Genome Compaction and Stability in Microsporidian Intracellular Parasites

Abstract

Microsporidian genomes are extraordinary among eukaryotes for their extreme reduction: although they are similar in form to other eukaryotic genomes, they are typically smaller than many prokaryotic genomes. At the same time, their rates of sequence evolution are among the highest for eukaryotic organisms. To explore the effects of compaction on nuclear genome evolution, we sequenced 685,000 bp of the Antonospora locustae genome (formerly Nosema locustae) and compared its organization with the recently completed genome of the human parasite Encephalitozoon cuniculi. Despite being very distantly related, the genomes of these two microsporidian species have retained an unexpected degree of synteny: 13% of genes are in the same context, and 30% of the genes were separated by a small number of short rearrangements. Microsporidian genomes are, therefore, paradoxically composed of rapidly evolving sequences harbored within a slowly evolving genome, although these two processes are sometimes considered to be coupled. Microsporidian genomes show that eukaryotic genomes (like genes) do not evolve in a clock-like fashion, and genome stability may result from compaction in addition to a lack of recombination, as has been traditionally thought to occur in bacterial and organelle genomes.

The microsporidian polar tube: A highly specialised invasion organelle

“…Both ptp1 and ptp2 are located on chromosome VI in E. cuniculi as a gene cluster, which has a conserved orientation and spacing in E. intestinalis and E. hellem ). Preservation of gene order has recently been described in studies of microsporidian genomes and may be due to constraints of evolution on these small eukaryotic genomes (Slamovits et al, 2004). All of the Encephalitozoonidae PTP2 s are basic proteins of about 30 kDa, with a maximal size difference of five residues (E. cuniculi PTP2 versus E. hellem PTP2) .…”

Shrink it or lose it: balancing loss of function with shrinking genomes in the microsporidia

“…Despite the differences detailed above, the overall gene content of diverse microsporidia is remarkably homogeneous. 7,8,11,13,18,30,31 Indeed, the overall degree of conservation in gene content serves to highlight what differences we do find. Nevertheless, these differences shuttle where they are absorbed by the alternative oxidase (AOX).…”

Comparative genomics of microsporidia

“…fr/externe/English/Projets/Projet_AD/AD.html) (Katin-ka et al 2001), a partial genome sequence for Antonospora locustae (formerly Nosema locustae) (jbpc. mbl.edu/Nosema/index.html), genome sequence surveys (GSS) from Spraguea lophii (jbpc.mbl.edu/Spraguea-HTML/) (Hinkle et al 1997), Vittaforma corneae (Mittleider et al 2002), and Antonospora locustae (botany.ubc.ca/keeling/AntonosporaGSS.html) (Slamovits et al 2004a), and an expressed sequence tag (EST) survey for A. locustae (amobidia.bcm.umontreal.ca/pepdb/ pep.php) (unpublished data).…”