Incomplete induction of mixis in <Emphasis Type="Italic">Brachionus calyciflorus</Emphasis>: patterns of reproduction at the individual level

In the rotifer Brachionus calyciflorus mictic-female production is density-dependent and appears to be induced by a chemical—a quorum sensing molecule—produced by the females themselves. Even at the highest densities, however, populations never become entirely mictic: i.e., some amictic females continue to be produced. Surprisingly, the phenomenon also occurs in clonal laboratory populations with genetically identical individuals. Here, we study how this ecologically adaptive phenomenon is generated at the level of individual reproducing females. In a life-history experiment we subjected 123 amictic females of a clone of B. calyciflorus separately to a daily renewed stimulus of culture medium conditioned at a density of 30 females ml⁻¹. For each of these mothers we isolated the lifetime offspring individually and recorded whether these females were amictic or mictic. Mothers produced on average 16 offspring but none of the mothers produced 100% mictic offspring; the average proportion of mictic females was 30%, despite the extremely strong stimulus. The distribution of amictic vs. mictic offspring was not uniform over the mothers’ lifetime. Early and late offspring had a low probability of being mictic whereas mid-aged mothers produced the highest proportion of mictic daughters (up to 56%). We conclude that not all oocytes of B. calyciflorus can be turned into mictic females, even when the mictic-female-inducing stimulus is extremely high. Propensity to become a mictic female also depends on the rank of an egg within a female’s offspring production. Despite these regularities, we observed considerable stochastic variability with respect to individual mothers’ life histories. Guest editors: S. S. S. Sarma, R. D. Gulati, R. L. Wallace, S. Nandini, H. J. Dumont & R. Rico-Martínez. Advances in Rotifer Research.     

Endogenous regulation of environmentally induced sexuality in a rotifer: a multigenerational parental effect induced by fertilisation

SUMMARY 1. Sexual reproduction in the heterogonic life cycle of many rotifers occurs when amictic females, which produce diploid eggs developing parthenogenetically into females, are environmentally induced to produce mictic females. Mictic females produce haploid eggs which develop parthenogenetically into males or, if fertilised, into resting eggs – encysted embryos which develop into amictic females after an obligatory diapause. 2. A Florida strain of Brachionus calyciflorus was used to test the prediction that amictic females hatching from resting eggs (Generation 1), and those from the next few parthenogenetic generations, have a lower propensity to produce mictic daughters in response to crowding than those from later parthenogenetic generations. In 10 replicate clones, populations initiated by amictic females from generations 1, 5, 8, 12 and 18 were exposed to a standardised crowding stimulus, and the proportion of mictic females in the populations was determined. These proportions varied significantly across generations and clones. They were very low in the early generations and gradually increased to a mean of about 0.5 at Generation 12. 3. The mechanism for the transgenerational plasticity in response to crowding is not known. One possibility is that resting eggs contain an agent from their fertilised mictic mother's yolk gland that prevents development into mictic females and is transmitted in increasingly low concentrations through successive parthenogenetic generations of amictic females. 4. This parental effect may contribute to clonal fitness by ensuring that a clone developing from a resting egg will attain a higher population size through female parthenogenesis before maximising its commitment to sexual reproduction, even in the presence of a crowding stimulus from a high population density of other clones. Therefore, the number of resting eggs to which a clone contributes its genes should be maximised. 5. The clonal variation in propensity to produce mictic females in this strain indicates genetic variation in the trade‐off between maximising population growth via female parthenogenesis and increasing the probability of producing at least some resting eggs before local extinction from the plankton.     

Low crowding threshold for induction of sexual reproduction and diapause in a Patagonian rotifer

1. This study investigates the basis and ecological significance of the extremely high propensity for mixis (sexual reproduction) observed in laboratory populations of Brachionus calyciflorus from a temporary pond in Patagonia. 2. Experiments with stem females hatched from resting eggs showed that these females were exclusively amictic but produced mictic daughters even at very low population densities. In six experiments, newly hatched stem females were cultured individually in different volumes (1.5, 12, 40 and 150 mL). The percentage of mictic daughters (mixis ratio) was high in the smaller volumes (e.g. 44–83% in 1.5 mL) and lower in the largest volume (6–21% in 150 mL). A regression analysis combining the data from these experiments showed a highly significant decrease in mixis ratio with volume and indicated that the lowest population density at which mixis still occurs (mixis threshold) would be 3.4 females L^?1 (95% CL 2.9–4.0 females L^?1). This value is considerably lower than mixis thresholds for other rotifers (25–250 females L^?1 for many species and 9000–477 000 females L^?1 for some Spanish Brachionus plicatilis). 3. In three additional experiments, stem females and their amictic daughters were cultured individually in 150 mL. The percentage of mictic daughters produced by these two generations of females was not significantly different, showing that the mixis response to crowding is not inhibited in the stem‐female generation. 4. Laboratory experiments showed that two common predators of the Patagonian B. calyciflorus (the calanoid copepod Parabroteas sarsi and the backswimmer Notonecta vereertbruggheni) each ate 60–70 B. calyciflorus predator^?1 per day and cleared all rotifers from c. 250 mL per day. Thus, a very low mixis threshold and high maximal mixis ratio may ensure production of some resting eggs soon after colonisation of the pond and before complete removal from the plankton. 5. Two laboratory experiments showed that resting eggs of the Patagonian B. calyciflorus hatched at variable rates (28 and 81%) after a brief diapause when kept in the conditions under which they were produced and oviposited (20–21 °C; L: D 16: 8). Early hatching of resting eggs from pond sediment may allow multiple periods of colonisation and resting egg production in a season. This may offset the fitness cost of limited population growth through female parthenogenesis in the face of unpredictable and abrupt risk of extinction because of predators.     

Induction of mictic females in the rotifer Brachionus: oocytes of amictic females respond individually to population-density signal only during oogenesis shortly before oviposition

1. One at a time during the reproductive period of amictic females, oocytes fill with yolk and undergo a mitotic maturation division (oogenesis), are oviposited as single cells, and then develop parthenogenetically into females. Sexual reproduction in Brachionus and several other genera is initiated when amictic females are crowded and oviposit some eggs induced to differentiate into mictic females. Mictic females produce haploid eggs that can develop parthenogentically into males or be fertilised and develop into diapausing embryos called resting eggs. 2. This study examines the time when oocytes in amictic females respond to maternal population density. Is the fate of all oocytes in the germarium irreversibly determined during the early postnatal life of the mother, or is each oocyte labile until just before oviposition? In the former case, the probability of an amictic female producing a mictic daughter at any time throughout her reproductive period would reflect the population density she experienced while young and not that at the time she oviposited an egg. 3. Amictic females of two clones of a Florida strain of B. calyciflorus were cultured singly from birth at a low or high density (in a large or small volume) until about halfway through their reproductive period and then switched (experimental treatment), or not (control treatment), to the other density condition. The results indicate that the female fate of an oocyte is determined by maternal population density during oogenesis. Eggs oviposited soon after transfer from low to high density had the same, or a higher, probability of becoming mictic females compared with those produced by control females kept at the high density; eggs oviposited after transfer from the high to the low density had the same low probability of becoming mictic females as those produced by control females kept at the low density. 4. Control females kept at the high density were less likely to produce mictic daughters as they aged. This decline is not because of a decreased propensity of older females to respond to crowding, as older females responded maximally when transferred from a low to a high population density. 5. As oocytes in amictic females respond to maternal population density only during oogenesis, there is a negligible lag between the population‐density signal in the environment and the commitment to sexual reproduction. This minimises the obligatory two‐generation lag between this signal and production of resting eggs, and thus reduces the possibility that crowding will lead to food limitation before production of these eggs.     

Life history characteristics of three types of females in Brachionus calyciflorus Pallas (Rotifera) fed different algae

This study describes the life history characteristics of amictic, unfertilized mictic and fertilized mictic females of the rotifer Brachionus calyciflorus cultured individually on two different algae at 0.1 mg ml^–1 food concentration and 27 °C. The duration of the juvenile period of amictic females was significantly shorter on Chlorella pyrenoidosa Chick than on Scenedesmus obliquus Kütz or both algae together. The duration of the juvenile period of unfertilized mictic females was significantly longer, and the number of eggs produced by amictic females was significantly larger on Chlorella pyrenoidosa than on S. obliquus. When fed the same type of alga, the duration of the juvenile period of the fertilized mictic females was the longest among the three types of females, and the durations of the reproductive period of the amictic females and the post-reproductive period of the fertilized mictic females were longer than, or equal to those of the other two types of females, respectively. The number of eggs produced by an unfertilized mictic female was the largest among the three types of females, and that of amictic females was larger than or equal to that of fertilized mictic females, depending on the type of diet.     

长、短期连续孤雌生殖对萼花臂尾轮虫生活史和遗传特征的影响

应用单个体培养、聚丙烯酰胺凝胶垂直板电泳技术和特异性染色方法,以具有明显生化遗传差异的3克隆萼花臂尾轮虫（Brachionus calyciflorus）为对象,对长期（世代数大于200）和短期（世代数约为12）连续孤雌生殖后的轮虫进行了生活史特征和等位酶（PGM,GPI,MDH）分析。结果表明,除3克隆轮虫的生殖后期历时和每个雌体一生中所产的后代数在长期和短期连续孤雌生殖后的个体之间均无显著的差异外（P>0.05）,其它生活史特征在长期和短期连续孤雌生殖后的个体之间的差异均因轮虫克隆的不同而异。克隆对轮虫的胚胎发育时间、幼体阶段历时、生殖期历时、平均寿命、轮虫每个雌体一生中所产的后代数、幼体大小和幼体的耐饥饿时间均有显著的影响（P<0.05）;连续孤雌生殖历期对轮虫幼体阶段历时、平均寿命、后代混交雌体百分率和幼体的耐饥饿时间均有显著的影响（P<0.05）;而克隆和连续孤雌生殖历期间的交互作用对轮虫的胚胎发育时间、生殖期历时、平均寿命、幼体大小和幼体的耐饥饿时间均有显著的影响（P<0.05）。长期连续孤雌生殖后的轮虫幼体阶段历时和幼体的耐饥饿时间均显著短于短期连续孤雌生殖后的轮虫,平均寿命显著长于短期连续孤雌生殖后的轮虫,后代混交雌体百分率显著高于短期连续孤雌生殖后的轮虫。3克隆轮虫在长期和短期连续孤雌生殖后其3种等位酶的酶谱均未出现明显的改变。     

Optimal rates of bisexual reproduction in cyclical parthenogens with density-dependent growth

This work explores theoretical patterns of reproduction that maximize the production of resting eggs and the long-term fitness of genotypes in cyclical parthenogens. Our focus is on density-dependent reproduction as it influences the consequences of a trade-off between producing amictic daughters – which reproduce parthenogenetically and subitaneously – and producing mictic daughters – which undergo meiosis and bisexual reproduction. Amictic females increase competitive ability and allow the population to achieve a larger size; mictic females directly contribute to population survival through harsh periods by producing resting eggs. Although morphologically indistinguishable, the two types of females differ greatly in their ecological and reproductive roles. What factors underlie the differential allocation of resources to produce amictic and mictic females? Using a demographic model based on readily accessible parameters we demonstrate the existence of a frequency of mictic females that will maximize the population's long-term fitness. This frequency, termed the optimal mictic ratio, m_o, is 1 ? (q/b)^1/2, where q is the mortality rate and b is the maximum birth rate. Using computer simulation we compared the fitness of a population with this constant mictic ratio with populations having multiple switches from complete parthenogenetic growth to complete allocation in mixis (mictic ratio either 0 or 1). Two important conclusions for optimal mixis in density-dependent growth conditions are: (1) intermediate mictic ratios are optimal, and (2) optimal mictic ratios are higher when habitat conditions are better. Physiological cues responding to differences in birth and death rates are common so that it is possible that populations may adjust their relative rates of mictic and amictic female production in response to environmentally induced changes to the optimum mictic ratio. Our analysis demonstrates that different patterns of mixis are expected in different type of habitats. Since the optimal mictic ratio is sensitive to the effects of a variety of environmental challenges, our model makes possible a new means to evaluate life history evolution in cyclical parthenogens.     

Effect of food concentration on the production and viability of resting eggs of the rotifer Brachionus: implications for the timing of sexual reproduction

1. Life‐table experiments with Brachionus calyciflorus test several hypotheses related to the idea that sexual reproduction in monogonont rotifers should occur when food resources are favourable. 2. The food concentration necessary for a fertilised mictic female to produce one phenotypically normal resting egg was higher than that for an amictic female to produce one daughter. At the lowest concentration of Cryptomonas erosa (1.25 × 10³ cells mL^?1), the lifetime fecundity of these two types of females was 0.9 and 1.4, respectively. 3. The lifetime fecundity of both fertilised mictic females and amictic females increased with food concentration to 3.4 resting eggs and 15.2 daughters female^?1, respectively. The approach to maximal fecundity with increasing food concentration was more rapid for fertilised mictic females, such that their lifetime fecundity relative to that of amictic females gradually decreased from 0.64 (at 1.25 × 10³ C. erosa mL^?1) to 0.22 (at 2.5 × 10⁴ C. erosa mL^?1). 4. The probability of a fertilised mictic female producing one or more abnormal resting eggs during her lifetime was high (approximately 75%). The mean proportion of abnormal eggs produced per female varied among the different food‐concentration treatments (26–38%) but was not higher at the low food concentrations. 5. The proportion of normal resting eggs that hatched was high (51–71%); those produced at low food concentrations were no less likely to hatch than those produced at high food concentrations. No abnormal resting eggs hatched. 6. The probability of a fertilised mictic female producing an abnormal resting egg increased rapidly with her age at all food concentrations. The probability of a normal resting egg hatching declined with maternal age at the low food concentration in one of two experiments. 7. The results support the idea that induction of mictic females should occur when food resources are good. Coincidence of sexual reproduction with low food availability risks low production of resting eggs for several reasons. Population size may be small, with a low probability of encounters between young mictic females and males, and fertilised mictic females may be unable to mature and produce resting eggs.     

Spine development in Brachionus quadridentatus from an Australian billabong: genetic variation and induction by Asplanchna

Fertilized resting eggs of Australian Brachionus quadridentatus hatched 2–3 days after hydration into females with or, more frequently, without posterior lateral spines. These females then produced clones with short-spined or long-spined phenotypes. Asplanchna girodi induced females from two short-spined clones and one long-spined clone to produce daughters with significantly longer posterior lateral spines. In all clones, there were significant differences in spine development among offspring of mothers within Asplanchna and control treatments. The range of phenotypes reported in one short-spined clone is observed in the billabong and includes much of the variation described for the species, with mehleni (long-spined) phenotypes occurring with Asplanchna. In B. quadridentatus, the ecological significance of long-spined, basic phenotypes, and of the spine-development response to Asplanchna, is unclear. In laboratory cultures, females of all clones were attached to the substratum or water surface, and were safe from Asplanchna; in nature, females are epiphytic and probably rarely susceptible to Asplanchna. Most (96%) resting eggs produced in cultures and kept under culture conditions hatched after a 7-day latent period. This raises questions regarding natural conditions which might prevent hatching and allow accumulation of resting eggs in a sediment egg bank. Hatching of resting eggs in nature may be enhanced in sediments which dry and then become flooded after rains.     

Environmental and endogenous control of sexuality in a rotifer life cycle: developmental and population biology

Induction of mictic females, and hence initiation of sexuality, in the life cycle of some Brachionus requires an environmental stimulus associated with crowding. The inducing stimulus appears to be a taxonomically specific chemical released into the environment by the rotifers. Oocytes are induced to develop into mictic females before they are oviposited by their amictic mothers and begin cleavage divisions. Thus, the inducer affects the oocyte in the maternal body cavity either directly or indirectly by altering the physiology of its mother. The level of sexual reproduction expressed in populations of a Florida strain of B. calyciflorus is controlled by two types of endogenous factors and by the degree of crowding. First, some fraction of genetically identical oocytes in a clonal population fails to respond to even extreme crowding conditions, thus ensuring some potential for continued population growth by female parthenogenesis. Second, the propensity of amictic females to produce mictic daughters is extremely low when they hatch from fertilized resting eggs and then gradually increases to an asymptote after about 12 parthenogenetic generations. This multigenerational parental effect likely is due to a cytoplasmic factor in fertilized eggs that inhibits expression of the mictic-female phenotype and that is gradually diluted in successive parthenogenetic generations. The effect may increase a clone's genetic contribution to the resting-egg bank by increasing its population size through parthenogenetic generations before mictic females are induced.     

The Effect of Food Concentration on the Life History of Three Types of Brachionus calyciflorus Females

The effect of food concentration on the life history of three types of Brachionus calyciflorus females (amictic, unfertilized mictic and fertilized mictic female) was studied with replicated individual cultures at 25 ° and at four food concentrations (1.5, 3.0, 6.0 and 9.0 × 10⁶ cells mL^—1) of Scenedesmus obliquus. There were highly significant effects of both food concentration and female type, independently and in interaction on the duration of juvenile period of the rotifer, but neither were the effects on the duration of post‐reproductive period and mean life‐span. The duration of juvenile period of unfertilized mictic female at the food concentration of 9.0 × 10⁶ cells mL^—1 was the longest among all the food concentration‐female type combinations. Both food concentration and female type influenced significantly the duration of reproductive period and the number of eggs produced by each type of female per life cycle, respectively. There was, however, no significant interaction between food level and female type. Among the three types of females, the number of eggs produced by an unfertilized mictic female was the largest, and that of a fertilized mictic female was the smallest.     

镜湖萼花臂尾轮虫种群内不同基因型克隆的生活史特征

应用生命表统计学等方法,比较了15、20、25和30℃4个温度下镜湖萼花臂尾轮虫种群内13个不同基因型克隆的生命表统计学参数和后代混交雌体百分率等生活史特征．结果表明：轮虫的世代时间、平均寿命、出生时的生命期望值、内禀增长率、净生殖率和后代混交雌体百分率对升高温度的反应均因克隆的不同而存在差异;温度、克隆及两者间的交互作用对其也都具有极显著影响．表明水环境在温度等方面所具有的时间异质性可能是镜湖萼花臂尾轮虫种群丰富的遗传变异得以维持的原因之一;自然选择出现在共存的克隆中,但其强度较低或作用时间较短是克隆共存的重要原因;遗传漂变可能在种群的基因型频率变化中起重要作用．不同基因型轮虫克隆的存在对其种群在水环境中的持续存在具有重要意义．     

<Emphasis Type="Italic">Brachionus calyciflorus</Emphasis> is a Species Complex: Mating Behavior and Genetic Differentiation Among Four Geographically Isolated Strains

Four geographic strains of B. calyciflorus are investigated regarding their genetic similarity and ability to cross-mate. DNA sequence analysis of the mitochondrial cox1 gene (694 bp) and the nuclear ribosomal ITS region (735 bp) showed that the Florida and Georgia strains were very similar to each other (0.3% sequence divergence for the 1429 bp) and different from the Texas and Australia strains (~7% and 9% sequence divergence for the 1429 bp, respectively). Consistent with this genetic relatedness, cross-copulation occurred only between the Florida and Georgia strains. Thus, B. calyciflorus is a complex of cryptic species. While the Florida, Texas and Australia strains were reproductively isolated from one another, most combinations of cross-strain mating tests showed intense and prolonged male circling behavior following male–female encounters. This suggests that precopulatory male circling and copulation are two separate behaviors that may be controlled by different female chemicals and male coronal receptors. In some cross-strain mating tests, females regularly retracted their corona when circled by a male, indicating that they can recognize ‘foreign’ males and actively interfere with copulation.     

Reduced investment in sex as a cost of inducible defence in Brachionus calyciflorus (Rotifera)

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Asexual reproduction changes predator population dynamics in a life predator–prey system

Many organisms display oscillations in population size. Theory predicts that these fluctuations can be generated by predator–prey interactions, and empirical studies using life model systems, such as a rotifer-algae community consisting of Brachionus calyciflorus as predator and Chlorella vulgaris as prey, have been successfully used for studying such dynamics. B. calyciflorus is a cyclical parthenogen (CP) and clones often differ in their sexual propensity, that is, the degree to which they engage into sexual or asexual (clonal) reproduction. Since sexual propensities can affect growth rates and population sizes, we hypothesized that this might also affect population oscillations. Here, we studied the dynamical behaviour of B. calyciflorus clones representing either CPs (regularly inducing sex) or obligate parthenogens (OPs). We found that the amplitudes of population cycles to be increased in OPs at low nutrient levels. Several other population dynamic parameters seemed unaffected. This suggests that reproductive mode might be an important additional variable to be considered in future studies of population oscillations.     

孤雌生殖累积世代数和雌体年龄对萼花臂尾轮虫繁殖的影响

采用单个体培养方法,研究了孤雌生殖的累积世代数和雌体年龄对萼花臂尾轮虫混交雌体形成和产卵量的影响,结果表明,随着轮虫孤雌生殖累积世代数和增加,各代中的总混交雌体百分率呈减小的趋势,年轮的雌体可产生较多的混交雌体,非混交雌体所产后代中的总混交雌体百分率具有随其祖母年龄的增大而增大的趋势,孤雌生殖的累积世代数对轮对轮虫非混交雌体的平均产卵量无显著的影响,非混交雌体的年龄对其报代的平均产卵量亦无显著的影响。     

Effect of diazinon on life stages and resting egg hatchability of rotifer Brachionus plicatilis

The effects of organophosphate pesticide, diazinon, on life history parameters and hatchability of resting eggs of rotifer Brachionus plicalitis were assessed. Newly hatched (<1 h-old) neonates were individually cultured in six varying concentrations (0/control, 0.1, 1.0, 2.5, 5.0 and 10.0 mg/l) of diazinon. The life history parameters such as time (h) the rotifers bear first egg and release first neonate, reproductive period, net reproductive rate, mixis, intrinsic rate of population increase, and life span were evaluated. Results showed that among the life history parameters, the time the rotifers took to release neonates is the most sensitive, giving the lowest EC₅₀ value of 1.24 mg/l. The fecundity of maternal females, amictic and mictic daughters was also investigated. Rotifers exposed to 10.0 mg/l produced significantly fewer amictic daughters, and at this concentration, rotifers did not produce any mictic daughter. At 5.0 mg/l, the number of male offspring was significantly lower than the control. Furthermore, the hatchability of resting eggs produced by the rotifers was evaluated when exposed to diazinon: from birth until they produced resting eggs (early development); during late developmental stage of resting eggs (before diapause); and during diapausing stage. The hatchability of the resting eggs was not affected when exposure was timed at late developmental and diapausing stages. Overall results showed that even though amictic females reproduced normally in the presence of low-concentration of diazinon, sexual reproduction is severely affected, especially the hatchability of resting eggs when the exposure was timed on its early developmental stages. Guest editors: S. S. S. Sarma, R. D. Gulati, R. L. Wallace, S. Nandini, H. J. Dumont & R. Rico-Martínez Advances in Rotifer Research     

Individual- and population-level effects of antibiotics on the rotifers, <Emphasis Type="Italic">Brachionus calyciflorus</Emphasis> and <Emphasis Type="Italic">B. plicatilis</Emphasis>

The toxicity of three common antibiotics (streptomycin sulfate, tetracycline hydrochloride, and tylosin tartrate) to the freshwater rotifer Brachionus calyciflorus and brackish-water rotifer B. plicatilis was investigated using full-lifespan exposure durations. Effects of each antibiotic on lifespan, lifetime reproduction, and Malthusian parameter were assessed at seven nominal concentrations (ranging from 5.6 mg l⁻¹ to 2,000 mg l⁻¹) and a negative control. Lowest Observed Effect Concentrations (LOECs) were determined for reproduction and lifespan, while 1%, 10%, 25%, and 50% Inhibitory Concentrations (IC₁, IC₁₀, IC₂₅, IC₅₀) and 95% confidence intervals were estimated for all three endpoints. LOECs ranged from 5.6 mg l⁻¹ to 90 mg l⁻¹, with all LOECs less than 90 mg l⁻¹ occurring in B. calyciflorus. The lowest IC1 concentrations were 3.91 mg l⁻¹ for the effect of tetracycline on lifetime reproduction in B. calyciflorus and 4.06 mg l⁻¹ for the effect of tylosin on lifetime reproduction in B. plicatilis. Overall, lifetime reproduction was the most sensitive endpoint and the Malthusian parameter was the least sensitive. IC₁ values for lifetime reproduction were roughly one to two orders of magnitude lower than the corresponding IC₅₀ values. Guest editors: S. S. S. Sarma, R. D. Gulati, R. L. Wallace, S. Nandini, H. J. Dumont and R. Rico-Martínez Advances in Rotifer Research     

食物密度对镜湖萼花臂尾轮虫不同克隆生活史特征的影响

对镜湖萼花臂尾轮虫（Brachionus calyciflorus）夏季种群内4个生化遗传特征上互不相同的克隆（克隆A、B、C和D）在4个斜生栅藻（Scenedesmus obliquus）密度（1.0×10⁶、2.0×10⁶、4.0×10⁶和8.0×10⁶ cells·L^-1）下的生活史特征进行了研究.结果表明：食物密度对各克隆轮虫的存活率和繁殖率均有不同的影响.4克隆中克隆C的世代时间最短,克隆B的世代时间、生命期望和平均寿命最长,克隆A的后代混交雌体百分率最高;净生殖率和个体适合度在4克隆间无显著差异.镜湖萼花臂尾轮虫在2.0×10⁶ cells·L^-1的食物密度下净生殖率最低;在1.0×10⁶cells·L^-1的食物密度下平均寿命和生命期望最短,而后代混交雌体百分率却最高;在8.0×10⁶cells·L^-1的食物密度下种群内禀增长率最高,平均寿命和生命期望最长;在高食物密度（4.0、8.0×10⁶cells·L^-1）下个体适合度较大.克隆C的个体适合度在密度为3.9×10⁶cells·L^-1时最小,而克隆D的个体适合度在食物密度为6.34×10⁶cells·L^-1时最大.食物密度的变化可能是7月份之后镜湖萼花臂尾轮虫从水环境中消失的原因,而4克隆轮虫个体适合度的相似性则可能是镜湖轮虫共存于同一水体的原因之一.