Characterisation of Red Algal Parasite Mitochondria from Aotearoa

Red algal parasites have evolved over 120 times in the class Florideophyceae and are often closely related to their red algal hosts. Only four mitochondrial genomes of parasitic red algae have been sequenced and they are similar in size and are identical in gene order and content as free-living red algae. The mitochondrial genome of Pterocladiophila hemisphaerica, has shown evidence of elevated substitution rates, which may be an indication of early mitochondrial genome evolution. It is unknown if these observed patterns are present in other red algal parasites. This study assembled and annotated the mitochondrial genomes belonging to 11 red algal parasites and their hosts from Aotearoa. Generally, the mitochondrial genomes of these parasites were conserved in their genome size, gene content, and gene order when compared to their closest free-living red algal relative. Multi-gene phylogenetic analysis supports a continuum of relatedness between parasites and their hosts, with most parasites residing in different genera within the same family as their host. One parasite, Choreonema thuretii (Hapalidiales), resides in a different order to its host. Substitution rate (dN/dS) data was calculated for all shared mitochondrial genes. All red algal parasites included showed higher substitution rates when compared to free-living relative. This large comparison shows that mitochondrial genome architecture is highly conserved among parasitic red algae and has shown for the second time, evidence of increased selection in some mitochondrial genes, suggesting early mitochondrial genome evolution. This study sheds light on mitochondrial genome evolution in red algal parasites and uncovers knowledge of the high red algal parasite diversity in Aotearoa.