From ice age to present: Clonal expansion and genetic variation in the world's oldest conifer stand

Clonality is widespread among plants, yet the processes governing genetic diversity within the most ancient clones remains understudied. We investigated these dynamics using a reputed 10,500-year-old clonal stand of one of the world's longest-lived tree species, the paleoendemic conifer Lagarostrobos franklinii at Mt Read in Western Tasmania.

To quantify the clone's spatial extent and map its underlying genetic structure and gain insights into its origin, we used unmanned aerial vehicle (UAV) photogrammetry and two complementary genetic data sets including nuclear simple sequence repeats (nSSRs) and multiplexed intersimple sequence repeat genotyping by sequencing (MIG-seq), a reduced representation SNP discovery method.

The putative clonal stand at Mt Read has been severely impacted by fire, with living stems occupying approximately 5260 m², a ~65% reduction in the stand's previously estimated size. Contrary to the hypothesis that a single clone endures at Mt Read, both data sets revealed multiple, circumscribed patches, each with fixed multilocus genotypes (MLGs).

The Mt Read stand comprises three closely related MLGs, with the two largest clones being 3350 m² and 1861 m², respectively, they remain some of the largest conifer clones known. Although extremely low allelic diversity in the stand meant that nSSRs could not exclude a somatic origin for these MLGs, higher resolution MIG-seq markers indicate an inbreeding origin. Overall, this study demonstrates how ancient clones previously regarded as single genets can harbor cryptic diversity. Accounting for this hidden structure is crucial to accurately estimate clone size and longevity and unravel their evolutionary history.