When we consider the strangeness of life, we tend to focus on the macroscopic oddities, like the Yeti crab and amphibians with tentacles on their heads. But there's an exceedingly strange world that operates on the biochemical level in many of the single celled eukaryotes, one that violates a lot of what we think we know about the tidy goings-on inside a cell.

Those of you who caught a report on the cilliate Tetrahymena a while back read about one example of this strangeness. These organisms actually run around with two genomes. One of these is used for mating, while the second is a copy of the first after it's chopped into pieces, pared down, and then amplified up to about 50 copies. Today's Nature contains a description of another cilliate, Oxytricha trifallax, that takes this genome processing to excess: it actually rearranges pieces of the mating chromosomes in order to construct its non-mating genome.

This raises a big question: how do these cells know which pieces to put where while the reassembly of its primary genome is taking place? The non-mating DNA can't be used, because one of the early steps in mating involves digesting it. The results in Nature suggest an unexpected answer: it uses chromosome-sized RNA molecules as templates for the rearrangements.

The research team came to this conclusion based on a number of lines of evidence. They were able to detect large, double-stranded RNAs, much larger than the normal messenger RNAs, in the organism during mating. They also found that interfering RNAs that targeted post-rearrangement sequences for destruction would cause the rearrangements to produce a variety of aberrant products. Finally, injecting RNA that had unusual rearrangement patterns was able to direct the reassembly of indentically unusual chromosomes after mating. All of this evidence points to large RNAs having a key role in directing the reconstruction of the genome.

The authors conclude that the most likely scenario is that one of the first steps in the mating process is to create RNA copies of all the chromosomes in the non-mating genome, after which that genome is destroyed. These RNA molecules are used by enzymes to order the DNA segments that are produced from fragmented copies of the mating genome, which are then linked to form the adult chromosome. Their conclusion helps reinforce one of my long-standing conclusions: that these complex, single celled organisms have a wealth of unusual biology that we're only beginning to understand.

Nature, 2007. DOI: 10.1038/nature06452