Vesicles made with phospholipids containing two double bonds captured DNA twice as effectively as those with rigid membranes after repeated freeze-thaw cycles. This fusion process, driven by ice crystal formation, allowed separate compartments to mix their contents on an early Earth where organic molecules were scattered.
The earliest cell-like structures were simple lipid bubbles enclosing basic organic molecules. Modern cells are intricate systems with internal scaffolding and genetic instructions. The transition from one to the other remains a central question.
Researchers built model protocells using three types of phospholipids: POPC, PLPC, and DOPC. POPC forms rigid membranes, while PLPC and DOPC, with more unsaturated bonds, create more fluid membranes.
Tatsuya Shinoda, a doctoral student at the Earth-Life Science Institute (ELSI) and lead author:
"We used phosphatidylcholine (PC) as membrane components, owing to their chemical structural continuity with modern cells, potential availability under prebiotic conditions, and retaining ability of essential contents."
After three freeze-thaw cycles, vesicles rich in POPC clustered without merging. Those containing PLPC or DOPC fused into larger compartments. The more PLPC present, the more likely fusion occurred.
Natsumi Noda, researcher at ELSI:
"Under the stresses of ice crystal formation, membranes can become destabilized or fragmented, requiring structural reorganization upon thawing. The loosely packed lateral organization due to the higher degree of unsaturation may expose more hydrophobic regions during membrane reconstruction, facilitating interactions with adjacent vesicles and making fusion energetically favorable."
This fusion allowed the contents of separate compartments to mix, potentially bringing key ingredients together for chemical reactions. PLPC vesicles were better at trapping DNA than POPC vesicles even before the cycles, and continued to hold more DNA afterward.
The study suggests icy environments, with repeated freeze-thaw cycles over long periods, could have concentrated molecules and promoted vesicle fusion. However, fluid membranes that support fusion can also become unstable and leak during freeze-thaw stress.
For early protocells, maintaining a balance between stability and permeability was crucial. The most successful membrane compositions likely depended on environmental conditions.
Tomoaki Matsuura, Professor at ELSI and principal investigator:
"A recursive selection of F/T-induced grown vesicles across successive generations may be realized by integrating fission mechanisms such as osmotic pressure or mechanical shear. With increasing molecular complexity, the intravesicular system, i.e., gene-encoded function, ultimately may take over the protocellular fitness, consequently leading to the emergence of a primordial cell capable of Darwinian evolution."
Simple physical processes like freezing and thawing may have guided the transition from basic molecular compartments to the first evolving cells.