Nucleomorphs and Lateral Gene Transfer

Sharing of DNA between and across species, genus, family, and other lines can happen in many ways. I'm reminded of this by a forthcoming PNAS paper (Lane et al., below) that characterizes the genome of a nucleomorph found in the cryptophyte Hemiselmis andersenii. Nucleomorphs are small DNA-containing nuclei found in the plastids of certain cryptomonads (flagellated unicellular plants). They are thought to represent the remnants of ancient endosymbionts.

The authors of the PNAS paper explain: "The nucleomorphs of cryptophytes and chlorarachniophytes are derived from red and green algal endosymbionts, respectively, and represent a stunning example of convergent evolution: their genomes have independently been reduced and compacted to under one megabase pairs (Mbp) in size." The authors found that the two nucleomorph genomes they studied encoded no introns. Moreover, proteins encoded by nucleomorph DNA "are significantly smaller than those in their free-living algal ancestors."

I think a larger point that bears remembering here is that unicellular plants have no business having flagella in the first place. Not to put too fine a point on it, but: The existence of something like Hemiselmis andersenii is not easily explained in evolutionary terms without invoking a theory of lateral gene transfer.

Lane et al., "Nucleomorph genome of Hemiselmis andersenii reveals complete intron loss and compaction as a driver of protein structure and function" in PNAS, December 6, 2007, 10.1073/pnas.0707419104.