Phylogenomics and the rise of the angiosperms

Angiosperms are the cornerstone of most terrestrial ecosystems and human livelihoods1,2. A robust understanding of angiosperm evolution is required to explain their rise to ecological dominance. So far, the angiosperm tree of life has been determined primarily by means of analyses of the plastid genome3,4. Many studies have drawn on this foundational work, such as classification and first insights into angiosperm diversification since their Mesozoic origins5,6,7. However, the limited and biased sampling of both taxa and genomes undermines confidence in the tree and its implications. Here, we build the tree of life for almost 8,000 (about 60%) angiosperm genera using a standardized set of 353 nuclear genes8. This 15-fold increase in genus-level sampling relative to comparable nuclear studies9 provides a critical test of earlier results and brings notable change to key groups, especially in rosids, while substantiating many previously predicted relationships. Scaling this tree to time using 200 fossils, we discovered that early angiosperm evolution was characterized by high gene tree conflict and explosive diversification, giving rise to more than 80% of extant angiosperm orders. Steady diversification ensued through the remaining Mesozoic Era until rates resurged in the Cenozoic Era, concurrent with decreasing global temperatures and tightly linked with gene tree conflict. Taken together, our extensive sampling combined with advanced phylogenomic methods shows the deep history and full complexity in the evolution of a megadiverse clade

Low summer temperatures: a potential mortality factor for high arctic soil microarthropods?

Throughout the summers of 1992–1994 the low temperature performance of soil microarthropods at Ny Ålesund, Spitsbergen (78 °56′N 10 °53′E), was investigated. Species studied were the Collembola Hypogastrura tullbergi (Schäffer), Onychiurus arcticus (Tullberg) and Onychiurus groenlandicus (Tullberg) and the mites Diapterobates notatus (Thorell), Hermannia reticulata (Thorell), Camisia anomia Colloff and Ceratoppia hoeli (Thor). The results show that: (i) The supercooling ability of these animals decreased rapidly on regaining activity in spring. For example, the supercooling point (scp) of H. tullbergi when heat extracted from frozen ground, decreased from −20 to −8 °C within 4 h. Population scp profiles of all species determined throughout the summer showed distinct bimodal distribution; (ii) starvation for 14 days, desiccation or a combination of both, resulted in little change in the mean scp of the collembolan O. arcticus; (iii) survival of the animals after a brief exposure to a sub-zero temperature was poor, in either humid or dry atmospheres. For example, 77% of H. tullbergi died after cooling to −5 °C at 1 °C min−1. Comparison with scp data indicates that animals died before they froze; (iv) all species examined showed some locomotory ability at temperatures approaching −3 °C; (v) polyols occurred in low concentrations, although elevated levels of glucose were observed in early spring and late autumn in O. arcticus; and (vi) soil temperature declined to −29.6 °C in the winter of 1992/93 and remained below zero for up to 289 days and the animals can be encased in ice for 75% of the year. Average daily soil temperatures for July and August rarely exceed 8 °C and were typically in the range 3–6 °C. Estimation of previous years soil temperatures from screen temperature records indicate that July /August ground surface temperatures < 0 °C occurred on 25 and 28 occasions between 1969–1993 at the polar semi-desert and tundra heath sites respectively; but, that soil temperatures at a depth of 3 cm are buffered against temperature extremes and temperatures below 0 °C are rarely encountered. The consequences for the soil microarthropod fauna of such extended periods of low temperature and the effects of climate change on these species are discussed

Thermal adaptation in the Arctic collembolan Onychiurus arcticus (Tullberg)

Ecophysiological characteristics, including survival at high and low temperatures, locomotory activity at sub-zero temperatures, supercooling ability and oxygen consumption rates, were investigated for the Arctic springtail Onychiurus arcticus (Tullberg) (Collembola, Onychiuridae). Individuals had a mean (± SE) fresh weight of 428.2±107.6 μg which contained 74.0±10.2% body water. Survival at high temperatures was humidity dependent. After 3h exposure at 100% relative humidity and 30°C, >80% of the animals survived, but at >32.5°C no individual survived. 70% of the animals survived a 1 h exposure at 32.5°C but at 35.0°C all animals died. At 0% relative humidity there were no survivors after 3 h at >25.0°C. At sub-zero temperatures, 60% of the springtails survived for 84 days at −3.0°C, but at −5.0°C survival was reduced to 35%. Individual collembolans showed locomotor activity down to −4°C. O. arcticus was freezing-intolerant and individuals supercooled to −6.1±0.1°C before freezing. This relatively high mean (±SE) supercooling point was stable throughout summer and was unaffected by acclimation temperature. A non-linear relationship existed between oxygen consumption and temperature. Between 0 and 10°C the Q10 was high at 7.0. It declined to 1.6 over the temperature range 10 to 30°C, increasing to 5.8 at higher temperatures. O. arcticus possesses ecophysiological characteristics suited to life in the upper layer of soil and surface vegetation, and beneath snow cover. However, it appears to be poorly adapted to survive severe winter temperatures being intolerant of freezing and with little supercooling ability. Such features may restrict its present distribution in the Arctic, but it seems likely that it would benefit by an increase in environmental temperature

Effects of temperature elevation on a field population of Acyrthosiphon svalbardicum (Hemiptera: Aphididae) on Spitsbergen

A manipulation experiment was carried out on a field population of the aphid Acyrthosiphon svalbardicum near Ny Ålesund, on the high arctic island of Spitsbergen, using cloches to raise temperature. An average rise in temperature of 2.8 deg. C over the summer season markedly advanced the phenology of both the host plant Dryas octopetala and the aphid. Advanced aphid phenology, with concomitant increases in reproductive output and survival, and successful completion of the life-cycle led to an eleven-fold increase in the number of overwintering eggs. Thermal budget requirements in day degrees above 0°C were calculated for key life-cycle stages of the aphid. Temperature data from Ny Ålesund over the past 23 years were used to calculate thermal budgets for the field site over the same period and these were compared with the requirements of the aphid. Each estimated thermal budget was then adjusted to simulate the effect of a +2, +4, and −2deg. C change in average temperature on aphid performance. This retrospective analysis (i) confirms that the life-cycle of A. svalbardicum is well suited to exploit higher summer temperatures, (ii) indicates that the annual success of local populations are sensitive to small changes in temperature and (iii) suggests that the aphid is living at the limits of its thermal range at Ny Ålesund based on its summer thermal budget requirements

Extreme adaptive life-cycle in a high arctic aphid, Acyrthosiphon svalbardicum

1 The year-round biology of a high arctic aphid is described for the first time. 2 The life-cycle is shown to be genetically determined, and thus markedly different to temperate species where the observed polymorphism is governed primarily by external environmental cues. 3 The fundatrix, which emerges from the overwintering egg, gives birth directly to sexual morphs, a phenomenon previously undescribed in the Aphidinae. This process is essentially prevented in temperate aphids by an endogenous mechanism, the interval timer. 4 In addition to the sexual morphs, the fundatrix produces a small number of parthenogenetic individuals (viviparae) that give rise to a third generation. This last generation consists exclusively of oviparae and males that would increase the number of overwintering eggs provided there is sufficient thermal budget for them to mature and oviposit before conditions become adverse. 5 The position of particular morphs in the birth sequences of the second and third generations maximize the chances of survival in harsh conditions, whilst enhancing the likelihood that individuals from the third generation will add to the number of overwintering eggs. 6 Guaranteed egg production combined with an in-built flexibility to produce an extra generation in particularly favourable seasons, confer adaptations to the high arctic environment, and ideally suit this aphid to exploit elevated temperatures in an era of climate change