Life history parameters and biocontrol potential of the mealybug parasitoid Coccidoxenoides peregrinus (Timberlake) (Hymenoptera: Encyrtidae): Asexuality, fecundity and ovipositional patterns

Properties relevant to the ovipositional activity and lifetime productivity of Coccidoxenoides peregrinus (Timberlake) were assessed in the laboratory, to determine the potential of this species as a biocontrol agent against the citrus mealybug, Planococcus citri (Risso). In general, this species has not performed well in orchards, except for a few localities on different continents. The mode of reproduction of C peregrinus is almost entirely thelytokous, with males produced sporadically and at low frequency. The females have both pro-ovigenic and synovigenic traits, which raises questions of the utility of this distinction. The females have a high reproductive potential with 10-20 eggs per day available within the first two days (after a short (12 h) pre-oviposition period), and 80-150 eggs per day thereafter until death at about eight days. Mean lifetime fecundity was 239.2 +/- 34.3 eggs. C peregrinus oviposits across a range of P. citri instars, but productivity relies predominantly on second instar hosts. Second stage (N2) hosts received most eggs in choice (about 52%) and no-choice (about 50%) tests. Most eggs deposited into N2 hosts (82%) reached adult stage whereas only a few of those deposited into N1 and N3 (about 5% each) developed successfully. The haemolymph of parasitised reproductive mealybugs contained granular structures and no parasitoid eggs were found 24 h after exposure to ovipositing wasps. Also, no wasps emerged from parasitised adult hosts that were kept alive. Parasitoid eggs deposited into adult hosts were presumed encapsulated and destroyed, as control mealybugs (not exposed to female wasps) had no granular structures in their haemolymph. Wasps exposed to an abundance of hosts soon started ovipositing, but only for a relatively short time each day (about 2.5 h out of a 7 h exposure). They stopped ovipositing despite eggs judged to be mature in their ovaries. The reproductive output of C peregrinus is discussed in relation to the ecological factors that could influence this output, and the implications for biocontrol are discussed. (C) 2003 Elsevier Inc. All rights reserved.

Genetic evidence for gonochoristic reproduction in gynogenetic silver crucian carp (Carassius auratus gibelio bloch) as revealed by RAPD assays

Sex evolution has been a debating focus in evolutionary genetics. In lower vertebrates of reptiles, amphibians, and fish, a species or a bioform reproduces either sexually or asexually but never both. A few species were found to consist of all females in fish. These all-female species can propagate by asexual reproduction modes, such as gynogenesis and hybridogenesis. However, the coexistence of sexuality and asexuality in a single species was recently noted only in a cyprinid fish silver crucian carp, Carassius auratus gibelio. This fish had been demonstrated to be capable of gynogenesis stimulated by sperm from other related species. Surprisingly, natural populations of this fish consist of a minor but significant portion (approx. 20%) of males. As different clones with specific phenotypic and genetic characteristics have been found, and RAPD markers specific to each clone have recently been identified, this fish offers many advantages for analyzing whether or not genetic recombination occurs between different clones. In this study, artificial propagation was performed in clone F and clone D. Ovulated eggs from clone F were divided into two parts and respectively inseminated with sperm from a clone D male and from a red common carp (Cyprinus carpio) male. The control clone D individuals were selected from gynogenetic offspring of clone D activated by sperm of red common carp. The phenotype and sex ratio in the experimental groups were also observed. Using RAPD molecular markers, which allow for reliable discrimination and genetic analysis of different clones, we have revealed direct molecular evidence for gonochoristic reproduction in the gynogenetic silver crucian carp and confirmed a previous hypothesis that the silver crucian carp might reproduce both gynogenetically and gonochoristically. Therefore, we conclude that the silver crucian carp possesses two reproductive modes, i.e., gynogenetic and gonochoristic reproduction. The response mechanism of two reproductive development modes may be the first discovery in vertebrates. Additionally, we discuss the evolutionary implication between gynogenetic and gonochoristic reproduction modes and the contribution of the minor proportion of males to genetic flexibility in the gynogenetic silver crucian carp.

Cryptic sexual populations account for genetic diversity and ecological success in a widely distributed, asexual fungus-growing ant

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A Brazilian population of the asexual fungus-growing ant Mycocepurus smithii (Formicidae, Myrmicinae, Attini) cultivates fungal symbionts with gongylidia-like structures

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Avessos do excesso: a assexualidade

Pós-graduação em Psicologia - FCLAS

Deleterious mutations destabilize ribosomal RNA in endosymbiotic bacteria

In populations that are small and asexual, mutations with slight negative effects on fitness will drift to fixation more often than in large or sexual populations in which they will be eliminated by selection. If such mutations occur in substantial numbers, the combined effects of long-term asexuality and small population size may result in substantial accumulation of mildly deleterious substitutions. Prokaryotic endosymbionts of animals that are transmitted maternally for very long periods are effectively asexual and experience smaller effective population size than their free-living relatives. The contrast between such endosymbionts and related free-living bacteria allows us to test whether a population structure imposing frequent bottlenecks and asexuality does lead to an accumulation of slightly deleterious substitutions. Here we show that several independently derived insect endosymbionts, each with a long history of maternal transmission, have accumulated destabilizing base substitutions in the highly conserved 16S rRNA. Stabilities of Domain I of this subunit are 15–25% lower in endosymbionts than in closely related free-living bacteria. By mapping destabilizing substitutions onto a reconstructed phylogeny, we show that decreased ribosomal stability has evolved separately in each endosymbiont lineage. Our phylogenetic approach allows us to demonstrate statistical significance for this pattern: becoming endosymbiotic predictably results in decreased stability of rRNA secondary structure.

Rates of nucleotide substitution in sexual and anciently asexual rotifers

The class Bdelloidea of the phylum Rotifera is the largest well studied eukaryotic taxon in which males and meiosis are unknown, and the only one for which these indications of ancient asexuality are supported by cytological and molecular genetic evidence. We estimated the rates of synonymous and nonsynonymous substitutions in the hsp82 heat shock gene in bdelloids and in facultatively sexual rotifers of the class Monogononta, employing distance based and maximum likelihood methods. Relative-rate tests, using acanthocephalan rotifers as an outgroup, showed slightly higher rates of nonsynonymous substitution and slightly lower rates of synonymous substitution in bdelloids as compared with monogononts. The opposite trend, however, was seen in intraclass pairwise comparisons. If, as it seems, bdelloids have evolved asexually, an equality of bdelloid and monogonont substitution rates would suggest that the maintenance of sexual reproduction in monogononts is not attributable to an effect of sexual reproduction in limiting the load of deleterious nucleotide substitutions.

Rare sex or out of reach equilibrium? The dynamics of F-IS in partially clonal organisms

Background: Partially clonal organisms are very common in nature, yet the influence of partial asexuality on the temporal dynamics of genetic diversity remains poorly understood. Mathematical models accounting for clonality predict deviations only for extremely rare sex and only towards mean inbreeding coefficient (F-IS) over bar < 0. Yet in partially clonal species, both F-IS < 0 and F-IS > 0 are frequently observed also in populations where there is evidence for a significant amount of sexual reproduction. Here, we studied the joint effects of partial clonality, mutation and genetic drift with a state-and-time discrete Markov chain model to describe the dynamics of F-IS over time under increasing rates of clonality. Results: Results of the mathematical model and simulations show that partial clonality slows down the asymptotic convergence to F-IS = 0. Thus, although clonality alone does not lead to departures from Hardy-Weinberg expectations once reached the final equilibrium state, both negative and positive F-IS values can arise transiently even at intermediate rates of clonality. More importantly, such "transient" departures from Hardy Weinberg proportions may last long as clonality tunes up the temporal variation of F-IS and reduces its rate of change over time, leading to a hyperbolic increase of the maximal time needed to reach the final mean (F-IS,F-infinity) over bar value expected at equilibrium. Conclusion: Our results argue for a dynamical interpretation of F-IS in clonal populations. Negative values cannot be interpreted as unequivocal evidence for extremely scarce sex but also as intermediate rates of clonality in finite populations. Complementary observations (e.g. frequency distribution of multiloci genotypes, population history) or time series data may help to discriminate between different possible conclusions on the extent of clonality when mean (F-IS) over bar values deviating from zero and/or a large variation of F-IS over loci are observed.