Rapid ‘Ōhi‘a Death Research

The native ‘ōhi‘a tree is a keystone and culturally important species in Hawai‘i

‘Ōhi‘a (Metrosideros polymorpha) are a keystone species and the dominant native forest tree in Hawai‘i. They are pioneer species on new lava flows, and provide key habitat to many other native and endemic species. They also hold great significance in indigenous Hawaiian culture. The ‘ōhi‘a is the subject of many moʻolelo (Hawaiian legends), and its vibrant lehua blossoms are often used in making lei worn during hula.

The ‘ōhi‘a is the dominant forest tree, founder species on new lava fields, provides habitat to other native species, and is used in Hawaiian cultural practices, for example making lei po‘o

The Rapid ‘Ōhi‘a Death (ROD) disease has devastated ‘ōhi‘a forests

Hawai‘i’s forests are currently threatened by Rapid ‘Ōhi‘a Death (ROD), a disease of the native ‘ōhi‘a tree. ROD was first characterized in 2014 and has since devastated ‘ōhi‘a forests, with >90% mortality in some areas on Hawai‘i Island. Researchers found that ROD is caused by two novel, invasive fungal pathogen species, Ceratocystis lukuohia and Ceratocystis huliohia. Although much work has been done to characterize this unprecedented disease and develop practical methods for detection and prevention, we don’t yet understand how these two distinct pathogens evolved to infect the ‘ōhi‘a upon introduction to Hawai‘i.

Symptoms and consequences of Rapid ‘Ōhi‘a Death

Producing genomic resources for studying the evolutionary history of ROD in Hawai‘i

My work will characterize the evolutionary history of ROD through comparative genomic analyses. Over the years since 2014, my collaborators at the USDA-ARS in Hawai‘i have isolated and archived fungal samples from infected ‘ōhi‘a throughout the Hawaiian Islands. I will generate long-read genome sequencing datasets of these isolates, assemble high quality genomes, and investigate their diversity and evolutionary history. Understanding the evolutionary histories and invasion dynamics of these two pathogens will help to inform appropriate management strategies of ROD and increase preparedness for handling future diseases of the ‘ōhi‘a.

Map of the isolation location and year of ROD isolates archived by Dr. Lisa Keith and team at the USDA in Hilo, HI.

Progress so far

Thus far, I have optimized fungal culture, high molecular weight DNA extraction, and library preparation methods for the ROD species, and produced an initial set of long-read genome assemblies.

One of these has been published as a genome announcement, representing the first publically available long-read genome of the C. huliohia species. For more details and to see how I used this genome for the training of fellow graduate students, see this poster.

• Ceratocystis huliohia: isolate C25-5 genome announcement paper
• Ceratocystis huliohia: isolate C25-5 (GCA_054512535.1) on NCBI

My preliminary analyses of this initial set of genomes show that C. lukuohia has a larger genome size and greater repeat content than C. huliohia. Additionally, analysis of single nucleotide polymorphisms (SNPs) and larger scale structural variants (SVs) show greater diversity in C. huliohia than C. lukuohia. However, diversity analyses also show that further sequencing is necessary to fully capture the variation within both species.

Preliminary results: A) ROD isolate locations and dates for preliminary set of genomes. B) Preliminary genome set phylogeny, genome sizes, TE contents. C) Diversity (SNP and SV) rarefaction curves for both species, showing non-saturation.

Next steps

I plan to produce many more C. lukuohia and C. huliohia isolate genomes in order to capture the genetic diversity of these species and perform population-scale comparative analyses. I am looking forward to continuing my work on this project and bringing attention to the issue of ROD for the conservation of the native ‘ōhi‘a tree. Stay tuned!

Sources

Jacobi JD, Boehmer HJ, Fortini LB, Gon SMO III, Mertelmeyer L, Price J. 2024. ‘Ōhi‘a Lehua (Metrosideros polymorpha): a most resilient and persistent foundation species in Hawaiian forests. Pacific Science 77.

Keith LM, Hughes RF, Sugiyama LS, Heller WP, Bushe BC, Friday JB. 2015. First report of Ceratocystis wilt on ʻŌhiʻa (Metrosideros polymorpha). Plant Dis 99:1276.

Luiz BC, Giardina CP, Keith LM, Jacobs DF, Sniezko RA, Hughes MA, et al. A framework for establishing a rapid ‘Ōhi‘a death resistance program. New For. 2023;54: 637–660.

Anne Nakamoto, Lisa Keith, Qingyi Yu, Lionel Sugiyama, Xiaohua Wu, Blaine Luiz, MaryAnn Villalun, Jodie Jacobs, Russ Corbett-Detig, Ariana Cisneros, Harrison D. Heath, Cole Shanks, Faith Okamoto, Alexis Abigail Aroma Albura, Kyle Henricson, Yi Jun Lan, Henry Moore, William Seligmann, Yulia Zybina. Genome sequence of Ceratocystis huliohia, a fungal pathogen of the native ‘ōhi‘a tree in Hawai‘i. Microbiology Resource Announcements, 0:e00236-26.