Abstract Global warming is causing ice retreat in glaciers
worldwide, most visibly over the last few decades in some
areas of the planet. One of the most affected areas is the region
of Tierra del Fuego (southern South America). Vascular plant
recolonisation of recently deglaciated areas in this region is
initiated by Gunnera magellanica, which forms symbiotic
associations with the cyanobacterial genus Nostoc, a trait that
likely confers advantages in this colonisation process. This
symbiotic association in the genus Gunnera is notable as it
represents the only known symbiotic relationship between
angiosperms and cyanobacteria. The aim of this work was to
study the genetic diversity of the Nostoc symbionts in Gunnera
at three different, nested scale levels: specimen, population and
region. Three different genomic regions were examined in the
study: a fragment of the small subunit ribosomal RNA gene
(16S), the RuBisCO large subunit gene coupled with its promoter
sequence and a chaperon-like protein (rbcLX) and the
ribosomal internal transcribed spacer (ITS) region. The identity
of Nostoc as the symbiont was confirmed in all the infected
rhizome tissue analysed. Strains isolated in the present study
were closely related to strains known to form symbioses with
other organisms, such as lichen-forming fungi or bryophytes.
We found 12 unique haplotypes in the 16S rRNA (small
subunit) region analysis, 19 unique haplotypes in the ITS
region analysis and 57 in the RuBisCO proteins region
(rbcLX). No genetic variability was found among Nostoc
symbionts within a single host plant while Nostoc populations
among different host plants within a given sampling site revealed
major differences. Noteworthy, interpopulation variation
was also shown between recently deglaciated soils and
more ancient ones, between eastern and western sites and
between northern and southern slopes of Cordillera Darwin.
The cell structure of the symbiotic relationship was observed
with low-temperature scanning electron microscopy, showing
changes in morphology of both cyanobiont cells (differentiate
more heterocysts) and plant cells (increased size).
Developmental stages of the symbiosis, including cell walls
and membranes and EPS matrix states, were also observed.