Quantitative trait analysis and geographic variability of natural populations of Zaprionus indianus, a recent invader in Brazil

Five natural samples of a recent South America invader, the drosophilid Zaprionus indianus, were investigated with the isofemale line technique. These samples were compared to five African mainland populations, investigated with the same method. The results were also compared to data obtained on mass cultures of other populations from Africa and India. Three quantitative traits were measured on both sexes, wing and thorax length and sternopleural bristle number. We did not find any latitudinal trend among the American samples, while a significant increase in body size with latitude was observed in the Indian and, to a lesser degree, in the African populations. American populations were also characterized by their bigger size. Genetic variability, estimated by the intraclass correlation among isofemale lines, was similar in American and African populations. The intraline, nongenetic variability was significantly less in the American samples, suggesting a better developmental stability, the origin of which is unclear. A positive relationship was evident between intraline variability of size traits and the wing-thorax length correlation. Altogether, our data suggest that the colonizing propagule introduced to Brazil had a fairly large size, preventing any bottleneck effect being detected. The big body size of American flies suggests that they came from a high-latitude African country. The lack of a latitudinal dine in America seems to be related to the short time elapsed since introduction. The very rapid spread of Z. indianus all over South America suggests that it might rapidly invade North America.     

Variability in life history traits of the aphid,Acyrthosiphon pisum (Harris), from sexual and asexual populations

Many aphid species have shown remarkable adaptability by invading new habitats and agricultural crops, although they are parthenogenetic and might be expected to show limited genetic variation. To determine if the mode of reproduction limits the level of genetic variation in adaptively important traits, we assess variation in 15 life history traits of the pea aphid, Acyrhosiphon pisum (Harris), for five populations sampled along a north-south transect in central North America, and for three traits for three populations from eastern Australia. The traits are developmental times and rates as affected by temperature, body weights as affected by temperature, fecundity, measures of migratory tendency, and photoperiodic responses. The most southerly population from North America is shown to be obligately parthenogenetic, as are the Australian populations, and the four more northerly North American populations are facultatively parthenogenetic with the number of parthenogenetic generations per year increasing from north to south. The broad-sense heritabilities of life history traits varied from 0.36 to 0.71 for nine quantitive traits based on a comparison of within-and between-lineage variances. Using these traits, 7–13 distinct genotypes (i.e. clones) were identified among each of the 18 lines sampled from the North American populations, but the number did not differ significantly among populations. The level of genetic variation differed from trait to trait. For 4 of 12 quantitative traits, the level of variation in the obligately parthenogenetic population from North America was lowest, but significantly lower than all the sexual populations for only 1 trait. The obligately parthenogenetic population had the highest level of genetic variation for two traits, and had intermediate levels for the others. The most northerly population, which was sexual and had relatively few parthenogenetic generations each year, had the lowest level of variation for 5 of 12 traits and the highest level of variation for 2 traits. There was no decline in variability from north to south correlated with the increase in the annual number of parthenogenetic generations. The Australian populations showed no less variation than the North American populations for two of three traits, although the pea aphid was introduced to Australia only 5 years prior to the study, whereas the aphid has been in North America for at least 100 years. The mode of reproduction has not had a substantial impact on the level of genetic variation in life history traits of the pea aphid, but there are population-specific factors that effect the level of variation in certain traits.     

Comparison of native and non‐native predator consumption rates and prey avoidance behavior in North America and Europe

Novel predator–prey interactions can contribute to the invasion success of non‐native predators. For example, native prey can fail to recognize and avoid non‐native predators due to a lack of co‐evolutionary history and cue dissimilarity with native predators. This might result in a competitive advantage for non‐native predators. Numerous lady beetle species were globally redistributed as biological control agents against aphids, resulting in novel predator–prey interactions. Here, we investigated the strength of avoidance behavior of the pea aphid (Acyrthosiphon pisum) toward chemical cues of native lady beetles and non‐native Asian Harmonia axyridis and European Coccinella septempunctata and Hippodamia variegata in North America, hypothesizing that cues of non‐native lady beetles induce weaker avoidance behavior than cues of co‐evolved native lady beetles. Additionally, we compared aphid consumption of lady beetles, examining potential predation advantages of non‐native lady beetles. Finally, we compared cue avoidance behavior between North American and European pea aphid populations and aphid consumption of native and non‐native lady beetles in North America and Europe. In North America, pea aphids avoided chemical cues of all ladybeetle species tested, regardless of their origin. In contrast to pea aphids in North America, European pea aphids did not avoid cues of the non‐native H. axyridis. The non‐native H. axyridis and C. septempunctata were among the largest and most voracious lady beetle species tested, on both continents. Consequently, in North America non‐native lady beetle species might have a competitive advantage on shared food resources due to their relatively large body size, compared to several native American lady beetle species. In Europe, however, non‐native H. axyridis might benefit from missing aphid cue avoidance as well as a large body size. The co‐evolutionary time gap between the European and North American invasion of H. axyridis likely explains the intercontinental differences in cue avoidance behavior and might indicate evolution in aphids toward non‐native predators.     

Genetic differentiation between Australian and North American populations of the monarch butterfly Danaus plexippus (L.) (Lepidoptera: Nymphalidae): an exploration using allozyme electrophoresis

Allozyme analysis was used to address the question of the source of the Australian populations of the monarch butterfly Danaus plexippus (L.). The study had three major aims: (1) To compare the levels of diversity of Australian and Hawaiian populations with potential source populations. (2) To determine whether eastern and western North American populations were sufficiently divergent for the Australian populations to be aligned to a source population. (3) To compare the differentiation among regions in Australia and North America to test the prediction of greater genetic structure in Australia, as a consequence of reduced migratory behaviour. The reverse was found, with F _ST values an order of magnitude lower in Australia than in North America. Predictably, Australian and Hawaiian populations had lower allelic diversity, but unexpected higher heterozygosity values than North American populations. It was not possible to assign the Australian populations to a definitive source, although the high levels of similarity of Australian populations to each other suggest a single colonization event. The possibility that the Australian populations have not been here long enough to reach equilibrium is discussed. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 75 , 437–452.     

Bill and body size in the peregrine falcon,north versus south: is size adaptive?

Visually, the bill size on southern hemisphere peregrine falcons ( Falco peregrinus ), especially from Australasia and the Philippines, appears appreciably larger for their body size than in other peregrine populations. Accordingly, we measured the bill 'size' or 'volume' (length, width, depth) as a function of body mass on a sample of peregrines. We used both wing and tarsal length as indicators of body mass. We compared bill volume between two northern hemisphere groups, a medium-sized tundra breeding group and the largest of all peregrines ( F.p. pealei ) from the Aleutian Islands of North America, and also among three southern hemisphere groups, South American, Australian and Melanesian/Philippine. Finally, we com- pared northern hemisphere and southern hemisphere birds. Southern hemisphere peregrines have a more massive bill relative to body mass than those of the northern group, and can be distinguished from the northern hemisphere birds using ratios of bill volume to wing or tarsus length. Absolute bill volume generally shows a stabilizing selection and, except for tundra birds which are smaller, all bills are statistically the same by sex while body size or mass changes geographically showing directional selection. It is not clear why body mass is consistently smaller in the southern hemisphere than in north temperate regions nor why the convergence in bill volume to body mass among southern groups should be so evident for such isolated geographical locations, especially since there is apparently no gene flow. Further, habitats and thus foods that might alter bill structure differ among locations.     

ORIGINS OF GENOTYPIC VARIATION IN NORTH AMERICAN DANDELIONS INFERRED FROM RIBOSOMAL DNA AND CHLOROPLAST DNA RESTRICTION ENZYME ANALYSIS

Restriction enzyme analysis of ribosomal DNA (rDNA) and chloroplast DNA (cpDNA) is used to assess the relative contribution of hybridization and mutation as sources of genotypic variation in weedy asexual dandelions, with focus on the dandelion flora of North America. Of 318 North American dandelions surveyed, 145 rDNA-cpDNA clones are detected. The combined rDNA-cpDNA genotypes show that most of the polymorphic rDNA and cpDNA restriction sites or lengths in these plants are also present in weedy asexual dandelions collected from natural populations in Europe and in asexual and diploid taxa (microspecies) chosen to represent diverse Eurasian members of the genus. However, of 222 combined rDNA-cpDNA genotypes found in 427 asexual plants surveyed, only 9 genotypes are found in both North American and Eurasian dandelions. Two rDNA and three cpDNA characters are unique to individual plants in North America and are consistent with mutational origins of genotypic variation in asexual lineages. But the array of genotypic diversity, characterized by different combinations of the rDNA and cpDNA characters, show that multiple hybridization events are a more important source of genotypic variation than mutation in the asexual polyploids. The rDNA and cpDNA data also indicate polyphyletic origin of several asexual Taraxacum taxa.     

The lack of genetic bottleneck in invasive Tansy ragwort populations suggests multiple source populations.

Jacobaea vulgaris (Asteraceae) is a species of Eurasian origin that has become a serious non-indigenous weed in Australia, New Zealand, and North America. We used neutral molecular markers to (1) test for genetic bottlenecks in invasive populations and (2) to investigate the invasion pathways. It is for the first time that molecular markers were used to unravel the process of introduction in this species.The genetic variation of 15 native populations from Europe and 16 invasive populations from Australia, New Zealand and North America were compared using the amplified fragment length polymorphisms (AFLP's). An analysis of molecular variance showed that a significant part (10%) of the total genetic variations between all individuals could be explained by native or invasive origin.Significant among-population differentiation was detected only in the native range, whereas populations from the invasive areas did not significantly differ from each other; nor did the Australian, New Zealand and North American regions differ within the invasive range. The result that native populations differed significantly from each other and that the amount of genetic variation, measured as the number of polymorphic bands, did not differ between the native and invasive area, strongly suggests that introductions from multiple source populations have occurred. The lack of differentiation between invasive regions suggests that either introductions may have occurred from the same native sources in all invasive regions or subsequent introductions took place from one into another invasive region and the same mix of genotypes was subsequently introduced into all invasive regions.An assignment test showed that European populations from Ireland, the Netherlands and the United Kingdom most resembled the invasive populations.     

Latitudinal clines in the timing and temperature-sensitivity of photoperiodic reproductive diapause in Drosophila montana

Reproductive diapause is a primary mechanism used by arthropods to synchronize their life cycle with seasonal changes in temperate regions. Our study species, Drosophila montana, represents the northern insect species where flies enter reproductive diapause under short day conditions and where the precise timing of diapause is crucial for both survival and offspring production. We have studied clinal variation in the critical day length for female diapause induction (CDL) and their overall susceptibility to enter diapause (diapause incidence), as well as the temperature sensitivity of these traits. The study was performed using multiple strains from four latitudinal clines of the species – short clines in Finland and Alaska and long clines in the Rocky Mountains and the western coast of North America – and from one population in Kamchatka, Russia. CDL showed strong latitudinal clines on both continents, decreasing by one hour per five degrees decline in latitude, on average. CDL also decreased in all populations along with an increase in fly rearing temperature postponing the diapause to later calendar time, the effects of temperature being stronger in southern than in northern population. Female diapause incidence was close to 100% under short day/low temperature conditions in all populations, but decreased below 50% even under short days in 19°C in the southern North American western coast populations and in 22°C in most populations. Comparing a diversity of climatic data for the studied populations showed that while CDL is under a tight photoperiodic regulation linked with latitude, its length depends also on climatic factors determining the growing season length. Overall, the study deepens our understanding of how spatial and environmental parameters affect the seasonal timing of an important biological event, reproductive diapause and helps to estimate the evolutionary potential of insect populations to survive in changing climatic conditions.     

Sexual selection accounts for the geographic reversal of sexual size dimorphism in the dung fly, sepsis punctum (Diptera: Sepsidae)

Sexual size dimorphism (SSD) varies widely across and within species. The differential equilibrium model of SSD explains dimorphism as the evolutionary outcome of consistent differences in natural and sexual selection between the sexes. Here, we comprehensively examine a unique cross-continental reversal in SSD in the dung fly, Sepsis punctum. Using common garden laboratory experiments, we establish that SSD is male-biased in Europe and female-biased in North America. When estimating sexual (pairing success) and fecundity selection (clutch size of female partner) on males under three operational sex ratios (OSRs), we find that the intensity of sexual selection is significantly stronger in European versus North American populations, increasing with male body size and OSR in the former only. Fecundity selection on female body size also increases strongly with egg number and weakly with egg volume, however, equally on both continents. Finally, viability selection on body size in terms of intrinsic (physiological) adult life span in the laboratory is overall nil and does not vary significantly across all seven populations. Although it is impossible to prove causality, our results confirm the differential equilibrium model of SSD in that differences in sexual selection intensity account for the reversal in SSD in European versus North American populations, presumably mediating the ongoing speciation process in S. punctum.     

Phylogenetics and evolution of the eastern Asian-eastern North American disjunct aphid tribe,Hormaphidini (Hemiptera: Aphididae)

A conspicuous biogeographic pattern of the Northern Hemisphere is the disjunct occurrence of related taxa on different continents. Perhaps best studied in plants, this pattern includes disjunct distributions of genera in eastern Asia and eastern North America. Such continental disjunctions are thought to be the remnants of a mostly continuously distributed, mixed mesophytic forest dating to the Miocene, which subsequently became fragmented by geological and climatic changes. Some highly host-specific insects, namely aphids, live on descendants of the mixed mesophytic forest taxa and exhibit the same disjunct distributions as that of their host plants. We estimated the phylogeny of Hormaphidini aphids, which host-alternate between witch-hazel (Hamamelis; an eastern Asian-eastern North American disjunct genus) and birch (Betula). Based on partial nuclear elongation factor 1alpha and mitochondrial tRNA leucine/cytochrome oxidase II sequences, trees inferred from maximum-parsimony and maximum-likelihood showed strong support for two monophyletic genera (Hamamelistes and Hormaphis), each containing a clade of Japanese and a clade of North American species. The estimated divergence dates of Asian and North American clades in both genera was 20-30 million years ago, consistent with the idea that aphids may have experienced the same vicariance events as those of their host plants.     

角倍蚜各虫态蜡腺的分布与结构

【目的】角倍蚜Schlechtendalia chinensis是五倍子的主要生产种,具有复杂的生活史,需要经历干母、干雌、秋迁蚜、越冬若蚜、春迁蚜和性蚜6种虫态,在盐肤木Rhus chinensis和藓类植物上转主寄生,经历有性和无性生殖、瘿内和瘿外世代的交替才能完成生活史。通过对角倍蚜蜡腺的研究,有助于深入了解角倍蚜的生态适应性,为角倍蚜培育技术改进和产量提高提供依据。【方法】以采自云南盐津田间的角倍蚜为虫源,通过室内饲养获得的角倍蚜各虫态标本。采用光学显微镜、电子显微镜和超薄切片技术,对角倍蚜各虫态蜡腺的分布、形态和结构进行分析。【结果】角倍蚜各虫态(1龄干母除外)均有蜡腺,沿背部中线左右对称分布,其中头部2列2排、胸部4列1排、腹部6列8排,共56个蜡腺;1龄干母(瘿外生活)没有蜡腺,2龄干母(瘿内生活)在腹部背面有2列8排共16个蜡腺。每个蜡腺由2~22个多边形凹陷组成,凹陷的数量和形态在不同虫态间差异显著,其中干雌、春迁蚜和秋迁蚜的蜡腺最为复杂,越冬若蚜次之。性蚜各部位的蜡腺结构差异明显,靠近背板中线的脊柱板和胸膜板的蜡腺结构简单,而靠近边缘线上的蜡腺结构复杂;2龄干母的蜡腺结构也较简单。与各虫态蜡腺的分布相对应,角倍蚜各虫态(1龄干母除外)的体表均有蜡粉,其中干雌、春迁蚜和秋迁蚜分泌大量的蜡粉,越冬若蚜次之,性蚜体表有少量蜡粉,1龄干母没有蜡粉,2龄干母有少量蜡粉。【结论】角倍蚜各虫态蜡腺和体表蜡粉的数量、排列和发达程度存在差异,且这些差异与其生活微环境和各虫态的生物学特性密切相关,可能是角倍蚜对环境条件长期适应的结果。     

Admixture between native and invasive populations may increase invasiveness of Mimulus guttatus

Self-fertilization and admixture of genotypes from different populations can have major fitness consequences in native species. However, few studies have addressed their potential roles in invasive species. Here, we used plants of Mimulus guttatus from seven native North American, three invasive Scottish and four invasive New Zealand populations to address this. We created seeds from self-fertilization, within-population outcrossing, between-population outcrossing within the same range, and outcrossing between the native and invasive ranges. A greenhouse experiment showed that native and invasive plants of M. guttatus suffered to similar degrees from inbreeding depression, in terms of asexual reproduction and biomass production. After outcrossing with plants from other populations, M. guttatus benefited from heterosis, in terms of asexual and sexual reproduction, and biomass production, particularly when plants from native and invasive populations were crossed. This suggests that, when novel genotypes of M. guttatus from the native North American range will be introduced to the invasive ranges, subsequent outcrossing with M. guttatus plants that are already there might further boost invasiveness of this species.     

High genetic diversity in French invasive populations of common ragweed, Ambrosia artemisiifolia, as a result of multiple sources of introduction

Ambrosia artemisiifolia is an aggressive North American annual weed, found particularly in sunflower and corn fields. Besides its economic impact on crop yield, it represents a major health problem because of its strongly allergenic pollen. Ragweed was imported inadvertently to Europe in the 18th century and has become invasive in several countries, notably in the Rhône Valley of France. It has recently expanded in both the Provence-Alpes-Côte-d’Azur and Bourgogne regions. As first steps towards understanding the causes and mechanisms of ragweed invasion, genetic variability of French and North American populations was analysed using microsatellites. Overall genetic variability was similar in North America and in the Rhône-Alpes region, but within-population levels of genetic variability were surprisingly lower in native than in invasive French populations. French populations also exhibited lower among-population differentiation. A significant pattern of isolation by distance was detected among North American populations but not among French populations. Assignment tests and distribution of rare alleles did not point to a single origin for all French populations, nor for all individuals within populations and private alleles from different North American populations were found in the same French populations. Indeed, within all French populations, individual plants were roughly equally assigned to the different North American populations. Altogether, these results suggest that the French invasive populations include plants from a mixture of sources. Reduced diversity in populations distant from the original area of introduction indicated that ragweed range expansion probably occurred through sequential bottlenecks from the original populations, and not from subsequent new introductions.     

Latitudinal trend in the reproductive mode of the pea aphid Acyrthosiphon pisum invading a wide climatic range

The maintenance of sexuality is a puzzling phenomenon in evolutionary biology. Many universal hypotheses have been proposed to explain the prevalence of sex despite its costs, but it has been hypothesized that sex could be also retained by lineage‐specific mechanisms that would confer some short‐term advantage. Aphids are good models to study the maintenance of sex because they exhibit coexistence of both sexual and asexual populations within the same species and because they invade a large variety of ecosystems. Sex in aphids is thought to be maintained because only sexually produced eggs can persist in cold climates, but whether sex is obligate or facultative depending on climatic conditions remains to be elucidated. In this study, we have inferred the reproductive mode of introduced populations of the pea aphid Acyrthosiphon pisum in Chile along a climatic gradient using phenotypic assays and genetic‐based criteria to test the ecological short‐term advantage of sex in cold environments. Our results showed a latitudinal trend in the reproductive mode of Chilean pea aphid population from obligate parthenogenesis in the north to an intermediate life cycle producing both parthenogenetic and sexual progeny in the southernmost locality, where harsh winters are usual. These findings are congruent with the hypothesis of the ecological short‐term advantage of sex in aphids.     

QTL mapping reveals a striking coincidence in the positions of genomic regions associated with adaptive variation in body size in parallel clines of Drosophila melanogaster on different continents

Latitudinal genetic clines in body size are common in many ectotherm species and are attributed to climatic adaptation. Here, we use Quantitative Trait Loci (QTL) mapping to identify genomic regions associated with adaptive variation in body size in natural populations of Drosophila melanogaster from extreme ends of a cline in South America. Our results show that there is a significant association between the positions of QTL with strong effects on wing area in South America and those previously reported in a QTL mapping study of Australian cline end populations (P < 0.05). In both continents, the right arm of the third chromosome is associated with QTL with the strongest effect on wing area. We also show that QTL peaks for wing area and thorax length are associated with the same genomic regions, indicating that the clinal variation in the body size traits may have a similar genetic basis. The consistency of the results found for the South American and Australian cline end populations indicate that the genetic basis of the two clines may be similar and future efforts to identify the genes producing the response to selection should be focused on the genomic regions highlighted by the present work.     

Maternal death relaxes developmental inhibition in nymphal aphid defenders

In certain aphids, first-instar nymphs defend their gall by attacking intruding arthropod predators. One correlate of such defensive behaviour is a lengthened duration of the first nymphal stadium during the galling phase of the life cycle. A prolonged first stadium allows a large army of first-instar defenders to accumulate, which may be advantageous for gall defence. The factors determining developmental delay have been unclear, however. Our field experiment with Pemphigus obesinymphae, a North American gall-forming aphid with defensive first-instar nymphs, tests whether first-stadium duration is influenced by the death of the colony''s fundatrix (mother). We killed fundatrices in certain galls, left those in control galls alive, and counted aphids in each stadium in each gall. Galls in which fundatrices were killed contained a lower proportion of first-instar defenders and more late-instar nymphs than did galls with living fundatrices, indicating that maternal death dramatically increased developmental rate of nymphs. Possibly nymphal aphids respond adaptively to environmental cues that signal a threat to the colony''s welfare. Alternatively, the fundatrix actively suppresses offspring development in order to maintain a large army of soldiers to protect her gall. The results add a new layer of complexity to our understanding of social aphid systems.     

An asexual strain of Lysiphlebus fabarum (Hymenoptera: Braconidae) gains fitness in superparasitised hosts,but a sexual strain pays costs

1. The fitness impacts of two levels of superparasitism were compared in a sexual and an asexual strain of Lysiphlebus fabarum (Marshall) developing in black bean aphids. 2. Asexual females were larger than sexuals, had higher eggs loads, and better adult emergence in two of three treatments, but parasitised fewer available aphids in 24 h (80% fewer when foraging alone). 3. Superparasitism resulted in wasps that were larger than those emerging from singly parasitised aphids, and asexual females had larger ovaries, without delayed development. In contrast, sexual wasps took about 2 days longer to develop in superparasitised hosts, and females had smaller ovaries with lower egg loads. 4. The growth of host aphids bearing asexual larvae was not reduced relative to healthy aphids, except when heavily superparasitised, whereas sexual larvae reduced aphid growth in all treatments. 5. Elimination of supernumerary larvae was virtually complete at 4.5 days after parasitism by sexual females, but no elimination occurred among asexual larvae. The evolution of diminished aggression in asexual larvae may be facilitated by greater genetic similarity, without necessarily leading to gregarious development, assuming the extended life of supernumeraries somehow improves survivor fitness. 6. Sons of virgin sexual females developed faster than sons of mated females, implicating parental effects that slowed development of the latter. 7. It is concluded that asexual females of this strain use superparasitism to improve host quality, and thus the fitness of their daughters, whereas females of the sexual strain prioritise offspring number.     

Density,parasitism, and sexual reproduction are strongly correlated in lake Daphnia populations

Many organisms can reproduce both asexually and sexually. For cyclical parthenogens, periods of asexual reproduction are punctuated by bouts of sexual reproduction, and the shift from asexual to sexual reproduction has large impacts on fitness and population dynamics. We studied populations of Daphnia dentifera to determine the amount of investment in sexual reproduction as well as the factors associated with variation in investment in sex. To do so, we tracked host density, infections by nine different parasites, and sexual reproduction in 15 lake populations of D. dentifera for 3 years. Sexual reproduction was seasonal, with male and ephippial female production beginning as early as late September and generally increasing through November. However, there was substantial variation in the prevalence of sexual individuals across populations, with some populations remaining entirely asexual throughout the study period and others shifting almost entirely to sexual females and males. We found strong relationships between density, prevalence of infection, parasite species richness, and sexual reproduction in these populations. However, strong collinearity between density, parasitism, and sexual reproduction means that further work will be required to disentangle the causal mechanisms underlying these relationships.     

Population genetic structure of sexual and parthenogenetic damselflies inferred from mitochondrial and nuclear markers

It has been postulated that obligate asexual lineages may persist in the long term if they escape from negative interactions with either sexual lineages or biological enemies; and thus, parthenogenetic populations will be more likely to occur in places that are difficult for sexuals to colonize, or those in which biological interactions are rare, such as islands or island-like habitats. Ischnura hastata is the only known example of natural parthenogenesis within the insect order Odonata, and it represents also a typical example of geographic parthenogenesis, as sexual populations are widely distributed in North America, whereas parthenogenetic populations of this species have only been found at the Azores archipelago. In order to gain insight in the origin and distribution of parthenogenetic I. hastata lineages, we have used microsatellites, mitochondrial and nuclear DNA sequence data, to examine the population genetic structure of this species over a wide geographic area. Our results suggest that sexual populations of I. hastata in North America conform to a large subdivided population that has gone through a recent spatial expansion. A recent single long distance dispersal event, followed by a demographic expansion, is the most parsimonious hypothesis explaining the origin of the parthenogenetic population of this species in the Azores islands.     

Shifting from clonal to sexual reproduction in aphids: physiological and developmental aspects

Developmental biology is one of the fastest growing and fascinating research fields in life sciences. Among the wide range of embryonic development, a fundamental difference exists between organisms with sexual or asexual development. Aphids are unusual organisms which display alternative pathways of sexual and asexual development, the orientation of the pathway being determined by environmental conditions. These insects offer an adapted system in which to study developmental plasticity, because a side-by-side comparison of sexual and asexual development can be made in individuals with the same genotype. In this review, we describe the developmental mechanisms that have evolved in aphids for alternative sexual and asexual reproduction. In particular, we discuss how environmental cues orientate the reproductive mode of aphids from signal perception to endocrine regulation, and propose a comparative analysis of sexual and asexual gametogenesis and embryogenesis, which has been possible due to the development of molecular methods. As a result of the recent development of genomic resources in aphids, we expect these species will permit major advances in the study of the genomic basis underlying the choice of developmental fate and multiple reproduction strategies.