Habitat fragmentation strongly restricts gene flow in endangered
ectomycorrhizal fungal populations: evidence from Rhizopogon
togasawarius, specific to Pseudotsuga japonica, across the
entire distribution range
Abstract
Habitat fragmentation is a major driver of genetic differentiation and
genetic diversity loss in endangered species due to restricted gene
flow, genetic drift, and inbreeding. However, the impact on
ectomycorrhizal (ECM) fungi remains unexplored, despite their critical
roles in forest ecosystems. In this study, we investigated the
population genetic structure of the ECM fungus Rhizopogon
togasawarius, which specifically colonizes Pseudotsuga japonica
across its entire distribution range. Both symbionts are found only in
small, fragmented forests in Japan; they are designated as endangered
species on the IUCN Red List. We assessed the population genetic
structure of 236 R. togasawarius specimens collected from five
remaining populations, distributed in two regions separated by a sea
channel. Microsatellite analyses using 20 loci revealed strong genetic
differentiation among populations (FST = 0.255),
even in the nearest population pair separated by a distance of only 8 km
(FST = 0.075), indicating extremely limited gene
flow between populations. Population history analyses implied that
population divergence occurred approximately 2200 generations ago
between the two regions, and nearly 1000 generations ago between the
nearest populations within the Shikoku region. Because of prolonged
genetic isolation, significant inbreeding was confirmed in four of five
populations, where effective population sizes became very small
(Ne = 9.0–58.0). Although evaluation of extinction risks for
microorganisms is challenging, our conservation genetic results
indicated that habitat fragmentation increases extinction risk through
population genetic mechanisms, as demonstrated in plants and animals,
and therefore should not be overlooked in biodiversity conservation
efforts.