McGill, Lorraine M.
A Molecular and Phylogenetic
Analysis of Cryobiosis in Nematodes
of the Genus Panagrolaimus.
PhD thesis, National University of Ireland Maynooth.
Many organisms are able to survive freezing temperatures through the development of
biochemical and physiological adaptations. These biochemical adaptations may
include the synthesis of proteins such as antifreeze proteins or cryoprotectants such as
trehalose, the elimination of ice nucleators, and the expression of stress associated
proteins (such as molecular chaperones, antioxidants, late embryogenesis abundant
(LEA) proteins). Physiological adaptations include the ability to undergo
cryoprotective dehydration. The molecular mechanisms underlying freezing stress
tolerance are poorly understood. One of the main aims of this project was to employ
phylogenetic, proteomic, and transcriptomic approaches to gain insights into the
adaptations that aid the survival of the free- living cryotolerant nematode
Panagrolaimus sp. from temperate, subpolar, polar and continental geographic
regions show a range of freezing ability from strains that show high survival upon
direct exposure to -80 oC to those that are freezing sensitive. Acclimation significantly
improves the freezing survival of temperate, subpolar, polar and continental
Panagrolaimus strains and species. They also undergo anhydrobiosis and a
correlation exists between desiccation tolerance and freezing tolerance. A
phylogenetic study did not find a relationship between freezing phenotype,
biogeography and phylogeny. The freezing and desiccation tolerance of ten new
tropical Panagrolaimus strains was investigated. These strains are desiccation tolerant
but not freezing tolerant, suggesting that freezing survival requires some specialised
adaptations. A phylogenetic study of all the Panagrolaimus strains used in this study
showed that the desiccation tolerant tropical Panagrolaimus strains are more
divergent from the other strains and species in this study.
Protein extracts from freezing tolerant Panagrolaimus sp. can inhibit the growth of
ice along specific planes of an ice crystal, resulting in hexagonal bipyrimidal ice
crystals. This ice faceting capacity was considered most likely to be due to the
presence of ice binding proteins. An ice affinity purification method was implemented
to purify ice-binding proteins from P. superbus. Several proteins found to be enriched
in the ice fraction were identified by mass spectrometry. As none of the identified
proteins was an obvious ice binding protein, it was not possible to determine whether
these proteins had ice-binding protein activity.
The divergence times for five Panagrolaimus strains and species were estimated
using the relaxed molecular clock approach. The Panagrolaimus sp. were found to
have diverged from other nematodes 70.12 million years ago. The Antarctic nematode
P. davidi diverged from its Californian sister species PS1579 approximately 17.18
million years ago and the Arctic nematode P. superbus diverged from its
Pennsylvanian sister species Panagrolaimus sp. AF36 9.97 million years ago.
The genes that are differentially expressed in response to a period of cold acclimation
were determined using the next generation sequencing method RNA-seq. A large
number of novel genes were significantly up-regulated (P-value <0.01) including
those involved in the oxidative stress response, transporting, membrane modification,
metabolism, signalling and cytoskeleton remodelling.
||Molecular; Phylogenetic Analysis; Cryobiosis; Nematodes; Genus Panagrolaimus;
||Science & Engineering > Biology
||27 Sep 2012 11:07
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