Complete genome sequence of a phage hyperparasite of Candidatus Xenohaliotis californiensis (Rickettsiales) - a pathogen of Haliotis spp (Gasteropoda)

Limited genomic signatures of population collapse in the critically endangered black abalone (Haliotis cracherodii)

The black abalone, Haliotis cracherodii, is a large, long-lived marine mollusc that inhabits rocky intertidal habitats along the coast of California and Mexico. In 1985, populations were impacted by a bacterial disease known as withering syndrome (WS) that wiped out >90% of individuals, leading to the closure of all U.S. black abalone fisheries since 1993. Current conservation strategies include restoring diminished populations by translocating healthy individuals. However, population collapse on this scale may have dramatically lowered genetic diversity and strengthened geographic differentiation, making translocation-based recovery contentious. Additionally, the current prevalence of WS remains unknown. To address these uncertainties, we sequenced and analysed the genomes of 133 black abalone individuals from across their present range. We observed no spatial genetic structure among black abalone, with the exception of a single chromosomal inversion that increases in frequency with latitude. Outside the inversion, genetic differentiation between sites is minimal and does not scale with either geographic distance or environmental dissimilarity. Genetic diversity appears uniformly high across the range. Demographic inference does indicate a severe population bottleneck beginning just 15 generations in the past, but this decline is short lived, with present-day size far exceeding the pre-bottleneck status quo. Finally, we find the bacterial agent of WS is equally present across the sampled range, but only in 10% of individuals. The lack of population genetic structure, uniform diversity and prevalence of WS bacteria indicates that translocation could be a valid and low-risk means of population restoration for black abalone species' recovery.

Withering syndrome induced gene expression changes and a de-novo transcriptome for the Pinto abalone, Haliotis kamtschatkana

In the abalone and Candidatus Xenohaliotis californiensis (Ca. Xc) system, the Ca. Xc bacterium infects abalone digestive tissues and leads to extreme starvation and a characteristic "withering" of the gastropod foot. First identified in black abalone in California after an El Niño event, withering syndrome (WS) has caused large declines in wild black and captive white abalone on the northeastern Pacific coast, but disease resistance levels are species-, and possibly population-specific. This study compared gene expression patterns in the digestive gland of Ca. Xc-exposed and unexposed (control) Pinto abalone (Haliotis kamtschatkana), a particularly susceptible species. Lab-induced Ca. Xc infections were followed over 7 months and RNAseq was used to identify differential gene expression. Exposed Pinto abalone showed distinct changes in expression of 68 genes at 3 and 7 months post-infection relative to those in control animals. Upregulation of an orexin-like receptor (which is involved in feeding signaling) and a zinc peptidase-like region (many amino peptidases are zinc peptidases) in animals infected for 7 months indicates that animals with Ca. Xc infection may be starving and upregulating processes associated with feeding and digestion. Other groups of differentially expressed genes (DEGs) were upregulated or downregulated across control and exposed individuals over the 7-month experiment, including DEG groups that likely correspond to early disease state and to general stress response of being held in captivity. No patterns emerged in genes known to be involved in molluscan immune response, despite this being an expectation during a 7-month infection; digestion-related genes and unannotated DEGs were identified as targets for future research on potential immune response to WS in abalone.

Differing responses of red abalone (Haliotis rufescens) and white abalone (H. sorenseni) to infection with phage-associated Candidatus Xenohaliotis californiensis

The Rickettsiales-like prokaryote and causative agent of Withering Syndrome (WS)-Candidatus Xenohaliotis californiensis (Ca. Xc)-decimated black abalone populations along the Pacific coast of North America. White abalone-Haliotis sorenseni-are also susceptible to WS and have become nearly extinct in the wild due to overfishing in the 1970s. Candidatus Xenohaliotis californiensis proliferates within epithelial cells of the abalone gastrointestinal tract and causes clinical signs of starvation. In 2012, evidence of a putative bacteriophage associated with Ca. Xc in red abalone-Haliotis rufescens-was described. Recently, histologic examination of animals with Ca. Xc infection in California abalone populations universally appear to have the phage-containing inclusions. In this study, we investigated the current virulence of Ca. Xc in red abalone and white abalone at different environmental temperatures. Using a comparative experimental design, we observed differences over time between the two abalone species in mortality, body condition, and bacterial load by quantitative real time PCR (qPCR). By day 251, all white abalone exposed to the current variant of Ca. Xc held in the warm water (18.5 °C) treatment died, while red abalone exposed to the same conditions had a mortality rate of only 10%, despite a relatively heavy bacterial burden as determined by qPCR of posterior esophagus tissue and histological assessment at the termination of the experiment. These data support the current status of Ca. Xc as less virulent in red abalone, and may provide correlative evidence of a protective phage interaction. However, white abalone appear to remain highly susceptible to this disease. These findings have important implications for implementation of a white abalone recovery program, particularly with respect to the thermal regimes of locations where captively-reared individuals will be outplanted.