INBREEDING DEPRESSION, INCREASED PHENOTYPIC VARIANCE, AND A TRADE-OFF BETWEEN GONADS AND APPENDAGES IN SELFED PROGENY OF THE APHID PROCIPHILUS ORIENS

Effects of Radiation From Contaminated Soil and Moss in Fukushima on Embryogenesis and Egg Hatching of the Aphid Prociphilus oriens

Radiation-contaminated soils are widespread around the Fukushima Daiichi Nuclear Power Plant, and such soils raise concerns over its harmful effect on soil-dwelling organisms. We evaluated the effects of contaminated soil and moss sampled in Fukushima on the embryogenesis and hatching of aphid eggs, along with the measurement of the egg exposure dose. Cs-137 concentration in soil and moss from Fukushima ranged from 2200 to 3300 Bq/g and from 64 to 105 Bq/g, respectively. Eggs of the eriosomatine aphid Prociphilus oriens that were collected from a non-contaminated area were directly placed on the soil and moss for 4 or 3 months during diapause and then incubated until hatching. The total exposure dose to the eggs was estimated as ca. 100-200 mGy in the 4-month soil experiment and 4-10 mGy in the 4-month moss experiment. There was no significant difference in egg hatchability between the contaminated soil treatment and the control. No morphological abnormalities were detected in the first instars that hatched from the contaminated soil treatment. However, we found weak effects of radiation on egg hatching; eggs placed on the contaminated moss hatched earlier than did the control eggs. On the contaminated soil, the effects of radiation on egg hatching were not obvious because of uncontrolled environmental differences among containers. The effects of radiation on egg hatching were detected only in containers where high hatchability was recorded. Through the experiments, we concluded that the aphid eggs responded to ultra-low-dose radiation by advancing embryogenesis.

Frequency-dependent selection acting on the widely fluctuating sex ratio of the aphid Prociphilus oriens

Frequency-dependent selection is a fundamental principle of adaptive sex ratio evolution in all sex ratio theories but has rarely been detected in the wild. Through long-term censuses, we confirmed large fluctuations in the population sex ratio of the aphid Prociphilus oriens and detected frequency-dependent selection acting on these fluctuations. Fluctuations in the population sex ratio were partly attributable to climatic factors during the growing season. Climatic factors likely affected the growth conditions of host plants, which in turn led to yearly fluctuations in maternal conditions and sex ratios. In the process of frequency-dependent selection, female proportion higher or lower than ca. 60% was associated with a reduction or increase in female proportion, respectively, the next year. The rearing of aphid clones in the laboratory indicated that mothers of each clone produced an increasing number of females as maternal size increased. However, the mean male number was not related to maternal size, but varied largely among clones. Given genetic variance in the ability to produce males among clones, selection should favour clones that can produce more numerous males in years with a high female proportion. Population-level sex allocation to females was on average 71%-73% for three localities and more female-biased when maternal conditions were better. This tendency was accounted for by the hypothesis of competition among foundresses rather than the hypothesis of local mate competition. We conclude that despite consistent operation of frequency-dependent selection, the sex ratio continues to fluctuate because environmental conditions always push it away from equilibrium.

Spatiotemporal fluctuations in natural selection acting on the gall-parasitic aphid Tetraneura sorini

The measurement of the selection gradient is crucial for understanding the magnitude of selection acting directly on a trait and predicting the evolutionary trajectory of that trait. This study evaluated the selection gradient acting on the morphology of the gall-parasitic aphid Tetraneura sorini during the galling process and compared the strength among populations. Gall formers (first instars) frequently fight with conspecifics or heterospecifics for usurping incipient galls using their well-developed hind legs. First instars that successfully acquired galls were found within galls, whereas those that failed were found dead on leaf surfaces. Selection gradients were estimated using logistic stepwise regression and partial least square (PLS) regression. Calculated selection differentials indicated that first instars that secured galls were larger in body size than failed individuals through all populations. However, selection gradients on weapon traits varied largely among populations or among years in the same population. We confirmed microevolutionary changes in the relationship between traits, which accorded with the expectation from changes in the selection gradients. When gall formers were transferred onto developing buds individually, individuals that successfully induced galls had smaller body size than failed individuals. Available evidence suggests that the selection gradient on body size becomes higher with an increasing proportion of T. sorini in the Tetraneura species community. Thus, we concluded that more intense fighting with conspecifics leads to stronger selective pressure on body size, but that selective pressure for each trait is variable depending on differences in the tactics and species composition among populations.

Morphological abnormalities in gall-forming aphids in a radiation-contaminated area near Fukushima Daiichi: selective impact of fallout?

To evaluate the impact of fallout from the Fukushima Daiichi Nuclear Power Plant accident on organisms, this study compared the morphology and viability of gall-forming aphids between the Fukushima population and control populations from noncontaminated areas. This study, in particular, focused on the morphology of first-instar gall formers derived from the first sexual reproduction after the accident. Of 164 first instars from Tetraneura sorini galls collected 32 km from Fukushima Daiichi in spring 2012, 13.2% exhibited morphological abnormalities, including four conspicuously malformed individuals (2.4%). In contrast, in seven control areas, first instars with abnormal morphology accounted for 0.0-5.1% (on average, 3.8%). The proportions of abnormalities and mortality were significantly higher in Fukushima than in the control areas. Similarly, of 134 first instars from T. nigriabdominalis galls, 5.9% exhibited morphological abnormalities, with one highly malformed individual. However, of 543 second-generation larvae produced in T. sorini galls, only 0.37% had abnormalities, suggesting that abnormalities found in the first generation were not inherited by the next generation. Although investigation is limited to one study site, this result suggests that radioactive contamination had deleterious effects on embryogenesis in eggs deposited on the bark surface, but a negligible influence on the second generation produced in closed galls. Furthermore, analysis of both species samples collected in spring 2013 indicated that the viability and healthiness of the aphids were significantly improved compared to those in the 2012 samples. Thus, the results of this study suggest the possibility that a reduced level of radiation and/or selection for radiation tolerance may have led to the improved viability and healthiness of the Fukushima population.

Host races of the pea aphid Acyrthosiphon pisum differ in male wing phenotypes

The evolution of reproductive isolation without geographic isolation (sympatric speciation) has recently gained strong theoretical and empirical supports. It is now widely admitted that many host-specific phytophagous insect species have arisen through shifting and adapting to new plants. The pea aphid Acyrthosiphon pisum has received considerable attention in this context and is now considered as a probable case of incipient sympatric speciation through host specialization. In Europe, three host races have been described so far, one on annual plants (pea and broad bean) and two on perennial plants (red clover and alfalfa, respectively). These host races are genetically differentiated and exhibit strong ecological specialization affecting their preferences and performances on alternative plants. Here, we investigate whether other life-history traits of ecological importance are associated with host specialization in the species. In particular, because A. pisum shows a genetically determined male wing variation, we tested if its host races also differ in their proportion of winged/wingless male phenotypes. We used a large collection of pea aphid lineages sampled on pea, broad bean, red clover and alfalfa and analyzed their male production by placing them in conditions inducing the sexual phase in A. pisum. Striking differences in the frequency of male dispersal genotypes were found between host populations; aphids producing winged males were in high proportion among lineages from annual hosts, while those producing wingless males were in high proportion on perennial ones. The evolutionary maintenance and ecological consequences of this association between habitat specialization and male wing variation are discussed.

Short-term population differences in the genetic architecture of life history traits related to sexuality in an aphid species

One of the most important factors that determine the evolutionary trajectory of a suite of traits in a population is the structure of the genetic variance-covariance matrix (G). We studied the cyclically parthenogenetic aphid Rhopalosiphum padi, whose populations exhibit two types of reproductive lineages respectively specialized in sexuality (that is, cyclically parthenogenetic lineages) and in asexuality (that is, obligate parthenogenetic lineages). We compared the quantitative genetics of life histories in these two lineage types. Our results suggest that both, the elements and the whole structure of the resulting G matrices differ in the very short term, between lineage types. This would involve the evolution toward different evolutionary optima in the same population, depending on whether sexual or asexual lineages predominate. Since sexual and asexual lineages vary seasonally in their abundance, a fluctuating selective regime has been proposed for this species, which would contribute to the maintenance of the reproductive polymorphism that these populations exhibit.