The palaeoendemic conifer Pherosphaera hookeriana (Podocarpaceae) exhibits high genetic diversity despite Quaternary range contraction and post glacial bottlenecking

Glacial relict plants are often endangered because extant populations can
be small, geographically isolated and persist in suboptimal environments,
leading to increased clonality and reduced genetic diversity putting their
survival at further risk.  This study examines how restriction to
interglacial refugia has impacted the genetic diversity and structure of
the threatened Tasmanian palaeoendemic, Pherosphaera hookeriana W. Archer
bis.  This species is a poorly dispersed, dioecious conifer that,
having once been a major component of Last Glacial vegetation, is now
limited to 30 known populations.  Genetic diversity and structure
were assessed using fifteen nuclear and nine chloroplast SSRs in 23
populations representing the species’ entire range.  Changes in
distribution and abundance from the Last Glacial to present were
investigated by examining the fossil record, approximate Bayesian
computation (ABC) and species distribution modelling.  Despite
fossil and ABC based evidence for a postglacial bottleneck, species-level
genetic diversity (He = 0.56 and Ne = 2.86) exceeded that of some conifers
with far wider distributions.  Significant genetic structure (Fst
= 0.127, Jost’s D = 0.203) was present, with most populations dominated by
distinct nuclear SSR genetic clusters and having unique chloroplast
haplotypes.  Unexpectedly, clonality plays only a small role in
population level regeneration.  Genetic diversity has likely been
maintained due to dioecy, persistence in multiple parts of its range and
extant populations being directly descended from proximate glacial
populations.  Protecting populations from the mounting threat of
fire will remain crucial for the in situ conservation of P.
hookeriana.