Mechanistically informed predictions of binding modes for carbocation intermediates of a sesquiterpene synthase reaction.

Sesquiterpenoids comprise a class of terpenoid natural products with thousands of compounds that are highly diverse in structure, generally containing a polycyclic carbon backbone that is constructed by a sesquiterpene synthase. Decades of experimental and computational studies have demonstrated that these enzymes generate a carbocation in the active site, which undergoes a series of structural rearrangements until a product is formed via deprotonation or nucleophile attack. However, for the vast majority of these enzymes the productive binding orientation of the intermediate carbocations has remained unclear. In this work, a method that combines quantum mechanics and computational docking is used to generate an all-atom model of every putative intermediate formed in the context of the enzyme active site for tobacco epi-aristolochene synthase (TEAS). This method identifies a single pathway that links the first intermediate to the last, enabling us to propose the first high-resolution model for the reaction intermediates in the active site of TEAS, and providing testable predictions

Tardigrade remains from lake sediments

Remains of tardigrades have rarely been reported to preserve in sediments, resulting in the absence of important ecological and biogeographic information that they could provide. However, a study of faunal microfossils in Antarctic lake sediment cores has shown that tardigrade eggs and occasionally exuvia can be abundant. Eggs from at least five tardigrade species were identified in sediment cores from six lakes from across the continent, with abundances up to 6,000 (g(-1) dry wt.). It is likely that the cold temperatures and absence of benthic grazers in Antarctic lakes results in particularly good preservation conditions, though it may also be a function of population density. The conservation of tardigrade eggs and exuvia in lake sediments enables a better understanding of paleodistributions and effects of environmental changes for this phylum that cannot otherwise be obtained

Life history traits and reproductive mode of the tardigrade Acutuncus antarcticus under laboratory conditions: strategies to colonize the Antarctic environment

Global climate change has become an important issue, particularly for organisms living in the Antarctic region, as the predicted temperature increase can affect their life history traits. The reproductive mode and life history traits of one of the most widespread species of tardigrades in Antarctica were analyzed. Specimens of the eutardigrade Acutuncus antarcticus from a temporary freshwater pond at Victoria Land (Antarctica) were individually cultured. This species reproduced continuously by thelytokous meiotic parthenogenesis. Its life cycle was short (60–90 days) and the reproductive output was low, with a short generation time (25–26 days). The maternal effect can be responsible of the phenotypic plasticity observed in life history traits of the three analyzed generations that may be seen as a bethedging strategy, as also observed in other animals inhabiting stochastic environments. These traits, along with the cryptobiotic capability of A. antarcticus, are advantageous for exploiting the conditions suitable for growth and reproduction during the short Antarctic summer, and can explain its wide distribution on the Antarctic continent. These results open new avenues of research for determining the role of bet-hedging strategy in organisms living in unpredictable environments

Environmental Adaptations:Encystment and Cyclomorphosis

Stressful environmental conditions generally limit animal survival, growth, and reproduction and may induce dormancy in the form of various resting stages. Tardigrades represent one of a few animal phyla in which different forms of dormancy are frequently encountered. One of these forms, cryptobiosis, a quick response to sudden changes in the environment, has gained a great deal of attention, whereas much less is known of the slower emerging form of dormancy, diapause. In this review we present the current knowledge of diapause in tardigrades. Diapause in tardigrades, represented by encystement and cyclomorphosis, is likely controlled by exogenous stimuli, such as temperature and oxygen tension, and perhaps also by endogenous stimuli. These stimuli initiate and direct successive phases of deep morphological transformations within the individual. Encystment is characterized by tardigrades that lie dormant\u2014in diapause\u2014within retained cuticular coats (exuvia). The ability to form cysts is likely widespread but presently only confirmed for a limited number of species. In tardigrades, cyclomorphosis was first reported as a characteristic of the marine eutardigrade genus Halobiotus. This phenomenon is characterized by pronounced seasonal morphological changes and in Halobiotus involves stages with an extra protecting cuticle. Cyst formation in moss-dwelling limnic species may also occur as part of a seasonal cyclic event and can thus be viewed as part of a cyclomorphosis. Therefore, whereas diapause generally seems to be an optional response to environmental changes, it may also be an obligate part of the life cycle. The evolution of encystment and cyclomorphosis finds its starting point in the molting process. Both phenomena represent an adaptation to environmental constraints. Notably, the evolution of diapause is not necessarily an alternative to cryptobiosis, and some tardigrades may enter both forms of dormancy. The simultaneous occurrence of several adaptive strategies within tardigrades has largely increased the resistance of these enigmatic animals toward extreme environmental stress