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Novel Nucleomorph Genome Architecture in the Cryptomonad Genus Hemiselmis CHRISTOPHER E. LANE and JOHN M. ARCHIBALD
 

Summary: Novel Nucleomorph Genome Architecture in the Cryptomonad Genus Hemiselmis
CHRISTOPHER E. LANE and JOHN M. ARCHIBALD
Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, NS B3H 1X5, Canada
ABSTRACT. Cryptomonads are ubiquitous aquatic unicellular eukaryotes that acquired photosynthesis through the uptake and retention
of a red algal endosymbiont. The nuclear genome of the red alga persists in a highly reduced form termed a nucleomorph. The nucleo-
morph genome of the model cryptomonad Guillardia theta has been completely sequenced and is a mere 551 kilobases (kb) in size, spread
over three chromosomes. The presence of three chromosomes appears to be a universal characteristic of nucleomorph genomes in
cryptomonad algae as well as in the chlorarachniophytes, an unrelated algal lineage with a nucleomorph and plastid genome derived from
a green algal endosymbiont. Another feature of nucleomorph genomes in all cryptomonads and chlorarachniophytes examined thus far is
the presence of subtelomeric ribosomal DNA (rDNA) repeats at the ends of each chromosome. Here we describe the first exception to this
canonical nucleomorph genome architecture in the cryptomonad Hemiselmis rufescens CCMP644. Using pulsed-field gel electrophoresis
(PFGE), we estimate the size of the H. rufescens nucleomorph genome to be $ 580 kb, slightly larger than the G. theta genome. Unlike
the situation in G. theta and all other known cryptomonads, sub-telomeric repeats of the rDNA cistron appear to be absent on both ends of
the second largest chromosome in H. rufescens and two other members of this genus. Southern hybridizations using a variety of nucleo-
morph protein gene probes against PFGE-separated H. rufescens chromosomes indicate that recombination has been a major factor in
shaping the karyotype and genomic structure of cryptomonad nucleomorphs.
Key Words. Cryptophyceae, genome evolution, genome rearrangement, genome reduction, secondary endosymbiosis.
THE process of endosymbiosis has had a profound impact on
the evolution of photosynthetic life on Earth. The primary
endosymbiotic origin of plastids (chloroplasts), whereby a cyano-

  

Source: Archibald, John - Department of Biochemistry and Molecular Biology, Dalhousie University

 

Collections: Biology and Medicine