Drop in the Ocean: Molecular Approaches for Exploring the Diversity and Distribution of the Ascetosporea

Abstract

The protistan class Ascetosporea (Rhizaria, Endomyxa) comprises five orders of parasites of aquatic invertebrates: Haplosporida, Paramyxida, Mikrocytida, Paradinida and Claustrosporida. The group includes a number of species known for their devastating effects as pathogens of economically significant bivalve species, the most notorious of which are the oyster pathogens Marteilia refringens (Paramyxida), Bonamia ostreae (Haplosporida) and Mikrocytos mackini (Mikrocytida). Due to its significance to aquaculture, interest in the group is growing, and a number of recent environmental DNA (eDNA)-based studies indicate vast, uncharacterised diversity within the class. Traditionally species discovery and description has relied on microscopy studies of infected invertebrate tissue, and as such our understanding of the group is biased towards pathogens of commercially exploited or easily studied invertebrates, with comparatively little known about the diversity of Ascetosporea outside such hosts. Many species, particularly those infecting less “important” hosts, have not been molecularly characterised, hindering phylogenetic studies and limiting our understanding of the relationship between morphology and phylogeny. This study uses primer-based screens of a wide range of environmental and invertebrate-associated samples to explore the diversity, distribution and host range of the four best-known ascetosporean orders, and in doing so links sequence data to described species, and allows the molecular and morphological characterisation of novel haplosporidian species in bivalve hosts. PCR screens also uncovered a large radiation of novel sequence types within Paradinida, in littoral and coastal environmental samples, and demonstrated the abundance and diversity of haplosporidians in freshwater and terrestrial sample types. This study also focuses on the development and application of sensitive complementary molecular and microscopy-based methods for the detection of parasite life-stages in host tissues, and consideration given to the role of molecular methods in facilitating rapid, accurate diagnosis of pathogens in important hosts.