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Brevipalpus californicus, B. obovatus, B. phoenicis, and B. lewisi (Acari: Tenuipalpidae): a review of their biology, feeding injury and economic importance.

The genus Brevipalpus includes most of the economically important species of Tenuipalpidae. Many Brevipalpus species reproduce by theletokous parthenogenesis while other species reproduce by male fertilization of female eggs. Previous researchers have determined that Brevipalpus californicus (Banks), B. obovatus Donnadieu, and B. phoenicis (Geijskes) females were haploid with two chromosomes. The life cycle and developmental times for these three species are reviewed. Longevity of each Brevipalpus species is two to three times greater than corresponding longevities of various tetranychid mites. Brevipalpus mites inject toxic saliva into fruits, leaves, stems, twigs, and bud tissues of numerous plants including citrus. Feeding injury symptoms on selected plants include: chlorosis, blistering, bronzing, or necrotic areas on leaves by one or more Brevipalpus mites. Premature leaf drop occurred on 'Robinson' tangerine leaves in Florida (USA). Leaf drop was observed in several sweet orange and grapefruit orchards in Texas (USA) that were heavily infested with Brevipalpus mites feeding on the twigs, leaves, and fruit. Initial circular chlorotic areas appear on both sweet orange and grapefruit varieties in association with developing populations of Brevipalpus mites in Texas. These feeding sites become progressively necrotic, darker in color, and eventually develop into irregular scab-like lesions on affected fruit. Russeting and cracking of the fruits of other plant hosts are reported. Stunting of leaves and the development of Brevipalpus galls on terminal buds were recorded on sour orange, Citrus aurantium L., seedlings heavily infested with B. californicus in an insectary. The most significant threat posed by these mites is as vectors of a potentially invasive viral disease called citrus leprosis.

Animals↗

Viewing the difference between the diploid and the polyploid in the light of the upland cotton aneuploid.

The aneuploidy of Gossypium hirsutum L. (upland cotton) aneusomatics were obtained by induced parthenogenesis. These aneuploids could grow and set seeds normally. In the process of meiosis there appeared large quantities of heteromorphic pairs and multivalent chromosomes and many cases of cytomixis and multisperm fertilization occurred. The aneuploids produced offsprings through sexual propagation. We explored penetratingly the questions how and why these aneuploids could survive. Through this research, we found that the upland cotton possessed an immense latent capacity to adapt to adverse environments. More importantly, in the case of the upland cotton, we discovered that the genetic pattern of the polyploid differs in some respects from that of the diploid.

Aneuploidy↗

Ethanol-induced chromosomal abnormalities at conception.

Preliminary findings have indicated that mouse eggs exposed briefly in vivo or in vitro to a dilute solution of ethanol activate parthenogenetically. Cytogenetic analysis of the first-cleavage chromosomes of haploid parthenogenetic embryos indicated that up to 20% of this population were aneuploid as a result of non-disjunction. Anaesthetics also can induce parthenogenesis of rodent eggs, and in studies using anaesthetics, colchicine and colcemid, abnormal chromosome segregation and heteroploidy of rodent embryos have been observed. I now report that when recently mated female mice are given a dilute solution of ethanol by mouth, non-disjunction can be induced in the female-derived, but apparently not in the male-derived, chromosome set of fertilized eggs. Taken together, these findings suggest that ethanol consumption (as well as exposure to other 'spindle-acting' agents) at the time of conception may be the cause of certain types of chromosomal defects commonly observed in human spontaneous abortions.

Abortion, Spontaneous↗

Male mutation rates and the cost of sex for females.

Although we do not know why sex evolved, the twofold cost of meiosis for females provides a standard against which postulated benefits of sex can be evaluated. The most reliable benefit is sex's ability to reduce the impact of deleterious mutations. But deleterious mutations may themselves generate a large and previously overlooked female-specific cost of sex. DNA sequence comparisons have confirmed Haldane's suggestion that most mutations arise in the male germ line; recent estimates of a, the ratio of male to female mutation rates, are ten, six and two in humans, primates and rodents, respectively. Consequently, male gametes may give progeny more mutations than the associated sexual recombination eliminates. Here I describe computer stimulations showing that the cost of male mutations can easily exceed the benefits of recombination, causing females to produce fitter progeny by parthenogenesis than by mating. The persistence of sexual reproduction by females thus becomes even more problematic.

Animals↗

Peg1/Mest imprinted gene on chromosome 6 identified by cDNA subtraction hybridization.

Parthenogenesis in the mouse is embryonic lethal partly because of imprinted genes that are expressed only from the paternal genome. In a systematic screen using subtraction hybridization between cDNAs from normal and parthenogenetic embryos, we initially identified two apparently novel imprinted genes, Peg1 and Peg3. Peg1 (paternally expressed gene 1) or Mest, the first imprinted gene found on the mouse chromosome 6, may contribute to the lethality of parthenogenones and of embryos with a maternal duplication for the proximal chromosome 6. Peg1/Mest is widely expressed in mesodermal tissues and belongs to the alpha/beta hydrolase fold family. A similar approach with androgenones can be used to identify imprinted genes that are expressed from the maternal genome only.

Amino Acid Sequence↗

Mammalian Emi2 mediates cytostatic arrest and transduces the signal for meiotic exit via Cdc20.

Fertilizable mammalian oocytes are arrested at the second meiotic metaphase (mII) by the cyclinB-Cdc2 heterodimer, maturation promoting factor (MPF). MPF is stabilized via the activity of an unidentified cytostatic factor (CSF), thereby suspending meiotic progression until fertilization. We here present evidence that a conserved 71 kDa mammalian orthologue of Xenopus XErp1/Emi2, which we term endogenous meiotic inhibitor 2 (Emi2) is an essential CSF component. Depletion in situ of Emi2 by RNA interference elicited precocious meiotic exit in maturing mouse oocytes. Reduction of Emi2 released mature mII oocytes from cytostatic arrest, frequently inducing cytodegeneration. Mos levels autonomously declined to undetectable levels in mII oocytes. Recombinant Emi2 reduced the propensity of mII oocytes to exit meiosis in response to activating stimuli. Emi2 and Cdc20 proteins mutually interact and Cdc20 ablation negated the ability of Emi2 removal to induce metaphase release. Consistent with this, Cdc20 removal prevented parthenogenetic or sperm-induced meiotic exit. These studies show in intact oocytes that the interaction of Emi2 with Cdc20 links activating stimuli to meiotic resumption at fertilization and during parthenogenesis in mammals.

Animals↗

Spatial and temporal genetic variability in French populations of the peach-potato aphid, Myzus persicae.

The peach-potato aphid, Myzus persicae (Sulzer), has a complex reproductive mode in which some lineages reproduce by continuous parthenogenesis, whereas others reproduce sexually once a year. The climate is thought to act directly on the reproductive mode, because sexual eggs are the only form that can resist frost in cold regions. Sexual reproduction necessitates an obligatory host alternation that may result in long-distance dispersal. Here, we examined the genetic variability at seven microsatellite loci of populations of M. persicae in France, where both reproductive modes occur. We provide clear genetic evidence that the breeding system affects genotypic variability, as cyclically parthenogenetic aphids are far more variable than their obligately parthenogenetic counterparts. A temporal decrease in genetic variability and a temporal genetic differentiation effect suggest the existence of selective factors that play an important role in shaping the genetic structure of M. persicae populations. Lastly, differences in the population structure between reproductive modes suggest that the migration associated with the change of host during sexual reproduction lowers the level of population differentiation.

Animals↗

Cytogenetic mechanism and genetic consequences of thelytoky in the wasp Trichogramma cacoeciae.

In Hymenoptera, complete parthenogenesis, that is thelytoky, is a common phenomenon where virgin females produce only daughters. Thelytoky is often induced by bacteria of the genus Wolbachia, but can also be genetically determined by the insect itself, as in the genus Trichogramma where both forms exist. In order to compare these two forms of thelytoky, chromosome behaviour analysis in young eggs and genetic analysis of microsatellite markers were carried out in the wasp Trichogramma cacoeciae, where thelytoky is genetically determined. Microscopic studies revealed that during female gamete formation meiotic cells undergo only a single equational division followed by the expulsion of a single polar body. This absence of meiotic recombination and reduction corresponds well with the high levels of heterozygosity observed in females collected from the field and a nonsegregation pattern in the offspring of heterozygous females. We therefore concluded that diploidy in T. cacoeciae is maintained through an apomictic cloning mechanism and that the incidence of thelytoky under genetic control of the wasp differs entirely from the mechanism induced by Wolbachia infection, where thelytoky is restored through gamete duplication.

Animals↗

Host-associated populations in the lettuce root aphid, Pemphigus bursarius (L.).

Pemphigus bursarius is a host-alternating aphid in which annual rounds of sexual reproduction on its primary host, Populus nigra, are interspersed with parthenogenesis on a range of secondary hosts. Evidence was sought for the existence of genetically distinct populations, associated with different secondary hosts, in P. bursarius. Microsatellite markers revealed that genetically distinct populations were present on three different secondary host species. Microsatellites were also used, in conjunction with mitochondrial DNA sequence variation, to investigate the relationships between aphids on Populus, following sexual reproduction, and those on the secondary hosts. Evidence was found for a distinct, cyclically parthenogenetic population that exploited Lactuca sativa as its secondary host. In contrast, populations associated with Matricaria inodora appeared to be largely composed of obligate parthenogens or may even have been another species of Pemphigus. Populations on Lapsana communis appeared to be a mixture of cyclical and obligate parthenogens and were more genetically heterogeneous than those on other secondary hosts, possibly due to founder effects. Experiments to measure the performance of P. bursarius clones on different secondary hosts were inconclusive, failing to demonstrate either the presence or absence of adaptations to secondary hosts.

Animals↗

Genetic structure and clonal diversity of an introduced pest in Chile, the cereal aphid Sitobion avenae.

In Chile, the aphid Sitobion avenae is of recent introduction, lives on cultivated and wild Poaceae, and is thought to reproduce by permanent parthenogenesis. In order to study the genetic variability and population structure of this species, five microsatellite loci were typed from individual aphids collected from different cultivated and wild host plants, from different geographical zones, and years. Chilean populations showed a high degree of heterozygosity and a low genetic variability across regions and years, with four predominant genotypes representing nearly 90% of the sample. This pattern of low clonal diversity and high heterozygosity was interpreted as the result of recent founder events from a few asexually reproducing genotypes. Most geographical and temporal variation observed in the genetic composition resulted from fluctuations of a few predominant clones. In addition, comparisons of the genotypes found in Chile with those described in earlier surveys of S. avenae populations in Western Europe led us to identify 'superclones' with large geographical distribution and high ecological success, and to make a preliminary exploration of the putative origin(s) of S. avenae individuals introduced to Chile.

Agriculture↗

Rickettsia associated with male-killing in a buprestid beetle.

Many populations of the buprestid leaf-mining beetle, Brachys tessellatus, from central South Carolina, USA, show highly skewed sex ratios, ranging from 1.3 to 6.0 females per male. We have identified a Rickettsia bacterium that is associated with sex ratio distortion (SRD) and selective killing of male embryos in B. tessellatus. Molecular assays of infection by this bacterium are highly associated with SRD within families, and treatment with an antibiotic (tetracycline) increases the number of male eggs that hatch and develop. The 16S rDNA sequence indicates that this is a novel Rickettsia, most closely related to Rickettsia bellii (a tick-associated bacterium) and a pea-aphid Rickettsia. It is also related to a Rickettsial bacterium that causes male-killing in an unrelated ladybird beetle species. Low levels of parthenogenesis are also observed in this system (about 10% of females) and may be the result of selection due to male rarity, or a direct result of infection by the Rickettsia.

Animals↗

Macrogeographic patterns of breeding system diversity in the Daphnia pulex group from the United States and Mexico.

Studies on the biogeographical patterning of reproductive systems promise to extend understanding of the factors which modulate breeding system transitions. Two closely allied cladoceran crustaceans, Daphnia pulex and D. pulicaria, show varied modes of reproduction, with populations reproducing by either cyclic or obligate parthenogenesis. Prior studies have provided a detailed understanding of their breeding system diversity in the polar and cold temperate regions of North America. The present investigation extends this analysis, characterizing breeding systems and clonal diversity at sites throughout the United States and Mexico. Genotypic diversity in these southern areas was high, but only diploids were detected, indicating that polyploids are restricted to the north. F(1) hybrids and their two parental species were present in most areas, although their frequencies varied geographically. Hybrids invariably reproduced by obligate asexuality, but both parental taxa showed regional shifts in their breeding system. The complexity of these latter patterns suggests that they reflect the interplay of historical factors and selection.

Animals↗

Reproductive mode and population genetic structure of the cereal aphid Sitobion avenae studied using phenotypic and microsatellite markers.

As French populations of the aphid Sitobion avenae exhibit a range of reproductive modes, this species provides a good opportunity for studying the evolution of breeding system variation. The present analysis combined ecological and genetic investigations into the spatial distribution of variation in reproductive mode. Reproductive mode was characterized in 277 lineages of S. avenae from France, and these aphids were scored for five microsatellite loci. The analyses revealed strong geographical partitioning of breeding systems, with obligate asexuals mostly restricted to the south of France, while lineages producing sexual forms were more common in the north. Contrary to what might be anticipated for organisms with frequent parthenogenesis, there was substantial genic and genotypic diversity, even in the obligately asexual lineages. More than 120 different genotypes were detected among the 277 aphid lineages, with an average of 5.9 alleles per locus (range four to 16) and heterozygosity of 56.7%. As with previous studies of allozyme variation in aphids, most loci showed heterozygote deficits, and disequilibrium was common among allelic variants at different loci, even after removal of replicate copies of genotypes that might have been derived through clonal reproduction. Our results suggest that selection is important in structuring reproductive systems and genetic variation in French S. avenae. Canonical correspondence analysis was employed to examine the associations between genotypic and phenotypic variables, enabling the identification of alleles correlated with life-history traits.

Animals↗

Microsatellite variation in Daphnia pulex from both sides of the Baltic Sea.

Despite large genetic differentiation among neighbouring populations of many freshwater zooplankton species, a macrogeographical homogeneity of allozyme variation is generally observed. A study on breeding systems in Scandinavian populations of Daphnia pulex suggested a latitudinally related cline in breeding system with both diploid cyclic parthenogens and diploid obligate parthenogens at the latitude of 60-61 degrees N. Variation at neutral markers may be more affected by selection at linked loci in such species than in strictly sexual species. In this paper I present a study of variation at five microsatellite loci in a total of 34 populations from small ponds and rockpools on both sides of the Baltic Sea at 60-61 degrees N. Two major groups, which may represent different species of the D. pulex complex, are defined with the microsatellites. Neighbouring populations show both similar and well differentiated genetic composition. Populations separated by larger geographical distances show only a large differentiation and a macrogeographic pattern. The large differentiation observed at small distances can be explained with small effective population size: variation at the microsatellite loci has been shaped by population bottlenecks followed with expansion in size, and possibly by selection. No conclusive evidence is found for obligative parthenogenesis.

Animals↗

DNA replication cycle in parthenogenetically developing eggs of the starfish Asterina pectinifera.

Starfish oocytes artificially activated by a calcium ionophore will develop normally if the formation of polar bodies is suppressed. In the present paper, schedules of the DNA replication period (S phase) of these parthenogenotes were explicitly timed using 5-bromo-2'-deoxyuridine (BrdU) and anti-BrdU monoclonal antibody. Their schedule of S phase was identical to that of fertilized eggs. Consequently, an S phase regulation system is triggered even in parthenogenotes raised by dual treatment of egg activation and polar body suppression. The S phase schedule of parthenogenotes confirms the temporal pattern of chromosome duplication, observed by other researchers, leading to tetraploid parthenogenotes. The S phase determination also provides a basis for argument concerning the number of centrioles participating in parthenogenetic development. If polar body formation of activated eggs was not suppressed, the first S phase was normal, but the second S phase did not recur on time. A rigidly regulated system of DNA replication cycle, which should be an essential prerequisite for parthenogenesis, thus requires the content of polar bodies.

Animals↗

Developmental constraints on the mode of reproduction in the facultatively parthenogenetic cockroach Nauphoeta cinerea.

Considerable work in evolutionary biology has focused on the question of why sex persists. Both advantages to sex and constraints limiting a return to asexual reproduction are hypothesized to maintain sex once it evolves. Developmental constraints would limit asexual reproduction from a sexual species if it were difficult for females to switch from making eggs that do not develop without fertilization to making zygotes that are capable of developing in the absence of fertilization. Nauphoeta cinerea is an ovoviviparous cockroach in which some females are capable of switching from a sexual mode of reproduction to an asexual mode when isolated from males. Yet, while facultative parthenogenesis can occur in individuals, few females make the switch. Thus, this cockroach provides an ideal system for examining the potential role of developmental constraints in maintaining sex. Here we compare the cytogenetics and embryonic development of sexual and parthenogenetic offspring in N. cinerea. We find that deviations from normal ploidy levels are associated with abnormal development. All viable N. cinerea embryos exhibit typically hemimetabolous insect embryogenesis. Although there is no variation among embryos in development within a sexually produced clutch, we see extreme variation in asexually derived clutches. These results suggest that developmental constraints limit the success of asexual reproduction in this facultatively parthenogenetic cockroach. Our data further suggest that the specific constraint occurs in the switch from a meiotic mode of reproduction requiring fertilization to diploid zygotes that develop in the absence of fertilization.

Animals↗

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.

Animals↗

Amino acid metabolism of bovine blastocysts derived from parthenogenetically activated or in vitro fertilized oocytes.

The uptake and synthesis of 19 amino acids by fresh or frozen-thawed bovine blastocysts produced by parthenogenesis (PT) or in vitro fertilization (IVF) were compared in the present study. Fresh blastocysts, 180 h after IVF or PT activation, and frozen-thawed blastocysts, 168 h old and cultured for 12 h post-thawing, were cultured in synthetic oviduct fluid medium (SOFM) containing polyvinyl alcohol (PVA) with both essential and non-essential amino acids (EAA and NEAA, respectively) (Medium 1: M1) or SOFM containing PVA with only EAA (Medium 2: M2). In Experiment 1, when fresh or frozen-thawed PT blastocysts were cultured in M1, the uptake of glutamate (in fresh only), aspartate and arginine, and the synthesis of glutamine and alanine were significantly enhanced. In the culture with M2, serine, asparagine, glutamate, glutamine, glycine, arginine and alanine were significantly taken up. It was found that the glutamine concentrations was significantly higher (P < 0.001) in the culture medium drops containing embryos than in the drops without embryos. In Experiment 2, when PT blastocysts were cultured in M1, the uptake of aspartate and synthesis of alanine were greater (P < 0.01) than those by IVF blastocysts. When M2 was used, a significant (P < 0.01) production of serine, asparagine, glutamate, glutamine and alanine, and the uptake of arginine by PT blastocysts were observed. In Experiment 3, when IVF blastocysts were cultured in M1, fresh blastocysts depleted more aspartate and glutamate, and produced more glutamine and alanine than frozen-thawed blastocysts. When cultured in M2, frozen-thawed blastocysts depleted more threonine (P < 0.01) than fresh blastocysts. These results indicate that the uptake and synthesis of amino acids were different in fresh or frozen-thawed bovine blastocysts derived from PT or IVF. These differences in amino acid metabolism may be related to the viability of the blastocysts.

Alanine↗