异迟眼蕈蚊在不同植物上的生长发育及种群参数

【目的】异迟眼蕈蚊Bradysia difformis Frey的幼虫取食为害作物的地下部分,影响作物的品质,为了明确韭菜、蚕豆、生菜、白菜和甘蓝5种植物对异迟眼蕈蚊生长发育以及繁殖的影响。【方法】本试验采用室内人工饲养测定的方法,研究了5种不同植物对异迟眼蕈蚊生长发育,繁殖力和存活率的影响,并统计了其对异迟眼蕈蚊种群参数的影响。【结果】结果表明:卵到蛹的发育历期依次为甘蓝、白菜、韭菜、生菜、蚕豆;5种植物对雌雄虫寿命影响不显著,对雌虫产卵量以及蛹重均有影响,其中在韭菜上的产卵量最大,甘蓝最少,在韭菜上蛹最重,生菜上蛹最轻;异迟眼蕈蚊的存活率随着生长发育降低,总体在韭菜上的存活率高于其他寄主植物,在生菜上的存活率均最低。统计分析不同植物对异迟眼蕈蚊种群参数的影响,净增殖率和内禀增长率在韭菜上最大而在甘蓝上最小;平均世代周期在蚕豆上最短,甘蓝上最长;种群加倍时间在韭菜上最短,而在甘蓝上最长。【结论】由此可知,异迟眼蕈蚊均可以在韭菜,蚕豆,生菜,白菜和甘蓝上完成生长发育及繁殖,其对5种植物的适应性依次为:韭菜、蚕豆、白菜、甘蓝和生菜。     

北京地区韭菜迟眼蕈蚊种群动态及越夏越冬场所调查研究

【目的】为明确北京地区不同栽培管理模式下韭菜田全年韭菜迟眼蕈蚊Bradysia odoriphaga种群动态的发生规律及其越夏越冬场所。【方法】分别在2014—2015年通过黄色板对露地和温室韭菜田块的韭菜迟眼蕈蚊成虫进行了监测,并通过挖根和网捕的方式调查韭菜迟眼蕈蚊的越夏越冬场所及虫态。【结果】北京地区,露地韭菜田块韭菜迟眼蕈蚊每年发生3~4代,温室内可全年发生,主要为害高峰期在春秋两季;韭菜迟眼蕈蚊幼虫主要分布在0~5 cm的土壤深处;夏季韭菜迟眼蕈蚊虫口基数偏低,但主要在本地韭菜田块越夏;冬季韭菜迟眼蕈蚊主要以4龄老熟幼虫在鳞茎内或鳞茎附近的土壤中越冬。【结论】本研究阐明了北京地区不同栽培管理模式下,韭菜迟眼蕈蚊周年发生的种群动态规律及越夏越冬生物学特性,为韭菜迟眼蕈蚊的预测测报和综合防治提供理论参考依据。     

韭菜迟眼蕈蚊雄虫日龄对其交配能力和雌虫生殖力的影响

为了提高韭菜迟眼蕈蚊性信息素监控水平,本文在25℃、RH70%±5%、光周期L∶D=14∶10的条件下研究韭菜迟眼蕈蚊不同日龄雄虫的交配能力及其对雌虫生殖力的影响。结果表明,未交配雄虫平均寿命4.5d,雌虫4.2d,雌雄成虫的平均寿命无显著差异。雄虫一生最多可交配13次,且随着雄虫日龄增加,雌虫交配成功率逐渐减少。随着雄虫交配经历的增加,雌虫交配的时间延长,最长达97min。雄虫的日龄并不影响与之交配雌虫的产卵量和卵孵化率,但是雄虫的交配经历与雌虫的产卵量和卵孵化率相关,尤其当雄虫交配经历超过8次时,与之交配的雌虫的产卵量和卵孵化率显著下降。研究阐明了韭菜迟眼蕈蚊的生殖行为特征,为其性信息素应用提供了参考。     

韭菜养根期干旱处理对韭菜迟眼蕈蚊种群动态的影响

【目的】本研究旨在明确韭菜养根期干旱处理对韭菜迟眼蕈蚊Bradysia odoriphaga种群动态的影响。【方法】通过控制浇水调控土壤含水量,调查15, 20, 25和30 d干旱胁迫下砂土和壤土栽培的韭菜中韭菜迟眼蕈蚊F1代成虫的发生量,并通过两性生命表方法分析干旱胁迫30 d对韭菜迟眼蕈蚊F₁代种群参数的影响。【结果】研究结果表明,不论砂土还是壤土,干旱胁迫均抑制了韭菜迟眼蕈蚊F1代成虫的发生: 随干旱胁迫时间的增加,韭菜迟眼蕈蚊成虫的数量显著下降;其中干旱胁迫30 d时,韭菜迟眼蕈蚊成虫的发生量最低,壤土或砂土中的F₁代成虫羽化量仅为1头,与其他干旱胁迫时间(15, 20和25 d)相比发生量差异显著。经历30 d干旱胁迫的韭菜迟眼蕈蚊F₁代1, 2, 3和4龄幼虫发育历期、未成熟期、雌成虫产卵前期和总产卵前期较对照组显著延长,分别延长0.82, 253, 2.82, 0.68, 6.32, 1.36和8.69 d,而净生殖率(R₀)、内禀增长率(r_m)以及周限增长率(λ)则显著低于对照组,分别较对照组下降32.38%, 37.50%和5.13%。【结论】在韭菜养根期进行干旱处理可有效抑制韭菜迟眼蕈蚊的发生。     

韭菜迟眼蕈蚊在不同温度-i下的实验种群生命表

组建了韭菜迟眼蕈蚊Bradysia odoriphaga在15℃、20℃、25℃和30℃温度下的实验种群生命表.结果表明,韭菜迟眼蕈蚊各虫态的发育速率随温度的升高而加快,在15℃下卵-成虫的发育历期最长(72.4天),而30℃时仅为21.2天,20℃和25℃时分别为27.3天和23.9天;韭菜迟眼蕈蚊卵、幼虫,蛹和卵-成虫的发育起点温度分别为5.9℃、8.7℃,3.3℃和7.8℃,有效积温分别为77.7、267.2、75.7和418.2日@度;韭菜迟眼蕈蚊成虫寿命随温度升高而逐渐缩短,雌虫寿命在20℃时最长,为11.7天,在30℃下最短,仅存活4.1天,除30℃外在其它温度下雌蚊寿命均长于雄蚊;单雌产卵量在20℃时最高,平均为159.9粒,30℃时最低,为114.7粒.20℃～25℃是韭菜迟眼蕈蚊的最适生长温度,成虫寿命长、繁殖力强、种群趋势指数大,温度过高或过低均不利于种群的繁衍.     

韭菜迟眼蕈蚊在不同温度下的实验种群生命表

组建了韭菜迟眼蕈蚊Bradysia odoriphaga 在15℃、20℃、25℃和30℃温度下的实验种群生命表。结果表明,韭菜迟眼蕈蚊各虫态的发育速率随温度的升高而加快,在15℃下卵-成虫的发育历期最长（72.4天）,而30℃时仅为21.2天,20℃和25℃时分别为27.3天和23.9天;韭菜迟眼蕈蚊卵、幼虫,蛹和卵-成虫的发育起点温度分别为5.9℃、8.7℃,3.3℃和7.8℃,有效积温分别为77.7、267.2、75.7和418.2日·度;韭菜迟眼蕈蚊成虫寿命随温度升高而逐渐缩短,雌虫寿命在20℃时最长,为11.7天,在30℃下最短,仅存活4.1天,除30℃外在其它温度下雌蚊寿命均长于雄蚊;单雌产卵量在20℃时最高,平均为159.9粒,30℃时最低,为114.7粒。20℃～25℃是韭菜迟眼蕈蚊的最适生长温度,成虫寿命长、繁殖力强、种群趋势指数大,温度过高或过低均不利于种群的繁衍。     

韭菜迟眼蕈蚊雌成虫对其不同虫态气味的行为反应

【目的】明确韭菜迟眼蕈蚊Bradysiaodoriphaga Yang et Zhang雌成虫对不同虫态的行为趋性及起作用的活性物质,为开发高效引诱剂诱杀成虫的绿色防控技术提供基础。【方法】采用Y型嗅觉仪测定未交配或已交配韭菜迟眼蕈蚊雌成虫对2龄幼虫、4龄幼虫、蛹和卵块挥发物的趋性,应用固相微萃取和气-质联用仪对有引诱活性虫态挥发物进行定性和定量分析,然后进一步采用Y型嗅觉仪测定不同化学成分及比例对雌虫的引诱活性。【结果】结果表明,未交配或者已交配韭菜迟眼蕈蚊雌成虫明显嗜好4龄幼虫,而对2龄幼虫、蛹和卵块无明显趋性。韭菜迟眼蕈蚊4龄幼虫体表挥发物主要成分是二丙基二硫醚和2,2-二甲基-1,3-噻烷等二硫化物,前者含量是后者的10倍。50 mg二丙基二硫醚单体对韭菜迟眼蕈蚊交配雌成虫具有明显吸引作用,50 mg和5 mg 2,2-二甲基-1,3-噻烷单体对韭菜迟眼蕈蚊交配雌成虫具有一定吸引作用;二丙基二硫醚与2,2-二甲基-1,3-噻烷以1︰1(50 mg︰50 mg)和10︰1(50 mg︰5 mg)混合物对韭菜迟眼蕈蚊交配雌成虫具有明显吸引作用;而将二丙基二硫醚和2,2-二甲基-1,3-噻烷以10︰1(50 mg︰5 mg)混合后与50 mg二丙基二硫醚单体比较,前者对韭菜迟眼蕈蚊交配雌成虫的吸引作用更明显。【结论】韭菜迟眼蕈蚊雌成虫明显嗜好4龄幼虫,其体表挥发物二丙基二硫醚等二硫化物对韭菜迟眼蕈蚊雌成虫具有吸引作用,2,2-二甲基-1,3-噻烷对二丙基二硫醚单体吸引韭菜迟眼蕈蚊交配雌成虫具有增效作用。     

韭菜迟眼蕈蚊规模化饲养技术

【目的】韭菜迟眼蕈蚊Bradysia odoriphaga Yang et Zhang是为害韭菜等蔬菜的重要害虫,规模化饲养提供大量虫源是其他研究工作得以开展的基础。本研究旨在解决目前韭菜迟眼蕈蚊饲养规模小、材料和劳动力成本高等问题。【方法】本实验室利用土豆、花生、大豆为食物进行了规模化饲养韭菜迟眼蕈蚊的研究,测定了取食上述饲料的韭菜迟眼蕈蚊的成活率、繁殖力、羽化节律和性比。【结果】取食土豆、花生、大豆等食物的韭菜迟眼蕈蚊的成活率和繁殖力与取食人工饲料和韭菜的无显著差异,卵到成虫的发育历期为20~23 d,雌雄性比为0.8︰1~1.2︰1。在规模化饲养过程中,土豆、花生、大豆等饲料有发霉长菌现象,经分子鉴定,主要种类为雅致放射毛霉、巴克斯毛霉、黄曲霉和赭曲霉,均为腐生菌,不影响韭菜迟眼蕈蚊的存活和繁殖。【结论】该技术省工省时,成本低,特别适合实验室种群的维持和大量试虫的饲养。     

营养物质和水分对韭菜迟眼蕈蚊成虫繁殖和寿命的影响

【目的】研究韭菜迟眼蕈蚊Bradysia odoriphaga成虫对营养物质的选择补充习性,找出对其产卵繁殖起重要作用的关键因子,为其有效防控提供依据。【方法】通过室内成虫单独配对试验,研究不同营养物质(花粉、蜂蜜、维生素、糖、醋、酒)和水分对韭菜迟眼蕈蚊成虫繁殖和寿命的影响。【结果】随着花粉、蜂蜜和维生素浓度的增加,成虫的繁殖力下降;糖醋酒液单一成分和混合液对成虫繁殖和寿命的影响与水分相近;补充供试营养物质和水分对韭菜迟眼蕈蚊的繁殖力均明显高于干燥对照。补充水分的成虫繁殖力最高,产卵率超过70%,单雌平均产卵量达70粒,雌雄成虫寿命最长均超过3 d。土壤湿度对产卵有显著影响,土壤相对湿度为40%和50%时,成虫的落卵率超过25%,落卵量大于480粒,显著高于其它湿度(P<0.05)。【结论】水分是影响韭菜迟眼蕈蚊成虫繁殖和寿命的关键因子,供试的营养物质对韭菜迟眼蕈蚊繁殖没有促进作用,因此在韭菜迟眼蕈蚊成虫盛发期可以通过控制田间土壤湿度来影响韭菜迟眼蕈蚊繁殖。     

韭菜迟眼蕈蚊Bradysia odoriphaga嗅觉行为反应

【目的】韭菜迟眼蕈蚊Bradysia odoriphaga Yang et Zhang是韭菜生产中最主要的害虫。通过嗅觉趋性试验测定了3龄幼虫和雌虫对多种味源植物的寄主选择性及行为反应,为进一步研发绿色高效的生态防控技术提供理论依据。【方法】利用"Y"型嗅觉仪测定幼虫对健康韭菜、灰霉菌侵染韭菜、平菇、大葱和小白菜等几种寄主植物的嗅觉趋性反应,利用四臂嗅觉仪测定雌虫对健康韭菜、机械损伤韭菜、腐殖质和浸泡大豆4种味源材料的行为反应。【结果】"Y"型嗅觉仪测定结果表明健康韭菜、平菇和大葱对3龄幼虫的引诱作用较强;四臂嗅觉仪测定结果表明韭菜迟眼蕈蚊雌虫对机械损伤韭菜和腐殖质表现出较强趋性。【结论】应用嗅觉仪观测了韭菜迟眼蕈蚊3龄幼虫、雌虫对不同味源材料在不同条件下的定向选择行为,此项工作可为韭菜迟眼蕈蚊植物源引诱剂或驱避剂的进一步研发奠定基础。     

Experimental induction and termination of non-reciprocal reproductive incompatibilities in a parahaploid mite

Two experiments were conducted to investigate the relationship between rearing temperatures, the presence or absence of Wolbachia endosymbionts, and non-reciprocal reproductive incompatibility in inbred lines of the parahaploid phytoseiid mite Metaseiulus occidentalis (Nesbitt) (Acari: Phytoseiidae).Heat-treated females crossed with infected males reared at room temperature produced few eggs and high proportions of shriveled eggs. No female progeny were produced. The reciprocal cross was normal. A second experiment showed that the incompatible cross from the first experiment could be made compatible if the infected line was heat-treated and those males crossed with the original heat-treated females. Furthermore, a new incompatibility was induced in a formerly compatible cross when the newly heat-treated females were crossed with males from their base colony. Heat-treatment was correlated with the loss of Wolbachia in both experiments. Wolbachia may thus affect non-reciprocal reproductive incompatibility in M. occidentalis, and may produce a unique incompatibility phenotype in this parahaploid species, including both reduced numbers of male progeny and a lack of female progeny.     

Sex ratios in offspring of sex-reversed sea bass and the relationship between growth and phenotypic sex differentiation

Groups of sexually undifferentiated sea bass Dicentrarchus labrax were fed with the androgen 17α-methyltestosterone (MT) during sex differentiation. MT treatment increased males from 79±3% in the controls (the usual 3:1 male:female sex ratio of cultured sea bass) to 100±0%, implying that in the treated groups one out of each five resulting males was a masculinized female (neomale). Thirteen males from the MT treated groups were taken as the parental generation and their sperm used to individually fertilize a pool of eggs from unrelated females. The probability of having at least one neomale was 95% and most probably two or three of the males used were neomales. The offspring from each family were reared separately under the same environmental conditions. Samples were taken at 11 and 15 months of age, during and after sex differentiation, respectively. Results showed that females predominated among the larger fish whereas males and undifferentiated fish predominated among the smaller ones. Intersexes exhibited an intermediate size. All fish with a body length smaller than 12 cm were undifferentiated. These results suggest that sex differentiation is more dependent on length than on age. At 15 months, sex ratios were male-biased in all families, except one (females ranged from 5 to 50%) and only two families had sex ratios not significantly different from 1:1, suggesting that the mechanism of sex determination in the sea bass is not of a XX/XY or ZW/ZZ type since no family exhibited a female-biased progeny, as would be expected from both types. Results support the hypothesis that factors other than genetic, i.e., environmental, may act epigenetically on the sex determination mechanisms of sea bass, as has been demonstrated in other fishes.     

Effects of parent age at mating on reproductive response of Glyptapanteles flavicoxis (Hymenoptera: Braconidae), a larval parasitoid of the gypsy moth (Lepidoptera: Lymantriidae)

Glyptapanteles flavicoxis (Marsh) (Hymenoptera: Braconidae) is a gregarious larval parasitoid of the Indian gypsy moth Lymantria obfuscata (Walker) (Lepidoptera: Lymantriidae), that is believed to have potential for inundative releases against gypsy moth populations, because it can be reared in large numbers with few hosts. Unfortunately, sex ratios in laboratory reared G. flavicoxis are usually male-biased, hindering efforts to mass release this species for biological control by making the production of females costly. Because parental age at time of mating is known to affect the sex ratio in some Braconidae, we crossed haploid males and virgin females at 0, 1, 4, 9, and 16 d old with at least 10 trials for each of the 25 combinations. Numbers and sex ratios of progeny produced by females each day were recorded. Both progeny and sex ratios (percentage of females) among progeny produced by ovipositing females of G. flavicoxis decreased markedly over time, so only the first days production need be used in mass rearing. The reduction in the proportion and numbers of females among progeny as females aged is consistent with sperm depletion. Approximately 30% of females in all age classes mated to newly emerged males (day 0) produced all male progeny, whereas only 10-15% of those mated to older males failed to produce any daughters. When crosses with only male progeny were excluded from the analysis, females mated to males 1 d old had higher sex ratios in progeny than those mated to males in other age classes. In addition, females mated the day that they emerged tended to have progeny with the highest sex ratios.     

Strong male/male competition allows for nonchoosy females: high levels of polygynandry in a territorial frog with paternal care

Our knowledge about genetic mating systems and the underlying causes for and consequences of variation in reproductive success has substantially improved in recent years. When linked to longitudinal population studies, cross-generational pedigrees across wild populations can help answer a wide suite of questions in ecology and evolutionary biology. We used microsatellite markers and exhaustive sampling of two successive adult generations to obtain population-wide estimates of individual reproductive output of males and females in a natural population of the Neotropical frog Allobates femoralis (Aromobatidae), a pan-Amazonian species that features prolonged iteroparous breeding, male territoriality and male parental care. Parentage analysis revealed a polygynandrous mating system in which high proportions of males (35.5%) and females (56.0%) produced progeny that survived until adulthood. Despite contrasting reproductive strategies, successfully reproducing males and females had similar numbers of mating partners that sired the adult progeny (both sexes: median 2; range 1-6); the numbers of their offspring that reached adulthood were also similar (both sexes: median 2; range 1-8). Measures of reproductive skew indicate selection on males only for their opportunity to breed. Reproductive success was significantly higher in territorial than in nonterritorial males, but unrelated to territory size in males or to body size in both sexes. We hypothesize that female polyandry in this species has evolved because of enhanced offspring survival when paternal care is allocated to multiple partners.     

The effects of differential prenatal and postnatal social enviroments on sexual maturation of young prairie deermice (Peromyscus maniculatus bairdii).

We compared the influence of different social enviroments on sexual maturation of young prairie deermice. Between 12 and 48 hr after parturition, one-half of a litter from a population was exchanged with one-half of a litter from a bisexual pair. In each situation the remaining half-litter served as controls for the exchange procedure. The testes and vesicular weights of population-reared males were significantly larger than those of males reared by bisexual pairs, irrespective of prenatal enviroment. Generally, population-reared females also had larger reproductive organs than those reared by bisexual pairs, although these differences were not statistically significant. There were no significant differences in the adrenal weights of males or females, but females reared by bisexual pairs had larger absolute and relative adrenals than females reared in populations.     

Precocene causes male determination in the aphid Myzus persicae

When adult apterous viviparous females of Myzus persicae, reared in short night conditions at 21–23°C, are treated with the precocene analogue 6-methoxy-7-ethoxy-2,2-dimethylchromene, they deposit males towards the end of their reproductive lives. The first-born normal daughters of the treated females also deposit some males at various times in their reproductive lives. Karyotypic analysis was used to investigate the sequence of male and female embryos in the ovarioles of precociously metamorphosed aphids. The experiments support the hypothesis (Mittler et al., 1979) that juvenile hormone level controls the sex determination process in aphids. Since male aphids have an XO sex chromosome constitution, this implies that juvenile hormone level influences the behaviour of the X-chromosomes at or before the single maturation division of the egg. At this division one X-chromosome is eliminated from eggs which will develop as males. Aphids provide the first example of a specific endocrine influence on chromosome behaviour in sex determination.     

Inheritance of microsatellite DNA in bisexual gynogenesis complex of Fangzheng silver crucian carp,Carassius auratus gibelio (Bloch)

Five gynogenetic progeny groups of silver crucian carp Carassius auratus gibelio were produced and sex ratios (males:total progeny) of each of the progeny groups were analysed. About 110 males and 366 females were genotyped at 15 microsatellite loci for comparison with their parents to (1) verify the gynogenesis status of Fangzheng C. auratus gibelio, (2) detect the incorporation of paternal genetic material into the offspring and (3) study the possible association of genetic exchange at microsatellite loci with the existence of sex. The sex ratios in progenies of five groups were highly variable, but all had significant female bias. The sex ratio ranged from 0 to 0·37. Significant differences in the sex ratio within and between groups were also found. Microsatellite genotyping at 15 loci showed that 100 and 97% of the progeny shared the same genotype with the mother in four groups and in one group, respectively, confirming that gynogenesis is the general mechanism of reproduction in C. auratus gibelio. However, 0·63% of all offspring did show incorporation of paternal genetic material. No single loci tested were associated with the occurrence of male progeny, indicating unknown genetic mechanisms for sex determination in C. auratus gibelio.     

ANDROGENETICS AND TRIPLOIDS FROM AN INTERACTING PARTHENOGENETIC HYBRID AND ITS ANCESTORS IN STICK INSECTS

Populations of unisexual organisms are often assumed to be genetically invariant (clones) and destined to a short existence on an evolutionary timescale. Unisexual organisms are most often obligate parthenogens and, by definition, ought to be completely isolated reproductively from related bisexual organisms. The assumption of complete reproductive isolation between amphimictic ancestors and thelytokous hybrids is common to most hypotheses on the evolution of sex and its adaptive significance. Stick insects of the genus Bacillus however provide evidence for reproductive interactions between allodiploid parthenogens and their ancestors, because pure species progeny (androgenetics) and triploid descendants are produced. These findings demonstrate that, through androgenesis, offspring of parthenogenetic hybrid females can contribute specimens of both sexes to the fathering species when fertilized by syntopic ancestral males and the parthenogenetic egg of strictly clonal females, when fertilized, allows a third genome to be added to the allodiploid chromosome set. These triploid genomes promote further genetic diversification and evolution of the unisexual populations through the formation of new clones by recombination during the changed maturation mode of allotriploid eggs. All this argues for much more complex breeding systems and evolutionary pathways than are usually assumed for hybrid unisexual organisms.     

Effect of prey density on sex ratio of two predacious mites, Phytoseiulus persimilis and Amblyseius womersleyi (Acari: Phytoseiidae)

The sex ratios of two phytoseiid mites, Phytoseiulus persimilis and Amblyseius womersleyi, were observed under various prey conditions. Upon consumption of abundant prey, both phytoseiids produced progeny in a female-biased sex ratio (approximately 0.8 females). When few prey were consumed, the sex ratio was lowered to 0.5 (the unbiased sex ratio). Under the conditions in which the unbiased sex ratio was observed, male and female progeny appeared in an alternating sequence. To determine the change in the sex ratio and the sequence of progeny, the size of eggs deposited by females under various prey conditions was first compared. Survivorship and developmental rate of progeny (immatures) hatched under ample and poor prey conditions were also examined. The eggs deposited under poor prey conditions were smaller than those deposited under ample prey conditions. This is an indication that the phytoseiid females did not invest extra energy into the eggs to secure survival of their progeny under poor prey conditions. The male and female progeny from the small eggs developed slowly, probably due to the small egg size. However, hatchability of the small eggs and survival of the immatures were the same as those of the normal eggs when the immatures were reared under ample prey conditions. The immature survivorship was little affected by the prey consumption rate of their mothers when the immatures were reared under poor prey conditions. We concluded that the sex ratio of phytoseiid mites is not determined by the characteristics of the progeny, but by the nutritious condition of the females.Exp Appl Acarol 22: 709723 © 1998 Kluwer Academic Publishers     

Reproductive behavior and the maintenance of all-female Poecilia

Synopsis There are four members involved in the breeding complexes of poeciliid fishes found in the freshwaters of northeastern Mexico: males and females of a bisexual species, and diploid and triploid unisexuals. Both unisexuals reproduce by gynogenesis, i.e., an asexual type of reproduction where the sperm triggers egg development but the male genome is excluded to produce clonal offspring. The three types of females are closely related, which suggests that they are potential competitors since all three require the service of the same males. The potential for competition is compounded by a highly skewed sex ratio in favor of females. On the average the unisexuals comprise about 30% of the Poecilia females. This high frequency coupled with a close genetic relatedness to their bisexual hosts, raises the question of how the unisexuals are maintained in nature.Other investigators who work with bisexual/unisexual complexes in the related genus, Poeciliopsis, have postulated that male dominance hierarchies are responsible for restricting the access of subordinate males to their conspecific females. Consequently, these subordinate males mate with unisexual females. The current report tests whether or not this hypothesis applies to bisexual/unisexual complexes of Poecilia.We have found that linear dominance hierarchies appear to function in the defense of home ranges and do not restrict access of males to females. Dominant males exhibit less mating activity than subordinate males towards females. Previous reports showed that males are reproductively competent throughout the year, whereas females show striking asynchrony in their reproductive readiness. Such asynchrony limits the proportion of receptive females at any one time. Consequently, there are more males ready to mate than there are females receptive to their mating attempts. This may lead to mating frenzies. We postulate that these indiscriminate matings maintain the fertility of both unisexuals. When the relative reproductive outputs of adult females are compared, both unisexuals appear as fit as their bisexual congeners.