Comparative genomics of Chlamydomonas

Abstract

Despite its fundamental role as a model organism in plant sciences, the green alga Chlamydomonas reinhardtii entirely lacks genomic resources for any closely related species, obstructing its development as a study system in several fields. We present highly contiguous and well-annotated genome assemblies for the two closest known relatives of the species, Chlamydomonas incerta and Chlamydomonas schloesseri, and a third more distantly related species, Edaphochlamys debaryana. We find the three Chlamydomonas genomes to be highly syntenous with similar gene contents, although the 129.2 Mb C. incerta and 130.2 Mb C. schloesseri assemblies are more repeat-rich than the 111.1 Mb C. reinhardtii genome. We identify the major centromeric repeat in C. reinhardtii as an L1 LINE transposable element homologous to Zepp (the centromeric repeat in Coccomyxa subellipsoidea) and infer that centromere locations and structure are likely conserved in C. incerta and C. schloesseri. We report extensive rearrangements, but limited gene turnover, between the minus mating-type loci of the Chlamydomonas species, potentially representing the early stages of mating-type haplotype reformation. We produce an 8-species whole-genome alignment of unicellular and multicellular volvocine algae and identify evolutionarily conserved elements in the C. reinhardtii genome. We find that short introns (<~100 bp) are extensively overlapped by conserved elements, and likely represent an important functional class of regulatory sequence in C. reinhardtii. In summary, these novel resources enable comparative genomics analyses to be performed for C. reinhardtii, significantly developing the analytical toolkit for this important model system.