Why Sequence Guillardia theta and Bigelowiella natans?

Scanning electron micrographs of Guillardia theta (left) and Bigelowiella natans. Images courtesy of Dr. Geoff McFadden (University of Melbourne, Australia).

The process of endosymbiosis, where one organism lives inside another, has been a monumental force in the origin and diversification of eukaryotic life. The primary endosymbiotic origin of plastids (chloroplasts) occurred more than a billion years ago and spawned three lineages--the green algae (and their land plant relatives), red algae, and glaucophytes--whose energy-generating capabilities paved the way for a transformation of the biosphere. The photosynthetic organelles of red and green algae have spread to unrelated eukaryotes by secondary endosymbiosis--the engulfment and retention of an algal cell inside a nonphotosynthetic host. Secondary endosymbiosis has given rise to some of the most abundant and ecologically significant aquatic photosynthesizers on the planet, including the heterokonts (e.g., diatoms and giant kelp), haptophytes (e.g., Emiliania), and the 'red tide'- causing dinoflagellate algae, as well as a variety of eukaryotic microbes of critical importance to human health (e.g., the malaria parasite Plasmodium). Despite its obvious significance, very little is known about the process of secondary endosymbiosis and its impact on the molecular and cell biology of secondary plastid-containing algae.

The goal of this proposal is to sequence the nuclear genomes of two microbial eukaryotes of pivotal evolutionary and cell biological significance, the cryptomonad Guillardia theta and the chlorarachniophyte Bigelowiella natans. These organisms are unique among secondary plastid-containing algae in that they still possess the nucleus (nucleomorph) and cytoplasm of their algal endosymbionts in a highly reduced and simplified form. Despite striking similarities in the size and structure of their nucleomorph genomes, the cryptomonads and chlorarachniophytes are the product of independent secondary endosymbiotic events involving different endosymbionts (red and green algae, respectively) and unrelated eukaryotic host cells. The limited coding capacity of cryptomonad and chlorarachniophyte nucleomorphs and plastids indicates that their nuclear genomes have been repositories for thousands of endosymbiont-derived genes throughout their evolutionary history. Comparing and contrasting these sequences will provide an unprecedented window into the process of secondary endosymbiosis and the integration of their respective hosts and endosymbionts at the genetic, biochemical, and cellular levels. These genome sequences will also provide critical insight into the evolutionary origins of two of the six currently recognized 'supergroups' of eukaryotes, the Chromalveolates and Rhizaria, to which the cryptomonads and chlorarachniophytes belong.

Principal Investigators: John M. Archibald (Dalhousie Univ.)