The strange case of the armored scale insect and its bacteriome

Is chimerism associated with cancer across the tree of life?

Chimerism is a widespread phenomenon across the tree of life. It is defined as a multicellular organism composed of cells from other genetically distinct entities. This ability to 'tolerate' non-self cells may be linked to susceptibility to diseases like cancer. Here we test whether chimerism is associated with cancers across obligately multicellular organisms in the tree of life. We classified 12 obligately multicellular taxa from lowest to highest chimerism levels based on the existing literature on the presence of chimerism in these species. We then tested for associations of chimerism with tumour invasiveness, neoplasia (benign or malignant) prevalence and malignancy prevalence in 11 terrestrial mammalian species. We found that taxa with higher levels of chimerism have higher tumour invasiveness, though there was no association between malignancy or neoplasia and chimerism among mammals. This suggests that there may be an important biological relationship between chimerism and susceptibility to tissue invasion by cancerous cells. Studying chimerism might help us identify mechanisms underlying invasive cancers and also could provide insights into the detection and management of emerging transmissible cancers.

Taxon-specific ultraconserved element probe design for phylogenetic analyses of scale insects (Hemiptera: Sternorrhyncha: Coccoidea)

Scale insects (Coccoidea) are morphologically specialized members of the order Hemiptera, with 56 families recognized to date. However, the phylogenetic relationships within and among families are poorly resolved. In this study, to further characterize the phylogenetic relationships among scale insects, an ultraconserved element (UCE) probe set was designed specifically for Coccoidea based on three low-coverage whole genome sequences along with three publicly available genomes. An in silico test including eight additional genomes was performed to evaluate the effectiveness of the probe set. Most scale insect lineages were recovered by the phylogenetic analysis. This study recovered the monophyly of neococcoids. The newly developed UCE probe set has the potential to reshape and improve our understanding of the phylogenetic relationships within and among families of scale insects at the genome level.

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.

Molecular phylogenetics of Aspidiotini armored scale insects (Hemiptera: Diaspididae) reveals rampant paraphyly, curious species radiations, and multiple origins of association with Melissotarsus ants (Hymenoptera: Formicidae)

The armored scale insect tribe Aspidiotini comprises many pest species that are globally invasive and economically damaging. The taxonomy of scale insects is based almost solely upon morphological characters of adult females, and little prior work has been done to test the classification of aspidiotines against molecular evidence. To address these concerns, we reconstruct a molecular phylogeny for aspidiotine armored scales that expands greatly upon taxonomic and character representations from previous studies. Our dataset includes 127 species (356 terminal taxa) and four gene regions: 28S, EF-1α, COI-COII, and CAD. Nearly 50% of the species treated are identified as pests and several more may represent emerging pests. Phylogenetic data were analyzed in a Bayesian framework using MC³ iterations. The majority of sampled aspidiotine genera are not monophyletic as currently defined. Monophyly constraints for 'worst offenders' were imposed on the phylogeny and stepping-stone MCMC was performed to calculate marginal likelihood scores. Comparisons of marginal likelihoods from runs with constrained vs. informative priors support the interpretation that pest-rich genera are not monophyletic. We use character mapping to illustrate signal and convergence for selected traits that have been used to define or recognize genera and evaluate consistency and retention indices for these traits. The phylogeny illustrates a pervasive pattern in which extremely polyphagous pests - typically having large populations and wide geographical distributions - are frequently intertwined with range-limited specialists on the phylogeny. Finally, the phylogeny recovers three origins of ant association among the Aspidiotini. The history of ant/diaspidid symbioses involves periods of sustained partner fidelity, spanning multiple speciation events, which have been punctuated by opportunistic switches to novel partners.

Maternal Inheritance of a Single Somatic Animal Cell Displayed by the Bacteriocyte in the Whitefly Bemisia tabaci

Bacteriocytes are insect cells harboring symbiotic bacteria that are required by their insect host and are transmitted vertically via the female ovary [1]. In most insect groups, the bacteria are released from the bacteriocytes and transferred to the ovary [2, 3], but in whiteflies, maternal bacteriocytes migrate to each egg [4, 5, 6], where they have been reported to lyse, releasing the symbionts [1]. To investigate bacteriocyte inheritance in whiteflies further, we applied microsatellite genotyping and genomic analysis to a genetically diverse population of Bemisia tabaci, and we observed the fate of the bacteriocyte in embryos. Surprisingly, the microsatellite profile of the bacteriocytes was uniform, and insect cross experiments demonstrated that the bacteriocytes have a stable genotype that differs from the genotype of the insect head (which lacks bacteriocytes). Comparative genomic analysis indicates that genomes of the bacteriocyte and whitefly head are distinct. Interestingly, the bacterioyte genome contains the canonical arthropod telomere repeats TTAGG, and the bacteriocytes express telomere maintenance genes that may underlie cellular immortality in animal cells [7]. Microscopy observations confirmed that a single bacteriocyte transmitted to each egg is retained and divides once just before egg hatch, yielding two bacteriocytes in the neonate insect. These data demonstrate the maternal inheritance of an absolutely required somatic insect cell, violating the developmental separation of germline and soma [8, 9]. Future investigation on the mechanism and phylogenetic distribution of maternally inherited bacteriocytes will shed light on the developmental origins and evolutionary diversification of bacteriocytes [10] and the processes underlying cellular immortality [11].

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Whitefly bacteriocytes have different microsatellite alleles from other somatic cells

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The bacteriocyte genotype is stable over three sexual generations of the insect

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Bacteriocyte genomes in different insects are genetically very similar

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The single bacteriocyte transmitted to the sexual egg persists through embryogenesis

Beyond sex allocation: the role of mating systems in sexual selection in parasitoid wasps

Despite the diverse array of mating systems and life histories which characterise the parasitic Hymenoptera, sexual selection and sexual conflict in this taxon have been somewhat overlooked. For instance, parasitoid mating systems have typically been studied in terms of how mating structure affects sex allocation. In the past decade, however, some studies have sought to address sexual selection in the parasitoid wasps more explicitly and found that, despite the lack of obvious secondary sexual traits, sexual selection has the potential to shape a range of aspects of parasitoid reproductive behaviour and ecology. Moreover, various characteristics fundamental to the parasitoid way of life may provide innovative new ways to investigate different processes of sexual selection. The overall aim of this review therefore is to re-examine parasitoid biology with sexual selection in mind, for both parasitoid biologists and also researchers interested in sexual selection and the evolution of mating systems more generally. We will consider aspects of particular relevance that have already been well studied including local mating structure, sex allocation and sperm depletion. We go on to review what we already know about sexual selection in the parasitoid wasps and highlight areas which may prove fruitful for further investigation. In particular, sperm depletion and the costs of inbreeding under chromosomal sex determination provide novel opportunities for testing the role of direct and indirect benefits for the evolution of mate choice.

A new embryonic pattern in parasitic wasps: divergence in early development may not be associated with lifestyle

Comparative embryogenesis of Encarsia formosa and Encarsia pergandiella (Hymenoptera Aphelinidae), two endoparasitoids of whiteflies (Hemiptera Aleyrodidae), revealed two strongly diverging developmental patterns. Indeed, the centrolecithal anhydropic egg of E. formosa developed through a superficial cleavage, as it occurs in Nasonia vitripennis, Apis mellifera, and Drosophila melanogaster. In contrast, the alecithal hydropic egg of E. pergandiella developed through holoblastic cleavage within a specialized extra-embryonic membrane (EEM). Since this developmental pattern evolved independently in several lineages of hymenopteran endoparasitoids, departures from the superficial cleavage mode have been argued to be strongly canalized in response to a shift from ecto- to endoparasitic lifestyle. Coexistence of both developmental patterns in two congeneric species suggests that alterations of early embryonic development may not be correlated with lifestyle. In addition, embryogenesis of E. pergandiella exhibited the following developmental novelties compared to other species possessing a hydropic egg: (i) polar body derivatives early acquired a cytoskeletal boundary prior to any other cellularization event; (ii) cellularization was asynchronous, starting with an early differentiation of a single apical blastomere at the end of the third cleavage; (iii) appearance of cytoskeletal boundaries of embryo blastomeres occurred between the third and fourth cleavages; (iv) the EEM originated through asynchronous participation of three separate lineages of cleavage nuclei, one of which associated with the polar body derivatives in a syncytium. Our results confirm a scenario of high plasticity in the early developmental strategies of hymenopteran endoparasitoids.

Chlorophyll content and chlorophyll fluorescence in tomato leaves infested with an invasive mealybug, Phenacoccus solenopsis (Hemiptera: Pseudococcidae)

Herbivore injury has indirect effects on the growth and performance of host plants through photosynthetic suppression. It causes uncertain reduction in photosynthesis, which likely depends on the degree of infestation. Rapid light curves provide detailed information on the saturation characteristics of electron transport as well as the overall photosynthetic performance of a plant. We examined the effects of different intensities of infestation of the invasive mealybug, Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae), on the relative chlorophyll content and rapid light curves of tomato Solanum lycopersicum L. leaves using a chlorophyll meter and chlorophyll fluorescence measurement system, respectively, under greenhouse conditions. After 38 d of P. solenopsis feeding, relative chlorophyll content of tomato plants with initial high of P. solenopsis was reduced by 57.3%. Light utilization efficiency (α) for the initial high-density treatment was reduced by 42.4%. However, no significant difference between initial low-density treatment and uninfested control was found. The values of the maximum electron transport rate and minimum saturating irradiance for initial high-density treatment were reduced by 82.0 and 69.7%, respectively, whereas the corresponding values for low-density treatment were reduced by 55.9 and 58.1%, respectively. These data indicated that changes were induced by P. solenopsis feeding in the relative chlorophyll content and chlorophyll fluorescence of infested tomato plants. The results indicating that low initial infestation by P. solenopsis caused no change in relative leaf chlorophyll content or light utilization efficiency could have been because the plants rapidly adapted to P. solenopsis feeding or because of compensatory photosynthesis.

Genomic conflict in scale insects: the causes and consequences of bizarre genetic systems

It is now clear that mechanisms of sex determination are extraordinarily labile, with considerable variation across all taxonomic levels. This variation is often expressed through differences in the genetic system (XX-XY, XX-XO, haplodiploidy, and so on). Why there is so much variation in such a seemingly fundamental process has attracted much attention, with recent ideas concentrating on the possible role of genomic conflicts of interest. Here we consider the role of inter- and intra-genomic conflicts in one large insect taxon: the scale insects. Scale insects exhibit a dizzying array of genetic systems, and their biology promotes conflicts of interest over transmission and sex ratio between male- and female-expressed genes, parental- and offspring-expressed genes (both examples of intra-genomic conflict) and between scale insects and their endosymbionts (inter-genomic conflict). We first review the wide range of genetic systems found in scale insects and the possible evolutionary transitions between them. We then outline the theoretical opportunities for genomic conflicts in this group and how these might influence sex determination and sex ratio. We then consider the evidence for these conflicts in the evolution of sex determination in scale insects. Importantly, the evolution of novel genetic systems in scale insects has itself helped create new conflicts of interest, for instance over sex ratio. As a result, a major obstacle to our understanding of the role of conflict in the evolution of sex-determination and genetic systems will be the difficulty in identifying the direction of causal relationships. We conclude by outlining possible experimental and comparative approaches to test more effectively how important genomic conflicts have been.

Polar bodies--more a lack of understanding than a lack of respect

Polar bodies are as diverse as the organisms that produce them. Although in many animals these cells often die following meiotic maturation of the oocyte, in other organisms they are an essential and diverse part of embryonic development. Here we highlight some of this diversity and summarize the evolutionary basis for their utility.

The evolution and suppression of male suicide under paternal genome elimination

Different genetic systems can be both the cause and the consequence of genetic conflict over the transmission of genes, obscuring their evolutionary origin. For instance, with paternal genome elimination (PGE), found in some insects and mites, both sexes develop from fertilized eggs, but in males the paternally derived chromosomes are either lost (embryonic PGE) or deactivated (germline PGE) during embryogenesis and not transmitted to the next generation. Evolution of germline PGE requires two transitions: (1) elimination of the paternal genome during spermatogenesis; (2) deactivation of the paternal genome early in development. Hypotheses for the evolution of PGE have mainly focused on the first transition. However, maternal genes seem to be responsible for the deactivation and here we investigate if maternal suppression could have evolved in response to paternally expressed male suicide genes. We show that sibling competition can cause such genes to spread quickly and that inbreeding is necessary to prevent fixation of male suicide, and subsequent population extinction. Once male-suicide has evolved, maternally expressed suppressor genes can invade in the population. Our results highlight the rich opportunity for genetic conflict in asymmetric genetic systems and the counterintuitive phenotypes that can evolve as a result.

Duplicated proteasome subunit genes in Drosophila and their roles in spermatogenesis

The proteasome is a large, multisubunit complex that acts as the cell's 'protein-degrading machine' in the ubiquitin-mediated proteolytic pathway for regulated protein turnover. Although proteasomes are usually thought of as being homogeneous structures, recent studies have revealed their more dynamic and heterogeneous nature. For example, in a number of plant and animal species, multiple isoforms of several proteasome subunits, encoded by paralogous genes, have been discovered, and in some cases, these alternative isoforms have been shown to be functionally distinct from their conventional counterparts. A particularly striking example of this phenomenon is seen in Drosophila melanogaster, where 12 of the 33 subunits that make up the 26S proteasome holoenzyme are represented in the genome by multiple paralogous genes. Remarkably, in every case, the 'extra' genes are expressed in a testis-specific manner. Here, we describe the extent and nature of these testis-specific gene duplications and discuss their functional significance, and speculate on why this situation might have evolved.

The visual system of male scale insects

Animal eyes generally fall into two categories: (1) their photoreceptive array is convex, as is typical for camera eyes, including the human eye, or (2) their photoreceptive array is concave, as is typical for the compound eye of insects. There are a few rare examples of the latter eye type having secondarily evolved into the former one. When viewed in a phylogenetic framework, the head morphology of a variety of male scale insects suggests that this group could be one such example. In the Margarodidae (Hemiptera, Coccoidea), males have been described as having compound eyes, while males of some more derived groups only have two single-chamber eyes on each side of the head. Those eyes are situated in the place occupied by the compound eye of other insects. Since male scale insects tend to be rare, little is known about how their visual systems are organized, and what anatomical traits are associated with this evolutionary transition. In adult male Margarodidae, one single-chamber eye (stemmateran ocellus) is present in addition to a compound eye-like region. Our histological investigation reveals that the stemmateran ocellus has an extended retina which is formed by concrete clusters of receptor cells that connect to its own first-order neuropil. In addition, we find that the ommatidia of the compound eyes also share several anatomical characteristics with simple camera eyes. These include shallow units with extended retinas, each of which is connected by its own small nerve to the lamina. These anatomical changes suggest that the margarodid compound eye represents a transitional form to the giant unicornal eyes that have been described in more derived species.

Male biased sex ratio in the Mediterranean fruit fly Ceratitis capitata, an example of Y-chromosome meiotic drive

A case of Y-chromosome meiotic drive is reported in the Mediterranean fruit fly Ceratitis capitata. It arose in an irradiated male and results in excess of males. Male excess is inherited strictly from father to son. A Y-linked factor MP (male producer) is proposed. Higher drive can be selected, but distortion declines rapidly in the absence of selection. Hybrid males from crosses between driving males and nondriving females also show drive but to a reduced extent, suggesting the action of suppressors. Sex ratio distortion is independent of postzygotic mortality, and is not associated with an obvious chromosome arrangement. Spermiogenesis in driving males is characterised by abnormalities in sperm tails and reduced numbers in some sperm cysts, whereas neighbouring cysts of the same MP testis are essentially wild type. The average number of missing sperms plus deformed sperms approximates to the average depression in female recovery among the progenies of siblings, suggesting that most of the missing or abnormal sperms would have given rise to females, that is, they would have been X-bearing. To explain the heterogeneity between neighbouring cysts, a theory is proposed that links it to variation in X-chromosome sensitivity to MP, arising by random suppression of the genetic basis of sensitivity during the six mitotic divisions in the origin of the cyst from its stem cell before meiosis.

Differential patterns of apoptosis in response to aging in Drosophila

Several lines of evidence suggest that programmed cell death may play a role in the aging process and the age-related functional declines of multicellular organisms. To pave the way for the use of Drosophila to rigorously test this hypothesis in a genetic model organism, this work examines the pattern of apoptosis in the adult fly during aging. The analysis across the lifespan of caspase activity and DNA fragmentation shows that apoptosis occurs in adult flies at all ages and that it is linked to physiological age. The results establish that under normal conditions, fly aging is coupled with a lifelong gradual increase of apoptosis within muscle cells and an activation of apoptosis in fat cells of old flies. The nervous system does not show signs of apoptosis. These time- and tissue-specific changes indicate that aging influences the levels and the nature of the cells that commit to apoptosis. The comparison with the apoptotic response to starvation and oxidative stresses strongly suggests that the lifelong increase in flight and leg muscles results from the accumulation of oxidative damage associated with aging. This finding presents an attractive mechanism to account for the decline of locomotor functions and muscle loss in the elderly and opens the way for the genetic analysis of sarcopenia in Drosophila.

The genetics of cell death: approaches, insights and opportunities in Drosophila

Cell death is ubiquitous in metazoans and involves the action of an evolutionarily conserved process known as programmed cell death or apoptosis. In Drosophila melanogaster, it is now uniquely possible to screen for genes that determine the fate - life or death - of any cell or population of cells during development and in the adult. This review describes these genetic approaches and the key insights into cell-death mechanisms that have been obtained, as well as the outstanding questions that these techniques can help to answer.