Anhydrobiosis and Freezing-Tolerance: Adaptations That Facilitate the Establishment of Panagrolaimus Nematodes in Polar Habitats

Anhydrobiotic animals can survive the loss of both free and bound water from their cells. While in this state they are also resistant to freezing. This physiology adapts anhydrobiotes to harsh environments and it aids their dispersal. Panagrolaimus davidi, a bacterial feeding anhydrobiotic nematode isolated from Ross Island Antarctica, can survive intracellular ice formation when fully hydrated. A capacity to survive freezing while fully hydrated has also been observed in some other Antarctic nematodes. We experimentally determined the anhydrobiotic and freezing-tolerance phenotypes of 24 Panagrolaimus strains from tropical, temperate, continental and polar habitats and we analysed their phylogenetic relationships. We found that several other Panagrolaimus isolates can also survive freezing when fully hydrated and that tissue extracts from these freezing-tolerant nematodes can inhibit the growth of ice crystals. We show that P. davidi belongs to a clade of anhydrobiotic and freezing-tolerant panagrolaimids containing strains from temperate and continental regions and that P. superbus, an early colonizer at Surtsey island, Iceland after its volcanic formation, is closely related to a species from Pennsylvania, USA. Ancestral state reconstructions show that anhydrobiosis evolved deep in the phylogeny of Panagrolaimus. The early-diverging Panagrolaimus lineages are strongly anhydrobiotic but weakly freezing-tolerant, suggesting that freezing tolerance is most likely a derived trait. The common ancestors of the davidi and the superbus clades were anhydrobiotic and also possessed robust freezing tolerance, along with a capacity to inhibit the growth and recrystallization of ice crystals. Unlike other endemic Antarctic nematodes, the life history traits of P. davidi do not show evidence of an evolved response to polar conditions. Thus we suggest that the colonization of Antarctica by P. davidi and of Surtsey by P. superbus may be examples of recent “ecological fitting” of freezing-tolerant anhydrobiotic propagules to the respective abiotic conditions in Ross Island and Surtsey.     

River health assessment in a large river: Bioindicators of fish population

Large river systems are some of the most important aquatic resource for human beings. Unfortunately, ecological integrity of large rivers has been altered severely by human activities. To monitor and manage these ecosystems, a large number of fish-based indexes, especially at community-level have been developed in the past decades. However, community-level methods have presented unique challenges in large river assessment. Comparatively, the population-level method could not only avoid the challenges, but also show many other advantages. Therefore, the aim of this paper is to develop and validate a population-based method applicable for large river. The present study was carried out at the Dongjiang River, with Xenocypris davidi and Hemibarbus labeo being selected as the sentinel indicators. In developing our method we sought population metrics, including growth, survival and reproduction related, to reflect ecological characteristics. Candidate biological metrics were screened for stability, responsiveness and redundancy. The metrics of standard length, average age and fecundity were finally selected for X. davidi, and standard length, weight, average age, and gonadosomatic index for H. labeo. Through the relationship between biological traits and catchments human disturbance information, Xunwu and Anyuan were ultimately chosen as the reference sites for X. davidi and H. labeo, respectively. The star plot areas, representing the degree of similarity with the reference conditions, showed a large variation among different sampling sites, ranging from 0.15 to 0.94 for X. davidi and 0.13 to 1.91 for H. labeo. It was indicated that all sampling sites except for XW could be assigned to the “impaired”. The publicly available census data were used as independent data set to validate our method. The high agreement of both methods suggested that our method could provide an accurate measure for river ecosystem condition. We believe our methodology would be helpful for water resource management of large river. The methodology established here may also serve as a reference of how to develop a population-based index for similar assessments in other large river systems.     

Molecular evolution in Panagrolaimus nematodes: origins of parthenogenesis, hermaphroditism and the Antarctic species P. davidi

Background  

As exemplified by the famously successful model organism Caenorhabditis elegans, nematodes offer outstanding animal systems for investigating diverse biological phenomena due to their small genome sizes, short generation times and ease of laboratory maintenance. Nematodes in the genus Panagrolaimus have served in comparative development and anhydrobiosis studies, and the Antarctic species P. davidi offers a powerful paradigm for understanding the biological mechanisms of extreme cold tolerance. Panagrolaimus nematodes are also unique in that examples of gonochoristic, hermaphroditic and parthenogenetic reproductive modes have been reported for members of this genus. The evolutionary origins of these varying reproductive modes and the Antarctic species P. davidi, however, remain enigmatic.     

The ability of the Antarctic nematode Panagrolaimus davidi to survive intracellular freezing is dependent upon nutritional status

The Antarctic nematode Panagrolaimus davidi is the best documented example of an animal surviving intracellular freezing and the only animal so far shown to survive such freezing throughout its tissues. However, a recent study found that after exposure to a freezing stress that produced intracellular freezing in a proportion of nematodes, the resulting survival levels could be explained if those nematodes that froze intracellularly had died. We have thus re-examined the survival of intracellular freezing in this nematode. The ability to survive a freezing exposure that is likely to produce intracellular freezing (freezing at ?10 °C) declines with culture age. In cultures that are fed regularly, the ability to survive freezing at ?10 °C increases, but in starved cultures freezing survival declines. Survival of intracellular freezing in fed cultures was confirmed using cryomicroscopy, staining of cells with vital dyes and by freeze substitution and transmission electron microscopy. We have thus confirmed that P. davidi can survive intracellular freezing and shown that this ability is dependent upon them being well fed. The effect of culture conditions on the nutrient status of the nematodes should thus be an important factor in the design of experiments.