Immunogenetics of Japanese frogs

世界のカエルの中であぶない「カエルツボカビ症」と言う真菌病（しんきんびょう）があります。ツボカビはカエルのはだをきずつけ入ります。たくさんのカエルのこたいすうが少なくなっています。ただし、東アジアのカエルはツボカビに耐性があります。ぼくのけんきゅうは、「なぜ日本のカエルはツボカビに耐性があるのか？」とさがしています。

Frogs are large and diverse group of amphibians which are cold blooded (ectothermic) tetrapods

The developing frog (tadpole) lives in an aquatic environment, while the adult frog lives in a semi-aquatic/ terrestrial environment. There is a possibility that the pathogens in the water and land may vary thus a developing individual may face a change in pathogens during it's lifespan.

As part of my research, I am interested to study the immune response of frogs, including

• which part of the life cycle does the adaptive immune system (antibodies, cell-mediate immunity, memory response) start working?
• immune association with diseases such as the deadly chytrid fungus.

My research will focus on the genetic basis of frog immunity, using Japanese native frog species as model organisms. Some immune genes I plan to study are those that code for MHC (major histocompatibility complex) and antimicrobial peptides (AMPs).

What is MHC?

Chytrid disease (Chytridiomycosis)

Source: PHOTOGRAPH BY JOEL SARTORE, NATIONAL GEOGRAPHIC

• A serious worldwide disease of amphibians
• Caused by chytrid fungus Batrachochytrium dendrobatidis (Bd)
  • First described by Berger et al (1998)
  • Linked to population declines and species extinctions
  • Higher Infection at cooler temperatures
  • Some strain diversity --> Globally emerging recombinant lineage (‘Bd-GPL’)

Chytrid life cycle

Source: Brian Gratwicke

• The skin is quite important in frogs
• Keratin is a major component of the skin
• Bd causes hyperplasia and hyperkeratosis, which causes abnormal electrolyte levels, and leads to death

Are all frogs susceptible to Bd infection?

No, there are frogs known to be carriers (infected but not susceptible/dead) and some species, populations or even indiiduals of specific genotypes that are considered resistant.

Carriers

African clawed frog (Xenopus laevis)

(c) Quintin Lau

American bullfrog (Lithobates/Rana catesbeiana)

Both alien species naturalised in Japan

Resistant (?, research is ongoing)

Specific populations?

Specific MHC genotypes?

East Asian frog species from South Korea and Japan?

So far, there are no reports of Bd-related infection or death in Japanese endemic frogs. One possible reason is that the Bd fungus may have originated in Asia, and thus East Asian frogs had a long time to build immune response to the deadly fungus.

In my research, I speculate that immune genes in 'resistant' Japanese frogs show some evidence of selection or 'adaptation'

Which Japanese frog species am I studying?

First, I studied three common Ranidae frog species カエルの三つの種：

Rana japonica Japanese brown frog 日本アカカエル Rana ornativentris Montane's brown frog ヤマアカガエル Rana tagoi tagoi Tago's brown frog タゴガエル

Study 1: MHC class I genes

Study 2: Transcriptome study + MHC class II genes

Study 3: Toll-like receptors