,而经溴氰菊酯筛选后kdr抗性基因频率则明显升高,达到98.9%。通过对kdr抗性基因频率和抗性水平进行相关和回归分析表明kdr抗性基因频率与家蝇对氯氟氰菊酯的LD₅₀呈对数关系,即LD₅₀值高的品系其kdr抗性基因频率相应的也较高。建议在家蝇防治中考虑轮换用药。     

黄腹山雀的鸣唱特征分析

2006年4—5月及2007年6月,在北京小龙门林区录制了黄腹山雀(Parus venustulus)的鸣唱,利用Avisoft-SASLab Pro鸟声声谱分析软件(德国)测量鸣唱特征参数后进行统计分析,发现该地区的黄腹山雀种群鸣唱句法简单,鸣唱句子均为相同音节的不断重复。所采集的音节曲目中包含了56种音节型,音节类型数与采样个体数(r=0.973,P=0.000<0.05)呈显著正相关,不同个体的领域性鸣唱存在显著差异。与同域分布的大山雀、褐头山雀、煤山雀、沼泽山雀相比较,黄腹山雀的鸣唱句子最短(Dv=0.83±0.48),频率较高(Fmax=7.64±1.01,Fmin=3.27±1.13),句子中音节的重复次数最少(Ns.v=2.0±0.2)。     

七种抑制剂对两种白蚁谷胱甘肽S-转移酶活性抑制作用的比较

利用分光光度酶动力学方法,确定了白蚁谷胱甘肽S-转移酶（GSTs）的最适反应条件,并进一步研究了7种抑制剂对黑翅土白蚁Odontotermes formosanus (Shiraki)和黑胸散白蚁Reticulitermes chinensis Snyder GSTs活性的体外影响。结果表明：白蚁GSTs测定的最适反应条件为pH 6.5,温度25℃,最适反应时间2 min。黑翅土白蚁GSTs的米氏常数(K_mCDNB和K_mGSH)分别为0.11±0.02 mmol/L和0.81±0.16 mmol/L,最大反应速度(V_maxCDNB和V_maxGSH)分别为425.92±19.67 nmol/(min·mg)和534.86±39.05 nmol/(min·mg)。黑胸散白蚁GSTs的米氏常数(K_mCDNB和K_mGSH)分别为0.12±0.03 mmol/L和1.03±0.31 mmol/L,最大反应速度(V_maxCDNB和V_maxGSH)分别为544.39±37.19 nmol/(min·mg)和715.45±83.68 nmol/(min·mg)。浓度为2×10^-5 mol/L时,槲皮素和辛硫磷对黑胸散白蚁GSTs活性的抑制作用要强于黑翅土白蚁,对黑胸散白蚁GSTs活性的抑制作用分别为62.28％和44.89％,对黑翅土白蚁GSTs活性的抑制作用分别为54.96％和28.36％。高效氯氰菊酯、甲氰菊酯、啶虫脒和单宁酸对黑翅土白蚁GSTs活性的抑制作用要强于黑胸散白蚁,对黑翅土白蚁GSTs活性的抑制作用分别为39.43％,72.07％,52.24％和82.19％;对黑胸散白蚁GSTs活性的抑制作用分别为14.96％,40.23％,39.96％和57.80％。阿维菌素对黑翅土白蚁和黑胸散白蚁GSTs活性的抑制作用没有显著差异,对黑翅土白蚁和黑胸散白蚁GSTs活性的抑制作用分别为76.21％和76.88％。这表明两种白蚁对药剂的敏感性完全不同。实验结果还表明,在3.2×10^-8～2×10^-5 mol/L内,上述植物次生物质和杀虫剂对两种白蚁GSTs活性的抑制率存在明显的剂量-效应关系。     

Kinetic properties of myosin heavy chain isoforms in mouse skeletal muscle: comparison with rat, rabbit, and human and correlation with amino acid sequence

Stretch activation kinetics were investigated in skinned mouse skeletal muscle fibers of known myosin heavy chain (MHC) isoform content to assess kinetic properties of different myosin heads while generating force. The time to peak of stretch-induced delayed force increase (t₃) was strongly correlated with MHC isoforms [t₃ given in ms for fiber types containing specified isoforms; means ± SD with n in parentheses: MHCI 680 ± 108 (13), MHCIIa 110.5 ± 10.7 (23), MHCIIx(d) 46.2 ± 5.2 (20), MHCIIb 23.5 ± 3.3 (76)]. This strong correlation suggests different kinetics of force generation of different MHC isoforms in the following order:MHCIIb > MHCIIx(d) > MHCIIa >> MHCI. For rat, rabbit, and human skeletal muscles the same type of correlation was found previously. The kinetics decreases slightly with increasing body mass. Available amino acid sequences were aligned to quantify the structural variability of MHC isoforms of different animal species. The variation in t₃ showed a correlation with the structural variability of specific actin-binding loops (so-called loop 2 and loop 3) of myosin heads (r = 0.74). This suggests that alterations of amino acids in these loops contribute to the different kinetics of myosin heads of various MHC isoforms. isoform structure-function relationship; stretch activation; muscle mechanics     

盐酸黄连素对蚜虫生物活性的研究

黄连素为黄连Coptis chinensis Franch体内的主要生物碱成份。用采自贵阳花溪附近的新鲜黄连主根提取盐酸黄连素,将粗品重结晶一次后,用 HITACHI-220紫外可见分光光度计测得一次重结晶产品含盐酸黄连素91%,黄连主根中的黄连素含量为11.56%。室内生物测定结果表明,盐酸黄连素对甘蓝蚜Brevicoryne brassicae (L.)具有很强的触杀作用和较好的拒食活性。1%盐酸黄连素的乙醇溶液点滴蚜虫,96 h死亡率高达92.5%,校正死亡率为91.4%, LC₅₀值为(2.289±0.0125)g/L,回归方程Y=7.712+1.027X,相关系数r为0.958,达显著水平;1%盐酸黄连素的乙醇溶液处理叶片,24 h处理和对照叶面的蚜虫栖息率分别为20.6%和79.4%,拒食效果为74.04%, AFC₅₀值为(4.055±0.0215) g/L ,回归方程Y＝9.115+1.719X,相关系数r为0.998,达极显著水平。     

球形芽孢杆菌对致倦库蚊的后致死作用

研究了球形芽孢杆菌Bacillus sphaericus C3-41菌株对致倦库蚊Culex quinquefasciatus幼虫的毒力及其后致死作用。生物测定表明,该菌株对目标蚊幼虫具有很高的毒力,其丙酮粉剂对3～4龄幼虫48 h的半致死浓度(LC₅₀)为(6.92±0.22) μg/L。用不同亚致死浓度处理2～3龄致倦库蚊幼虫,存活幼虫在后期发育中存在明显的延续死亡和损伤现象,经LC₃₀、LC₅₀、LC₇₀、LC₉₀和LC₉₈剂量的C3.41粉剂处理的致倦库蚊羽化前的总死亡率分别为84%、91%、95%、97%和100%,同时存活的幼虫、蛹和成蚊的发育和行为也受到一定的影响。这种后致死作用随处理浓度的升高而增强,可能同球形芽孢杆菌毒素蛋白对处理期间蚊幼虫中肠上皮细胞造成的损伤相关。     

Relationship between Coleoptile Elongation and Alcoholic Fermentation in Rice Exposed to Anoxia. II. Cultivar Differences

The relationship between coleoptile elongation and survivalvs. alcoholic fermentation of rice under anoxia is examinedusing eight cultivars differing in submergence tolerance. Anoxiawas imposed on either 1 or 4 d aerated seeds either by N₂ flushingsubmerged tissues or by incubating tissues in stagnant deoxygenatedagar at 0·1% w/v; the latter simulated the stagnant conditionsof waterlogged soil. Two cultivars that were most tolerant tosubmergence also had the greatest tolerance to anoxia, whilea submergence intolerant cultivar was also intolerant to anoxia. Coleoptile growth under anoxia was related to rates of ethanolsynthesis (R_E), however differences between growth during anoxiaand survival after anoxia indicated that post-anoxic injurymay also be important in rice seeds exposed to anoxia. The correlationbetween coleoptile growth and R_E measured on a tissue basisusing intact seeds was r² = 0·67 among six varietiesover 0-3 d anoxia. This correlation improved to about r² = 0·85using R_E of (embryos plus coleoptiles) over 0-3 d, or coleoptilesat 3 d after anoxia. Coleoptile growth of individual seeds wasusually poorly correlated to R_E in these cultivars at 2-3 dafter anoxia. When coleoptiles of similar lengths were obtainedfrom different cultivars using 4 d aerated seeds, there weredifferences in R_E and coleoptile growth which were related tocoleoptile growth during 3 or 5 d anoxia, either on a tissue(r² = 0·85) or a fresh weight basis (r² = 0·70-0·97respectively). Results are discussed in relation to factorswhich may limit ethanol synthesis in rice exposed to anoxiaand the importance of growth to the survival of seeds and matureplants during submergence in the natural environment.Copyright1994, 1999 Academic Press Anoxia, ethanol, alcoholic fermentation, Oryza sativa L., rice, submergence     

Effect of Photoperiod and Irradiance Changes Upon Development of Freezing Tolerance and Accumulation of Soluble Carbohydrate in Seedlings of Lolium perenne Grown at 2 {degrees}C

Two contrasting cultivars of Lolium perenne were exposed toa range of daily radiation integrals during hardening at 2°Cfor 15 d. The maximum induced freezing tolerance measured asLT₅₀ (temperature for 50 % kill) differed markedly between thecultivars. The observed LT₅₀ values were unaffected by changesin the radiation integral above 10 mol m^–2 d^–1,whereas accumulation of water-soluble carbohydrate showed astrong positive correlation with the radiation integral overthe entire range of the experiment. The correlation betweenLT₅₀ and soluble carbohydrate content at the end of the hardeningperiod was poor and showed no obvious connection with genotype.Fructan polymers and sucrose were the major components of thesoluble carbohydrates in both cultivars. The depression of freezingpoint attributable to the accumulation of soluble, osmoticallyactive carbohydrate was not sufficient to account for the observedchanges in LT₅₀ in the hardy genotype. These results are discussedin relation to the interactions between growth, photosynthesisand assimilate partitioning during hardening. Lolium perenne, hardening, freezing tolerance, irradiance, carbohydrate, fructan     

Lung resistance and elastance in spontaneously breathing preterm infants: effects of breathing pattern and demographics

Reported values of lung resistance(RL) and elastance (EL) in spontaneouslybreathing preterm neonates vary widely. We hypothesized that thisvariability in lung properties can be largely explained by both inter-and intrasubject variability in breathing pattern and demographics.Thirty-three neonates receiving nasal continuous positive airwaypressure [weight 606-1,792 g, gestational age (GA) of25-33 wk, 2-49 days old] were studied. Transpulmonary pressure was measured by esophageal manometry and airway flow by facemask pneumotachography. Breath-to-breath changes in RL andEL in each infant were estimated by Fourier analysis ofimpedance (Z) and by multiple linear regression (MLR).RL_MLR (RL_MLR = 0.85 × RL_Z 0.43; r² = 0.95) and EL_MLR(EL_MLR = 0.97 × EL_Z + 8.4; r² = 0.98) werehighly correlated to RL_Z andEL_Z, respectively. Both RL(mean ± SD; RL_Z = 70 ± 38, RL_MLR = 59 ± 36 cmH₂O · s · l¹)and EL (EL_Z = 434 ± 212, EL_MLR = 436 ± 210 cmH₂O/l)exhibited wide intra- and intersubject variability.Regardless of computation method, RL was found to decreaseas a function of weight, age, respiratory rate (RR), and tidal volume(VT) whereas it increased as a function ofRR · VT and inspiratory-to-expiratorytime ratio (TI/TE). EL decreasedwith increasing weight, age, VT and female gender andincreased as RR and TI/TE increased. Weconclude that accounting for the effects of breathing patternvariability and demographic parameters on estimates of RLand EL is essential if they are to be of clinical value.Multivariate statistical models of RL and ELmay facilitate the interpretation of lung mechanics measurements inspontaneously breathing infants.

     

GENETIC VARIATION IN SYMPATRIC AND ALLOPATRIC POPULATIONS OF HYBRIDIZING FRESHWATER SNAIL SPECIES(VIVIPARUS ATER AND V. CONTECTUS)

To estimate the geographical extent of introgression, we studiedthe genetic structure of sympatric and allopatric populationsof hybridizing freshwater snail species Viviparus ater and V.contectus in central Europe. Six allozyme loci which were variablein Lake Garda, Italy in a previous study (five nearly diagnosticloci between the two species and one highly polymorphic locusin V. contectus) were analyzed from ten sympatric locationsand four allopatric populations each for the two species. Presumablyintrogressed genes (low allele frequencies) were found fromat least one locus in seven out of the ten sympatric sites.These seven sites covered most of northern Italy. The data indicatethat introgression has occurred from Viviparus contectus toV. ater and vice versa. Therefore, there is a possibility ofwidespread introgression or mosaic zones in nature. However,we cannot rule out that the observed patterns are due to theshared ancestry. V. ater possessed low genetic variation (thejackknifed mean of Wright's F_ST±S.E. over four loci was0.041±0.004). On the other hand, V. contectus showedhigh genetic differentiation (the jackknifed mean of F_ST±S.E. over six loci was 0.546±0.166). Although introgressionmay have caused evolutionary changes in V. ater and V. contectus,it was not strong enough to level out the genetic differencesbetween the two species, which may have originated from isolationamong populations in V. contectus and a past bottleneck eventin V. ater. (Received 21 June 1996; accepted 31 January 1997)