Evidence for a race-specific resistance factor in some lettuce (Lactuca sativa L.) cultivars previously considered to be universally susceptible to Bremia lactucae regel

Summary Previously undetected race-specific resistance to Bremia lactucae (downy mildew) was located in many lettuce cultivars hitherto considered to be universally susceptible to this disease. This resistance factor(s) may also be widely distributed in other cultivars known to carry combinations of already recognised factors R1 to R11. Specific virulence to match this resistance is almost invariably present in pathogen collections. This situation may be either a relic of the evolutionary history of the B. lactucae — L. sativa asssociation or may reflect a rare mutation in B. lactucae for avirulence on all but a few specialised L. sativa genotypes.     

Trade-offs between sexual and asexual reproduction in the genus Mimulus

Summary Six estimates of resource allocation to sexual reproduction (nectar concentration, nectar volume, sugar production, pollinator visitation rates, fruit-set, and seed production) and a single estimate of resource allocation to asexual reproduction (the rate of rooted-branch production) were examined for five species of Mimulus. There were significant interspecific differences for all paramaters. With the exception of nectar concentration, there was 1) a consistent positive correlation among the parameters measuring allocation to sexual reproduction, and 2) a significant negative correlation between parameters measuring allocation to sexual reproduction and the rate of rooted-branch production (asexual reproduction). The results indicate that Mimulus species which produce the highest volumes of nectar 1) receive the most pollinator visits, 2) have the highest fruit-set, 3) produce the most seeds, and 4) produce the fewest rooted branches.     

Density-dependent sexual reproduction in natural populations of the rotifer asplanchna girodi

The monogonont rotifer Asplanchna girodi was continuously present in daily and bi-daily plankton samples of Golf Course Pond during the spring of 1977. Two cycles of sexual reproduction occurred during this period. By isolation and culture of females it was possible to determine the reproductive type of collected individuals. Data thus obtained suggest that environmental cues associated with population density are responsible for the production of sexual females.     

Photoperiodic control of sexual morph production by the currant-sowthistle aphid, Hyperomyzus lactucae

Sexual morph production in Hyperomyzus lactucae (L.) is under photoperiodic control. In southern England oviparae are not produced in night lengths of less than 10 h 40 min at 15 ° C but males are produced in all night lengths between 8 and 14 h with a peak between 10 and 12 h. Old clones produce sexual morphs after fewer generations and via a limited sequence of morphs compared to young clones, but the restraining mechanism in young clones is very variable in the extent of its operation.

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Résumé La production des sexués chez Hyperomyzus lactucae est sous un déterminisme photopériodique, mais les mécanismes qui provoquent la production des ovipares et des mâles sont indépendants et contrôlés par des stimulus photopériodiques différents. La durée de la scotophase pour la production des ovipares est déterminante et d'environ 10 h 40 pour les clones étudiés. Il n'y a pas de production d'ovipares à 15° C en scotophase plus courte. Par contre, des mâles sont produits à toutes les scotophases étudiées entre 8 et 14 h, atteignant un pic entre 10 h 20 et 12 h, mais avec diminution de leur apparition encore plus nette quand les nuits se prolongent.La diversité des conditions tendant à produire des sexués chez les jeunes clones fait contraste avec le nombre limité de possibilités chez les clones plus âgés et prouve qu'un mécanisme réduit dans certains cas la sensibilité à la scotophase tôt dans l'année. Mais, même dans un seul clone, la variation causée par un tel mécanisme est considérable.Il semble que la longueur de la scotophase critique chez cet insecte suffise seule à empêcher la production de sexués au printemps dans le sud de l'Angleterre. Ceci peut donc expliquer les variations du mécanisme restrictif qui, vraisemblablement, ne doit pas être important pour la survie des clones de cette région.

     

Interpopulation variation in Chrysoperla carnea reproduction: implications for mass-rearing and storage

To determine whether diverse populations and biotypes of Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae) can be mass-produced and stored under uniform conditions, we assessed reproduction in five North American populations, four from the carnea biotype and one from the mohave biotype. Non-diapause adults from the carnea biotype varied significantly in only one aspect of reproduction, viz., the preoviposition period, whereas the duration and rate of oviposition, as well as fecundity, did not vary significantly. Relative to the carnea populations, the mohave population typically had delayed oviposition and a lower rate of oviposition; however, its oviposition period was long, and thus its fecundity differed only from that of the most productive carnea populations.The shelf-life (= survival during storage, and subsequent survival and reproduction after storage) was evaluated for four populations of the carnea biotype. Survival of diapausing adults during the first 18 weeks of storage was high among all four populations; adults from three populations also showed high survival after 35 weeks of storage. A high incidence of females from all four populations laid some fertile eggs after being stored for up to 31 weeks or longer. After six or more weeks of storage, average fecundity was high and ranged from 400–900 eggs/female. Duration of storage (up to 39 weeks) was a significant determinant of fecundity in two of the four populations.In practice, diverse populations of C. carnea can be efficiently mass-produced (with or without storage) under uniform conditions in the insectary. Thus, the choice of which populations to mass-produce and use in augmentation can be based primarily on the characteristics (e.g., climate, habitat, crop) of the intended area of release, rather than on considerations concerning mass-rearing.     

Isolation of myosin XI genes from the Closterium peracerosum-strigosum-littorale complex and analysis of their expression during sexual reproduction

Myosins comprise a large superfamily of molecular motors that generate mechanical force in ATP-dependent interactions with actin filaments. On the basis of their conserved motor domain sequences, myosins can be divided into at least 17 classes, 3 of which (VIII, XI, XIII) are found in plants. Although full sequences of myosins are available from several species of green plants, little is known about the functions of these proteins. Additionally, sequence information for algal myosin is incomplete, and little attention has been given to the molecular evolution of myosin from green plants. In the present study, the Closterium peracerosum-strigosum-littorale complex was used as a model system for investigating a unicellular basal charophycean alga. This organism has been well studied with respect to sexual reproduction between its two mating types. Three types of partial sequences belonging to class XI myosins were obtained using degenerate primers designed to amplify motor domain sequences. Real-time polymerase chain reaction analysis of the respective myosin genes during various stages of the algal life cycle showed that one of the genes was more highly expressed during sexual reproduction, and that expression was cell-cycle-dependent in vegetatively grown cells.     

Selective advantage for sexual reproduction with random haploid fusion

This article develops a simplified set of models describing asexual and sexual replication in unicellular diploid organisms. The models assume organisms whose genomes consist of two chromosomes, where each chromosome is assumed to be functional if it is equal to some master sequence σ₀, and non-functional otherwise. We review the previously studied case of selective mating, where it is assumed that only haploids with functional chromosomes can fuse, and also consider the case of random haploid fusion. When the cost for sex is small, as measured by the ratio of the characteristic haploid fusion time to the characteristic growth time, we find that sexual replication with random haploid fusion leads to a greater mean fitness for the population than a purely asexual strategy. However, independently of the cost for sex, we find that sexual replication with a selective mating strategy leads to a higher mean fitness than the random mating strategy. The results of this article are consistent with previous studies suggesting that sex is favored at intermediate mutation rates, for slowly replicating organisms, and at high population densities. Furthermore, the results of this article provide a basis for understanding sex as a stress response in unicellular organisms such as Saccharomyces cerevisiae (Baker’s yeast).     

The influence of temperature and photoperiod on the development, survival and reproduction of the sowthistle aphid, Hyperomyzus lactucae

The development time, survivorship and age-specific fecundity of apterous and alate virginoparae of the sowthistle aphid, Hyperomyzus lactucae (L.) (Homoptera: Aphididae), reared on Sonchus oleraceus L., were measured under various temperature and light regimes. Within the temperature range of daily means of 12.5–24°C, speed of development and reproductive rate increased, while life span and total fecundity decreased, with an increase in temperature. The relationship between temperature and rate of development was nearly linear, with a notional development threshold estimated at about 2°C. The intrinsic rate of increase (r_m) was positively correlated with temperature when calculated on a daily basis, but was inversely related to temperature when measured on a physiological time scale. Alatae generally had a longer development time, and achieved a lower reproductive rate and life-time fecundity, than apterae reared under identical conditions.Comparison of aphid performance under constant and alternating temperature regimes, and between successive generations under the same regime, showed that (1) the conditions experienced by the aphid as an embryo within its mother are important in determining survival and development and (2) tolerable temperature limits vary with the pattern and amplitude of the fluctuations of temperature.At 22°C, changes of photoperiod within the range of 12D:12L to 8D:16L had little effect on aphid performance.

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Résumé La durée du développement, le taux de survie et la fécondité en fonction de l'âge ont été suivis chez les virginipares aptères et ailés du puceron: Hyperomyzus lactucae L., élevé sur Sonchus oleraceus L., et soumis à différentes conditions de température et de lumière. A l'intérieur de la gamme de températures moyennes de 12,5 à 24°C, la vitesse de développement et le rythme de reproduction augmentent, tandis que la longévité et la fécondité totale diminuent. La relation entre la température et la durée du développement est presque linéaire, avec un seuil théorique de développement évalué à environ 2°C. Le taux intrinsèque d'accroissement est lié positivement à la température calculée sur une base quotidienne, mais la relation est inversée quand la température est mesurée sur une échelle physiologique. Pour des conditions d'élevage identiques, les ailés ont eu, dans l'ensemble, une durée de développement plus longue, un taux de reproduction et une fécondité totale plus faibles que les aptères.La comparaison des performances des pucerons sous thermopériodes ou températures constantes, ou entre les générations en conditions identiques, a montré (1) que les conditions subies par l'embryon dans la mère conditionnent la survie et le développement; (2) que les limites des températures tolérables changent en fonction du type et de l'amplitude des fluctuations de température.A 22°C, des modifications de la photopériode, dans la gamme; 12D:12L à 8D:16L, ont eu peu d'effet sur les performances des pucerons.

     

Bottlenecks and spatiotemporal variation in the sexual reproduction pathway of perennial meadow plants

Sexual reproduction is important for the growth of populations and the maintenance of genetic diversity. Several steps are involved in the sexual reproduction pathway of plants: the production of flowers, the production of seeds and the establishment of seedlings from seeds. In this paper we quantify the relative importance and spatiotemporal variability of these different steps for four grassland perennials: Centaurea jacea, Cirsium dissectum, Hypochaeris radicata and Succisa pratensis. We compared undisturbed meadows with meadows where the top soil layer had been removed as a restoration measure. Data on the number of flower heads per flowering rosette, the numbers of flowers and seeds per flower head, and the seedling establishment probabilities per seed were collected by field observations and experiments in several sites and years. Combination of these data shows that H. radicata and S. pratensis have higher recruitment rates (1.9 and 3.3 seedlings per year per flowering rosette, respectively) than the more clonal C. dissectum and C. jacea (0.027 and 0.23, respectively). Seedling establishment is the major bottleneck for successful sexual reproduction in all species. Large losses also occurred due to failing seed set in C. dissectum. Comparison of the coefficients of variation per step in space and time revealed that spatiotemporal variability was largest in seedling establishment, followed closely by flower head production and seed set.     

Effect of host age at parasitization by Aphidius sonchi on the development, survival, and reproduction of the sowthistle aphid, Hyperomyzus lactucae

Nineteen cohorts of Hyperomyzus lactucae (L.) (Homoptera: Aphididae) reared at various temperaturelight regimes were exposed to attack by Aphidius sonchi Marshall (Hymenoptera: Aphidiidae) at different ages of their development. Irrespective of host age at the initial parasitization and rearing conditions, the rates of reproduction, development and survival were unaffected until the parasites within had completed approximately 30, 40 and 55% of their total development respectively. As a result, aphids parasitized in the first or early second instar failed to reach maturity. Thereafter, the numbers of young produced by parasitized aphids increased rapidly as the age of the hosts at initial parasitization increased, and aphids parasitized 3 days after the final moult or later showed similar potential to contribute to future population increase.In comparison with the effects on host moulting, those on wing development of the potentially alatiform nymphs appeared much sooner after the beginning of parasitization, but again with the time lags being more or less independent of host age at the initial parasite attack. Consequently, the extent of wing development depended entirely on the age of the hosts when attacked.These results showed that the age of the host at the initial parasite attack is crucial in determining the consequences of parasitization on the host.

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Résumé Dix-neuf cohortes d'Hyperomyzus lactucae L. (Homoptera: Aphididae), élevées dans différentes conditions de lumière et de température, ont été exposées à différents moments de leur développement aux attaques d'Aphidius sonchi Marsh. (Hym. Aphidiidae). Quels que soient les conditions d'élevage et l'âge des pucerons quand ils ont été parasités, la survie, le développement et la reproduction n'ont été affectés que lorsque les parasites avaient accompli 30, 40 et 55% de leur développement total. Ainsi, les pucerons parasités au premier stade ou au début du second ne parvenaient pas à atteindre la maturité. Par la suite, le nombre de jeunes produits par les pucerons parasités a cru rapidement au fur et à mesure que l'âge de ces derniers au moment où ils avaient été parasités la dernière mue ou plus tard présentaient la même potentialité pour contribuer à l'accroissement ultérieur de la population.Comparés aux effets sur la mue, les effets sur le développement alaire des larves potentiellement ailées apparaissent beaucoup plus tôt après le moment où le puceron a été parasité, mais là aussi avec des délais plus ou moins indépendants de l'âge de l'hôte lors de la première attaque du parasite. Par conséquent, le degré de développement des ailes dépendait totalement de l'âge du puceron lorsqu'ila été parasité.Ces résultats ont montré que l'âge du puceron lors de la première attaque détermine les conséquences du parasitisme de l'hôte.

     

Resource allocation to sexual and vegetative reproduction in a forest herbSyneilesis palmata (Compositae)

The balance between sexual and vegetative reproduction inSyneilesis palmata was examined in relation to environmental conditions and the amount of reproductive resources, which is defined here as the total quantity of dry matter invested in both modes of reproduction. The allocation balance was measured for individual plants of two populations, with different densities in an open habitat, for 2 years, and those of two other populations, under different light intensities in a plantation forest (forest floor and edge), for 3 years. Relative allocation to sexual reproduction decreased with increasing reproductive resources in all populations except for the forest edge, which showed a constant allocation balance. The high density population showed lower relative allocation to sexual reproduction than the low density population, irrespective of the amount of reproductive resources. However the between-year comparison of the high density population suggested that under extremely high density, plants with a small amount of reproductive resources enhanced sexual reproduction, while plants with a large amount of reproductive resources reproduced vegetatively. On the forest floor, plants with small amounts of reproductive resources had higher relative allocation to sexual reproduction than the forest edge population, while plants with large amounts of reproductive resources had a somewhat lower one. The adaptive significance of such allocation patterns are discussed, based on qualitative data on the characteristics of both types of offspring.     

Unlocking variability: inherent variation and developmental traits of garlic plants originated from sexual reproduction

Recent collections of fertile garlic (Allium sativum) accessions from Central Asia allow a detailed study of seedling developments and the evaluation of inherent variations. We hereby provide a comprehensive account of the ontogenesis of a population of garlic seedlings and their vegetative and reproductive traits. A nucleotide binding site profiling marker technology was applied to provide conclusive evidence for the cross-pollination nature of garlic, and to compare the levels of polymorphism between progeny derived from a single mother clone fertilized by several pollinators. The seedlings’ population demonstrates a large variation in vegetative and reproductive characters, including bulbing ability, bulb color and size, clove number, and response to environmental conditions, similar to that of the genepool of vegetatively propagated garlic clones. In addition, a large variation in flowering and seed production ability was recorded. The understanding of garlic physiology, the availability of the large variability unleashed by sexual reproduction, and the possible utilization of sexual hybridization opens the way for genetic studies and breeding work. Haim D. Rabinowitch and Rina Kamenetsky contributed equally to this paper.     

夜光藻有性繁殖研究进展

夜光藻是全球最主要的赤潮生物之一,也是我国近海常见的浮游甲藻。根据营养方式分为异养的红色夜光藻和混合营养的绿色夜光藻,前者广泛分布于温带和亚热带近岸水域,后者仅分布于热带西太平洋、阿拉伯海、阿曼湾和红海。夜光藻的生活史包括无性繁殖和有性繁殖过程。少部分营养细胞自发转变为配子母细胞,启动了有性繁殖。每个配子母细胞可形成大量配子,具有横沟、纵沟和2根鞭毛,形态与裸甲藻接近。配子两两融合形成合子,合子不经过休眠孢囊阶段直接发育成新的营养细胞。目前,对配子母细胞形成的调控机制、合子发育的影响因素的认识还存在分歧。研究发现,营养细胞经过一定次数的二分裂后都会转变为配子母细胞,而配子的存在能够中止此过程,使营养细胞继续进行二分裂。因此,有性繁殖可能通过产生新个体对种群增长做出贡献,还可能通过释放配子维持无性繁殖,进而促进种群增长。配子在相模湾水域全年都有分布,其丰度峰值与营养细胞丰度峰值同步或提前出现,配子的大量出现可能是赤潮形成的必要条件。对有性繁殖的研究佐证了夜光藻在甲藻的系统进化中处于较为古老的地位。此外,还简单介绍了研究夜光藻有性繁殖的主要方法,回顾了国内的夜光藻研究,并对相关研究进行了展望。     

Sexual selection and intelligence: Does sexual reproduction drive the evolution of intelligence?

The basal hypothesis discussed here is the idea that brain architecture could be plastic on a very basal, genetic level due to sexual recombination and reassortment of alleles of genes related to brain development, e.g., neuronal cell adhesion molecules (NCAMs) and others.The role of sexual reassortment leads the study of brain development, species behavior and intelligence to a new version of the so-called “Red Queen Hypothesis”: using the mechanism described here, a kind of runaway selection mechanism seems to arise. Even if NCAMs are almost constant within an individual, they seem to act very differently at the population level and so the role of reassorting polymorphic NCAM- (and other) genes gets particularly clear. If several NCAM-NCAM combinations cause extreme behavior and intelligence variability in a population, these combinations also represent a use of sexual selection. This mechanism of NCAM allele assortment seems to be important for the process of speciation by mutual selection of individuals. Therefore NCAM variants and their associated behaviors are thought to be important for the development of intelligence, in that they promote the attraction of individuals with already high intelligence, leading to the speciation of super-intelligent groups.     

Sexual and vegetative reproduction in the aboveground part of a dioecious clonal plant, <Emphasis Type="Italic">Dioscorea japonica</Emphasis> (Dioscoreaceae)

In dioecious clonal plants, the reproductive effort required to set seeds will be responsible for the larger investment in sexual reproduction by females. If there will be a trade-off in resource allocation between sexual and clonal reproduction, this differential sexual reproduction will lead to sexual differentiation in the relative amount of clonal reproduction. To test this prediction, we studied differences between the sexes in their phenologies and investments in sexual and vegetative reproduction (clonal reproduction by means of bulbils) with respect to ramet size in a dioecious clonal plant, Dioscorea japonica Thunb. The period of bulbil production overlapped the period during which infructescences developed. Females flowered later, produced heavier inflorescences, and fewer flowers per inflorescence than did males. Regression analysis using the size of the individual plants demonstrated that large females made smaller investments in inflorescences and larger investments in sexual reproduction than did large males. In contrast, females invested fewer resources in vegetative reproduction than did males. However, the total investments in sexual and vegetative reproduction did not differ between the sexes. These results supported our hypothesis on the sexual differentiation in sexual and clonal reproduction.     

Asexual and sexual reproduction in the rotifer Brachionus plicatilis cultured at different salinities

Two strains of the rotifer Brachionus plicatilis (L-type) differing in the levels of mictic female and male production, were grown in batch cultures with the alga Nannochloropsis gaditana as food, at two low (2.5 and 10), and two high (40 and 50) salinities. While both the low (strain S-1) and the high (strain S-3) sexual reproducing strains developed similar growth cycles at 2.5 and 10, the population growth response at 40 and 50 showed that; 1) in strain S-1, mixis can be suppressed in conditions that still allow asexual reproduction, and 2) in strain S-3 mictic female and male production are possible at nearly zero asexual population growth rates. In strain S-3, a double log linear relationship between the densities of males and females was found. These results show that mixis can occur over a wide ranges of female population density, and support the hypothesis that sexual reproduction is a strain dependent component of the general reproductive response.     

Morphofunctional organization of reserve stem cells providing for asexual and sexual reproduction of invertebrates

Published and original data indicating evolutionary conservation of the morphofunctional organization of reserve stem cells providing for asexual and sexual reproduction of invertebrates are reviewed. Stem cells were studied in representatives of five animal types: archeocytes in sponge Oscarella malakhovi (Porifera), large interstitial cells in colonial hydroid Obelia longissima (Cnidaria), neoblasts in an asexual race of planarian Girardia tigrina (Platyhelmintes), stem cells in colonial rhizocephalans Peltogasterella gracilis, Polyascus polygenea, and Thylacoplethus isaevae (Arthropoda), and colonial ascidian Botryllus tuberatus (Chordata). Stem cells in animals of such diverse taxa feature the presence of germinal granules, are positive for proliferating cell nuclear antigen, demonstrate alkaline phosphatase activity (a marker of embryonic stem cells and primary germ cells in vertebrates), and rhizocephalan stem cells express the vasa-like gene (such genes are expressed in germline cells of different metazoans). The self-renewing pool of stem cells is the cellular basis of the reproductive strategy including sexual and asexual reproduction.     

The role of sexual and clonal reproduction in maintaining population in Fritillaria camtschatcensis (L.) Ker-Gawl. (Liliaceae)

Fritillaria camtschatcensis can reproduce by means of both sexual reproduction and clonal multiplication. Despite prolific seed production, no seedlings have been found for several years in natural populations on Mt Hakusan. The purposes of this paper are to clarify: (i) whether population size is maintained mainly by clonal multiplication; and (ii) if this is the case, to what extent occasional seedling establishment affects population growth rate and population structure. Two permanent quadrats were placed in subalpine meadows in 1992 on Mt Hakusan. Plant size, location and reproductive states for all ramets in the quadrats were recorded every year. Projection matrices were created based on field census, and computer simulation experiments were performed. Fritillaria camtschatcensis had two types of flower, male flower and cosexual flower, and they were changeable. This is the first report on sex lability in Fritillaria. Clonal growth was more closely correlated with life-history stages, especially with sexual states than with plant size. The population growth rate, , was 1.006 for the Mizuyajiri population and 1.047 for the Nanryu population, respectively. Seedlings were found in 1996 for the first time. These facts indicate that populations of F. camtschatcensis on Mt Hakusan can usually be maintained by clonal multiplication. However, it is not yet certain whether seeds germinate every year or whether a flush of seedling emergence occurs once in every few years in natural populations. Computer simulation revealed that: (i) there was a critical germination rate above which population growth rate suddenly increased; and (ii) occasional seedling establishment could provide almost the same contribution to population growth rate as that of annual seedling establishment. These results suggest that population size can be maintained mainly by clonal multiplication, and the role of sexual reproduction lies beyond maintaining the population size in F. camtschatcensis.     

Low genetic variation in Swedish populations of the rare speciesVicia pisiformis (Fabaceae) revealed with rflp (rDNA) and RAPD

Nine Swedish populations, 1–5 individuals/population, and one cultivated individual of the rare speciesVicia pisiformis were investigated for genetic variation. In hybridizations with two rDNA probes using 8 restriction enzymes, only two individuals belonging to one population were polymorphic. A map of the rDNA gene cluster was constructed for four of the restriction enzymes used. Two of the polymorphic sites were mapped and were found to be located outside regions coding for rRNA, presumably caused by single point mutations or small deletions. The repeat length of the rDNA region was c. 10,000 bp, which corresponds well with the size found for other species belonging toFabaceae. No length polymorphism was found in the intergenic spacer, contrary to the situation found for most other plant species investigated for rDNA variation. The haplotype diversity for the species (Hsp Shannon) was very low (0.055). Within-population values (Hpop) was 0 for all populations except the variable one, which had 0.301. PCR amplification with 6 random primers also revealed very low levels of genetic diversity. A polymorphism was observed in a limited number of individuals for four populations. Hsp was 0.065 and was 0.050. The average D value (Wetton) for the PCR haplotypes was 0.99.