Investigating diversity, evolution, development and physiology of red algal parasites from New Zealand
Authors:   , Preuss Maren 
Published: December 2021

<p>Red algal parasites have evolved independently over a 100 times and grow only on other red algal hosts. Most parasites are closely related to their host based on the similarity of their reproductive structures. Secondary pit connections between red algal parasites and their hosts are used to transfer parasite organelles and nuclei into host cells. Morphological and physiological changes in infected host cells have been observed in some species. Parasite mitochondrial genomes are similar in size and gene content to free-living red algae whereas parasite plastids are highly reduced. Overall, red algal parasites are poorly studied and thus the aim of this study was to increase the general knowledge of parasitic taxa with respect to their diversity, evolutionary origin, development, physiology, and organelle evolution. Investigation of the primary literature showed that most species descriptions of red algal parasites were poor and did not meet the criteria for defining a parasitic relationship. This literature study also revealed a lack of knowledge of many key parasitic processes including early parasite development, host cell “control”, and parasite origin. Many of these poorly studied research areas were addressed in this thesis. Phylogenetic analyses, using a range of markers from all three genomes (cpDNA: rbcL, nDNA: actin, LSU rRNA; mtDNA: cox1), showed different patterns of phylogenetic relationships for the four new red algal parasites and their hosts. The parasites Phycodrys novae-zelandiophila sp. nov. and Vertebrata aterrimophila sp. nov. closest relative is its host species. Cladhymenia oblongifoliophila sp. nov. closest relative is its host species based on nuclear and mitochondrial markers whereas the plastid markers group the parasite with Cladhymenia lyallii, suggesting that the parasite plastid was acquired when previously parasitizing C. lyallii. Judithia parasitica sp. nov. grows on two Blastophyllis species but the parasites’ closest relative is the non-host species Judithia delicatissima. Developmental studies of the parasite Vertebrata aterrimophila, showed a unique developmental structure (“trunk-like” cell) not known in other parasites, plus localised infection vi and few changes in infected host cells. High-throughput-sequencing revealed mitochondrial genomes of similar size, gene content and order in the parasite Pterocladiophila hemisphaerica to its host Pterocladia lucida, and a reduced non-photosynthetic plastid in the parasite. Mitochondrial (mt) and plastid (cp) genome phylogenies placed Pterocladiophila hemisphaerica on long branches, either as sister to Ceramiales (mt) or Gracilariales (cp). Further analyses, filtering non-elevated plastid genes grouped the parasite neither with the Gracilariales (mt) or Gelidiales (cp) on shorter branches but without support. Nuclear phylogeny grouped P. hemisphaerica as sister to the Gelidiales and other red algal orders and was the only phylogenetic relationship with support. Investigations of photosystem II capacity using PAM fluorometry, and quantifying chlorophyll a content in three pigmented parasites, showed different host nutrient dependencies. Rhodophyllis parasitica and Vertebrata aterrimophila are not able to photosynthesize and are fully dependent on host nutrients. Pterocladiophila hemisphaerica is able to photosynthesize independently, even though it has a reduced non-photosynthetic plastid genome, and therefore is only partially dependent on its host. This study advances our current understanding of red algal parasites and highlights many possibilities for future research including genome evolution and understanding parasite diversity.</p>