Temperature, age of mating and starvation determine the role of maternal effects on sex allocation in the mealybug Planococcus citri

Essential roles of histone lysine methyltransferases EZH2 and EHMT1 in male embryo development of Phenacoccus solenopsis

Paternal genome elimination (PGE) is an intriguing but poorly understood reproductive strategy in which females are typically diploid, but males lose paternal genomes. Paternal genome heterochromatin (PGH) occurs in arthropods with germline PGE, such as the mealybug, coffee borer beetles, and booklice. Here, we present evidence that PGH initially occurs during early embryo development at around 15 h post-mating (hpm) in the cotton mealybug, Phenacoccus solenopsis Tinsley. Transcriptome analysis followed by qPCR validation indicated that six histone lysine methyltransferase (KMT) genes are predominantly expressed in adult females. We knocked down these five genes through dsRNA microinjection. We found that downregulation of two KMT genes, PsEZH2-X1 and PsEHMT1, resulted in a decrease of heterochromatin-related methylations, including H3K27me1, H3K27me3, and H3K9me3 in the ovaries, fewer PGH male embryos, and reduced male offspring. For further confirmation, we obtained two strains of transgenic tobacco highly expressing dsRNA targeting PsEZH2-X1 and PsEHMT1, respectively. Similarly, fewer PGH embryos and fewer male offspring were observed when feeding on these transgenic tobacco plants. Overall, we present evidence that PsEZH2-X1 and PsEHMT1 have essential roles in male embryo survival by regulating PGH formation in cotton mealybugs.

Planococcus ficus (Hemiptera: Pseudococcidae) movement and demography: methods for generating cohorts for laboratory studies

Planococcus ficus (Signoret) is a worldwide pest of grapevine. Mealybugs overwinter under bark and move into the grape canopy as the season progresses. Because crawlers are more active than later stages, mealybug movement behavior is likely to be stage specific. To quantify P. ficus demography and movement behavior, a series of laboratory experiments were conducted. First, P. ficus populations were monitored on grapevine seedlings to describe survival, change in size, timing of male pupation, and timing of oviposition over a 6-wk period. Subsequently, cohorts of mealybugs were generated by infesting grapevines with crawlers and holding infested grapevines for a specified duration of 0 (crawlers), 1, 2, 3, or 4 wk. Crawlers (0-wk) were more likely to move upwards and towards a light source, than all other age cohorts tested. Further, mealybugs from 4-wk-old cohorts were more likely to move downward than all other age cohorts tested. Results suggest that crawlers are more likely to move to the top of grapevines by moving upwards and orienting towards either the sun or the moon than all other age cohorts tested, whereas older gravid females are more likely to move downward. Passive movement of mealybugs on farm machinery or animals requires surviving a host free period. To quantify risk of passive movement, establishment rates and effects of starvation on each age cohort were quantified. Larger and older mealybugs were more likely to establish on grapevines than smaller and younger mealybugs. Further, mealybug longevity in absence of food was greater for older cohorts compared to younger cohorts. Crawlers survived an average of 2 days without food, whereas females from 4-wk-old cohorts survived for an average of 11 days without food. Further, 70% of starved females from 4-wk-old cohorts deposited fertile eggs. In the absence of food, some mealybugs from cohorts aged 2-, 3-, and 4-wk formed pupa with viable males emerging. Adult males from starved nymphs lived for an average of 3 days post-emergence. Results provide methods for producing cohorts of mealybugs of predictable size and stage and provides insight into P. ficus demography and movement behavior.

H3K9 and H4K20 methyltransferases are directly involved in the heterochromatinization of the paternal chromosomes in male Planococcus citri embryos

Using a new method for bulk preparation of early stage embryos, we have investigated the role played by putative Planococcus citri H3K9 and H4K20 histone methyl transferases (HMTases) in regulating heterochromatinization of the imprinted paternal chromosomal set in male embryos. We found that H3K9 and H420 HMTases are required for heterochromatinization of the paternal chromosomes. We present evidence that both HMTases maintain the paternal "imprint" during the cleavage divisions when both parental chromosome sets are euchromatic. A testable model that accommodates our findings is proposed.

Sexual antagonism in haplodiploids

Females and males may face different selection pressures, such that alleles conferring a benefit in one sex may be deleterious in the other. Such sexual antagonism has received a great deal of theoretical and empirical attention, almost all of which has focused on diploids. However, a sizeable minority of animals display an alternative haplodiploid mode of inheritance, encompassing both arrhenotoky, whereby males develop from unfertilized eggs, and paternal genome elimination (PGE), whereby males receive but do not transmit a paternal genome. Alongside unusual genetics, haplodiploids often exhibit social ecologies that modulate the relative value of females and males. Here we develop a series of evolutionary-genetic models of sexual antagonism for haplodiploids, incorporating details of their molecular biology and social ecology. We find that: 1) PGE promotes female-beneficial alleles more than arrhenotoky, and to an extent determined by the timing of elimination - and degree of silencing of - the paternal genome; 2) sib-mating relatively promotes female-beneficial alleles, as do other forms of inbreeding, including limited male-dispersal, oedipal-mating, and the pseudo-hermaphroditism of Icerya purchasi; 3) resource competition between related females inhibits the invasion of female-beneficial alleles; and 4) sexual antagonism foments conflicts between parents and offspring, endosymbionts and hosts, and maternal-origin and paternal-origin genes. This article is protected by copyright. All rights reserved.

Maternal effect determines drought resistance of eggs in the predatory mite Phytoseiulus persimilis

The ability of an organism to adapt to short-term environmental changes within its lifetime is of fundamental importance. This adaptation may occur through phenotypic plasticity. Insects and mites, in particular, are sensitive to changes in temperature and humidity, especially during the juvenile stages. We studied the role of phenotypic plasticity in the adaptation of eggs to different relative humidity conditions, in the predatory mite Phytoseiulus persimilis, used worldwide as a biological control agent of the spider mite Tetranychus urticae. The biocontrol efficacy of P. persimilis decreases under dry conditions, partly because P. persimilis eggs are sensitive to drought. We exposed P. persimilis adult females from two different strains to constant and variable humidity regimes and evaluated the hatching rate of their eggs in dry conditions, as well as the survival and oviposition rates of these females. Whereas the eggs laid by P. persimilis females exposed to constant high humidity did not survive in dry conditions, females exposed to constant low humidity started laying drought-resistant eggs after 24 h of exposure. Survival and oviposition rates of the females were affected by humidity: females laid fewer eggs under constant low humidity and had a shorter lifespan under constant high and constant low humidity. The humidity regimes tested had similar effects across the two P. persimilis strains. Our results demonstrate that transgenerational phenotypic plasticity, called maternal effect, allows P. persimilis females to prepare their offspring for dry conditions.

Sexual attractiveness and reproductive performance in ageing females of a coccoid insect

Female age strongly influences reproductive success in various animals. Males are, therefore, expected to show preferential responses to sexual signals released from females of ages suitable for reproduction. Here, however, we report an unexpected and contradictory effect of ageing on sexual attractiveness and reproductive performance in a coccoid insect: the pheromone-based attractiveness of females increased with ageing, though their reproductive performance was in rapid decline. Surprisingly, senescent females continued releasing relatively high amounts of pheromone and maintained their sexual attractiveness, even at ages when they had almost completely lost fertility, with reduced densities of endosymbionts to support their physiology. Our dataset suggests a potential sexual conflict within a pheromone communication system, where females benefit at males' expense through deceptive signals of fertility.

Effects of delayed mating on the reproductive biology of the vine mealybug, Planococcus ficus (Hemiptera: Pseudococcidae)

The effect of increasing mating delay on the reproductive performance and population growth rates of the vine mealybug, Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae), was investigated under laboratory conditions. Virgin females were mated at 1, 3, 5, 7, 14, 21 and 28 days after emergence and reproductive and life table parameters were estimated. The pre-oviposition period (number of days between mating and the onset of oviposition) significantly decreased in females mated within 7 days, whereas females mated at older ages showed equivalent pre-oviposition periods (7 days, as shorter delays in mating did not reduce the population growth rates.

Mating disruption of Planococcus ficus (Hemiptera: Pseudococcidae) in vineyards using reservoir pheromone dispensers

Mating disruption field experiments to control the vine mealybug, Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae), were carried out in 2008 and 2009 in two commercial vineyards in Sardinia (Italy). The effectiveness of mating disruption was evaluated by testing reservoir dispensers loaded with 100 mg (62.5 g/ha) and 150 mg (93.8 g/ha) of the sex pheromone in 2008 and 2009, respectively. The number of males captured in pheromone traps, the P. ficus population density and age structure, the parasitism rate, the percentage of ovipositing females, and the crop damage were compared between disrupted and untreated plots. In both field trials, the number of males captured in mating disruption plots was significantly reduced by 86% and 95%, respectively. Mating disruption at the initial dose of 62.5 g/ha of active ingredient gave inconclusive results, whereas the dose of 93.8 g/ha significantly lowered the mealybug density and modified the age structure, which showed a lower percentage of ovipositing females and a higher proportion of preovipositing females. Mating disruption did not affect negatively the parasitism rate, which was higher in the disrupted than in the control plots (>1.5-fold). Crop damage at harvest was very low in both field trials and did not differ between treatments. Mating disruption was effective in wide plots protected with dispensers loaded with 150 mg of the sex pheromone, showing its potential to be included in the overall integrated control programs in Mediterranean wine-growing regions.

Mealybug chromosome cycle as a paradigm of epigenetics

Recently, epigenetics has had an ever-growing impact on research not only for its intrinsic interest but also because it has been implied in biological phenomena, such as tumor emergence and progression. The first epigenetic phenomenon to be described in the early 1960s was chromosome imprinting in some insect species (sciaridae and coccoideae). Here, we discuss recent experimental results to dissect the phenomenon of imprinted facultative heterochromatinization in Lecanoid coccids (mealybugs). In these insect species, the entire paternally derived haploid chromosome set becomes heterochromatic during embryogenesis in males. We describe the role of known epigenetic marks, such as DNA methylation and histone modifications, in this phenomenon. We then discuss the models proposed to explain the noncanonical chromosome cycle of these species.